structural foundation manual for low-rise buildings by atkinson

8/7/2019 Structural Foundation Manual for Low-Rise Buildings by Atkinson

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Contents

Preface viii2.7.4 Driven piles 5

Examples ix2.7.5 Driving precast piles 5

2.7.6 Test loading 5

Bibliography 6

1 Site investigations 1

1.1 Walk-over survey 1 3 Foundations in cohesive soils 6

1.2 Desk study 2 3.1 Introduction 6

1.3 Site investigation: field work 4 3.2 Settlements in cohesive soils 61.3.1 Trial pit logs 4 3.3 Consolidation settlement 6

1.3.2 Borehole record 43.3.1 Bearing capacity of cohesive soils 61.4 Site investigation procedure 4 3.3.2 Vertical stress distribution 7

1.4.1 Borehole logs 4 3.3.3 Construction problems on clay sites 71.4.2 Trial pit logs 4 3.3.4 Foundation" designs on clay soils 71.4.3 Groundwater 4 3.3.5 Settlements in clay soils 71.4.4 Standard penetration tests 4 3.4 Moisture movements 7

1.5 Interpretation of laboratory testing 9 3.4.1 Liquid limit test 761.5.1 Chemical tests 9 3.4.2 Plastic limit test 77

1.6 Solution features 11 Bibliography 79

1.6.1 Limestones 11

1.6.2 Chalk 11

1.6.3 Salt 13 4 Foundations in sands and gravels 81

.6.4 Gypsum 14 4.1 Classification of sands and gravels 81Case study 1.1 Investigation of former mining site, 4.1.1 Composite sands and gravels 81

Sheffield 14 4.1.2 Dilatant sands 82Bibliography 21 4.1.3 Calcareous sands 82

4.2 Relative densities of granular soils 82

4.2.1 Field density assessment 822 Foundation design 23 4.2.2 Visual observations 82

2.1 Introduction 23 4.2.3 Groundwater levels 83

2.1.1 Width of footing 23 4.2.4 The standard penetration test 83

2.1.2 Soft spots 23 4.2.5 Interpretation of SPT results 84

2.1.3 Stratum variation in excavation 23 4.2.6 Ultimate bearing capacities 84

2.1.4 Firm clays overlying soft strata 25 4.3 Construction problems in granular soils 86

2.1.5 Depth of footings 26 4.4 Foundation design in granular soils 87

2.2 Widened reinforced strip footings 29 4.5 Plate bearing tests 89

2.3 Reinforced strip footings on replacement 4.6 Piling into sands and gravel strata 89

granular fill 324.6.1 Bored piles 90

2.4 Trench fill foundations 33 4.6.2 Continuous flight auger piles 90

2.5 Raft foundations 34 4.6.3 Design of bored piles 90

2.6 Pad and pier foundation 44 4.6.4 Set calculations 92

2.6.1 Disused wells 44 4.6.5 Dynamic pile formula 92

2.7 Piled foundations 47 4.6.6 Re-drive tests 93

2.7.1 Bored piles 47 4.6.7 Base-driven steel tube piles 93

2.7.2 Design of a bored pile 48 4.6.8 Top-driven steel piles 93

2.7.3 Design of bored and driven piles 48 Bibliography 94

v



Contents

95 7.2.2 Additional weight of regrade fillBuilding in mining localities

7.2.3 Changes in the groundwater level or5.1 Coal mining, past and present 95

surface runoff5.2 Coal shafts 96

7.2.4 Excavations for deep drainage5.3 Shallow mineworkings 97

7.2.5 Removal of trees and vegetation5.4 Drilling investigations 99

7.2.6 Split-level housing5.5 Stabilizing old workings 99

7.3 Retaining systems5.5.1 Collapsed workings 99

7.3.1 Gravity type retaining systems5.5.2 Special conditions 100

7.3.2 Cantilever walls: reinforced concrete5.6 Foundations in areas with shallow workings 100 or brickwork5.7 Active mining 100

7.3.3 Gabions, crib walling, reinforced earth5.8 Future mining 100

7.3.4 Steel sheet piling5.9 Mitigating the effects of mining subsidence 101

7.4 Designing retaining walls5.9.1 Longwall mining (advancing system) 101

7.4.1 Active pressure on walls5.9.2 Designing buildings for future mining

7.4.2 Surcharge loadingsubsidence 101

7.4.3 Passive resistance (granular soils)5.9.3 CLASP system of construction 102

7.5 Cantilevered retaining walls5.9.4 Mining rafts 103

7.5.1 Mass brick or block walls5.9.5 Irregular-shaped units 104

7.5.2 Reinforced cavity walls5.9.6 Designing strip footings in active

7.5.3 Pocket-type wallsmining areas 107

7.6 Damp-proofing to retaining walls5.9.7 Movement joints 109

7.6.1 Type A structures: tanked protection

Bibliography 112 7.6.2 Type C structures: drained cavityconstruction

Bibliography6 Sites with trees 113

6.1 Foundation design 113

Building on filled ground.1.1 Climatic variation 119 8

6.1.2 Distances between trees and8.1 Opencast coal workings

foundations 1198.2 Foundations

6.1.3 Foundation depths related to8.2.1 Stiff raft foundations

proposed tree and shrub planting 1218.2.2 Piled foundations

6.1.4 Measurement of foundation depths 1218.3 Suspended ground-floor construction

6.2 Building on wooded sites 1228.4 Compaction of fills to an engineered

"

6.2.1 Piled foundations 122specification

6.2.2 Deep trench-fill concrete foundations 123 8.4.1 Procedure6.2.3 Deep strip footings with loose stone

8.4.2 Site testing before backfillingbackfill 124

8.4.3 Foundations6.2.4 Stiff raft foundations on a thick

8.4.4 Roads and drainagecushion of granular fill 124

8.4.5 Groundwater6.2.5 Deep pad and stem foundations 125

8.5 Ground improvement techniques6.3 Precautions to take when there is evidence of

8.5.1 Dynamic consolidationclay desiccation 125

8.5.2 Surcharge loading6.3.1 Suspended floors 126

8.6 Compaction of structural fills6.3.2 Drainage and services 126

8.6.1 Materials specification6.3.3 Protection to drainage 126

8.6.2 Definitions6.3.4 Precautions against clay heave 127

8.6.3 Suitable fillmaterials6.4 Foundations in granular strata overlying

8.6.4 Unsuitable materials

shrinkage clays 134 8.6.5 CompactionBibliography 136

8.6.6 Testing on site

Bibliography

7 Developing on sloping sites 137

7.1 Stability of slopes 1379 Ground improvement

Case study 7.1 Sloping site with clay fills over

9.1 Vibro-compaction techniquesoulder clay 141

9.1.1 Types of treatment.2 Developing on sloping sites 146

9.1.2 Ground conditions.2.1 Additional weight of dwellings 146

vi



Content

147 9.1.3 Engineering supervision 190 11.4.1 Asbestos 23

9.1.4 Design of vibro-compaction stone 11.4.2 Scrap yards 23

columns 193 11.4.3 Sewage treatment works 23

9.1.5. Foundations on vibro-cornpaction sites 197 11.4.4 Timber manufacturing and timber

9.2 Dynamic consolidation 198 treatment works 23

9.2.1 Testing 201 11.4.5 Railway land 23

9.3 Preloading using surcharge materials 201 11.4.6 Petrol stations and garage sites 23

9.4 Improving soils by chemical or grout injection 202 11.4.7 Gasworks sites 23

Bibliography 203 11.4.8 Metal smelting works 23

11.4.9 Old mineral workings 2311.4.10 Toxicological effects of

10 Building up to existing buildings 205 contaminants 23

10.1 Site investigation 20511.5 Landfill sites 23

10.2 Foundation types 20511.5.1 Gas migration 24

10.3 Underpinning 21211.5.2 Gas monitoring 24

11.5.3 Carbon dioxide 2410.3.1 Beam and pad solution 216

11.5.4 External measures 2410.3.2 Pile and needle beam solution 216

11.6 Desk study 24Case study 10.1 Investigation and underpinning

of detached house on made11.6.1 Local geological study 24

ground, York 21811.6.2 Industrial history of the site 24

Case study 10.2 Differential settlement, Leyburn 21911.6.3 Mining investigation 24

11.6.4 Site reconnaissance 24Case study 10.3 House on made ground, Beverley 222

11.7 Site investigation 24Case study 10.4 House founded on sloping

rock formation, Scarborough 22311.7.1 Trial pits 24

10.4 Shoring 22611.7.2 Boreholes 24

Bibliography 22711.7.3 Testing for toxic gases 24

11.7.4 Chemical analysis 24

11.7.5 Safety 24

11 Contaminated land 22911.7.6 Conclusion 24

Case study 11.1 246

ILl Contaminated sites 229 Bibliography 248

11.2 UK policy on contaminated land 229

11.3 Risk assessment 230

11.4 Industrial processes 233 Index 251. .

147

147

147

148

148

148

148149

150

150

150

151

152

152

152

156

156

169

169

172

174

175

177

178

178

178

179

181

181181

182

182

182

182

182

182

182

183

183

183

183

183184

184

185

186

186

187

vi

Site investigations

SITE SURVEYS LTD BOREHOLE RECORD BOREHOLE

Site Address: V<lle Avenue. LocationNUMBER

Wa.keFie1d . , W Yorks. Type of Dri ll ing: Qol-ru-~ i P€("cvs$./ve 5BORING SAMPLING RECORD OF STRATA Sheet No

1

DATEASING WA De~th Type NYC Nt Depth Level

Key Descr ip tion of stratam) TER (m % ROD. (m) (m)

0·00 10Q-o ADD

l 2 S 2 2 f ' i1o.de. 6roIJIld - Bla.c.k Ash

~"- FIrM. MediUM browl\. SQ.lldj~

2.·0 LI ~. " . s.~ CLAY w d - h ~- par 8s--.--

4-'cm~

...=-

.....I...I:Jk.oI- fo ~dill»1 e r ~ - brc11.Jl/l-

l- .......... F i 7Le t- o ~d l . u l l ' l (lro. . i .ne d. . ,......

f1a.53j . sANO~ONE:. .. ......Oo:.a. '$10 nal: ~hin b<Mc: is o r...... ...

.....

C l ~ o f ~sl-roJ-a.

r-.... .

f'''4s).....

B~ -.... ..... ..... ..... ..... .

WoJ-w ......... .

rE

'''jress I-q·oo...............

-... ... '"........ ..... ...... ..... .

\0'00. -" ..... .

f==MediUM fo DarK Er~-bro"'A

F==U"o/\ Sh~..t\ed !;~ S~i.~tJl).sTONE wiHI. iA!:erb Cicled

F= siJ1-s I-oi\e ba.t1dU!~F=

r-'?"50 r==

_~F==

MediUM to cia ..rk: ere3 -brow!\..!==i=== lool\. sl-cWt.ed ~~ s V i a . A ~

I- !== l"v\uOSTONE u . i c H t "'-f-eree.Clde..d -1=== 1 ? 1 c u : ~ c=J!:j .sl1c:d.e b:v1ds.t===i== -F==

I- !== .,

t===17·00 i==

i== Moo. i l /Vl to dark: (rE?j evld1==

r-" i=== bla .c .k r fodl-.j sVJ~ r l4 .ud.sJoAe _

F== uJi . I -~ bq_" d S o o F b a .ck

i=== co~ sho.le.s .

r=s=r

'2.1·0 f= t = I '1 Q ~ Borehol-e

Remarks and groundwater observations

Grool\d wal-er i¥l~re.ss 0} ~·OM. °3 .

Fig. 1.9 Typical borehole log.

6



SOILS SEDIMENTARY ROCKS

Site investigation procedure

Made ground Chalk

Limestone

Conglomerate

Breccia

Sandstone

Siltstone

Mudstone

Shale

Coal

Pyroclastic(volcanic ash)

Gypsum.

Rocksalt etc.

IGNEOUS ROCKS

Topsoil

Boulders and cobbles

I : . : . : 0~ · I_ T

Gravels

Sands

Silts

1 = - - = - = = 1 Clay

Peat

Note: Composite soil typeswil l be identif ied by

combined symbolse.g.

Dilty sand

METAMORPHIC ROCKS

~ Coarse-grained

~ Medium-grained

§Fine-grained

Fig. 1.10 Key to soil symbols in trial pit logs.

1 · · · · · · ·' 1..... - _ -o e ••••••

I : : : ; 1

I I

I ~ t t + t l . Coarse-grained

I'_·+~L·+ ...I

· . Medium-grained

IV"V.JV V.vV I

. Fine-grained

Site investigations

LABORATORY RESULTS: SYMBOLS

B Bulk sample: disturbed

Cohesion (KN/m2 )

Effective cohesion intercept (kN/m2 )

California Bearing Ratio

Jar sample: disturbed

Laboratory vane test

Coefficient of volume decrease (m2JkN)

Standard penetration test value (blows

/300mm)

Acidity / alkalini ty index

Plast icity index (%)

c

c'

CBR

D

LVT

mv

N

pH

P I

Soil strengths. Soil tests used on cohesive strata fall into

two main categories. The undrained triaxial test is carried

out on a soil which is stressed under conditions such that no

changes occur in the moisture content. This reproduces the

Table 1.1. SPT values for cohesive soils

Consistency Undrained shear

strength

(kN/m2)

Very stiff > 150

Stiff 75-150

Firm 40-75

Soft 20-40

Very soft <20

Nvalue

> 20

10-20

4-10

2--4

<2

Table 1.2. SPT values for sands and gravels

Couistency Nvalue

Very dense

Dense

Medium dense or compact

Loose

Very loose

> 50

30-50

10-30

4-10

<4

Table 1.3. Field assessment of soil s trengths

Consistency Method of testing Approximate undrained

of soil shear strength

(kN/m2)

Very soft Exudes between fingers <20

when squeezed in one's

handSoft Moulded by light finger 20--40

pressure

Firm Moulded by strong finger 40-80

pressure

Stiff Indented by thumb pressure 80-150

Very stiff Indented by thumbnail 150-300

Hard Difficult to indent with >300

thumbnail

8

Soluble sulphate content

Undisturbed 100 mm dia, samples

Undrained triaxial compression test

Water sample

Natural moisture content (%)

Liquid limit (%)

Plastic limit (%)

Bulk density (kg/m))

Dry density (kg/mJ)

Angle of shearing resistance (degrees)

Effective angle of shearing resistance (degrees)

Table 1.4. Typical ground bearing capaci ties

Maximum safe beari

(kN/m2) capacity

Types of rock and soil

10 700

Rocks

Igneous and gneissic rocks in sound

condition

Massively bedded limestones and hard

sandstones

Schists and slates

Hard shales, mudstones and soft

sandstones

Clay shales

Hard solid chalk

Thinly bedded limestones and sandstones

Heavily shattered rocks

Non-cohesive soils"

Compact , well-graded sands and

gravel-sand mixtures

Loose, well-graded sands and

gravel-sand mixtures

Compact uni form sands

Loose uniform sands

Cohesive soils"

Very st iff boulder clays and hard clays

with a shaly structure

St iff clays and sandy clays

Firm clays and sandy clays

Soft clays and si lts

Very soft clays and siltsd

Peats and made ground=

4300

3200

2200

1100

650

Dry Submerge

430-650 220-320

220-430 110-220

220--430 110-220

110-220 55-110

430-650

220--430

110-220

55-110

55-nil

a To be assessed a fter inspect ion

bWith granular soils the width of foundation is 10 be nOI le ss tha n 900 m

'Dry' means that the goundwater level is at a depth not less than 900 mmbe low the foundation ba se

C Cohes ive soi ls a re suscept ib le to long- leon conso lida tion

d To be determined a fter inves tiga tion

Source: BSI (1986) BS 8004.

conditions most likely to occur under the actu

foundations. The undrained test gives the apparent cohesio

Cu and the angle of shearing resistance I / I u . With saturate

non-fissured clays I / I u tends to zero and the apparen

free

ratio

values,30

non-

n using

hen the

classes.

d in the

9 relates

eness to

if

cement

ee

82 .1

SOIL SURVEYS LTDTRIAXIAL COMPRESSIONTEST RESULTS(QUICK UNDRAINED)

Client:

Location: Leeds Ave.

r s . c . J.;::.Q"Sf:.NCb~ Lb::L .

Date: 2.7'/' q27'+0OB N°·

.....(~.~()!.e. N .o .5 D ~.H 1... .Q .:§2 .M . OescripliM: S C M c l ( J C /~

A p.~ .. . .ohe; (9 . t. \ . 4 -5 kNj",~ A~I :e . . . . . f . . S .h~~ f < tJ s1 ~ .r q , . . . .':t.~...

Fig. 1.11 Mohr circle stress diagram.

1.6 SOLUTION FEATURES

Solution feature

Damage to structures can often occur because of severe

subsidence caused when certain types of strata below ground

are affected by water and become soluble. Materials such as

salt, gypsum, chalk and certain limestones can all be affected

in this way.

sink-holes. These sink-holes generally develop where joints

in the limestone intersect, and they can result in large open

galleries. The size of these galleries and sink-holes depends

on the geological structure and the existence of layers o

impervious strata .

1.6.2 Chalk

Limestones are generally very jointed and the action of

acidic ground waters can produce solution features known as

Chalk is a pure soft limestone, which usually contains

approximately 95% calcium carbonate. Solution features do

not generallyform underground in the form of caverns as the

chalk being softer than limestone collapses as it goes into

1.6.1 Limestones

11



Site investigations

1.6.4 Gypsum

Gypsum is more readily soluble than soft limestone, and

sink-holes and large caverns can therefore develop in thick

beds, especially when water is a dominant factor. The most

significant areas are in Ripon in Yorkshire and the surround-

ing areas north of Ripon where subsidence hollows have

been recorded along the outcrop of the gypsum beds within

the marl strata. Extensive geological mapping of these

solution features has been carried out by the British

Geological Survey. There is evidence that shows that a lot of

Investigate to provethe adequacy ofthe points of

structural support

Scope of hazard cannotbe fully defined

Design and constructspecial foundations

(address any residualhazard for externalareas and services)

subsidence took place many thousands of years ago,

there are areas where gypsum is still present and could b

problem in the future.

From the statistical evidence available it appears that

building in a hundred may be at risk in the Ripon area an

is considered prudent to provide stiff raft foundations in

floundering area designated by the British Geological Surv

Old subsidence hollows have often been filled with orga

silts and peats and in these situations the only solution i

use a piled raft with the piles taken below the gypsum.

assessment(geophysics,trial pitting,probing,

boring or drilling)

I Further work to I

L~I~e~e~~~...-----..___--,Design andconstructstandard

foundations(subject to

normalperformancecriteria)

Fig. 1.14 Decision flowchart for siteson chalk (Edmonds and Kirkwood, 1989).

Case study 1.1 Investigation of former mining site, Sheffield

INTRODUCTION

A large tract of land is to be offered for sale by treaty to a

major housebuilder and developer for future residential

development. The site is 11 km north of Sheffield city

centre, South Yorkshire, and the site location north of

Wortley Road isshown in Fig. LI5.

Preliminary enquiries with British Coal mining surveyors

have revealed that there may be unrecorded mine shafts and

possible shallow mine workings over part of the site. Other

14

shafts and spoil resulting from ironstone workings could a

be present on the site.A desk study report on the geological and past mini

situation has been commissioned by the developers. T

report is to outline the geological, geotechnical and p

mining problems that may be encountered in

development of the site. In addition the report shou

include a detailed soils investigation using trial pits a

recommendations as to the need for a borehole investigati

to establish the nature and extent of any shallow c

workings beneath the site.



ago, but

be a

that one

and it

ns in the

Survey.

organic

tion is to

ld also

t mining

ers. This

and past

in the

should

pits and

coal

Case study 1

Case study 1.1 (contd.)

RESEARCH SOURCES 2 . The British Coal Opencast Division at Burton upo

Trent;

3. Ordnance Survey County Series plans 1850 edition an

1905 edition;

4. The County Series geological maps and memoi

including consultations with the British GeologicSurvey at Keyworth. Nottingham;

In compiling this report we have examined information and

records and made enquiries from the following sources:

1. British Coal Mining Surveyors at Technical Head-

quarters. Burton upon Trent to examine the shaft registerheld in the archives;

Fig. 1.15 Case study 1.1: site location and layout.

.1



Site investigations

Case study 1.1 (cantd.)

SITE GEOLOGY. Sheffield City Technical Services Dept;

6, Mining Records Office at Rawmarsh, Rotherham, South

Yorkshire;

7. Plans of abandoned mines and quarries contained in

the archives of the Health and Safety Executive,London;

8. Mineral Valuer District Offices; Sheffield.

The 1:10560 scale County Series geological map NZ282

published by the British Geological Survey shows most

the site to be overlain by deposits of boulder clay of glacorigin. The thickness of these drift deposits is not record

on the geological maps but from information gleaned fro

:::::::8:,:;;:;;..................

Coal

PF

Silkstone rock

Coal

Sandstone

Silkstone coal

Sandstone

Sandstone

Sandstone

Coal

Sandstone

Sandstone

Whinmoor coal

Coal

16

Fig. 1.16 Case study 1.1: generalized vertical section of strata below the superficial deposits. Unlabelled areas are shales or mudstones.




Case study 1

previoussite investigations in the site locality, the boulder

clay has been proven up to depths of 8 m beneath the

existingsurfacelevels.

Research into the history of the site, described later in

this report, also established the existence of previous

colliery spoil heaps within the site boundaries. There is a

possibility therefore that some areas of the site could be

overlain by deposits of colliery spoil. In addition there may

be backfilledpits on site as a result of ironstone extraction;

2 82 SE

st of

acial

from

these were indicated on the 1905Ordnance Survey ma

The old Ordnance plans showthe Thorncliffe Ironworks

be close to the northern boundaryof the site. The bounda

is in fact marked by the old mineral line which served t

ironworksand isnowdisused.A local shaft isnoted to ha

been worked close to the ironworks from Thornclif

Colliery and a generalized vertical section of the site

shown in Figs 1.16 and 1.17 taken from the Thornclif

Colliery shaft records.

------

._-------

.....................

~ ~~ H ~ ~ ~ ~ E E E ~ ~ ~ [ E:: :::::::::::;:::::::........................................... ._- .:: :::::::::::::::::::....................... .:: ::::::::::::::::::::: ::;:::;::::::::::::................... _ ... .

Description of

strata

Bind

Descnption of strata fromBritish Geological Surveyof Mining Terms

~Brassband

Silkstone coal

Dirt

Silkstone coal

Spavin

Strongstone

Imperfectly laminatedMUDSTONESor any f inegrained rock

Iron pyrite

Soft shaley materialinterbedded with coal

Seatearth - coarse clayor mudstone withrootlets

Sandstone

Fig. 1.17 Case study 1.1:vertical section of strata at Thomcliffe colliery.

1

Site investigations


SOLID GEOLOGY

The published geologicalmaps show that the drift measures

are underlain by Middle and LowerCoal Measure strata of

the Carboniferous Age. These consist of sandsto

siltstones and mudstones with interspaced coal and fire

horizons. Several collieries existed close to the site, not

the Thorncliffe Colliery about 0.50 miles east of the

2·7M

(rial P i t : : No. 51

E

(3 ) Bcvlder c1~

Q ) SOIL

® CLAY

Sand4j topSOil

tO M

*5'O~E w

@ C~ / a.sh / bride fJ J

o " ' ILL ..: L-aose er't) ~ CI/ ld ask fill

® F I. .L: Loose lY"j ~ / o . . sk Ibicks ek

G) CLAY: 80vklu cia.LL darll brOL<ltt, sl1(f 5CU1~

Mai5t-, s " fP iM.e. c i. due to fill. OJ:ove

@ cLAY : Boolw ~ / daN< brow/!. I V€(3 sf-iff;

$ 1 i t J ~ m<Mt

'5~: BuLl: P a t K ® B . ®u q . . f i " o M . @

18

Fig. 1.18 Case study 1.1: trial pitlogs SI and S2.



Case study 1


fireclay

notably

site.

There was also an ironworks close to the site referred to as

the Thomcliffe Iron Works.

The Silkstone coal seam, average thickness 1.80 m, is

conjectured to outcrop south of Wortley Road with the

Whinmoor coal seam in excess of 40m below the site. Oth

workable coal seams are the Hard Bed coal seam (Ganister

the Coking coal seam and the Pot Clay coal seam. Howeve

it is considered that if these seams have been worked in t

Tri-o l Pi b N o . S3

)

® C.I~

C D FILL

@ eLAY: Baolrkr c~ r dark browll. I shff

sa.Yl~ wiHt . ahundcutt- 8 ( a . J . I e 1

TriaL P J .:: No. S'f-

1. T~

@ Top~aiL: SCU1~ EPpsoil

@ FILL louse 95: w(Jh ash. mist-ore:

@ cLAY 6aulckr c .la j - \/~ .s r- vf f / rna i l5r

Fig. 1.19 Case study 1.1:tr ial pit logs 53 and 54.

1



Site investigations

Case study 1.1 (cori td . )

past then any surface subsidence associated with the workings

will have long since ceased. British Coal have stated in their

mining report that no future mining isexpected to take place.

The coal seams are known to dip to the north at a

gradient varying between 1 in 5 and 1 in 15 and i

therefore considered that the Silkstone seam will be at s

a depth below the site that there will be sufficient rock co

over any possible workings.

1,101 Pib No. S5

W

1\-~_~)5M

~ = = = = = = = = = = = = = = = = = = = = ~ £1TopsoiL

2·30~

Bulk. froNt0 and: U4-

@ SOIL

C d ) FILL

@ CLAY

- r :o p 5 O " i.L / SCU1~

Le rose 8~ d~ ( l J ' " f c : ; { o. . .sk

BcrokW- c . .~ f ~IC brawl\.,

ver~ sl-iff wiJ-h. abUllda(rt

8rcweLs Q . ¥ I c L some coal

frOof:J~ts

Fig. 1.20 Case study 1.1:trial pit logs S5 and S6.

20




Bibliography

nd it is

at such

ckcover

Geological faults TRIAL PITS

The geological maps also show that the site locality is

traversedbynumerous geologicalfaults which have resulted

in localizedvariations in the stratification of the coal seams

in regard to their depth beneath the site and degree of dip.

The Silkstone Coal is present at depths which should

provide an adequate cover of competent rock over any

possibleold workings.

SITE HISTORY

The various editions of the Ordnance SurveyCounty Series

wereexamined. The 1850and 1905publications record the

positionof the old Thorncliffe Colliery northeast of the site,

and the Thorncliffe Iron Works.

The 1905Ordnance plans showseveral ironstone pits on

the site and an air shaft in the northwest comer.

The later Ordnance Survey plan also shows ponds

together with colliery spoil heaps in the northern

area adjacent to the mineral line. An inspection of the

site reveals that the spoil heaps have been removed or

regraded.

PAST MINING ACTIVITY

The available records showthat coalmining has taken place

beneath the site in several main seams. The depth of the

Silk~ne seam dips from approximately 16m south of

Wortley Road away to the north. It is likely that the

Silkstone coal seam has been mined by pillar and stall

methods in the past but it isconsidered that theypresent no

riskto stabilityon this site.

Researches ofmine records and topographical plans have

confirmed the positions of one air shaft within the site

area. The recorded position of this shaft has been

investigated and identified using a JCB backacting

excavator to carry out slit trenching under our supervision.

Detailed records of the shaft do not exist; it will require

capping off at rockhead.

Coalmining has ceased in the locality, the main collieries

having been closed. The possibility of future underground

mining for coal or any other mineral can reasonably bediscounted.

The trial pit investigation proved up to 1.50 m of collier

spoil and ironstone debris in various parts of the site

underlain by natural brown boulder clays. The maximum

thickness ofboulder clayrecorded in the trial pits was3.0m

There maybeother areas of the sitewhich are overlain b

colliery spoil and which have not been revealed in thi

investigation.

The trial pit records are included in the appendix to thi

report (see FigsU8-l.20).

CONCLUSIONS AND RECOMMENDATIONS

The site is underlain by superficial deposits which ar

variable in their thickness and lateral extent. These deposi

are generally natural boulder clays but parts of the site ar

shown to be overlain byapproximately 1.50m thickness o

colliery spoil and ironstone minerals. These fillscould be

result ofbell pit workingsfor ironstone.

In those areas of the 'site underlain directly by natura

boulder clays it is recommended that standard-width stri

footings be used for dwellings of 2-3 storeys. These

foundations should be reinforced with a nominal layer o

mesh type B283top and bottom.

Where colliery spoil is evident, fo~ndations will need t

be taken down below the s n into the natural boulder clayfor a minimum distance of300 mm.

Though the soluble sulphate content of the colliery spoi

waswithin the Class 1 range it isrecommended that a Clas

3 concrete mix be adopted owing to the lowpH values.Almortar belowground shouldusesulphate-resistingcement.

Ground floor construction should be a voided precas

beam and block system. .

We are of the opinion that the Silkstone coal seam is a

such a depth beneath the site that any abandoned mine

workingswithin these seams would not present a source o

potential surface instability.

It is therefore recommended that excavations during the

site strip are examined to ensure there are no other mine

shafts present on the site. Should any be encountered the

local British Coal mining surveyorshouldbe informed.

It is recommended that the old air shaft be capped off a

rockhead level. Should the depth to rockhead be excessiv

the shaft infill should be grouted down to rockhead prior to

capping at the surface.

BIBLIOGRAPHY

BRE(1991) BREDigest363: Concrete in sulphate-bearing soils

and groundwater, BuildingResearchEstablishment.

BSI (1992) BS 882: Part 2. Specification for aggregates from

natural sources for concrete, BritishStandardsInstitution.

BSI(1999) BS5930: Code of practice for site investigations, Britis

StandardsInstitution.

BSI (1986) BS 8004: British Standard code of practice for

foundations, BritishStandardsInstitution.

21



Site investigations

BSI (1990) BS 1377: Methods of testing for civil engineering

purposes, British Standards Institution.

Edmonds, C.N. and Kirkwood, J.P. (1989) Suggested approach to

ground investigation and the determination of suitable substructure

solutions for sites underlain by chalk. Proc. International Chalk

Symposium, paper 12, Thomas Telford, London.

Joyce, M.D. (1980) Site Investigation Practice, E. & F.N. Spon,

London.

22

NHBC (1977) NHBC Foundation Manual: Preventing Found

Failures in New Buildings, National House-Building Co

London (now rewritten as NHBC Standards Chapter 4.1)

Tomlinson, M.J. (1980) Foundation Design and Construction

edn, Pitman.



Foundation design

225mmminimumthickness

.,f'\' / ' 1 '

Widened reinforcedstrip footing.1.0 mwideC.25P Concrete

Well-compactedstone fill

II)

II >.~l'tl>

-5c.

~

6 . • ~. A.

Main wirestransverse

High-yield .I(

fabric mesh

./40 mm cover

J . , :. . •. . .Width varies and is dependent on

the ground strength

Fig. 2.1 Widened reinforced strip footing.

20 mm chipboardPolythene vapour barrier (500 g)

H Tfabric

Fig. 2.2 Pseudo-raft foundation.

Where rock is encountered it is essential that the house is

placed wholly on the rock stratum. If the rock stratum cannot

be excavated at a reasonable depth, i.e. less than 3 m, then a

different foundation solution should be considered. Figure

2.3 indicates the use of a raft slab on a thick cushion of

crushed stone fill. Alternatively, piles and ring beams o

and pad with ring beams are the usual options worth

sidering when these conditions are encountered. The u

concrete manhole rings to form mass concrete piers is a

tical, economic and safe construction method and, w

24



.

.

s or pier

rth con-

e use of

a prac-

where

Substantiatthickness of consolidated granularf il l placed in 200 mm layers

Introduction

~::::=.._!..----;----_ -__-_--. 1 - '_ - = - - - _ - - - _ . _ ~ -

___!--------;---- --\.-'.c--------------'-----;r 1 Rock stratum

Maximum depth of clay stratumover rock to be no morethan twice the thicknessof granular fill cushionto limit differential settlement

Fig. 2.3 Sloping rock formation.

Note:All foundations to. be placed on similar stratum

Quarry backfill /

\\

\ \Fig. 2.4 Pad and stem foundation.

only a few dwellings are affected, avoids the expense of

piling and avoids delays in the construction programme(Fig. 2.4).

2.1.4 Firm clays overlying soft strata

Avoid deep footings where a firm clay stratum overlies a soft

stratum which reduces in strength with depth. Where footings

have to be deepened to cater for existing trees or existing

deep drainage then it is preferable to adopt a pseudo-raft

foundation on a thick cushion of granular fill (Fig. 2 . 5 ) . If a

strip footing is deepened it is important to check that the

footing load does not overstress the softer strata at depth if

excessive settlements are to be avoided. In such ground con-

ditions the width of the footing should be kept as narrow as

possible so keeping the pressure bulb within the firm stratum.

25



Foundation design

Foundation width B

Excavation = 2 x B minimum

Well-consolidatedgranular fill placedin200 mm thicklayers and given4-6 passes perlayer with avibratory roller

Fig. 2.8 Double reinforced footing on consolidated granular fill.

.:=> •. • A·,.- . Made ground or .

,weak strata _-

/ > .

(a)

Firm natural

L ..........,r-7T--77.T:m:::!.V stratum for at~ ~, ~ least 1.50 times

Concrete to be a the foundationdense mix if fills widthcontain sulphates orground is acidic

oIDIZ

~

--oH=:foo_1-50-75 mm .Clayrnasterlow-density

_polystyrene _

(11kg/~

Desiccated-clays -

~

E P, . .0 E'6 0

0It)

(b )

Fig. 2.9 Trench-fill foundations: (a) filled ground; (b) catering for existing trees and vegetation.

28

;' ,/

/ /to/fi 'W

.~ ~~... < & • ••

'~A·

I: vReinforced concrring beam

~. . .'. '" . J

• •

A, I..-

- .A_ .--- v

I'"[;-Concrete stubcolumn at4-5 m centres

1-,-

1 < ' 4, E E, . .

E ::J

E.6 0'2I I • • 'T--r • C') 'E

a .. ..'-~

Widened reinforced strip footings

ete

MassC20Pconcrete inmanhole

Dowel barlinking beam

, to pier

- .

. ' .1I . " .

Fig. 2.10 Pad and stem foundations: (a) piers formed from formwork; (b) piers formed from concrete-filled manhole rings. Size of pad

foundation to suit bearing capacity of ground.

• Pad and stem foundations. Designed using ground

beams spanning between piers formed from concrete-filled

manhole rings or formwork (Fig, 2.10).

• Bored or driven piling with reinforced ring beams at

ground level.

2.2 WIDENED REINFORCED STRIP FOOTINGS

Designed in accordance with BS 8110.

Unfactored line load including self-weight of footing =40 kN/m

Allowable bearing capacity of ground taken as 40 kN/m2

Therefore footing width = 40/40 = 1,0 m.

Net uplift pressure = 40 - (0.225 x 24) = 34.6 kN/m2

Load factor = 1.50

I . 50 3 60 0,3752

U tirnate transverse moment = 1. x 4. x -- = 3.65 kN m.2

Using the Design Chart No.1 (Fig. 2.11):

M 3.65 X 106- =0.005

bd2 feu - 103 X 1702 X 25

Therefore lever arm, la =0.95d

From BS 8110: Part 3:

feu = 25 N/mm d = 170 mm

3.65x106

A . = 0,87x460xO.95xI70

. . 0.13x103 x225Minimum percentage 292 mm 2

100

Use A393 fabric mesh (which has the required minimum

percentage Of steel in both directions) in bottom. The fin

ished foundation is shown in Fig. 2.13.

29

Foundation design

1.0

~

1 \

\- ,1 \\~

\1\

1\

\

i\~

\~

\r\,.\ .. .,

1\

\

\

.~

f\\

l/0.156Iv

0.95

N

E~ttl~Q)

>~

II 0.90

1J> <. . l i e :

~o+-'

~

E~ttl

f f i 0.85

~

0.80

0.05 0.10 0.15M

bcf2 fcu

When Mlbcf2fcu exceeds 0.156, compression steel is required. .A s = M/O.87fyktd. fy = 460 up to 16 mm dia; fy =425 over 16 mm dia;fy =485 wire

Fig. 2.11 Design Chart No.1: lever ann curve for limit state design.

30

o

C\ I

t;~WW/N) m J

o io 0 io.q- C"') C"') C\ I

oLt)

.\ \\\ '\

---_.---_ . _ -_._-- --------_.-. - _ - " _

---.--. ----- -_ ..'--' _ - - _ ....• ._ _ ... -_ .. - _-

\\\1 \ \ ' f \ \ -- ---- ----- f---- 1-----1----

'\\ \~ ,\

-r-r

'\~\

~ \\

~\\,~~~,

~c----

"--"

~

_ \

-,

------- --_-

----1------- - --

\\

-,

, , , , \

o

Lt)

o

CD io o

oCD.q-

oC\ I

Lt) .,;ST""" . .,

,r:J

"0. ,u. . . .< 2t::

'u. . . .:>."@ jt::';;;

N0

e n 1) Zq ;:.0 'g

0 . < : :

0 U0 C

T""" bI)

T"""- ; ; ;.,Q

r'I. . . .N~fi:

LO

o

oo



Foundation design

2.3 REINFORCED STRIP FOOTINGS ON be used placed on a thick mat of replacement stone

REPLACEMENT GRANULAR FILL (Fig. 2.8). The bulb of pressure remains within the

depth and the underlying soft clays are not stressed.

Where a weak stratum exists with allowable bearing capa- method can often be utilized instead of a raft foundation

cities between 25 and 50 kN/m2 then a reinforced footing can important to ensure that the thickness of stone fill belo

Table 2.1. Design Chart No 3. Reinforcement: areas of groups of bars

Diameter Area (mm-) for numbers of bars

(mm)2 3 4 5 6 7 8 9 10 II

6 28 57 85 113 142 170 198 226 255 283 311

8 50 101 151 201 252 302 352 402 453 503 553

10 79 157 236 314 392 471 550 628 707 785 864

12 II3 226 339 452 566 679 792 905 1020 1130 1240 1

16 201 402 603 804 1010 1210 1410 1610 1810 2010 2210 2

20 314 628 943 1260 1570 1890 2200 2510 2830 3140 3460 3

25 491 982 1470 1960 2450 2950 3440 3930 4420 4910 5400 5

32 804 1610 2410 3220 4020 4830 5630 6430 7240 8040 8850 9

40 1260 2510 3770 5030 6280 7540 8800 10100 11300 12600 13800 15

50 1960 3930 5890 7850 9820 ll800 13700 15700 17700 19600 21600 23

Diameter Area (mm-) for spacings in mm

(mm)50 75 100 125 150 175 200 250 3

6 566 377 283 226 189 162 142 II3

8 1010 671 503 402 335 287 252 210

10 1570 1050 785 628 523 449 393 314

12 2260 1510 1130 905 745 646 566 452

16 4020 2680 2010 1610 1340 1150 1010 804

20 6280 4190 3140 2510 2090 1800 1570 1260 1

25 9820 6550 4910 3930 3270 2810 2450 1960 1

32 16100 10700 8040 6430 5360 4600 4020 3220 26

40 25100 16800 12600 10100 8380 7180 6280 5030 41

50 39200 26200 19600 15700 13100 II 200 9800 7850 65

Diameter

""'cmm) 6 8 10 12 16 20 25 32 40 5

Mass

(kg/m) 0.222 0.395 0.616 0.888 1.579 2.466 3.854 6.313 9.864 15.4

Fig. 2.13 Widened reinforced footing.

32

footings remains fairly constant. Shear vane result

40kN/m2 to depths of 3.0 m.

At a depth of 450 mm the line load of 40 kN/m

imposes a ground pressure of 40/0.60 = 66 kN/m2.

increased line load of 60 kN/m run, it will be necessacheck the ground pressures at critical levels.

Checking at underside offooting

qm =60/0.60 = 100 kN/m2

This exceeds the allowable and will require the weak c

be replaced by consolidated granular fill. 100 kN/m2

ceptable for a well-compacted granular fill placed in 20

layers.

filling

e stone

d. This

It is

e low the

12

340

604

942

1360

2410

3770

5890

9650

15100

23600

300

94

168

262

377

670

1050

1640

2680

4190

6550

50

15.413

sults are

run2• For an

ssary to

k clays to

is ac-

200mm

,Trench fill foundations

Table 2.2. Design Chart No.4

Mesh sizes Cross-sectional

(nominal pitch Wire sizes area per Nominal

BS of wires) metre width mass per

reference square metre

Main Cross Main Cross Main Cross

(mrn) (mm) (mm) (mm) (mm) (mm) (kg)

Square mesh fabric

A393 200 200 10 10 393 393 6.16

A 252 200 200 8 8 252 252 3.95

A 193 200 200 7 7 193 193 3.02

A 142 200 200 6 6 142 142 2.22

A98 200 200 5 5 98.0 98.0 1.54

Structural mesh fabric

B 1131 100 200 12 8 1131 252 10.9

B 785 100 200 10 8 785 252 8.14

B503 100 200 8 8 503 252 5.93

B 385 100 200 7 7 385 193 4.53

B 283 100 200 6 7 283 193 3.73

B 196 100 200 5 7 196 193 3.05

Long mesh fabric

C785 100 400 10 6 785 70.8 6.72C 636 100 400 9 6 636 70.8 5.55

C503 100 400 8 5 503 49.0 4.34

C 385 100 400 7 5 385 49.0 3.41

C 283 100 400 6 5 283 49.0 2.61

Wrapping fabric

D49 100 100 2.5 2.5 49.1 49.1 0.770

Checking 1.35m below ground level

Vertical pressure factor = 0.386 (pressure bulb, Fig. 2.14)

5.70sllQall=3.Q

whe~s =undrained shear strength,

Il=plasticity index correction factor

All . 5.7x4OxO.8 4 60 Nt 2owable beanng pressure = 5. k m3

Actual pressure at 1.35m depth = 0.386 x 100 = 38.60 kNtm 2

<45.60

Use a 600 mm wide by 225 mm thick double reinforced

concrete footing reinforced with fabric mesh B283 top and

bottom and specify a concrete mix of C25P.

2.4 TRENCH FILL FOUNDATIONS

These are only suitable if a good bearing stratum is known tobe present at an economic and practical depth. The stratum

below the base of the trench fill must remain competent for

at least 1.5 times the foundation width.

This foundation is useful for sites where deep fills, thick

soft strata or peat beds overlie firmnatural strata. Due regard

must be taken of the possibility of soluble sulphates in the fill

materials,or in acidic materials such as peats. In these situ-

ations the concrete should have a minimum cement content

based on BRE Digest 363 (1991 edition).

This type of foundation is also useful for meeting th

requirements of NHBC Chapter 4.2 where houses are bui

close up to existing mature trees or landscape planting. I

these situations itmust be used with caution especially if th

ground Within and adjacent to the house walls is in a desic

cated state. If the clays are desiccated the followingrecommendations should be adopted to prevent foundation

heave should the clays rehydrate.

1. Provide a fully suspended voided ground-floor construc

tion using full-span timber joists orjoists on sleeper wall

on their own foundation. In deep foundations the use o

sleeper walls can be uneconomic. Alternatively a precas

beam and block floor can be used.

2. Provide a low-density polystyrene material of suitable

thickness on the internal face of the trench fillto all foun

dation walls affected by desiccation of clays. The density

of the polystyrene should be no greater than 11 kg/m''. A

product such as Claymaster or Claylite would be suitable

3. Provide a slip membrane by using I200g polythenesheets on the external face of the trench fill. If th

existing trees are within a distance less than twice th

foundation depth then a suitable thickness of low-density

polystyrene should be provided on both sides of th

trench fill. The density of the polystyrene should no

exceed 11 kg/m",

4. Ensure that the width of the trench fillis maintained with

out any projections occurring at higher levels. If the clay

rehydrate, any projections are going to be subjected to

3



Foundation design

0.1048

Fig. 2.14 Vertical pressure under a rectangular footing.

Polytheneslip membrane Claymaster

or similarlow-densitypolystyrene

If . Eoomin

' l I \ "(a )

Fig. 2.15 Trench-fill foundations: (a) correct; (b) incorrect.

34

uplift pressures; these have been known to separat

upper wider sections of concrete from the lower sec

(Fig. 2.15).

2.5 RAFT FOUNDATIONS

Raft foundations are most suitable for use in ground

ditions such as soft clays and filled ground, in old mareas which may have a potential for instability, and in a

mining localities. The raft foundation has that inherent

ness, not available in strip footings, which by virtue

load-spreading capacity is more resistant to differential s

ments.

If the fill is old, well layered and does not contain s

ficant voids or organic matter, a pseudo edge beam raf

be used placed on a layer of granular fill which should ex

under and beyond the raft edge for about 0.5 m (Fig. 2

The raft should have edge beams about 450 mm deep

sufficient width under the load-bearing walls to suit

ground conditions and be capable of spanning 3.0 m

cantilevering 1.5 m at the comers.If the fills are variable and contain mixed materials

advisable to remove the poor fill and proof-roll the forma

prior to either replacing the fill if it is suitable for recom

tion or using imported granular materials of a suitable g

ing. This method is obviously only economic when depth

replacement fill are less than 1.5 m.

This foundation method is most suitable for use on

where old cellars are encountered and the minimum th

ness of stone fill over the old cellar walls should be 1.0

prevent a hard spot from developing.

The pseudo edge beam raft is also suitable for sites w

trees and heavy vegetation have been removed below pote

Upward heave onsides and projectiqnscan separate concreteatX-X

6,

_ i x

t t..

x

\ Avoid ledges inexcavatedtrench

"'. 0'

~~ l>

(b )

rate the

sections

nd con-

mining

in active

stiff-

ue of its

al settle-

signi-

raft can

extend

. 2.16).

ep with

suit the

m and

it is

mation

le grad-

pths of

on si tes

n thick-

1.0 m to

es where

potential

Raft foundation

house plots and where the upper clays have become desiccated

and could be affected by trees which are to remain. NHBC

Chapter 4.2 recommends that the thickness of stone fill below

the underside of the raft slab should be 50% of the depths

required from their tables, up to a maximum of 1.0m. Upfill in

e x c e s s of 1.0 m is only permitted if the NHBC can be sat-

isfied that the correct fill is being used and it is being

compacted properly. This generally requires the builder to have

the works supervised by qualified engineers who may carry out

various tests to confirm the adequacy of the compaction.

There are situations where pseudo rafts can be placed at a

lower level and the poorer fills removed can be replaced on

top of the raft. This type of construction is often used on

sites where a band of peat is present below firm soils and the

peat is underlain by soft alluvium or soft silty clays. The ad-

vantages of this form of construction are that no materials

require to be taken off site, and that the amount of imported

stone is reduced. Where the formation is very soft it is good

practice to place a layer of geotextile fabric down prior to

placing the stone cushion (Fig. 2.17).

Rafts are also used in areas where old shallow mine

workings are present and require grouting up. Where the old

workings have rock cover of less than six times the seam

thickness and have been grouted up there is still a risk

residual subsidence; a raft is better able to withstand loca

ized crown hole subsidence.

Example 2.1 Semi-rigidraft design

Consider a typical semi-detached dwelling of brick and blocconstruction to be built of a raft foundation placed on to wea

clayey sands which have a high water table. The ground floor pla

isshown in Fig. 2.18.

DESIGN INFORMATION

Loading: to BS 6399

Reinforced concrete design in accordance with BS 8110

Imposed loads on roof: 0.75 kN/m2 for 25°pitch

Imposed loads on floors: 1.50 kN/m2

Concrete: 35.0 N/mm2 at 28 days

Reinforcement: high-yield bars and high-yield fabric.

Face of edgebeam given twocoats of bituminouspaint

Two course clear cavity

Finished floor level

Construction jointif desirable

Fig. 2.16 Stepped-edge beam raft.

TImber joist orprecast concrete floor

Fill level orconcrete

oversite levelto be at orabove outsideground level

600mm min.to clayformations

l ~ p . . . . . . : : . t . . - 'r--...o.<~___;,"::""'--,,",--,,_,,:,,'---~_....:.</ No dpm required

_)":";"";"';'::"~II:,-'-r-L---,Y unless forprotection fromsulphates in theground150 mm minimum

stone bed

Fig. 2.17 Voided raft.

3



Foundation design

The site investigation reveals that the upper stratum of shallow fills

overlies weak clayey sands. In view of the high water table it is

considered prudent to adopt a pseudo or semi-rigid raft foundation

with edge beams designed to span 3 m and cantilever 1.5 m.

Total unfactored floor loads = 2.40 kN/m2

Total factored floor loads = 3.66 kN/m2

LOADINGS

Roof

External walls

(kN/m2)

2.25

1.25

0.25

3.75 x lAO

Service loads

(kN/m2)Tiles 0.55

Battens and felt 0.05

Trusses 0.23

Ceiling board 0.15

Insulation 0.02

1.00 x lAO

Imposed loads

Snow 25' pitch 0.75

Storage 0.25

1.0 x 1.60

Factored loads

(kN/m2)

Brick

Block (100 mm)

Plaster

1.40 Spine walls (kitchen/Ioonge)

100 mm blockwork

Plaster

1.25

0.50

1.75 x lAO

1.60

Total unfactored roof load =2.0 kN/m2

Total factored roof load =3.0 kN/m2Staircase wall

First floor100mm blockwork

Plaster

1.25

0.50

1.75 x lAO

(kN/m2)

0.10

0.15

0.15

0.50

0.90 x lAO

Imposed loads = 1.50 kN/m2 x 1.60

Factored

loads

(kN/m2)

Boarding (22mm)

Joists 225 x 50

Ceiling board

Partitions (stud)

Party wall

2 skins blockwork 2.50

0.50

3.0 x lAO

1.26

2.40

2 coats plaster

8.28 (12.10)kNlrn

First floor joists

( ) EE

4.900m

Lounge

100mm

Fig. 2.18 Example 2.1: ground-floor plan. Figures in brackets are factored loads.

36

= 5

2

2

4



WALL LOADINGS

Ignore windows and doors.

Front walls

Unfactored

loads

(kN/m)

5.25Roof

1.00 x 10.52

1.0 x 10.52

Walls

3.75 x 5.202.45

Raft foundations

Staircase wall

First floor 0.90 x 4.~0 2.20 x 1.40 3.08

100 mm block 1.25 x 2.4 = 3.00 x 1.40 4.20

Plaster 0.50 x 2.4 1.20 x 1.40 1.6

Factored6.40 8.96

loads

(kN/m)

Gable wall

5.25 x 1.40 7.35

5.25 x 1.60 8.40

19.50 x 1.40

30.00

Rear walls

(kN/m)

2.45Roof

I00 X10.5

. 2 5.25 x 1.40

1.0 x 10.52

5.25 x 1.60

4.20

Firstfloor

0.90 x 3.402

1.50 x 3.40-2- 2.55 x 1.60

1.53 x 1.40

. .Walls

3.75 x 5.20 19.50 x 1.40

34.08

Party walls

(kN/m)

Average height 6.50 x 3 = 19.50 x 1.40

First floor 2 x 0.90 x 2.5 = 4.50 x 1.40

First floor 2 x 1.50 x 2.5 = 7.50 x 1.60

31.50

Spine wall

First floor 0.90 x 32 4(kN/m)

1.53 x 1.40

1.50x 3:/ 2.55 x 1.60

100mm block 1.25 x 2.4 = 3.00 x 1.40

Plaster 0.50 x 2.4 1.20 x 1.40

8.28

27.30

43.05

(kN/m)

7.35

8.40

2.14

4.08

27.30

49.27

(kN/m)

27.30

6.30

12.00

45.60

(kN/m)

2.14

4.08

4.20

1.68

12.10

3.75 x6.5

(kN/m)

= 24.37 x 1.40

(kN/m

34.12

Total load (unfactored)

Front wall: 30.00 x 12.75

Rear wall: 34.08 x 12.75

Gable wall: 24.37 x 2 x 10

Spine wall: 8.28 x 2 x 6

Stair wall: 6.40 x 2 x 6.50

Party wall: 31.50 x 6.5

Party wall: 19.50 x 3.50

Total

(kN)

383

435

487

100

83

= 205

68

1761

4.8kN/m2

= 1.6kN/m2

6.40kN/m2

Self-weight of raft = 0.2 x 24

Edge thickenings

Area of raft

Weight of raft

= 10.90 x 13.150 = 143m2

143 x 6.40 915 kN

1761+915143

round pressure 18.71 kN/m2

Plus imposed load 1.50

20.21 kN/m2

Maximum line load = 34.08 kN/m

Consider edge beam and raft slab as composite with slab actin

as a tie. If overall width istaken as 900 mm:

Line load = 34.08 kN/m

Edge beam = 6.85 kN/m

40.93 kN/m

Pressure under edge strip = 40.93 = 46 kN/m2 unfactored0.9

This is less than the allowable ground bearing pressure o

50 kN/m2.

With the pseudo raft the centroid of loading on the walls a

foundation level generally coincides with the centroid of the edg

strip. The raft slab is therefore not subjected to any torsiona

moments. In practice, the ground pressure under the edge strip i

more likely to approach a triangular distribution with a higher edg

pressure. The slab therefore needs to have sufficient reinforcemen

in the top section to cater for this rotating force. Its main function i

to act as a structural t ie while atthe same time enhancing the edg

beam stiffrless.Where tHied ground occurs it is prudent toprovide

layer of bottom reinforcement to enable the raft slab to span ove

any soft spots.

To produce a theoretical edge pressure twice the allowable

would result in an equivalent eccentricity o f 150 mm. The slab wil

therefore haveto cater for an ultimate moment of 0.15 x 40.93 x 1.

=9.20 kN/m.

37



Corner cantilever

. 58.180 X 1.502

6545 kNUltimate design moment = =. m

2.0

M

bd2feu

65.45 X 106

=0.025

450 X 4002 x35

I.factor = 0.95.

&

65.45 x 10

62

Therelore A.= 0.87 x 460 x 400 x 0.95 = 430 mm

. . 252x500 2A252 Fabric 500 mm wide = =126 mm

1000

Two T16 bars in top =402

528mm2

Provide A252 Fabric plus two T16 bars in top and bottom of edge

beams at comers.

Place raft on a 1200 gauge polythene dpm laid on sand-blinded

stone f i l l .

Toe design

Design as a cantilevered slab, to carry shear loading from outer leaf

of cavity wall. Take height of wall as 6.8 m using a 150 mm thick

toe reinforced with A252 fabric mesh in bottom.

Consider 1.0 m length.

Therefore b;= 1000 mm

d = 150 - (40 + 3.5) = 106.50 mm

Loading / metre run = 110 mm brickwork= 2.30 x 6.8 =·15.64 kN/m

Toe self-weight = 23.6 x 0.15 x 0.45 = 1.60 kN/m

Therefore gx = 17.24 kN/m

Therefore design shear V = 1.40 x 17.24 = 24.136 kN/m.

' "Shear stress

V 24.136x103V=-= 0.23 N/mm2

b; d 1000 x 106.50

Reference BS 8110 clause 3.5.5 (shear resistance of solid slabs) and

clause 3.4.5.2 (shear stress in beams).

Permissible concrete shear stress

Reference BS 8110 Table 3.9:

100/A.= loox252 =0.236

bv d 1000 x 106.5

v is less than Ve• therefore from BS 8110 Table 3.17 no shear steel isrequired.

Check toefor bending

Upward design load from OBP = 1.50 x 46 x 0.45 = 31.05 kN/m

Downward design load (wall) = 1.40 x 15.64 = 21.89 kN/m

(toe self-weight) = 1.40 x 1.60 2.24

24.13

Raft foundation

Bending

31.05 x 0.45 _ 21.89 x 0.10 _ 2.24 x 0.45

2.0 2.0

=6.98 - 2.819 - 0.504 3.65 kN m

Reinforcement

Vsing design formulae method (BS 8110 Cl. 3.4.4.4):

By inspection k is greater than 0.95. therefore z = 0.95 d.

Therefore

M 3.65x106 2

A.=--= =90mm0.87/yz 0.87x46OxO.95xl06.5

Minimum percentage =0.13% = 195mm2, therefore use A25

Fabric in bottom of toe.

Internal ground beam (party wall)

Factored wall line load

Self-weight of beam 23.6 x 0.45 x 0.80 x 1.40

45.60

11.90

57.50kN/m

Ground beam considered to act as a partially fixed element:

VI· desi 57.50x3.02

5175 ktimate esign moment = . Nm10

Vse two layers of A252 in slab with one layer of A252

thickening.

Consider A252 in bottom plus two T16. Therefore:

Area of fabric = 700 176 mm21000

Two T16 bars 402mm2

578mm2Total

d = 450 - (40 + 12) = 398.0 mm

Mb = 800 mm, z = 0.95 d . Therefore: As = ---

0.87/yz

Therefore: Design moment of resistance = As 0.87/y Z

M = 578 x 0.87 x 460 x 0.95 x 398.0 x 10-6= 87.46 kN m.

OK> 51.75

Consider both layers of A252 in slab when checking cantileve

mode.

Ultimate design moment = 57.50 x 1.502

= 64.68 kNm2.0

Moment of resistance of two layers ofmesh

= 176 x 0.87 x 460 x 0.95 x415 x 10-6= 27.76kN m

= 176x 0.87 x 460 x 0.93 x 300 x 10-6= 19.65 kN m

Moment of resistance of two T16 bars = 402 x 0.87 x460 xO.95

405 x 10-6= 61.89 kN m

Total moment of resistance = 109.30 kNm

Use one layer of A252 in bottom of raft thickening with two layer

in slab with two T16 in top and bottom of beams (Fig. 2.21).

3



Foundation design

Table 2.3. Summary of ground beam loadings

Wall location Ground beam type Maximum

factored

wall line load

(kN/m)

External front

wall

43.05

External rear

wall

900 mm wide edge beam

with two T16 bars in

top and bottom with A252

fabric mesh in toe beam.T10 links at 250 mrn centres

900 mm wide edge beam

with two T16 bars in

top and bottom with A252

fabric mesh in toe beams.

T10 links at 250 mm centres

800 mm wide slab thickening

with A252 in base and two

layers of A252 in 200 mm

slab with two T16 top and

bottom

Party wall

Gable wall 750 mm wide edge beam

with two T16 bars intop and bottom with A252

fabric mesh in toe beam

T10 links at 250 mm centres

600 mm wide slab thickening

with A252 in base and two

layers of A252 in 200 mm

slab

Spine wall

Staircase

wall

As spine wall 8.96

42.27

45.60

34.12

12.10

Example 2.2 Structural calculations for three-

Morey flats

DESIGN INFORMATION

Design codes

• BS 6399 Part ILoading

• BS 648 1964 Schedules of weights of building materials

• BS 5628 Structural use of masonry

• BS 8110 Structural use of concrete

Foundation concrete: Fco = 35 N/mm2

Reinforcement: fy 460 N/mm2

Tf = 1.50 for dead plus imposed loads.

The si te investigation has revealed that the upper firm clays are

underlain at about 2 m below ground by a 100 mm band.of peat. In

addition, the site is in an area known to be affected by subsidence

arising from solution features in gypsum strata at depth. In view of

this, a stiff edge beam raft will be used.

LOADINGS

Roof

Concrete tiles

Battens and felt

40

Self-weight of trusses at 600 mm centres

Insulation

Plasterboard

Imposed load:

Dead load

Snow

Ceiling

0.23

0.02

0.15

1.00

0.75

0.25

1.0

2.00k

(kN/m2)

2.25

1.20

0.15

0.50

4.10

1.50

5.60kN

(kN/m2)

2.0

2.0

0.25

4.25

(kN/m2)

2.0

0.50

2.50

(kN/m2)

4.20

0.30

4.50

3.00

7.50kN

(kN/m)

38.25

33.60

71.85

External wall B

Wall 4.25 x 7.80 33.15

(kN/m2) Roof 8.13 x 2.08.13

0.552.0

0.05 Total 41.28

Therefore total roof load taken as 1.00 + 1.0

Floors

150mm deep beam and block floor

50 mm concrete screed

Plasterboard

Stud partitions

Dead load

Imposed load

Therefore total floor load taken as 4.10 + 1.50 =

External walls

100mm blockwork

102mm brickwork

12.50 mm plaster

Internal walls

l00mm blockwork

Two coats of plaster

Staircases

175 rnm in-situ concrete

Finishes

Imposed load

Total

Loadings to walls

External wall A

Wall 4.25 x 9.0

3 x 5.60 x4.02.0

Three floors

Total



1.00

m2)

2

2.0

2.0

2.0

m2)

Raft foundation

275 mm----+-i~cavity 22 mm chipboard on

vapour barrier on38 mm polystyrene

A252 mesh

- , , . '

'----1200 gauge polythenedpm on sand-blinded crushedstone fill252

mesh 900mm

(a )

275mmblockworkparty wall

A252 mesh

.,. 0

A252mesh

300mm

mesh

800mm~--~-- __

(b)

Fig.2.21 Example 2.1: reinforcement details to raft foundation. (a) External edge beam; (b) party wall thickening.

3 x 7.500 x 2

2.0

Spine wall C Three landings 22.50

Wall 2.50 x 8.0 20.0

Three floors 3 x 5.60 x 7.058.80

2.0

Total 78.80

Staircase wall

Wall 4.25 x 9.0 (average height) 38.25

Three floors 3 x 5.60 x 325.20

2.0

Total 85.25

G R O U N D FLO O R BLO C K W OR K D E SIG N

Maximum line load on spine wall = 78.80kN/m. Using ION

crushing strength blocks with mortar designation (iii). /k =·8.2.

Design vertical resistance of wall / unit length:

n = {3tAW t'm

41

floor to

)

erleaf to

level.

ature and

t of some

3.5 N blockwork

Precast floors

000

7.0 N blockworkE

Provide 35 mm x 5 mm ggalvanized steel N

restraint straps at 2.0 m C f c

(I .' •

10 N blockwork

8.00 ill 250mm

fixity. Maximum line load = 85.25 kN/m. With load factor of 1.50

for dead plus imposed loads the ultimate line load equals 1.50 x

85.25 = 127.875 kNm. Therefore

Ultimate moment (sagging) = 127.8;g x42

204.60 kNm

Ul· . 127.875x22

2557 kNnmate moment (hoggmg) = = 2 -. 5 m

Try 600 mm x 600 mm edge beam, d = 600 - 40 - 10 = 550 mm,From Fig. 2.11:

k

= -255.75 x 10

6

_ 0.04- bd2 f e u 600 X 5502 x35

Therefore lever arm factor =0.945, and therefore z = 0.945 x 550 =519mm.

MA.==-

0.87/z

255.75 x 106

= 1231mm2 -

0.87x460x519

Therefore use four T20 high-yield bars top and bottom (1256 mm-),

Shear

V = 85.25 x 2.0 x 1.50 = = 255.7 kN

v = = . I . . = 255.75 x 103=0.775 N/mm2 looA, =0.38

bd 550x6QO 6QOx550Vc = 0 .45 N/mm2

Therefore minimum links required

s=250mm

A s v > MObs 0.40 x 600 x 250 = = 150 Il1Ill2

0.87fy 0.87x 460

Therefore minimum two leg TI0 links at 250mmcentres. A s v =157mm2

Maximum spacing of links = 0.75 d = = 0.75 x 550 = 412mm.

Raft foundations

F.g. 2.23 Example 2.2: typical cross-section ofthree-storey flats.

Eo<D

N

EE

~-

Slab design

Total load on raft (kN)

8 x 71.85 x 1.0 575

8 x 78.80 x 1.0 630

8 x 85.25 x 1.0 = 682

7.50 x 41.28 x 2.0:: 620

2507

Load spread = 8,73 x 8.315 = 72.60 m2• Therefore average bearing

pressure below slab- = = 2507/72.60 = = 34.50kN/m2• Assume slab

spans simply supported. Maximum span = 4.0 m,

(kN/m2)51.75

6.72

45.03

Therefore net uplift pressure =45.03 kN/m2• Try two-way spanning

slab, simply supported at edges.

Provide spine beam across centre of raft.

Ix=4.0m, ly =4.0

1 1 l s . , = a . . x n I}msy= a.yn Ii

Ultimate design load

Less weight of slab

1.50 x 34.50

1.40 x 0.20 x 24 :::

From Table 3.14:

Iy= 4.0 =1.0

i, 4.0a s . r "=0.062

as, =0.062

m s x =0.062x45.03x42=44;66kNm

msy= 0.062 x 45.03x 42= 44.66 le N m

Iy =4.0 = 1.33 a s x = = 0.093t, 3.0

asy =0.055

m~,= 0.093x 15.03x32 =37.69 kNm

msy =0.055 xA5.03x32=22.28 kNm

43



Foundation design

4T20D • ~.

Fig. 2.24 Example 2.2: raft edge detail.

Forfell = 35 Njmm? andfy = 460 N/mm2 .

M 44.66 x 106k=--= 0.053

bd2 fe u 1000 X 155x 155x35

From Fig. 2.11, z = 0.93. Therefore:

.. 44.66 X 106

A . 77 5 mm? 1m= 0.87x155xO.93x460

Use fabric A393 supplemented with T10 at 200mm centres in top

of slab both ways with A393 in bottom of slab.

For the 4.0m x 3.0 m bay, moment = 37.69 kN m. Therefore:

M 37.69 x 106

=0.044

bd2 fe u = 103 x 155x 155X 0.935

Therefore z=0.935

Th f A 37.69xW6

650 mm'ere ore s = 0.87x460x155xO.935

Use fabric A393 supplemented with TIO at 300mm centres in top

in both directions with A393 in bottom of slab.

2.6 PADANDPIERFOUNDATION

This type of foundation (Fig. 2.25) can be used in situations

where piling is being considered. If only one or two dwell-

ings are affected the cost of piling can be prohibitive because

of the initial cost of getting the piling rig to the site. It is used

in situations where very soft clays, peat and fill materials

overlie firm or stiff strata at depths up to 3-4 m. On sites

where foundations are on rock strata and a deep face is

44

E

1LEEoIi)

.r250mm

150 mm min well-consolidatedgranular fill

encountered because of past quarrying or geological faultin

depths of up to 6 m are still more economic than piling an

construction delays can be reduced.

The piers can often be constructed using manhole rin

sections placed on a concrete pad foundation and filled wi

mass concrete. The top section of the pier can be reinforce

to form a connection for the reinforced concrete ring beamplaced at or close to ground level.

This method of construction can also be used where exis

ing drainage is too close to a proposed wall foundation. In

cases the pad foundations must be wholly on similar bearin

strata.

2.6.1 Disused wells

Quite often old disused wells are encountered on housin

sites, usually during excavation for the foundations. Wh

such wells are found it is wise to make the well safe witho

altering the water source which supplies the well. Filling

well with. mass concrete is not a recommended solution:could be very expensive and changes in the groundwate

regime could occur. The most suitable method if the well

deeper than 2 is to fill the well with 150mm single-size ston

and, if in a garden area, provide a reinforced concrete ca

twice the diameter of the well. If the well is under or ve

close to a foundation then, after filling, a beam system w

be required to span over the well. The beams should extend

sufficient distance beyond the well; this minimum distance

generally taken as the well diameter each side.



faulting,

iling and

ole ring

lled with

inforced

beams

e exist-

n. In all

r bearing

housing

When

without

Filling a

lution: it

e well is

ze stone

rete cap

or very

tem will

extend a

stance is

Pad and pier foundation

50mmC10Poversite concreteon 1200'gauge polythene

dpm

Timber suspended

ground floor

NoteAll foundations tobe on similar strata

Fig. 2.25 Padand pier foundations,

Example2.3 Disused well level. The well-was approximately 2.0min diameter and the to

2.0 m was brick-lined s :The well was positioned below the junctio

of the rear walland party wall. I t was notpossible to reposition th

dwelling so abeam system in the form of-a tee configuration w

adopted.

During excavation for a house footing a well was discovered

following removal of a large sandstone cover (Fig. 2.26). The well'

was found to be 6 mdeep and water was within 1.0m of the ground

E 494legTS

~L ~. centres

40 mm min._...jI - - - I 'cover W

Fig. 2.26 Example 2 .3 : . disused well - foundations. Concrete mix 30 N/mm2 at 28 days; reinforcement to be high-tensile bars; allowable

ground bearing pressure 80kN/m2 minimum.

45



while

y clause

should be

kN/m

with three

Therefore

100As _ 100 x 3220

/;;i- 600x540 0.99

Therefore Vc = 0.63, and hence A s v is less than Vc + 0.4 and

minimum links are required.

Hence

= 0.4 x 600 x 300 =180 mm'A s v 0.87 x 460

Provide four leg T8 at 300 mm centres.

PAD FOUNDATIONS

Maximum un f actored line load on party wall = 48 + 8 = 56 kN/m.

With a 600 mm wide footing, bearing pressure = 56/0.60 =

93kN/m2.

Maximum unfactored line load on external wall = 26.80 + 8 =

34.80 kN/m.

With a 450 mm wide footing, bearing pressure = 34.80 = 77 kN/m

0.45

In order that differential settlements between the pad foundation

and the strip footings can be kept within acceptable limits, a

maximum allowable bearing pressure of 100 kN/m2 should be

adopted under the pad foundations even though the natural stiff

clays are good for about 200 kN/m2. In addition some section of the

ground beam between the bearing pads and the well will be sup-

ported on natural ground, reducing ground bearing pressures even

further.

Party wall pad

. 6 3.50 98Unfactored reaction = 5 x -- = kN

2

Self-weight of base = 0.4x 24 = 9.6 kN/m2

Are.quired 98.0 = 1.084 m2.

100-9.60 'say 1.05 m x 1.05 m x 400 mm thick pad foundation.

External wall pad

Unfactored reaction = 34.80 x 7.0 + 98.0 = 170.80 kN2 2

A. d 170.80

rea require =100-9.60

1.88 m 2 = 1.40 m x 1.40 m x

400mm thick pad foundation

Base reinforcement

d::: 400-40 -10 = 350mm

Maximum ult imate moment

0402:::(100 - 9.6) x 1.5 x _._ = 10.848 kN m

2.0

_!:!__= 10.848xI06

0003

bd 2 fe u 103 X 3502 X 30 .

Therefore l. = 0.95d = 0.95 x 350 =332 mm

Th e & .d 1O.848xl06

82 2e re ro re • - s = = mm

0.87 x 460 x 332

M· . 0.13 3 4 5 2i rnmum percentage =-- x 10 x 350 = 5 mm100

Piledfoundation

Therefore use TI2 at 200 mm centres both directions in bottom.

Party wall pad

Maximum ultimate moment

0.2252= (100 - 9.6) x 1.5 x -- = 3.43 kN m

2

By inspection, nominal steel will be required.

Minimum percentage = 0.13 x 103 x350 =455 mm?

100Therefore use 12 mm at 200 mm centres each way.

2.7 PILED FOUNDATIONS

Where a lot of dwellings require special foundations and t

fills or weak:ground are not suitable for ground improveme

techniques, piling can be carried out at very little extra co

provided the design details are carefully considered. In mo

situations driven piles are used, driven to a predetermined s

and subjected to a random load test of 1.50 times t

working load.

Driven piles should be avoided in situations where tbedrock profile can vary over a short distance, as in infill

railway cuttings and backfilled stone quarries, for exampl

In these situations driven piles are prone to drifting out

plumb during the driving stage and often end up bein

damaged because of the eccentric loads applied.

If piling is being used then the ground beams should

kept as high as possible. It is often more economic to desig

a scheme based on large piles, especially if the piles a

taken down on to rock or hard clays. One of the problems

using small mini-type piles in yielding strata, such as cla

and sands, is that the end bearing component of the workin

load required is usually of a low magnitude and the pil

need to be driven deeper to pick up sufficient skin frictioWhat may seem to be an economic piling scheme based o

estimated driven lengths may, on final remeasure, end u

being very costly.

Where piles are driven or bored through filled ground st

settling under its own weight, due allowance must be mad

for the additional load arising from negative skin friction

Also, on sites where highly compressible strata such as pe

are likely to be loaded, due to the site levels being raise

additional loads will be transferred to the pile shaft.

2.7.1 Bored piles

These are generally formed using a simple tripod rig. Whe

there are groundwater problems, or very soft clays whic

may cause necking, then temporary casings or permanen

steel sleeves should be used. Great care must be exercise

when withdrawing temporary casings, and a sufficient hea

of concrete should always be maintained in the pile shaft

prevent necking or concrete loss.

Bored piles usually rely on end bearing and skin friction

support the pile loads. They are best suited to clay site

where no groundwater problems exist and the upper stra

4

Foundation design

are strong enough to maintain an open bore. Once steel

casings have to be considered, the bored pile can be uneco-

nomic compared with other faster systems.

Their main advantage is that they can be installed quietly

with minimum vibration; ideal when piling close to existing

buildings.

Where end bearing is required it is important to ensure

that the stratum below the pile toe remains competent for a

distance of at least 3 m. If no site investigation is available

this can be checked by overboring several piles.

Figure 2.27 shows the sequence of operations for

installing a bored pile. Different cutting tools are required for

the various soil conditions.

Safe loads for bored piles in clay soils are generally

calculated using the Skempton formulae which combine both

end bearing and frictional properties of the pile.

For varying soil strengths the skin friction is considered for

the separate elements of the pile shaft with negative skin

friction being considered when passing through filled ground.

2.7.2 Design of a bored pile

Adopt a factor of safety of 3 for end bearing and a factor ofsafety of 2 for skin friction. Qu = the ultimate resistance of the

pile = Qs + Qb. Qs is the ultimate value for skin friction =shaft

area X 0.45c = 1t dh x 0.45c. Qb is the ultimate value for end

bearing = base area x 9c = ~ 1t c f 2 x 9c where c isthe cohesionvalue of the clays determined by laboratory testing or by the

use of field tests: i.e. penetrometer or shear vane tests. Thus

Allowable working load for a pile = ~s + ~

I Ind2xge)

= -(ltdh x 0.45e) +--'-----'-2 3 4

ndc(2.7h+9d)

= . , 12

Where the clay cohesions vary, the pile shaft is split into

vertical elements using the appropriate values down the pile

length.

2.7.3 Design of bored and driven piles

Estimation of approximate working loads is as follows.

The ultimate load capacity Q u of a pile is

Qu=Qb+ Q s

where Qs = ultimate shaft resistance, Q b = ultimate base arearesistance, q = ultimate unit end bearing resistance, Ab =effective cross-sectional area of the base of the pile, /

=ultimate unit shaft resistance on sides of pile, As = effectivesurface area of pile shaft considered as loaded.

Base resistance: clay strata

The ultimate base resistance of a bored or driven pile in

cohesive strata is given by

q=N; Cb

48

where N; = bearing capacity factor, Cb = undrained she

strength of the cohesive strata at the pile base. Values of

for cohesive strata can be variable and are dependent on t

angle of internal friction, ¢. For estimating purposes a val

of 9 is generally used for pile diameters up to 450 m

diameter.

Base resistance: granular strata

The base resistance in a granular stratum is given by

q= rDNq

where Nq is the bearing capacity factor obtained from t

Berezantsev graph (Berezantsev, 1961) based on the angle

shearing resistance ¢ for the stratum. The value of ¢

usually determined from the standard penetration test resu

carried out in the field. For bored piles an Nq val

appropriate for loose soil conditions is recommended a s- t

boring operation loosens the strata, and ¢ values of 28-30

can be used. T = average effective unit weight of so

surrounding the pile, and D = depth to the base of the piThe value of q should not exceed I I MN/m2.

Shaft resistance in clay soils

The shaft resistance / is given by

f=aC,

where a = 0.45 for bored piles. Cs = undrained she

strength. a is taken as 1.0 for driven piles in contact wi

strata with C« < 50 kN/m2.

For strata with Cs > 50 kN/m2 the value of a lies betwe

0.25 and 1.0 and is dependent on the depth of penetratio

into the clay strata and the prevailing ground conditions.

The shaft resistance/in granular soils is given by

f=! reD + If) K, tan 0

where D = depth to base of pile or base of the granular stratwhichever is the lesser; d ;:: depth to the top of the granul

strata; (j = angle of friction between the granular strata athe pile shaft; K; = earth pressure coefficient dependent

the relative density of the soil.

Broms (I966) related the values of K s and (j to the e

fective angle of shearing resistance of granular soils ¢ f

various types of piles and relative densities. For driven pile

Pile

type

Low relative

density

High

relative

density

Steel

Concrete

0.5

1.0

1.0

2.0

¢is generally taken to be the value of ¢as obtained from t

SPT tests. For bored piles. values of (j =220 and K, = 1

should be used to cater for the loosening effect when borin

out.

shear

s of Nc

on the

a value

50 mm

om the

angle of

of tp is

t results

q value

as the

28-30°

of soil

the pile.

shear

ct with

etween

strata,

granular

rata and

den t on

the ef-

ls tp for

n piles:

rom the

= 1.0

boring

The expression! t(D + d) K, tan 0 can only be used for

penetration depths up to 10--20 times the pile diameter.

Between 10 and 20 times the pile diameter a peak. value of

unit skin friction is reached and this value is not exceeded at

greater depths of penetration. It is prudent to adopt a peak

value of 100 kN/m2 for straight-sided piles.

Tomlinson (1980) suggested that the following approx-

imate value can be adopted for f (kN/m2):

Relative density

<0.35

0.35--0.65

0.65--0.85

>0.85

Loose 10

Medium dense 10--25

Dense 25-70

Very dense 70 but < 110

A factor of safety of 2.5 should be adopted to these ultimate

values to obtain the allowable or safe working load on the pile.

Example 2.4 Bored piles

A site investigation has revealed that loose colliery waste fills about

4m thick overlie firm-to-stiff clays which are underlain by

weathered mudstones. The maximum pile loading is 300 kN but an

additional load resulting from negative skin friction has to be

catered for as the fil ls have only been in place for one year and are

still consolidating under their own weight. The strata are described

in Figs 2.28 and 2.29.

An exist ing culvert passes under the proposed dwell ing and its

condition is not known. It has been decided therefore to provide

bored piles down to the strong mudstones at about 6.50 m below

ground level.

Based on information from borehole no. I(Fig. 2.28), no test

result"are available for the firm/stiff brown clays and weathered

mudstones. Assume a value for c of 50 kN/m2 which is veryconservative (Fig. 2.30).

unit negative skin friction

Po = = effective overburden pressures

¢e = = the effective angle of shearing resistance

"act = = k Po tan ¢e

From Bjerrum, raet = = 0.20 po for clays of low plasticity.

The negative skin friction factor a==0.20

Skin friction ultimate == rhaAS2

Required pile capacity = = 300 kN.

NEGATIVE SKIN FRICTION

This is approximately = = ~"ac,poAs

0.20(18x3.80)= = 2 x(1tx0.45x3.80)=38 kN

Therefore total pile capacity required ==300 + 38 = = 338 kN.

END BEARING

For N values of 49 the unconfined compressive strength of the

mudstone equates to 13.30 x 49 = 652 kN / m2 (soft rock).

Piled reinforcement

652The shear strength = = - = = 326 kN/m2

2

Allowable end bearing pressure

N e Xc 9x326 =978 kN/m2Factor of safety 3.0

End bearing capacity = 978 X 1tx 0.62

= = 277 kN4 .

SKIN FRICTION

a ==adhesion factor = = 0.40.

u = cxA, == 0.40 x 50x (trxO.45 x 2.45)

S Factor of safety 2.0

==34kN

Therefore pile capacity = 277 + 34 ==311 kN.

With a 1.0 m penetration into the mudstone this value isconsidere

adequate for borehole no. 1.

Based on soils strata in borehole no. 2 (Figs 2.29, 2.31):

Required pile capacityNegative skin friction = == = 300 kN

0.2(18x 7.2) ( 5 72)2 x trxO.4 x .

+ 0.1(18 x 7.2 + 0.65 x 20) (z x 0.45 x 1.3)

= = 132 kN

26kN

Therefore pile capacity required

For N ==152, unconfined compressive strength

= 13.3 x 152

2022Shear strength =--

2

Allowable end bearing capacity

9 xlOll x(tr x 0.452)/4

3

== 458 kN

= = 2022 kN/m2

= lOll kN/m

=482 kN

This is greater than the 458 kN required. The piles must therefore

penetrate at least 1.0 m into the shaley mudstones, ideal ly below

the weathered zones. The minimum cement content in piles shoul

be 370 kg/m3 sulphate-resisting cement with a free water ratio no

exceeding 0.45 to cater for Class 2 sulphates in soils.

PILE REINFORCEMENT

N = 1.60 x 458 = 732 kN

f e u =40 N/mm2

With 75 mm tolerance on pile position the bending moment on the

pile shaft equates to

75-3 x (1.60 x 458) = 54.96 kN(ult)10

N 732 X 103

== 0.090d2f = 450x450x40

~ == 54.96 x 106 0.015.

d3 ! C U 450

3X 40

Therefore use 'nominal steel only. Provide seven R12 vertical bar

with nominal R6 helical binders at 150 mm pitch.

49



Foundation design

HUDDERSPIELD

Sheet 1. Borehole

I";S:;"o"""'ri:-n-g-:-M:-e""th-o"""d:-------------------------l of 1. No. 1

L. . /GHT CABLE PER.CuSSION AT 150 MM DIAfrtETE:R.. ~S;::;i:;:te:---=--...L.---==----I

Drillingcommenced GroundLevel 122·0 AoD

DescriptionfStrata

MADFi ! GlbUND: Co/IIU. j fAP..Sbe

w·ci1t 6(JMQ. c oa l: / bn c.ks

c/~ and: t-Lmh£r

MADE GR.oVND

DepthLegend(m )

Remarks

2.7.4 Driven piles

Fig. 2.28 Example 2.4: borehole 1.

In the housing field these usually consist of steel tubes filled

with concrete, precast concrete segmental piles, concrete

segmental shell piles or other similar types.

The main advantage of these types of pile is that they

be placed through weak or water-bearing strata without

changes occurring in their cross-section. They are gene

end bearing piles but additional loads resulting from

friction (adhesion) can be developed when driven thro

50

they can

ut any

om skin

through

Piled reinforcement

Sheet L I BoreholeBoring Method of 1 No·2

LIGHT CABI.E PEra:.USSION AT ISO NM DIAmETER. Site

HUDOERSF/cLDDrilling commenced Ground Level 12/·00 lor>

Samples/Tests StandI, ReducScale

ing edDescription of Strata Depth

Legendample Insitu Water Level (m)Type Test Level (m) (m)

f l l /AOr= GROUND Shak a/ld. /

= - Mudsh:mM, brick. froe~s,C U ' I d timr:w-

_g;Q_

~r = -

>(.-= -~~ '2 ·qo,6.0

~MADE 6ROVNO: ISrick.s J sfoI1es

~ tVand c..~s

~(4-)

~ 3·!i"C!

rS'o mADE GRoUND ; 8hA.ck . a.6h and~ brick fr"$~5~-

=. . 1 Q ; Q _

=~r : -....!bQ_ (,'0

- mADe GROc.JND : SCu1d..st-oNl. ~~

: aJK l b~c/( CA'J,t

= -...illL

7';ZO

= 6of~ d.a.rk c r~~v~ ~

.. . : _ : .

;-:::::-:-= - f!~~ ~5~~ rtrat:-;--

- - = - - - = -=16.0

<. n s;V(JrtS . ~ _ I

f{o..3~ -:-=---

:.. ~

2·50 ~

= - Oark: e " " ~ and. bto.Gk 1 . U € C A k ~;"'IAj~~ S'.<;;>n =

= uJ~ed. t ' t - r . i . n } : t _ b t Z - d d . u 1 .18.0

;: IVE;~

Mt.JCk;1-o

t (152)&r~~ SU$p~ ti1, sero":)= - ( ' IWcL~ !LeS ai: q.30 M.

-20.0

Remarks

Tn:u.e-s o f wa..ter enbered: bore~ cW~tnj drliW'lj cd 770 j\.(

Borehole c i r ; J onWtfk:J.rr:iwa)

o r GaSCM.J

Fig.2.29 Example 2.4: borehole 2.

the clays or gravels overlying the bearing strata.

These piles are usually driven to a predetermined set based

on the Hiley formula which is a dynamic criterion related to

the weight of the driving hammer and height of the drop.

When using this concept it is essential that a random pile b

load tested using kentledge or by jacking against tension

piles to confirm that the pile driving assumptions and

established set criteria are valid.

51

Foundation design

Ground level 122.00AOD

~".-, (?" '.f :0/'\\ 0.'

0" ~ 0,

t , . . . : - -- '' ' ' . . . .450mm "-..

dia ---to 1/ f--I I . . . . . . .

~

~r5')'0 ,~? V-- r ~ ~ " . : . . ' :...D_:_-+=-!;2~,8~0

t--

k : : ; . ~ \ r _M;;d;grc~~md •. < E

I:-- ",Clays, and coal D'...:....~

V F . 0 0 " b.. o_:" -T--" ,. ..D~~,80. . . . : : . _ _ : ; , _ ~? - - : - - - - . -Assume c

r--:' _ . . -----. --- =50kN/m2

~ 1°./, -===-Firm to stiff brown:==- . -=~~--=- 'V..· .' _clays C ' : 1-- r-.. ~ - __ ... -~----.

- -= V ..- --'., -==---=~--L5.10

_--...._ I/o, ====Weathered grey ..__ - - = = - - ~S;~~/~2

C / ~ - . _~:_~~ud_~t~n~ .. ' . E

~,i;;,---'- " : - + . . .~~-I. ' I>.~N =49 blows 0,2.5

-_.......___J

'YW= 18 kNlm3

E

Made groundcolliery shale, bricks

clay, timber etc.(N values = 4)

8CD

N

EoIi)

< D

~,50

Fig. 2.30 Example 2.4: ground conditions at borehole 1 .

«' ~n450mm dia.

'J'

1l

Made ground. ,

ash, claysE

'Yw= 18 kN/m30

'"C\l

, ,0 o r - : .

J •.

s:0.c~III

£c0

c0

~

E~c

0 ~Ii) III

.,; CD>iOlCI l

Z

o() .

d '

.0

-7.2

Eo

' C ' : 1

-8.5

/N=152

f

52

Fig. 2.31 Example 2.4: ground conditions at borehole 2.



loads are

h to which

no access

) is to be

o access is

.

d is zero.d load is

2 for a 3 0°

are taken

ve single

corridors,

kN/m2.

which they

Assessment of wall loading

30 4010 20 5 0

o

A . . . . . . .~ i = 1 l. .. i . i l i a 4 . : _ ~ · · : " . ·.! , . - ·. _ · . .• · _ t · · = _ · · . · . · . ; ; L ~.~ · · , · · ~ = : t ? d _ : . · _ T : · . .L · · . . . _ · L ; . • • . .· · · _ · · · ·. . . . . . ; . . .N!:":::~~"'_;_""i.-...I'''''''i'''''' .. L;- ;. i .

-2 ,-r- ;E· .. · · · . L , · · · ·.· - i . . · . . . . I ; ?~.

i~! - · ~ · : - · · ·· . · · : · - - : · :. ~ ' . · · · · - -. . .: · · ; ·- · · · -· · · · + :r : _ : . . : _ : : : : ~ ( _ : : : : ~ J ~ · · ~ : _ : : : : ~ : : I _. . . _ . . . + · I _ _ + . · - - · ~ · · · · ~ ·.. . - - · · · ~ · · ! - - · r · - · . .· - ~ : ~ _ ~ · i - - · ·.· . - . . . . . . .-.~.-..--+-~ ..

~ . , ..·1· .. + · + · + · · . 1 . . · · . . . . . . . I '~ . .. .. ! l· ....'.·.·....· ·. . -!- ..· · · -! - -! - - - ! - .

..... ··1 ..·.·.·...... .: 1 ' . . 1 · . ; . . . .

.+-~~--~-+~r-+--+~~+--r~~+-~~--+-~~~+-~-7--~~~--~;

. . .. · .. . . .. . . .. . . + ' : . .. : : : i ! : : , .. ! ! : . . : : : · ~ : : : : : : : : : r · : : : : : r :: : : : " r :: : : .: : : : . : : : : : : : . . . . . : . . . : · · · · I : : r . . . . . . ! · . . ..... .. -!- ·..·' ·I j j + + . . . . -! ' , . ' '.. " r : "j .

-8 ~ __~_. __~ __._~ __~j~~i __~' '~-L__~ __~~ __~~L-_. __~ __~~ __~ __~~ __~~ ___,

Load (tonnes)

Fig. 2.33 Load-sett lement plot.

;.- , .····-r·

.. ~.:::~::.'"

.... ; ..····1·······; ,.,.

' . : ! : "

r ' : ' "

1

2. . . 3

E 4g

5

E 6(I)

E 7

~ 8(J)

9

10Time (h)

Fig. 2.34 Graph of load and settlement versus time.

Roof Imposed loads

Dead loads(kN/m2)

0.75

0.25Factored 25° pitch snow load

Ceil ing access loads(kN/m2)

0.55

0.05

0 .23

0.15

0.02

1.00 x lAO = 1.40 kN/m2

Tiles

Battens and felt

Trusses

Plasterboard ceiling

Insulation

1.0 x 1.60 = 1.60 kN/m2

Total unfactored dead load + imposed load = 2.00 kN/m2

Total factored dead load + imposed load = 3.00 kN/m2otal =

55

!,""

Pile loadings

First floor2.4x4.0 4.80 7.32 Ground floor

2.4 x 4.04.80 7.32

2.0 2.0

shown in 2.4 x4.0 4.80 7.32 Walls 1.75 x 2.6Ground floor2.0

4.55 6.37

Walls 3.75 x 5.5 20.62 28.86Wall (F) total 14.15 21.01

Wall (Ct) total = 36.82 53.40 WallGFactored

(kN/m)Wall C2

100 mm blockwork = 1.75 x 2.6 = 4.55 x 1.4 = 6.37

1.50

7.3236.82 - (2 x 4.80) = 27.22 38.70

Wall H (as wall F)

Wa/lD=14.95x 1.4 =21.01

7.32

Unfactored FactoredThe calculated wall line loads are summarized in Fig. 2.36.

34.125 (kN/m) (kN/m)The pile layout is shown in Fig. 2.37.

Roof: nominal 0.5 m width =0.5 x2.0 1.00 1.50Ground beams: 600 mm x 400 mm wide

50.265 Self-weight 5.76 kN/mWalls = 3.75 x 6.5 24.375 34.125

Tie beams: 300 rom x 300 mm wideWall (D) total 25.375 35.625 Self-weight 2.16 kN/m

FactoredWallE

(kN/m) PILE LOADINGS

9.90 Unfactored Factored

2.4 x2.0(kN/m) (kN/m) Pile 1

3.66 First floor 2.40 3.662.0 Unfactored Factored

3.66 First floor 2.4 x4.0 4.80 7.32 (kN) (kN)2.0 34.975 x 2.875 50.27

50.625 x 2.87572.25

28.86 Ground floor2.4 x 2.0

2.40 3.662.0 2.0

2.0 27.22 x 4.175 56.8238.70 x 4.175

46.08 2.4 x 4.0 2.080.78

Ground floor 4.80 7.322.0

2.0 5.76 x (4.175 + 2.8751 20.30 8 x 3.525 28.20

Wall: 100mm blockwork = 1.75 x 2.6 4.55 6.372.0

38.70--

Total 127.39 = 181.23Wall (E) total 18.95 28.33

WallF Pile 2

Factored Unfactored Factored(kN/m)

Unfactored Factored

9.90(kN/m) (kN/m)

(kN) (kN)

First t\jpr2.4 x4.0 4.80 7.32 34.975 x

2.875= 50.27 50.265 x 2.875 = 72.25= =

2.0 2.0 2.0_. l~ ~ _.0 61 32.02 (46.08) 6227.22 (38.70)

T:c

I._.~C o(J1

~ »c. >

E 18.95 (28.33) ~ ~C o

_. 0>~

_. (J1

G 4.55 (6.37) A

~.(J1 I\) 3

0I\)

I~Oi

(J1

i : . > 2 10

~~ !> C!l(J1 0-

,EJw I~<0(J1

(J1

~J

I\)

~

I1

N IC1 36.82 (53.40) T e o C!l C2 27.22 (38.70) Co)

I

Fig. 2.36 Example 2.5: wall line loads (kN/m). Factored lineloads sho~n in brackets.

57



75 = 30.20

0 - 14.045 -

=239.07

Factored(leN)

13.29

16.70

30.20

= 36.50

25.40

43.62=

= 165.71

Factored

(leN)

80.78

5 =84.77

= 24.58 21.01 x 3.475= 36.502.0

4 15X 3.475I. 2.0

x(4.175 + 3.175 + 3.475) 31.175.76 2.0

8 x 5,41 = 43.28

=171.02otal =245.33

Pile 7

Unfactored Factored

(kN) (kN)

32.02 x 3.175 50.83 46.08 x3.175 =

73.152.0 2.0

32.02 x .l:2_ 48.03 46.08 x3.0

= 69.122.0 2.0

5.76x (3+3.175)

17.78 8 x 3.0875= 24.702.0

Total 116.64 = 166.97

PileS

Unfactored Factored(leN) (kN)

18.953.175

30.08 28.33 x3.175

44.972.0 2.0

18.953.0

28.42 28.33 x__lQ__

42.502.0 2.0

5.766.175

17.78 8 x6.175

24.70=2.0 2.0

2.164.175

4.50 3 x4.175

6.26--2.0 2.0

Total 80.78 118.43

Pile t j I J

Unfactored Factored

(kN) (kN)

36.826.175

113.68 53,406.175

164.87x2.0 2.0

5.766.175

17.78 86.175

24.70-- x --2.0 2.0

2.164.175

4.50 34.175

6.26x2.0 2.0

Total 135.96 195.83

Pile 10

3.0

2.0

2.175

2.0

5.76 x (3 + 2.175)

2.0

Unfactored

(kN)

48.03 46.08 x

Factored

(kN)

69.122.02 x3.0

2.0

35.625 x 2.175 38.745.375 x 27.592.0

8 x 2.584.90 20.70

Total 90.52 128.56

Ground beam analysis

Pile 11

Unfactored Factored

(leN) (kN)

25.375 x(2.175 +4.175)

80.56 35.625 x 3.175 = 113.12.0

18.95 x3.0

28.425 28.33 x3.0

42.502.0 2.0

5.76 x(2.175+4.175+3.0) =

26.92 8.0 x 4.67 = 37.362.0

Total = 135.90 192.96

Pile 12

Unfactored Factored

(kN) (kN)

25.375 x4.175

52.97 35.625 x4.175

74.362.0 2.0

36.823.0

55.23 53.403.0

80.10x2.0 2.0

5.76 x(4.175 + 3) =

20.66 8 x 3.58 = 28.702.0

Total = 128.86 = 183.1

The calculated pile loadings are summarized in Table 2.4.

Maximum unfactored pile loads have not taken into account th

additional load due to elastic shears. If these are to be considered

then it is appropriate to multiply the working loads by 1.25:

Maximum working load based on pile 6 = 171.02 x 1.25

. = 213.77kN

Use 165mm diameter steel tube piles driven to a predetermined se

to give a maximum working load of 225 kN.

A ll piles to be subjected to a re-strike and one random pile to b

load-tested using kentledge. The test load applied to b e 1.50 x 225

= 337.50kN.

GROUND BEAM ANALYSIS

This analysis assumes the ground beams are continuous and

designed to cater for bending moments top and bottom of wf2 / 10 .

Some engineers use a simple supported design philosophy with

the use of anti-crack reinforcement over the pile supports. This i

very conservative for the bottom steel but problems of service-

ability cracking over the supports may result which could affect the

durability of the concrete beams.

On sites where clay heave is unlikely the ground beams can be

cast against the earth face using 75 mm cover to the links or the

beams can be poured in shutter moulds (BS 8110 Clause 3.3.1.4)

A ll design in accordance with BS 8110 Part I (1985).

C 30 P mix feu = 30 N/mm2

High-yield bars fy = 460 N/mm2

b=400mm, h=600mm

Figure 2.38 shows a typical beam section.

From BS 8110 Table 6.1, Minimum cement content to be 370 k g /m?

with a water/cement ratio of 0.45. This gives Class 2 sulphate

protection (concrete exposed to sulphate attack).

59



Foundation design

As = area of steel in mrn?

d = effective depth of beam =600 - 40 - 8 - 1 { ! 1 2 = 552 - 12 =

540mm

z is limited to O.95d

Design ultimate moment, M = As x 0.87 xfy x Z

= 380.20 As d x IQ 6 kNm

z is less than O.95d

From BS 8110 Clause 3.4.4.4 :

Therefore

(.:. - 0.5)2 = 0.25 _ ~d 0.9

z2 z k

df"-d=- 0.9

Table 2.4. Calculated pile loadings for Example 2.5

Pile Service load

number (kN)

•127.39

2 150.84

3 139.58

4 166.97

5 114.39

6 171.02

7 116.64

8 80.78

9 135.96

10 90.52

II 135.90

12 128.86

Ultimate load

(kN)

181.23

214.95

198.71

230.07

165.71

245.33

166.97

118.43

195.83

128.56

192.96

183.16

D -EE E0 EL() 0

0CD

-.

\ . 400mm

Fig. 2.38 Example 2.5: typical beam section.

60

SubstitutingM M

0.S7fAforzand -?- fork:

bd-feu

M -M

0.87fyAsd 0.9bd2feu0.87fAd)2

Therefore

M

(0.87fA4 0.87fAd 0.9bd2feu

Therefore

0.9bfeu

Whenfy =460,[eu = 30 and h = 400, then

(0.87 x 460 , s ) 2M =0.87x460x As d '':i

. 400x30xO.9

Ths: M 400A,d -14.83A,l

ere, ore = 6 kN m10

This formula only applies when z is less than 0.95d.

Maximum span

Deflection criteria are as follows.

Clause 3.4.6.3 Table 3.10:

Span = 20 simply supportedEffective depth

Clause 3.4.6.5 Table 3.11: Modification factor for tension s

equal to

0.55+ 477-5fy/8

120(0.9+Mlbd2)

but no greater than 2.0.

Therefore

Maximum span = (0.55 + 1.57 2 )20d0.9+MI400d

Shear reinforcement

. (I00A Ibvdt\400ld)1/4Design concrete shear st ress, ve = 0.79-'--.::.'---'--'---....::.....-

'Y m

where 'Y m =1.25.

(0.25A )1/3(400)1/

4

v =0.63 --' - N/mm2e d d

Clause 3.4.5.2 Table 3.8:

Minimum shear steel using TS links

A = .0.4b,.s,.sv 0.87!.",

where by = 400 mm,[yy = 460 N/mm2, Asv = 100.5 mm-. Theref

100.50 x 0.S7 x 460 2s; = = 50 mm

0.4 x 400

Minimum links T8 at 250 mm centres.

With minimum links v = Ve+ 0.4 with v =V/byd. Therefore

V=vb d'= (vc+0.4)400dv 103



steel is

(50.625 + 8.0)t ' I [ 1 1 1 1 1 1 1 1 1 [ 1 1 1 1 1 1 1 1 1 1 1 1 \

2.875 m 1 3.475 m123

Beam 1-3

(46.08+ 8.0)

Loadings

f 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

10 3.00 m f7 3.175 m

fllllllllllllllllllllf [

Beam1-4-7-10

(53.40+ 8.0)

Beam3-6-9-12

(35.625 + 8.0)

1 t , [

(38.70+ 8.0)

I I I I I I I I4.175m

(38.70+ 8.0)

I I I III [ II I I I III I I I I f2.175m f11 4.175m12

Beam10-11-12

(28.33+ 8.0)

f i I II II1 I II f II II I I Illl11 3.0 m 8 3.175 m (4,5)

Beam 11-8-{4,5)

57.76 (21.01+ 8.0)

4 P I I I I II 7 0 :5

1 1 1 1 I I I I I I f2.875 m 3475 m 6• •••

Beam 4-5-6

Fig.2.39 Example 2.5: beam-loading diagrams.

Shear steel T8 links

A = bvsv(v-vJ

sv 0. 87 fy v

v = A sv x O. 87 fy v +v c

bv sv

Therefore

V A S Y xO .87fyvd vb d-----;;-'--+ -..£.._L_ kNSy X103 103

s, V

100 D Ad + O .4ved

125 D Ad + O Aved

150 D Ad + OAved

175 0.23d + OAved

200 0.20d + OAved

225 O. I S d+OAved

Beam design

Beam 1-3 (Fig. 2.39(a))

Maximum ultimate moment = 5S.265x 304752

70.358 kNm10

Beam 1-4-7-10 (Fig. 2.39(b))

M· 1· 46.7x4.175

2

ax imurn u nmate moment = SIAO kNm10

Beam 3-6-9-12 (Fig. 2.39(c))

. I· . 46.7 x 4.1752

Maximum u timate moment = 81040 kNm10

6

Foundation design

Ventilated void

'." .... ~. c" ,,· 50mmconcrete

r=-----.,..., '_":":"~~-__:.----~-----; oversite on1200 gauge polythenedpm

1+----40 mm min.cover to l inks

, . ."

2T16 '.

T10300 mm

centres

EE0

.. ',~.

0<0

~.

i -2T16 .1:.

.

~40mm cover

to l inks

400

I+----+-f----Reinforcement frompile

Fig. 2.40 Example 2.5: ground beam detail.

Beam 10-11-12 (Fig. 2.39(d))

M· I' 43.625x4.l75

27

aximum u trrnate moment = = 6.04 kN m10

Beam 11--8-(4, 5) (Fig. 2.39(e))

-M I' 36.33 x 3.1752

aximum u tim ate moment = 36.62 kN m10

Beam 4-5-6 (Fig. 2.39(f))

R. = 29.01 x 2.875 = 41.70 kN2

57.67 x _Q2_ =14.04 kN2.875

R. = 55.74kN

Rb = 41.70+(57.67 x 2.175)=85.32 kN2.875

1922Moment 4 - 5 = 55.74 x 1.92 - 29.01 x -._ = 53.55 kN m

2

Maximum shear = 101.23 kN

Using C30 N/mm2 concrete with 40 mm cover and Design Chart

No. 1 (Fig. 2.11):

k = __!!__ = 81.40 X 106

0.023

bd2fcu 400 x 540

2x30

Therefore lever arm factor = 0.95

A.=~= 81.40xl06

=397mm2

0.87 fy z 0.87 x 460 x 0.95 x 540

62

75mmprojection

t

Concrete or steeltube pile filledwith concrete

Use two TI6 mm bars top and bottom in all beams.

. 101.23xl03

Ma x im um shear stress = 0.468 N/mm2400x540

Vc = 0.36 N/mm2; provide minimum links throughout beams.

A = 0.40 x 400 x 300 =119 mrn?sv 0.87 x 460

Use T1 0 links at 300 mm centres throughout.

Example 2.6

Poor ground conditions on part of a housing site require

plots to be built off piled foundations. Maximum pile load

approximately 450 kN. Piles to be 200 mm square precast co

driven to a predetermined set and this set to be checked

striking the piles. Minimum factor of safety on piles to be 2.2

piles will be driven with a 4 tonne hydraulic hammer (Banut

with a 400 mm drop (Table 2.5).

Using the modified Hiley formula:

R = Eu S+C/2.0

Where R; = total load (450 kN) x factor of safety (21012.50kN;

E = transfer energy at top of pile = 0.85 x 104 kNmm;

C = temporary compression of pile and ground per blow (a

lOmm);

S = set per blow.

Therefore:

E C 8500 10S=---=-----=3.39 mm/blow

Ru . .2.0 1012.50 2

Therefore set for 10 blows = 34 mm or less.



several

ading is

concrete

by re-

2.25. The

nut type)

(2.25)

(assume

Pile loading

Table 2.5. Hammer transfer energy table. Rig type: hydraulic

hammer (Banut type)

Suspended in-situ concrete ground floor

200mmslab 4.70

Stud partitions 0.50 1.50

Total 5.20

External walls

Unfactored dead loads

(kN/m2)

102.5 mm brickwork 2.25

100 mm blockwork 1.25

Plaster 0.25

Total 3.75

Hammer weight

(tonnes)

Transfer energy

(tonne m)

Hammer drop

(mm)

300 400 500 600 700

0.25 0.35 0.450.55 0.70 0.90

0.85 1.10

1.05 1.40

1.503.0

4.0

5.0

LOADINGS

Floor plan is shown in Fig. 2.41.

RoofParty walls

215 mm brickwork 4.50

Plaster two sides 0.50

Unfactored Total 5.00

imposed loads

(kN/m2)

Walls A and D

Dead load Imposed load0.75 (kN/m) (kN/m)0.25

Roof and ceil ing1.00 x 8.50

4.251.0 x 8.50

= 4.21.00 2.0 2.0

First floor nominal 0.75 x 1.0 0.75 1.50 x 1.0 = 1.5

30° pitch

Unfactored dead loads

Concrete tiles

Battens and felt

Trusses

Insulation

Ceiling: plasterboard

(kN/m2)

0.55

0.05

0.23

0.02

0.15

Total 1.00

First floor

Tongued and grooved boarding 0.10

Plasterboard and skim 0.15

Stud partitions 0.50

Ground floor 5.20 x 5.0 x 0.33 = 8.58 1.50 x 5 x 0.33= 2.50

3.75 x 5.0 = 18.75

= 10.00

1.5 Wall

. . Total 0.75 Underbuild and ground beam

Total

II "

5.0m

" I WallB

A------1- Roof and ceil ing 1.00 x 0.60

I0.75 x 5.0I First floor

2.0

+~ 1 II Ground floor 5.20x2:Q

B C '0 E 2.0

I 0010a: ~

I Wall 3.75 x 6.0

First f loor span I Underbuild and ground beam

Ground floor

I Total

___ J.

42.33 8.25

0.63 1.0 xO.6pO = 0.60

1.50 1.50 x 5.0 = 3.752.0

= 13.00 1.50 x 5.0 = 3.752.0

= 22.50

= 10.00

47.97 8.10

oWall C (party wall)

a : . Party wail

IFig. 2.41 Example 2.6: floor plans.

Roof and ceiling 1.00 xO.6 x2

0.75x5.Ox22n.0

1.20 1.0 x 0.6 x 2 = 1.20

3.75 1.50 x 5.0 x 2 =7.502.0

First floor

63



Foundation design

5.20 x 5.0 x 2 = 24.50 1.50 x 5.0 x 2 =7.502.0 2.0

Ground floor

Wall 5.00 x 6.0 = 30.00

Underbuild and ground beam

Total

= 10.00

70.95 16.20

PILE LOADINGS

Pile I

(42.33+ 8.25)x 5.0 +(47.97+8.25) x 4.252.0 2.0

=245.60 kN

Pile 2

( 8 25)5.0 x 2.0 ( ) 4.25

42.33+. x---+ 70.95+16.20 x- =438.00 kN2.0 2.0

Pile 3

(47.97 + 8.10) x 4.25 = 238.30kN

Pile 4

(70.95 + 16.20) x 4.25 = 370.38kN

Pile 5

As Pile I = 245.60kN

Pile 6

As Pile 2 = 438.00kN

BEAMS

jearn moments

BeamsAandD

42.33 x lAO x 5.02

(kNm)

Ultimate moment 148.15105.02

33.00.25 x 1.60 x-10

Total 181.15

BeamB

47.97 x lAO x 4.252

(kNm)

Ultimate moment 121.3010

8.10 x 1.60 x 4.252

2304110

Total 144.71

BeamC

70.95 x lAO x 4.252

(kNm)

Ultimate moment 17904110

64

16.20 x 1.60 x 4.252

10

Total

Beam shears

Beams A andD

Ultimate shear 42.33 x lA O x~2

8.25 x 1.60 x ~2

Total

BeamB

Ultimate shear 47.97 x lAO x 4.252

8.10 x 1.60 x 4.252

Total

BeamC

4

22

14

1

(

1

1

Ultimate shear = (70.95 x lA O + 16.20 x 1.60)x 4.25 = 266.152

Beams A and D

Ultimate moment = 181.15 kNm Ultimate shear = 181.15 k

k=.....!!._= 181.15xl06

0.119bd 2 fe u 400 X 3902 x 25

Therefore lever arm factor =0.83

18U5 x 106A. - 1398 mm '- 0.87x460x390xO.83

Use five T20 mm bars (1571 mm-)

Fig. 2.42 Example 2.6: pile layout.

EL)

C\I

~



Beams A and D

2Tl0

I -

1.2Sm _ I e 1.2Sm

- I

2Tl0

1 i 14T2O

1.,

T8-200 J T8-2S0 I T8-1200 I T8-2S0

I I I I I

tT20

I14T20I

~ 4.2Sm ~::; 4.2Sm

IBeamB

2Tl0

Ell)

C\J

-.i

46.818

226.23

(kN)

148.155

33.00

181.15

(kN)

142.71

27.54

170.25

.15 kN

Beam

18U5x103Shear stress v = 1.16 N/mm2

400 x 390' tW~·'~1~ r '3 ; r - ' P4+ .---.-+~ ~(J) . ~

~ o.Z. t:.

I

- + - ( " . , . , ~ -1--

looA, = 100 X 1571

b,d 400 x 390

Therefore v, = 0.64

1.0

Because (v c + OA) is less than v provide nominallinks to beam

Using T8 links (Table 2.6) :

s = loo.50xO.87x460 =251 mm

, O AO x 400

Provide T8 links at 250 mm centres for full length of beam.

Fig. 2.43 Example 2.6: line loads (kN/m).

2T101.50 m I 1.50 m

I ! 5T20 I_2T10

TS-200 TS-250 \ TS-200 TB~ 200. J TS - 250

I 1.0 m .I .1 _ I 1.0m 1.0 m 'I I

I I

. ,

I ~ . . . I

l d ST20 ~ ~ 5T2DP P

. .p5.00m 5.DOm

BeamC

Fig. 2.44 Example 2.6: reinforcement details.

6



m from

length of

tion of

on Soil

ing soils

bearing

31.

erials,

1: Unre-

de of

andards

Code of

arts for

4th

r Site.

f!

I!i

I

t

3.1 INTRODUCTION

Foundations transmit the total load from a building on to the

ground by direct contact pressure. The foundation must dis-

tribute the building loads in such a way that it ensures that

the bearing stratum is not overstressed and that total settle-

ments are within acceptable limits. The foundation designer

therefore needs to have some knowledge of the type of strata

present below the foundations.

Site investigations seldom reveal an allowable bearing

capacity in simple terms. This is because the various strata

are composed of many different soil types, each having

different properties.

Cohesive soils are subjected to plastic deformation when

they are loaded. If the pressures applied to such soils are

just sufficient to cause shear failure, this pressure is de-

scribed as the ultimate bearing capacity of the soil. By

applying a factor of safety to this value, we have a reducedbearing pressure which is referred to as the safe bearing

capacity.

The allowable bearing pressure is the maximum net

loading that a soil can sustain, taking into account the safe

bearing pressure and the magnitude of settlement that the

building can accommodate safely.

The net bearing pressure is the difference between the

actual pressure below the foundation and the pressure from

the removed overburden. This principle is often adopted

when designing buoyant foundations ..

3.2 SETTLEMENTS IN COHESIVE SOILS

Soil conditions can change considerably from before to

during and following construction of foundations. Most

cases of excessive settlement arise because of unforeseen soil

conditions which suddenly arise. It is therefore useful to

examine the types of ground movement mechanism which

are potential causes of settlement in cohesive soils.

(a) Consolidation settlements. In cohesive soils which are

saturated, the effect of loading the soils is to squeeze out

1

Chapter 3

Foundations in

cohesive soils

some of the porewater. This is called consolidation. A

change of loading is required for this consolidation t

take place and it may take several years before it finishe

settling. The most susceptible strata are the normally

consolidated clays and silts, and organic clays such a

soft alluvium and clayey peats.

(b) Moisture movements. Some types of clay show

marked volumetric change as their moisture content i

changed. Clays which fall into this category are referred

to as shrinkable or expansive clays. They are usually

found in southern and eastern counties of the UK, bu

they can occur in other areas in localized pockets.

(c) Effects of trees and vegetation. A major factor which

can affect cohesive soils with medium-to-high plasticity

is the effects of trees and vegetation. The tree-roo

system abstracts water from the clays, resulting in

surface subsidence. If trees and vegetation are removed

the clays are allowed to rehydrate with the result thaswelling takes place. This subject is dealt with in more

detail in Chapter 6.

(d) Groundwater lowering. Clays containing a high wate

table can be affected if this water table level is drawn

down, by pumping for example. First, the resulting

reduction in moisture will cause the clay to shrink and

settle and, second, the weight of the overburden wil

increase as the soils are no longer buoyant. With sof

clays and peat strata this could result in further

consolidation stresses occurring because of the increase

in effective stresses.

(e) Temperature changes. Frost can cause severe ground

heave in sustained low-temperature conditions. Mossilts, fine sands and chalks are frost-susceptible. Grea

care must be taken when designing foundations for cold

storage buildings.

(f) Lateral displacement. This is often caused by deep

trenches being excavated parallel and too close to

existing foundations. In effect the clays are subjected to

a shear-slip type of failure similar to that experienced on

sloping sites.

(g) Mining subsidence. Settlement can occur at the surface

67



a bear-

the dif-

e limits.

ia may

ulated

soils

on case,

design

can be

ere the

e shear

normal

internal

ground

r a soil

where c = undrained cohesion,

N « =bearing capacity factor,

p = = total overburden pressures at foundation level.

When ~ = 0, N, = 2 + 1t = 5.14.In 1943 Terzaghi produced an equation for q« which

allowed for the effects of cohesion and friction under the

foundation base; this was applicable to shallow foundations,

i.e. where zl B is less than 1.0. For a strip footing Terzaghi's

equation isqu = c N; + 'Y z N q + 0.50 'Y B Nl

The values of Nc, Nq and Nl for various values of ¢can be

obtained from Fig. 3.1. The value of Nc increases to 5.70 for

a surface foundation due to the frictional allowance.

Therefore

quit = 5.70 C + 'Y z

The coefficient N q allows for the surcharge effects arising

from the overburden, and Ny allows for the size of the

foundation, B. When ~ = 0, Ny = 0, N; = 5.70 and Nq = 1.0.Skempton showed that N; increases with foundation depth

increase for ¢ = 0 soils and these values can be obtained

from Table 3.1.

Table 3.1. Nc values for depth factor in soils with ~ = 0 (after

Skempton)

Foundation Depth foundation width ratio, Z/B

type

0 0.50 1.0 2.0 4.0

Circle or square 6.2 7.1 7.7 8040 9.0

Strip footing 5.1 5.9 6.40 7.0 7.50

Using these coefficients, the ultimate net bearing capacity

of a strip footing is given byqn u = C Nc + p o (N q - 1 ) + 0. 5 'Y B Nl

For a square or circular foundation

qn u = 1.2 c N; + Po (N - 1) + 00 4 'Y B Ny

where y = the bulk density of the soil below the foundation,

c = the undrained shear strength of the soil,P o = the effective overburden pressure at foundation level,and B = the foundation width (or diameter).The ultimate bearing capacity Puis given by

Pu=Pnu +p

where p = the total overburden pressure at the foundation

level. If the water table is at or above the foundation levelthen the value for the density must be the submerged density.

When calculating p o, a similar density must be used when

the water table is at or above the foundation level.

Meyerhof (1952) modified the Terzaghi equations to make

allowance for the foundation shape, depth and roughness of

the base.

These values for N c, Nq and Ny are shown in Fig. 3.2.

When using Meyerhof values a shape factor ')..must be

applied; this can be obtained from Fig. 3.3. As for the

Consolidation settlemen

104

-Values of Nc -- "-ValuesofNq ---

. I) Ii~

(I

r/.

VIW /

V ~ A

/ /J / ./

/."/ 'it.V/? - -

.;~

~,.L

ff. V

~'./

~

Deep foundation(0)58)

Shallowfoundatio(0=8);E

"0C

to 10 3

;l

~o-0. l ! !

~ 10 2

li l0.tooOJ

.E 10to

~ 86

4

2

10

Surfacefoundation

(a )10 300 50 60

Angle of shearing resistance, 4l

104 h (

V,3 il

fI 1

/'I I

1 # V/1 1/

II/! J J

v/

{I I

Deep foundatio

0>58)

10 Surfaceoundation

;[

.9 10 2~~'0

§o 10

g > 8'm 6OJ 4

2

1. 00.80.6

0.3

0. 2

0.11 10 20 30 40 50

Angle of shearing resistance, 4l(b )

Fig. 3.2 (a) Values of N « andN q for strip footings; (b) values ofN

for strip footings (after Meyerhof, 1952).

Terzaghi formula, the submerged density must be used if th

water table is at or above the foundation level. For pur

cohesive soils, i.e , ~= 0, N q = 0 and Ny = O.Therefore valueof N; canbetaken from Fig. 3.4.

Using the Skempton formula, the allowable bearing

capacity qa is given by

6



4.0

Fig. 3.4 N, values for strip footings on soils with ~ = 0 (Meyerhof,

1952).

Foundations in cohesive soils

oShape factor. A

1 2 3

~~1~~~ \~\~ ~~-+--t---l

2~~--+-~~~~~~--~

~ 3 ~ + - - + - - + - t I - l - t t l + - l - H - I \ . . . . . . . . - t - - - t~ \ \ \ \e 4~+-~~-;;-~H-\~

~ 5~~--+--H~-'~~~--~

~ 6~~--+--H-r-a~~~~~s:

a .~ 7~~--+--H-+-H-r~-i+-~

9~~--+--H~-H-+~~~~

10

.p = 45° 20 10 5 2 1uriedfootings

.p = 40° 10 5 2 1 0

.p =35° 5 0 (circle)

.p =30° 1

Length/width ratios ofrectangular foundations

Fig. 3.3 Values of shape factor Afor strip footings (afterMeyerhof, 1952).

5

Bearing capacity factor, Nc

6 7 8-_~~

\

r -1\

Theoretical \Experimental --- \.

~ 1.0

ao

~~ 2.0"0

~a~ 3.0

o

9

where Nc = bearing capacity factor, Cu = average undrainedshear strength of soils below the foundation, Po = vertical

pressure applied to the soil at footing level, F = factor of

safety (usually a minimum of 3.0). Therefore

qne t=Nc Cu

F

For a strip footing, Nc = 6.50 for zlB = 0.75/0.60 = 1.25;

therefore

6.50Cu 215 CqUl t=~=. u

Since the bearing capacity factor N; is never less than 5-{)

then a reasonable net allowable bearing pressure for shallow

70

strip footings can be obtained by using the undrained shea

strength Cu x 2.0.

3.3.2 Vertical stress distribution

When a foundation load is applied to a soil a pressure bulb i

generated. The stress on the ground decreases with depth and

by using graphs the values of vertical pressure can be

obtained from Table 3.2.

Table 3.2. Vertical pressure factors

BIZ Factor for Factor for vertical

shear stress pressure

0 0 0

0.1 0.032 0.065

0.2 0.063 0.127

0.5 0.15 strip 0.30

0.6 footings 0.358

0.8 0.22 0.46

1.0 0.25 0.55

1.50 0.30 0.71

2.0 0.32 0.82

2.2 0.31 0.88

3.0 0.29 0.92

3.50 0.27 0.94

4.0 0.25 0.96

4.50 0.23 0.97

5.0 0.22 0.978

105.50 0.20 0.985

6.0 0.19 0.988

6.50 0.18 0.99

7.0 0.17 0.991

8.0 0.15 0.994

9.0 0.13 0.996

10.0 0.12 0.997

100.0 0.01 1 .0

q =contractpressure;B =foundationwidth;Z=depthof soilelementbelowfoundationbase.Shearstress=q x shearfactorVerticalpressure=q x verticalpressurefactor

3.3.3 Construction problems on clay sites

Some clay soils are very variable. They often contain water-

bearing lenses of sands, gravels and silts as a result of past

glaciation. When these are encountered in an excavation,

many building inspectors ask the groundworks foreman to

excavate deeper in the hope of finding clays at a lower level

Quite often, excavating deeper can lead to a costly founda-tion. If the clays are not encountered within a reasonable

distance and the sands are water-bearing, or contain perched

water, the sides of the trench will collapse, and a large, sof

'h{)lewiIl result. The only solution left is to pump out the

excavation and fill it with mass concrete to within 900 mm of

the ground level.

It will be then be possible to compact granular fill in

discrete layers over the mass concrete and provide a raft

foundation or wide reinforced stiff ground beam.

shear

bulb is

th and

can be

water-

past

an to

und a-

nable

soft

ut the

fill in

a raft

To avoid this situation occurring on site, always excavate

a trial hole about 3 m away from the foundation trench to

determine whether good clays are present at shallow depth

below the sand. If they are not present at a depth of 1.2 m

then consideration should be given to using either a raft

foundation or a wide reinforced stiff ground beam.

When excavating for deep trench-fill foundations, pre-

cautions should be taken to prevent collapse of the trench

sides due to heavy ingress of water from wet sandy lenses.Generally, such flows can be controlled by pumping from a

lower sump at one end of the excavations.

When constructing foundations in clays with medium to

high plasticities the base of the excavation should be con-

creted immediately following excavation to reduce the risk

of swelling from seepage or rainwater. Failure to protect the

base of the excavation can result in the clay's bearing capa-

city being reduced as the water makes it more compressible.

Trench excavations which are left unconcreted in dry

weather for long periods can be subject to swelling as the wet

concrete restores the moisture levels. If the soil's moisture

content increases, the soil will heave and upward movement

of the concrete will occur. It is therefore prudent on clay sites

to install as much as possible of the main drainage so that in

wet weather the working conditions are improved.

Clay sites can give rise to problems of slope stability,

especially if major cut-and-fill operations are taking place on

the site. The placing of fills on to clay slopes should be

avoided as the natural- slope drainage is blocked, and an

unstable slope could result.

3.3.4 Foundation designs on clay soils

Where firm or stiff clays overlie soft clays, soft silty

alluvium which reduce in strength with depth, then it is

recommended that a detailed trial pit or borehole

investigation be carried out and shear strengths obtained for

the various strata at depths likely to be affected by the

proposed foundation loads. In trial pits the use of a hand

shear vane tester with extended rods is recommended, as

trial pits in excess of 1.20 m should not be entered,

especially if soft ground or peats are suspected.

When using the shear vane it is most important to take a

range of readings at each level so that an average value can

be obtained. This is particularly important in the soft to very

soft clays. In such clays account may need to be taken of the

adhesion of the clay on the barrel of the vane tester and this

value can be determined by using the dummy vane and

deducting the amount obtained.Once these in-situ undrained shear values are obtained the

allowable bearing capacities can be determined from

Terzaghi's equations. These can then be compared with the

calculated stresses at foundation level and in the stressed levels

below the foundation base within the pressure bulb (Table 3.4).

A correction factor 11 can be used depending on the

plasticity index of the clays. These fall within the range

shown in Table 3.3. For a strip footing q. = 1.90SIl,

neglecting overburden pressures.

Consolidation settlement

Table 3.3. Correction factor, J l

Clay category Undrained shear strength J l(kN/m2)

Very stiff ,/-;_150"'\.r '110' e l l 1.0

Stiff 100-150 r;~ 0.90

Firm to stiff 75-100 0.75

Firm 50-75 0.65

Soft to firm 40-50 0.60

Soft 20-40 / 0.55Very soft <20 0.50

Table 3.4. Proposed allowable bearing values for clays (after

Terzaghi and Peck (1968»

Description N c qrn

of clay

Square Strip,

Very soft <2 < 13.50 <32 <24

Soft 2-4 13.50-27 32--M 24-48

Medium 4-8 27-54 64-128 48-96

Stiff 8-15 54-107 128-260 96-190

Very stiff 15-30 107-215 260-515 190-385Hard > 30 >215 > 515 >385

N=numberof blowsperfootin standard penetration.est; .c =cohesion(kN/m2);qm = propo s ed normalal lowable bear ing value ( l eN/m2);F = factorof saf e ty withrespectto ba s e failure

Example 3.1

f ' A v - - t ( : < t l~(IV'Vt,-y {

Strip footing on clay soil

A strip footing 1.00m wide isplaced at a depth below ground level

of 1.00 m. Vane test readings down to 3.0 m show that the clays are

firm down to 2.0 m with shear strengths of 60 kN/m2 changing to

30 kN/m2 from 2.0 m down. Determine the allowable bearing

capacity of the soils at foundation level, and check that the widthused is acceptable to carry a line load of 60 kN/m along its length.

Using Meyerhof values for Nc, Nq for I'l=0:

N « = 5.14, Nq = 1.0, Ny = 0

Therefore net ultimate bearing capacity = 5.14 Cll = 5.14 x 60 =308kN/m2

Using a factor of safety of 3.0:

Allowable bearing pressure =~~~= 103kN/m2

Actual pressure = l ~ g=60 kN/m2 < 103 OK.Check at 2.0 m depth:

Allowable bearing pressure = 5.1~~ 30 51.50 kN/m2

From Table 3.2, vert ical pressure factor for zlB = 1.0/1.0 = 1.0

equals 0.55.

Actual pressure =60 x 0.55 = 33 kN/m2, < 51.50.

The safe bearing capacity = Cll NJ3.0 + overburden pressures yz .

Therefore at 2.0 m, safe bearing capacity = 51.50 + 18 x 2 = 87.50kN/m2. Should' groundwater levels rise these pressures should be

halved. The width of the foundations is therefore satisfactory.

71



kN/m2

h, total

ost irn-

settle-

of such

ess of

posed

tables

and

soil).

c, the

t

ading.

elow a

r each

of the

ly over

such

value

the in-

nsoli-

cor-

lue if

p:= J 1 Pood

Values of 1 1 are given in Table 3.3 for the different types of

clay.

Because of the variations which can occur in soils, settle-

ment calculations can only be considered an approximate

guide.

Differential settlements are generally taken as one half of

the total settlements calculated. Though this is only a rule of

thumb, it is adequate for simple structures on fairly uniform

strata.

For sites where the strata consist of soft uniform clays to

depths in excess of twice the foundation width, there are

quick approximate methods available for calculat ing the total

consolidation settlements. Consider a simple strip footing

loaded to give a contact pressure of p kN/m2 at the base of

the footing. The vertical normal stress beneath the centre of

the footing can be considered to be in the form of a tri-

angular dispersal as illustrated in Fig. 3.6.

f(

1.58

Fig. 3.6 Strip footing on uniform soils: vertical pressure

distribution.

1.678

Directly below the footing the contact pressure will be p,

which equals O'z . At a depth of 1.50 B below the footing, O' z

is approximately equal to 0.1 p. Therefore the maximum

depth of stressed soil is equal to

1.50B = 16 7 Bm0.90 .

For an average pressure O' z of 0.50 p the maximum total

settlement is given by

P o e d = 1.67 B x 0.50 p x m y = 0.835 my B mm

This is an approximation. On a soil of infinite thickness with

m y decreasing with the total normal vertical pressures, it

would not be realistic to consider settlements deeper than 2B

below the footings.

Consolidation settlement

Flexiblesquarefoundation

; ; - . 1

2.56 2.08 8

58'~---+----4-----L---~--~28 8 0

I

Fig. 3.7 Vertical pressure under a uniformly loaded square

foundation.

(b) Influence line/actors method

Where a footing is on an infinitely thick layer of soft com-

pressible stratum which has a constant strength it is possible

to calculate the total settlements using depth influence

factors.

As m; decreases with depth (as the vertical pressures

decrease) the settlements between ZI and Z2 (variable depths)

can be obtained by using the coefficients obtained from

Fig. 3.8:

Pood =m y B p (h-[I)

where IIand li are the influence factors for depths z , and Z2

respectively.

Example 3.2 Settlements on clay soil

A house foundation 600 mm wide supports a three-storey gable

wall . The applied line load equals 65 kN/m run. Vane test results

taken at various depths in the soils directly below the footing indi-

cate that there is a stiff desiccated clay crust for 1.0m below ground

level underlain by a soft silty clay alluvium. The values of the vane

shear strengths are 75 kN/m2 down to 1.0m depth and 30 kN/m2

down to a depth of 3.0 m below ground level . Determine the total

settlements under the gable foundation.

73

o020 040

Influence factor, I

060 080 1 0 120 140

- - ~ r--r-- . _ _~

~ ~

r-;~ ~ <

\'\1 \ ' \ .~ ~ ~t,.,;,,~)

\ \\ -, <,r-,

~

\ r -

K \r- ~ -,

1\ II

tIJ- . . . .-

~'iii' '& \" E

I f \ \ ]

1.0

~~ 2.0Ql

o

3.0

4.0

Fig. 3.8 Influence line factors I at the centre of a foundation.

Stress applied at footing level:

65x103 = 108 kN/m2600

Consider the loading to be on an infinite footing length. The vertical

stress distribution graph is shown in Fig. 3.10. The values of pv are

obtained as follows:

!...= 0.25 = 0.41B 0.60

Therefore

Pv =0.75P

pv = 0.75p = 0.75 x 108= 81 kN/m2

!...= 0.75 =1.25 r, =0.50p=0.50x108=54kN/m2

B 0.60

!.. .= 1.25 = 2.08 P: = 0.30p = 0.30xl08 = 33 kN/m2B 0.60

!...= 1.75 =2.91 Pv =0.20p=0.20x108=21 kN/m2B 0.60

!...= 2.25 =3.75 Pv =0.14p=0.14xl08=15 kN/m2B 0.60

The stiff clay has an undrained shear strength of 75 kN/m2.

Therefore an approximate value of Evl (the modulus of

compressibility, = lImv) is given by

Ev l = 130Cu = 130 x 75 = 9750kN/m2

The soft clay has an undrained shear strength of 30kN/m2• Therefore

Evl = 130 Cu = 130 x 30 = 3900 kN/m2

The total settlement is equal to the area of the pressure diagram in

Fig. 3.10. For an approximation take a triangular distribution. Then

74

108+81 0.25xl03Cil=--2- x-

9-7-5-0-

81+54 0.50X!03

Ci 2 =-2-x 3900

54 + 33 0.50 x !O 3Ci 3 =--2-x 3900

33+21 0.50xI03

Ci4 =-2-x 3900

21+15 0.50x103

Ci5 =-2-x 3900

=2.42 mm

=8.65mm

=5.57mm

=3.46 mm

=2.30 mm

Total settlement =22.40 mm

This is less than 25 mm and is considered acceptable. A corr

factor J 1 could be applied where J 1 = 0.55-{).6 which would re

a settlement of about 14mm.

Using the quick approximate method:

P =0835B =0.835xO.60xIXI08X103

=138~ . ~P ~oo .7mm

3.4 MOISTURE MOVEMENTS

One of the commonest problems with cohesive soils

effect of the soil's drying out as a result of extreme

weather or from moisture abstraction by roots of large tThe slow volume changes which occur when mo

evaporates from a clay soil can be predicted by assumin

lower limit of the soil's moisture content to be the shrin

limit. Desiccation beyond this value will not bring abou

further reduction in volume.

Many clay soils in the UK, especially in the southe

England, possess a large potential for slow volumetric ch

However, the mild damp climate which generally pre

means that any significant deficits in soil moisture co



ult in

s is the

e dry

trees.

ing the

rinkage

out any

ast of

change.

content

Moisture movements

qlQ

o 1 02 03 04 05 06 07 0.8 0.9 10Or-~--~--~~--'---r-~--~~~~~--.-~~~--'---r--T--.-~---r--.

0.51--t_-+_-t-_t--f'+_t-;. e~~/ I ./

/o v V V/ ~\~e'l'l'V~-+--t_+--t-+---l--I

J i _ jV' / )::'; /1.01---+-+---i-['_~++--+-..y___'-+V~""-+-+-I--+-+--+-+--l---1

UJ V .ffL .,I / ij~--h./~~-f--+--+--+--+--t--t--t----i,.---J

~~---+-~~~~+-~~~~--~~--+-~---t---+---t--t---t--~--+---t--;I If 0°fLV

zlB I V

J

1 J I2.01--+---+---I+/I-I---+-/+,-lI-+--I--I--t--+----l-+---+---+--I---I--t---t----t

V I

1 / / /

/ 1

4.01--4-+~-~-4-~--1--~--+_-;--~-+_-;~-+-+--4~~-+__t-_r__i

Fig. 3.9 Distribution of vertical stress beneath a long strip footing.

0.75

Soft silty

dayalluvium

which occur in the summer months are generally limited to

the top 1.0--1.50 m below ground level, and these soils gen-

erally recover over the winter period. However, it is recog-

nized that deeper permanent deficiencies can be caused by

large high water demand trees.

Driscoll states that the moisture content of a clay at valuesof suction eF equal to 2 and 3 are 0.50 liquid limit and 0040

liquid limit. These provide a crude estimate of the moisture

content at the beginning of the desiccation process and when

it becomes significant.

In addition to shrinking, these types of clay are also prone

to swelling when the clays rehydrate. Poor or inadequate

drainage can introduce excess water into the soils and weaken

them. Expansive clays can be identified from their plasticity

characteristics. One of the soil properties most widely used

2.0m

~.

-.k~:::!'!..

Fig. 3.10 Example 3.2: vertical stress distribution.

75



s-

~»>

->_,

~

./'->

->v

~~

. . - - -

52 53 54Moisture content{%)

Fig. 3.12 Liquid limit graph using cone penetrometer test. The liquid limit is the'water content corresponding to 20 mm penetration.

55%.


to predict swelling potential is the activity of the clay. This

was researched by Skempton in 1953. A clay's activity is

defined as

Plasticity indexActivity = .:...__

Clay percentage

Clays with large activities are referred to as active clays; they

show plastic properties over a wide range of moisture content

values. Figure 3.11 (After Skempton) indicates the rela-

tionship between the plasticity index and clay percentage.

100

~80

0:

~ 60

~:5'40

~ , : ~ ~ ~ ; ; ~ ~ : : : = = = = = = = ~ ~ ~ i ~ n ~ : e : : ~ ~ ~20 40 60

% Clay fract ion> 2 ( . 1 1 1 1

Fig.3.11 Clay 'activity' graph (after Skempton).

80

To design foundations in cohesive soils. such that the

effects of moisture depletion or excess moisture do not result

in settlements or heave, it is essential to determine the Atter-

berg limits of the clays. These tests will enable the clays to

be fully classified. They were designed by Atterberg in 1911.

The tests determine the various values of moisture content at

which changes in a soil's strength characteristics occur; as a

26

25

24

23

22

1 2 1co~ 20

Q i

: Ii 19c.G)

§ 18o

17

16

15

14

50 51

76

silt or clay dries out its strength increases and it becom

compressible.

The moisture content at which the clays stop acting

quid and start acting as a plastic solid is known as th

limit. As further moisture is removed from the

becomes possible for the clays to resist large s

stresses. Eventually the soil simply fractures with no

deformation taking place. The limit at which plastic

changes to brittle fracture is referred to as the plastic

The plasticity index is the range of moisture conten

in which a clay is plastic. The finer the grain particles

soils the greater is its plasticity index.

Plasticity index = Liquid limit - Plastic limit

PI=LL-PL

100

3.4.1 Liquid limit test

BS 1377 specifies two methods for determining the

limit of a clay sample. Field samples for these tests

be a minimum of 2 kg or greater.

(a) Conepenetrometer

The clay sample to be tested is first kiln-dried and tho

ly mixed. 200 g of the sample are then sieved thr

425 micron sieve and placed on a glass slide. The sa

then mixed with enough distilled water to form a p

standard metal mould, approximately 55 mm in diame

40 mm deep. is filled with the clay paste and levelled

55 56 57 58

s less

as a li-

liquid

lays it

plastic

t with-

s in the

liquid

should

rough-

a

ple is

ste. A

ter and

.off at

the surface. The cone penetrometer is placed at the centroid

of the sample and level with it. The cone is then released so

that it penetrates into the sample and the full penetration

depth over a period of 5 s is measured.

This test is repeated by lifting the cone clear, filling in the

first depression with more paste and allowing the cone to fall

again. If the difference between the two measurements is less

than 0.50 mm then the test is considered to be valid. The

average penetration is noted and the moisture content of thesample is determined in the normal way.

The procedure is repeated at least four times with increas-

ing moisture contents. The amount of water added to the

samples should be just sufficient to produce depths of pen-

etrations within the range of 15to 25mm.

The liquid limit is then found by plotting the variation of

cone penetration on the vertical scale against the various

moisture content values on the horizontal scale. A best-fit

straight line should be drawn through the points on the graph.

The liquid limit is taken to be the moisture content which

corresponds to a cone penetration of 20mm (Fig. 3.12).

(b) Casagrande apparatus

This method was superseded by the cone penetrometer, but it

is still widely used. The process of drying the sample and

making a paste is similar to that for the cone method. The

paste is then placed in a special brass cup and a 2mm wide

groove is cut in the top of the sample using a special profiled

grooving tool. The brass cup is then inserted into the

apparatus and the handle is turned at a rate of 2 rev/so This

actuates the cam which causes the brass cup to lift 10mm

and then fall on to the base plate. The number of blows to

close the 2 mm gap over 13mm is recorded and the moisture

content is determined in the usual way (Fig. 3.13).

Rubber base

(a )

13 mmt ~C

(b)

Fig. 3.13 Casagrande apparatus: (a) Casagrande liquid limit test

apparatus; (b) grooving tool.

The test is repeated at least four times and the moisture

contents are plotted on the vertical scale against the number

of blows on the horizontal scale, using a log scale. The mois-

ture content which corresponds to 25 blows is the liquid

limit, expressed as a whole number (Fig. 3.14).

Moisture movements

8 0

. . . . . . . .<

. . . . . . . . . . . . .

~Uquid limit r - . . . . . . . . .r--- rr-r-,I

~l <t - - . . .\t

l

~:::-70§c: :oo 60

~LLiii

~ 50

40o 10 15 20 25 30 40 50 60

Number of blows (log scale)

Fig. 3.14 Graph for l iquid limit determination using Casagrande

apparatus.

3.4.2 Plastic limit test

Take 20-25g of the clay sample after it has been kiln dried.

The sample is then placed onto a glass plate and sufficient

water is mixed with the sample to form a paste which can berolled out between the palm of the hand and the glass plate.

The sample is said to be at its plastic limit when itjust begins

to crumble at a thread diameter of 3 mm. The moisture

content of the sample is determined and the test is repeated

several times.

Once the soil plasticity characteristics have been found.

the clays can be classified by using the Casagrande plasticity

chart (BS 5930:1981) which will enable comparisons to be

made.

(a) Casagrande plasticity chart

To use the plasticity chart (Fig. 3.15) the coordinates forplasticity index and the corresponding liquid limits are plot-

ted. The sample can then be classified from its position on

the chart relative to the A line: an empirical boundary be-

tween inorganic clays which come above the line and

organic silts and clays which come below the line. The A

line is drawn through the baseline where the PI is equal to

zero and the liquid limit is 20%.

The main soil types are given specific designation letters

and additional designatory lettering is used to denote the

grading and plasticity (Table 3.6).

(b) The triaxial test (undrained compressive test)

This test, to determine the values of the total shear strength

parameters of a soil, is carried out in the triaxial test appar-

atus but the sample is prevented from draining during shear-

ing and is therefore sheared immediately after the application

of the cell pressure. The test is quick and the results are

expressed in terms of total stress.

The shear strength of a clay soil is made up of

two components: cohesion and frictional resistance. Samples

of the clay are subjected to quick undrained triaxial

77



rate

circles

dc to

st gaveof

circles

on th e

drawn

Bibliography

03=70 _ I

J01 = 107

JI I

03 = 1401

I01 = 258

03 = 210

01 =340 ~I

Fig.3.16 Determination of c and j!j from Mohr stress circles .

Shear stress

[ : L _ _ _ _ _ . , L _ _ ~ ~ e ~Normal stress

I :

~ I

~ I1

Fig. 3.17 Construction of Mohr's circle of stress.

BIBLIOGRAPHY Carter, M. (1983) Geotechnical Engineering Handbook, Pentech

Press. London.

Casagrande, A. (1947) Classification and identification of soils.

Proc. American Society of Civil Engineers. No. 73.

Department of the Environment (1991) Approved Documents.

HMSO, London.

BSI (1999) BS 5930: Code of practice for si te investigations,

British Standards Insti tute.

BSI (1990) BS 1377: Method of testing for civil engineering

purpose, British Standards Institute.

79




Meyerhof, G.G. (1952) The ultimate bearing capacity of founda-

tions. Geotechnique, 2 (4), 301-332.

Peck, R.B., Hanson, W.E. and Thombum, T.H. (1974) Foundation

Engineering, John Wiley, New York.

Powell, M.J.V. (ed.) (1979) House-Builder's Reference Book,

Newnes-Butterworth, London.

80

Skempton, A.W. (1951) The bearing capacity of clays. Bu

Research Conference, Institution of Civil Engineers, Div.

Terzaghi, K. and Peck, R.B. (1968) Soil Mechanics in Engin

Practice, 2nd edn, John Wiley, New York.

Tomlinson, M.J. (1980) Foundation Design and Constructio

edn, Pitman.



E(') 0 ('\10100 E~ ~ (') o co~ 0 (')100 (;jg ~~ 0 ....

0..:;co -e-- E"":N c? 10P , .. .. , .. .. (\I N~ L COr-

01/

(3)/ 2) (1 )

0

/-

0"..

0/

/./

v - I

"../

o f . I . I T I ) 2 8 20 60 200 600 2 8 20 60 200(m

Clay:Fine I Medium Coarse r Rne IMedium r Coarse I Fine I Medium I Coarse I Cobbles

Silt I Sand I Gravel I

i/ding

,180.

4th

4.1 CLASSIFICATION OF SANDS AND GRAVELS

Sands and gravels are classified in the laboratory by carrying

out a sieve test. In this test the soil samples are washed, kiln

dried, and then run through a set of graded sieves. The soil

mass retained on each sieve is recorded and the results are

plotted on a particle size distribution chart as shown in

Fig. 4.1. From these grading curves it is possible to

determine for each soil sample the total percentage of a

particular particle size and the percentage of particle sizes

larger or smaller than any particular particle size.

• A sand or gravel is deemed to be well graded if the curve

on the chart is not too steep and is constant over the full

range of the soil's particle sizes with no excess or defi-

ciencies of any particular size of particle.

• A sand or gravel is deemed to be poorly graded if the

majority of the curve on the chart is too steep, the soils

10

9

8

2

Chapter 4

Foundations in

sands and gravels

have a limited particle size distribution and most of t

particles tend to be about the same size.

• If the curve on the chart shows a large percentage of larg

and smaller particles with only a small fraction of theinte

mediate sizes then the sample is deemed to be gap-graded

Figure 4.1 illustrates the curves for three samples usin

sieve analysis:

I . a gap-graded sandy gravel;

2. a uniformly graded sand;

3 a fine silty clay.

4.1.1 Composite sands and gravels

Gravels laid down in the form of alluvial deposits are usual

mixed with sands in various proportions. Table 4.1 lists t

required description for such mixed soils based on the

composition.

British Standard Sieve Sizes

m)

Fig. 4.1 Particle size distribution chart. Sample 1:gap-graded sandy gravel. Sample 2: uniformly graded sand. Sample 3: fine, silty sand.

8

ere is

ty or

ed by

when

test

in thesands

trial

ushed

pick.

than

g into

a soft

). A

hand

of a

n the

after

sides

soils

Such

es of

t too

2. Was any groundwater evident? The level of groundwater

in granular soils can affect the allowable bearing capacity

and settlement criteria by a factor of 2. The level of the

groundwater lowers the effective stress parameters of the

soils, thus reducing their ult imate bearing capacity.

3. Did the sands flow laterally because of water ingress?

Fine sands in a wet loose condition are susceptible to

lateral movement when excavated. Major problems can

occur on sites where drainage excavations follow on afterfoundations are placed resulting in a loss of confinement

of the sands below the foundation base with resulting

subsidence.

4. Did the sands contain soft clay or soft silty lenses? Such

soils, if encountered, will require the bearing capacity of

the sands to be re-assessed and most likely reduced to

avoid excessive settlements.

In general, therefore, well-graded dry sands or gravels of

medium dense or dense composition have higher ultimate

bearing capacities than most cohesive soils. In addition, the

settlements under load take place quickly but these settle-

ments can increase if the groundwater levels rise to within a

distance equal to the foundation width.

4.2.3 Groundwater levels

It is most important to make sure that the full effect of arising water table due to seasonal movements is allowed for

in the foundation design. Trial pits excavated in a dry sum-

mer period may not be so dry during a wet winter period. If

there is any doubt, it should be assumed that groundwater

could rise and consequently the allowable bearing capacity

should be halved.

Table S in BS 8004:1986 gives the density classification

for granular soils based on standard penetration test results.

These are shown in soil reports as N values.

4.2.4 The standard penetration test

This test is the most widely used method of determining the

relative density of a granular soil. The test involves placing a

split spoon sampler with a bottom steel driving shoe on to

the drilling rods. When the borehole has been sufficiently

advanced the split spoon sampler is lowered down the hole

and driven into the soil by means of hammer blows on the

top of the drilling rods. The hammer weighs 63.S0 kg and is

dropped a distance of 760 mm. The number of blows re-

quired to drive the sampler through three ISO mm intervals is

recorded. The sum of the number of blows required to drive

the last two ISO mm increments is recorded as the N value.

The first ISO mm increment is a seating in allowance and is

disregarded (Fig. 4.2).

SPT values are usually obtained at 1.50-2.0 m intervals.

Though used predominantly for granular soils the test can be

used in granular fills, mixed soils and clays, but the results

Obtained must be tempered with caution.

Table 4.3 illustrates the relative densities of sands and

gravels based on SPT results.

Relative densities of granular soi

1"-- Drilling rod

Holes for tommy bar

~-- Split barrel

~H--- Open cutting shoe

50mm

I--l

~

60° cone shoeadded for takingSPT readings

Fig. 4.2 SPT apparatus, showing drilling rod, split barrel sampler

and SPT cone shoe.

Table 4.3. Relative densities of sands and gravels based on S PTresults

Relative density N, blow count/300 mm

Very loose

Loose

Medium dense

Dense

Very dense

<4

4-10

10-30

30-50

>50

The SPT is an empirical test, based on experience, and

suitable precautions need to be taken when carrying out the

test to ensure accurate results. The base of the hole must b

carefully cleaned out, removing any disturbed soil. When

drilling in sands below the water table, the positioning of the

casing can be critical to obtaining accurate results. If the

casing is not advanced far enough, the wet sands can surge

into the borehole, which will result in low N values. If the

casing is extended too far, the sands may be compacted and

high N values will result.

Interpretation of the test results is based on experience,

and- many researchers in soil mechanics have produced

correlations to be applied for various soil conditions. A

83

Foundations in sands and gravels

rCorrection factor = corrected N value

measured N value2 3 4

Ie~~100~------~Yr----------t----------1~a.c

'E:>

£~~ 150~--~L---1-----------~--------4>U

~

250L---------~--------~L---------~

Fig. 4.3 Depth-correction values for SPT N vaiues (Gibbs and

Holtz, 1957).

shallow depths the recorded N values tend to be under-

estimated, and correction factors were derived by Gibbs and

Holtz (1957). These can be obtained from Fig. 4.3 to take

account of the overburden pressures.

Silty sands and saturated silts usually produce an over-

estimation of the relative density and a modified N value, Nm ,can be obtained from the formula

Nm =15+.!_(N-15)2

where N> 15. IfN < 15then no correction is required.

4.2.5 Interpretation of SPT results

Terzaghi and Peck (1968) produced correlations for bearing

capacity factors based on the relative density of the soils

obtained from standard penetration results. Figure 4.4 illus-

trates the factors N q and N.., derived by Terzaghi, Peck and

Hanson using the angle of shearing resistance c p . In addition,the SPT results can be used in Fig. 4.5 to determine the

.allowable bearing pressures for foundations in excess of

1.0m width based on settlements not being greater than

25 mm as indicated in BS 8004 Table 1.

However, w h e r e foundations narrower than 1.0 mare

being used in sands the allowable bearing capacity must be\

checked, as research has shown that these reduce rapidly and 1bearing capacity failure becomes the criterion with a suitable.r

factor of safety applied.

A set of charts are indicated in Figs 4.6, 4.7 and 4.8 based

on different ratios of depth to foundation width and based on

84

14

13

12

0

0 ....... J Ny

<, I'0

" ' "0

r-, ,<,

I!Nq

0~ I J

0

II " '\.0

'J \0 I ,f\

1 /l l'

.Iy

A

1/'~.

.:./

11

;:(10""0c<II 9~~ 8

~~ 7>.

~ 6a.

~ 50Clc'iij 40

ID 30

20

10

o28 30 32 34 36 38 40 42 44 46

Angle of shearing resistance, Ijl

o410

20

Very

Lo

Mede

De

V

de

Relden

30

40

50

60

70

Fig. 4.4 Correlation of values of e, N q and Ny with SPT testsPeck, Hanson and Thornburn).

.0 3Footing width (m)

Fig. 4.5 Allowable bearing pressures on sands based on SPT

N values (25 mm settlement criterion) .

5

a straight-line relationship for simplicity. These valu

for dry soils and based on a factor of safety of 2. Shou

water levels rise to within a distance equal to the foun

width, then these values should be halved.

4.2.6 Ultimate bearing capacities

For granular soils where the dissipation of pore

pressures is usually fairly rapid, the effective shear str



e

y

s are

the

water

650~,--------.--------,--------.--------,

N=50

5401~-------+--~----~------~--------~

N=40

~4301~-------+~------A--------,--------~

}e.'iii0-

~3251~------~~----f-----+-~------~--------~

BC)

c

ss~ 215,~--~L---*---~-----h~-------+--------~

1'aQ ;

z

N=30

N=20

N= 15

300 60Q _ . '" 900

\ \.'£, 'foundation widfR, B (mm)

65,lli--------r-------.------~------~

N=50

N=40

'"4301r---------~--~----~------~--------~e . .

s>.:t:o

~3251r_------~~------_r~------~L-----~~

~

1£2151r_-----+~--~~--_hL-----~~------~

~O JQ ;

z N= 10

108J--I--J.~L~~4:~::=::==I=====t

N=30

N=20

N= 15

N=5

600 900 1200Foundation width, B (mm)

Fig.4.7 Net allowable bearing capacity for zl B =0.50.or

foundation settlements not exceeding 25mm. Factor of safety = 2.0.

Relative densities of granular soi

Fig. 4.6 Net allowable bearing capacity for zl B = 1.0. For

1200 I. foundation settlements not exceeding 25mm. Factor of

~ 1- . l , . , ( , ( ' safety = 2.0.

6501,----.----.------.------,---

N=50

108N= 10

N=5

300 600 900Foundation width, B (mm)

Fig. 4.8 Net allowable bearing capacity for zl B =0.25.or

foundation settlements not exceeding 25mm. Factor at safety =2.

1200

8



·~

/~

/~

~

·~v

·

v/r--

).. .V -~

0. . . .

,/. . . . . . . ./

Ny, V.

_//

. , . , i - - " Vv-I- Nq/- /


45

40

-Go 35

mo& i 301 ii

8 i~ 25c:

~~ 20til

'0Q)

"Ol15

.& :

10

S

0.2 0.3 0.40.S 1.0 2

Fig. 4.9 Bearing capacity factors Ny and Nq for sands and gravel strata. Ny and Nq values based on SPT N values andj1j

relationship.

Table 4.4.

Value o f e

(degrees)

Relative Round grain Angular grain Silty Sandy Inorganic

density Uniform Well graded sands gravels silts

27-33 35

30-34 50

Loose

Dense

27.50

34

33

45

27-30

30-35

are used when considering the allowable bearing capacities.

Because of the difficulty in obtaining undisturbed samples in

the field for laboratory testing, the ground strength para-

meters are usually obtained from SPT results using the

various correlations.

Using Terzaghi equations, the ultimate net bearing

capacity of a shallow foundation is given by:

Strip footings

_ ( ) yBNyPnu-eNc +r; Nq -1+-2-

Pad foundations

P nu = 1. 2 e N; +P« (N q - 1) + 0. 4 yB N y

where e = the shear strength of the soil;"(= the bulk density of the soil;

B = the foundation width;Po = the effective overburden pressure;N c, Ny and N q are Terzaghi bearing capacity factors.

When the soils are granular, and e =zero, eN c = O .

Figure 4.9 and Table 4.4 can be used to determine the Nq

and Ny based on l/ J values.

86

3 4 S 20 30 40S0 2000 100

4.3 CONSTRUCTION PROBLEMS IN GRANULA

SOILS

Granular deposits make good founding strata if they ar

medium or dense state of compaction. In fact, such sand

gravels perform better than firm clays in that the found

settlements occur immediately they are loaded.

The major construction problems are caused

granular deposits have water-bearing levels either in the

of wet lenses or as a standing water table. Any excav

below the water table will cause instability in the sides

excavation. In addition the base of the excavation can

if the sands are very loose and start to flow.

If the water ingress into excavations is not too hea

may be possible to control itby pumping from a lower s

Consideration must be given to the effects of tempo

lowering the groundwater level especially if there

existing buildings close by. Any groundwater low

could remove the fine particles from the surroun

locality and result in subsidence of existing buildings. I

be necessary to consider the use of chemical inje

methods which are suitable for fine-to-coarse sands

gravels.

On. open sites where deep drainage is to be insthrough water-bearing granular soils, well pointing c

adopted if the soils have a fine-to-coarse grading.

system can also be used closer to existing building a

filtering system does not remove as much of the

particles as occurs when pumping from open sumps. W

the soils have a grading less than 0.06 mm, such as

silt, well pointing is not a suitable method of groundw

lowering and it may be necessary to use electro-osmos

ground-freezing methods.

in a

s and

hen

form

boil'

it

are

ding

may

and

n be

This

s the

finer

fine

or

If water levels on a site cannot be lowered owing to

various circumstances it will be necessary to consider using a

! /':""Iraft foundation or piles. If a raft is used the main housey .

;,_,...-; ;, d rainage should be installed prior to forming the raft

formation. If piling is used, it is preferable and more eco-

nomic to use a precast concrete driven pile or steel pile than

a bored pile with a temporary casing. Alternatively a con-

tinuous flight auger pile can be used if wet granular bands

are evident at depth.

4.4 FOUNDATION DESIGN IN GRANULAR SOILS

The allowable bearing capacity of granular soils is usually

limited by settlement considerations. The allowable bearing

pressures shown in Fig. 4.4 are based on a maximum

settlement of 25 mm with ,nofactoI:s gf safety included for.

.They a re also based ontheassumpti6n'that the wafer table is

ata depth of at least Bbelow the foundation base.

For foun(iations l:ss than' ~.O.~~~.peatj~gcapaCity should be""checkeci, wffii a factor of safety of

~eirigappUe4agamst'1)eaniig"Capacity"'Iarrure~~'"

' U e a r r n g " c a p a c i t y ,failure of soil below a foundation is

generally accompanied by high settlements and rotational

movements, It i§ differential settlements that give rise to

most structural failures, and to a~~~'ii~Cturr~';lcesiiief o u n : d a t i ; ; ~ o ( sh~~i(fbt'designed to keep total settlements

Iwithin acceptable limits. In situations where sands a re

" 'Yaterlo~<!!I..m2:!J2!!.Q4~~p.Qwg~gjx~~t.~L!LWi.r:!i'i~'·", capacity failure.

~ .. ..._.~ - _'_ '__ ' ' _ - -" ,_ ' " • ' - _:_ • c _'

T For most practical uses it is sufficiently accurate to

obtain the values of N q and N y from the SPTresults and

obtain c jl from Table 4.4 and Table 4.5 derived by Terzaghi

andPeck (1968). "

0.4

Foundation design in granular soils

Table 4.5. Factor Nq (from Terzaghi and Peck)

SPTblow Angle of internal Nqcount.N friction, l : ' l

(degrees)

lO 30 18

20 33 26

30 36 37

40 39 55

50 41 72

~ ~ _ \ ~ f ~ L · \ ~ 1 o

Example4.1 Stri~;g on granu.l~rsoil

" ",'),~'\A strip footing for a fact'\?ry,..i$o ~ 5 0 ' mm wide. The maximum

line loading on the wall ~ 5~kN Per"metre run. Field tests have

shown that the site is und'eFlain by medium dense sands with an

angle of internal friction of 350 with average N values of 15 at

1.20m depth. Determine the net allowable bearing pressures and

check the foundation width if the,depth of the foundation is 1.20 m

below ground. Bulk density of soil,iS 18.0 kNho3.

FrolI\ Fig. 4.4 , Nq'';' 30 and Ny '; ' 3 0 . Therefore

' [ N ' D }(netult)= 2 '1 + ( N q - l . O ) ; B

where Nq and Ny are bearing capacity factors;

D r = depth of foundation below ground level;

B =foundation width;

'Y =bulk density of soil below foundations.

Therefore'

q(net ult .)=[ 30 +(30-1.0) 0·60'18XO.752.0 0.75J.

=(15+23.20)x13.50=515 kN/m2

With a factor of safety 'of 3.0 the allowable bearing capacity =515/3.0 = 170kN/rIi2• " ,

Actual bearing pressure =55:010.75 =13kNho2•

D,

Water table

0.5O.3L---------~--__--~~---- __----__~~~_,---------------

o 1.0Dw

Dl+B

Fig. 4.10 Correction factor to SPT blow count for depth of water table (after Peck, Hanson and Thornbum).

87

.: I t

73.0 690 ~p= 0.47x45xO.50 =. mm p

.' o,'ty. \ ,~ 1 - _ ;'\ll-" ( ( \ \ A f r rt(\!- \";H't'f Example 4.3\ ' ~~

When excavations on site result in variations in the formation

bearing stratum with sands alternating with f i r m clays it is only

necessary to provide a bottom layer of mesh reinforcement. say

B283 as required in BS 8004.


As a rise in the groundwater level could result in the allowable

bearing capacity being halved to 85 kN/m2 this is a suitably sized

foundation which will keep settlements within acceptable limits.

Calcul~ted settleIl}!;~:''''\ / ,,~'\ \:" ('/

, " P : ' ; ' ' f A A l,",".'v",~, - \\''''Y

settlement p=-- 't ' ( ,\0.47N' ) """ J . , \

where P = applied bearing pressure; N = SPT blow count.

Overburden pressure = 18 x 1.20 = 21.60 kN/m2

From Fig. 4.3, correction factor = 3; therefore modified blow count

N' isgiven by

N'=3 x 15 =45

Assume groundwater can rise up to foundation level, and apply a

correction factor of 0.50 from Fig. 4.10. Then

Example 4.2 Pad foundation on sand

A pad foundation 3.0 x 3.0 m is founded at a depth of 1.0 m in a

thick sand layer which has been assessed as being in the dense

category. The value of shearing resistance = 35° and the sand's

in-situ bulk density is 19 kN/m3. Determine the safe bearing

capacity of the sands using a factor of safety of 3.0. .

Safe bearing capacity = qunet + r Z3.0

qunet=rZ(Nq -1)SqDq +0.5rBNySyDy

where Sq= shape factor = I + (B/L) tan (from De Beer);

D q = depth factor = 1+ 2 tan (1- sin < 1 > ) 2 (z /B) for z/ B = < 1.0;

Sy= shape factor = I - 0.4 B/L;

Dy= depth factor = 1.0 for zl B = < 1.0.Using Table 4.6 for = 35°, Nq=33.30 and Ny= 48.03:

Sq =1+~tan35=I+lxO.70=1.703.0

Sy = 1- 0.4 x 3.0 = 0 .603.0

D q =1+2xO.70(1-0,573)2 x~B

= 1+ (0.427)2 x.!. =1.0863

quit = 19x 1.0(33.30-1)1.70x 1.086

+0.5 x 19x3.0x48.03xO.6xl.O

= 1133+ 821.31= 1954.31 kN/m2

S & be . . 1954.31 19 10ale anng capacity = --- + x.3.0

=651+19 = 670 kN/m2

This value would be halved if the water table rose to within 3.0 m

of the foundation base, i.e. 335 kN/m2.

88

Table 4.6. Typical bearing capacity factors

p

(degrees)

o5

10

15

20

2530

35

40

45

50

5.13

6.50

8.34

10.98

14.83

20.7230.14

46.12

75.31

133.87

266.88

1.0

1.57

2.47

3.94

6.40

10.6618.40

33.30

64.20

134.87

319.06

(~

Strip footing on sand, high

A continuous strip footing 1.0m wide is founded at a

1.0m in a well-graded angular sand which has a bulk de

18.50 kN/m3• The water table is known to fluctuate to

foundation level. Determine the ultimate bearing capacit

sands if the soil strength parameters are based on SPT resul

12at 1.0m depth and N = 15at 1.20m depth.

Consider that < jl = 30° for N = 12 and = 31° for N = 1

continuous footing with < jl = 30, Nc = 30.14. Ny = 22.40 a

18.40:

= 18.50 x 1.0 x 17.40 + 0.5 x 18.50 x 1.0x 22.10

= 321.90 + 207 = 529 kN/m2

Applying a factor of safety of 3.0:

529Safe bearing capacity = - + r Z

3.0

= 176+ 18.50 x 1.0 = 194.50 kN/m2

As the groundwater level can rise to the foundation ba

prudent to halve this value and use a figure of93 kN/m2•

Example 4.4 Bearing pressure of granular

A granular stratum was tested at depths of 2.0 m and 3.0 m

N blow counts recorded were 18. Groundwater was measu

depth of 1.30m below ground level. The saturated sands ha

density of 19 kN/m3.

A strip footing is to be placed at a depth of 1.0 m and isreqbe 1.20 m wide. Determine the allowable bearing pressure.

Corrected N value:

n;=15+t(N-15)=15+t(18-15)

= 16.50

Depth correction factor:

Overburden pressure = 2 x 19= 38 kN/m2

From Fig. 4.2 the depth correction factor = 2.50. Therefore



o0.10

1.22

2.65

5.39

pth of

of

within

of the

of N=

. For a

ndNq =

e it is

soil

nd the

ed at a

a bulk

red to

Steel beam grillage

Thick steelplate beddedon 25 mm sand

Fig. 4.11 Plate bearing test apparatus.

N' = 16.50 x 2.50 = 41

For N' = 41 and B = 1.20 using Fig. 4.4:

Allowable bearing pressure = 490 kN/m2

This value is for a dry sand and should be halved to 245 kN/m2 to

allow for the groundwater levels rising.

4.5 PLATE BEARING TESTS

The allowable bearing pressure of a granular soil can be

determined by carrying out a plate bearing test (Fig. 4.11).

However, such tests must be carried out with full knowledge

of the underlying strata as the plate will only stress a limited

depth of ground below the foundation level and it is most

important that the groundwater level is known.

The main drawback in attempting to determine settlements

with this method is the effect of the small plate stressing ashallower zone of stratum than that stressed by a wider

foundation. The plates therefore should be as large as

possible and never less than 300 mm.

The test plate should be rigid enough to avoid bending and

can be between 400 mm and 800 mm, square or circular. The

kentledge should be placed in incremental stages, each

increment of load being about one fifth of the proposed

bearing pressure. The loading is then increased up to two or

three times the proposed loading, and settlement readings

should be recorded for each stage. Where there is no defini-

tive failure point the ultimate bearing capacity is taken to be

the pressure which causes a settlement equal to one fifth of

the plate width. The results of settlement against load inten-sity should be plotted on a log scale to determine the failure

point.

Terzaghi established that the settlement of a 300 m square

plate at a given load can be related to the settlement of a

foundation by using the formula

(2B ) 2

S 2 =SI --I+B

where SI = the settlement of a foundation of width B, where

Piling into sands and gravel strata

. 1Dial gauges supportedclear of the test area

B is taken as the ratio of foundation width to plate width;

S2= the settlement of a test plate.The ultimate bearing capacity of the foundation can be

assessed from that of the plate by applying the following

formula for granular soils:

Q2 = B 2

Q 1 s ,

where Q2 and QI are the ultimate bearing capacines of

foundation and plate, respectively, and B2 and B I are their

respective widths.

Settlement predictions from plate bearing tests are often

inaccurate but Peck, Hanson and Thornburn (1974) deve-

loped analysis based on field experience of small diameter

plates and actual foundations and their conclusions were that,

for foundations on granular strata:

p

p= 0.47N

where P = applied bearing pressure in kN/m2; N = SPT blow

count; p = settlement in mm. The SPT values should be thetest results with correction factors for depth from Fig. 4.3

and for groundwater levels from Fig. 4.10.

4.6 PILING INTO SANDS AND GRAVEL STRATA

Piles in sands and gravel strata can be bored, in-situ cast in

place driven type, precast or steel driven or continuous flight

auger (CFA) piles. Generally the preferred system is thedriven pile, which in driving increases the density of the

granular strata. Inaddition the risk of having a poorly formed

pile is ruled out when steel sections or precast concrete piles

cast in a factory are used.

Bored piles and CFA piles can be very useful on sites

where there are existing buildings and vibrations need to be

kept to a minimum. CFA piles are generally used when

water-bearing or very soft strata are encountered and a bored

type of pile is needed.

89




4.6.1 Bored piles

These can be formed using a conventional three-leg tripod

rig. For large-diameter piles a special rig design is generally

required, especially if the piles are to be under-reamed, to

enlarge the pile base.

Where the piles have to pass through very weak soils or

water-bearing strata, the piling contractor may need to use

temporary or permanent casings. If the casing is temporary

and the pile is an in-situ concrete system in which the casing

is extracted during the construction of the pile shaft, it is

important to ensure that a sufficient head of concrete is

maintained in the pile shaft to prevent 'necking' of the pile

during withdrawal of the casing.

Great care must be exercised when withdrawing the pile

casings to avoid lifting the pile reinforcement cage and

surrounding concrete up with the casing. To prevent this

from being a problem the concrete should be a rich mix and

have a high workability.

In extreme situations where groundwater inflows are high

it may be desirable to form the pile shaft using a tremie pipe

operation. Concrete placed by tremie operation should be

easily workable, have a slump between 100 and 175 mm,and should have a high cement content of at least 400 kg/m",

In some situations the casing can be replaced by using a

bentonite slurry. The use of a tremie pipe in these situations

requires that an adequate head of concrete is kept in the

tremie pipe to overcome the pressure of the bentonite mud

which has to be displaced by the outflowing concrete.

4.6.2 Continuous flight auger piles

These piles are very useful in soils such as soft alluvium, wet

sands and peat soils. They should only be used when a good

site investigation is available. The auger on the piling rig has

a central core down which a cementitious mortar or fineconcrete can be pumped prior to and during removal of the

pile spoil on the auger. If, on removal of the auger, it is

evident from the soils on the auger tip that the pile has not

been formed in suitable bearing strata, then it is necessary to

replace the auger and reform a deeper pile.

4.6.3 Design of bored piles

In 1976 Meyerhof determined bearing capacity factors for

deep foundations. Similar work was carried out by Bere-

zantsev in 1961 and by Hansen and Vesic, and these factors

are listed in Table 4.7.

Ultimate load capacity Qu = Qb + Q s

where Q b : :;: ultimate end bearing component, and Q s

ultimate skin friction component. Now

Q b = qbAb =a'; NqAb

where a'; = the effective overburden pressure at the pile toe;Nq : :;:the bearing capacity factor (Table 4.7); Ab = area of pileat base. Now

Qs=/sA,

90

Table 4.7. Bearing capacity factors (after Meyerhof)

p No Nq

(degrees)

0 5.14 1.0

5 6.49 1.60

10 8.34 2.50

15 10.97 3.90

20 14.83 6.40

25 20.71 10.70

26 22.25 11.80

28 25.79 14.70

30 30.13 18.40

32 35.47 23.20 2

34 42.14 29.40 3

36 50.55 37.70 4

38 61.31 48.90 6

40 75.25 64.10 9

45 133.73 134.70 26

SO 266.50 318.50 87

where I s : : ; :average value of skin friction developedembedded length of the pile shaft; As ::;:surface are

embedded pile length of the pile shaft. The average

Is is given by

Is =K,c 'v tan Ii

where K ; ::; :he coefficient of lateral earth pressure; (

of friction between the pile shaft and the surroundin

Values for K; and (jare listed in Table 4.8 (derived b

in 1966).

Table 4.8. Typical values for 8 and K, (Broms, 1966)

s,

Pile material Relative density o

Loose

Steel 20° 0.50

Concrete 0.75 p' 1.0

Timber 0.67 p 1.50

\!t' =Angle of shearing resistance in respect of effective stress valu

The values ofIs are limited for pile lengths between20 times the pile diameter or pile width. For practical

maximum is taken as 100 kN/m2. Meyerhof determin

qb is approximately equal to 14N D lB where N: : ; : SP

count; B ::;:pile diameter or pile width; D = embedded

of pile in the end bearing strata; Is is approximately0.67 N kN/m2•N = the average uncorrected N valueshaft length considered.

When constructing bored piles in sands and gra

granular strata will be loosened during removal of

material. In view of this it is prudent to adopt a c

approach and use values of ¢ and N q based on lo

conditions to determine the ultimate end bearing a

friction values. Adopting this approach will resul

ultimate bearing capacity lower than that achieve

Ny

1.10

er the

of the

alue of

angle

soils.

Broms

soil

Dense

1.0

2.0

4.0

10 and

that

blow

length

ual to

er the

s the

core

soil

skin

in an

by a

driven pile in the same strata but the loosening effect of the

boring operations on the base and pile shaft in granular soils

can be quite significant.

Example 4.5 Bored piles

A dwelling is to be supported on bored piles which are required

because of the closeness of an old existing building. The maximum

unfactored line load for the three storey dwelling is 60 kN per metre

run. The soil conditions below the site consist of approximately

4.0 m of very loose to loose ash fill which is still undergoing

consolidation settlement and, below the fill, a medium dense sand

with recorded SPT values of 22. The density of the ash fill is

1300kg/m! and all the boreholes revealed dry conditions down to

15.0 m depth. The density of the sand is 1850kg/m>.

With piles at 4.0 m centres, and using continuously designed ring

beams, the maximum working load on each pile will equal 4 x 1.20

x 60 = 288 kN. Ultimate skin friction on pile shaft, I s = K; 'Y d tano.

Since boring will loosen the sands, use Table 4.8 for K, and o .K,= 1.0, c P =33° and 0= 0.75 x 33 = 24.75. Therefore

Unit skin friction at top of sand = 1.0 x 1.30 x 9.80 x 4 x tan 24.75

= 23.49 kN/m2

Ifwe assume piles will be approximately 10m long:

Unit skin friction at pile toe = 1.0 x (1.3 x 4.0 + 1.85 x 6)x 9.8 x tan 24.75

= 73.64 kN/m2

Assuming 400 mm diameter concrete piles:

Total skin friction = (23.49 + 73.64) x 6 x 1£x 0.40 = 366 kN2.0

Assuming that during the boring the medium dense sands wil l be

loosened and the value of c p will be reduced to 32°. Using the

Berezantsev chart (Fig. 4.12):

Nq for DIB = 6.0/0.40 = 15 is 33

1£X0.402

)End bearing resistance = (1.30x4.0+1.85x6 x9.8x334

=662.50 kN

300r-----~-----r-----r----~----~250200

2~~LL~_LLi_LLl~LL~~~~LL~20° 25· 30· 35° 40° 45°

Angle of shearing resistance, 4 >

Fig.4.12 Values of Nq for pile formula (after Berezantsev, 1961).

Piling into sands and gravel strata

Therefore

Ultimate resistance = 662.50 + 366 = 1028.50 kN

Adopting a combined factor of safety of 3.0, the maximum allow

able working load = 1028.50/3.0 = 342 kN.

Because the fi lls are st ill set tling under their own weight, th

effect of negative skin friction must be allowed for in the pi

design. This value of negative skin friction must be added to th

pile working load.

Assume the negative skin fric tion acts over the top 4.0 m of th

pile. The peak value of negative skin frict ion will not at any tim

act over the whole length of the pile shaft embedded in the fill and

will therefore be necessary to make a reasoned assessment of th

magnitude of the drag-down forces to be used in the design.

Negative skin friction = el y K tan C P ' a-

where el y = effective vertical stress, and K tan c p ' . is assumed to bconstant for the pile's length.

Unit skin friction at the top of the sand was calculated

23.49 kN/m2. This value can be used as the peak value of th

negative skin friction at the base of the fill, as it will b

approximately equal. Therefore total negative skin friction on th

top 4.0 m of the pile shaft equals

(0+23.49)X1£X0.40X4.0=59.0 kN2.0

Therefore

. 1028.50 2 96Factor of safety on pile = ---- = .

288+59

This is slightly less than the recommended value of 3.0 but is accep

able because of the low values adopted for the density of the sands

Using a drivel' pile

Using a 275 mm x 275 mm precast concrete pile driven through th

fills into the medium dense sands (N = 25 blows) to a calculated se

a factor of safety of 2.50 can be adopted.

Maximum pile working load = 288 + 59 = 347 kN.

Therefore ultimate load capacity requires to be 347 X 2.50

867.50 kN. For a SPT blow count of 25, < p = 36°. Using th

Berezantsev chart (Fig. 4.l2), for a.1O m pile length:

! : : _ = 6000 = 21.80B 275

Therefore

Nq=60

Qs=lsxAs

Is = K, d y tan 0

where K; = 2.0 and 0= 0.75 x 36 = 27. Therefore

Qs = 2.0x(4.0rl +6.0rz )tan27x4xo.2752.0

= 2.0 x(4.0X 13.0+6 x 18.50)x 0.509 x 1.10 = 91.26 kN2.0

and

Qb=Pb(Nq-I)Ab

where Pb is the effective overburden pressure at the base of the pil

and Ab is the area of the pile base = 0.275 x 0.275 = 0.075 m

Therefore from the Berezantsev chart

9

L 6000B = 275 =21.80 Therefore V, =60

Q b = 163(60 -l)X 0.075 = 727kN

Q u = 727 +91.26 = 818kN

Though this is slightly less than 867.50 kN, the pile will be driven

to a calculated set and will most likely penetrate the sands for less

than the 6.0 m available.

4.6.4 Set calculations

Using the modified Hiley formula for precast piles,

R; = ultimate load = 347 x 2.50 = 867.50 kN;E = Transfer energy at pile top = 0.70 x 104,

c = temporary compression of pile and ground per blow, say12mm;

s = set blow count.Therefore

E c 7000 12s=---=----

Ru 2 867.50 2

= 8.06 - 6 = 2 rum/blow

Therefore adopt a set of 20 mm for 10 blows or less. In

practice.j, is taken as 100 kN/m2 maximum.

For driven piles in granular soils there are approximate

formulae derived by Meyerhof in 1976 to calculate pile

capacities.

40ND ()qb =-B- but<400N kN/m2

where N = SPT blow count, D = embedded length of pile inbearing strata, and B =diameter or width of the pile.

Consider example 4.5:

Qb = q b X Ab = 400 x 25 X 0.2752 = 756 kN

andj, =N kN/m2 = the average uncorrected N value over theembedded length of the pile in the bearing stratum. Therefore

Qs = 25 A s = 25 x 6 x 0.275 x 4 = 165 kN.

For a bored pile in granular soils

qb = 14ND kN/m2

B

and Is =0.67N kN/m2

Example 4.6 Working load of precast concrete

piles

A 3.0 m thick layer of loose sands and gravels overlie a thicker

deposit of dense sands and gravels. SPT tests in the dense sands

produced values of 35 from the base of the loose sands to a depth of

12.0 m below ground level at 1.0 m intervals. Using a 275 mm x

275 mm precast concrete pile and adopting a factor of safety of 2.50

determine the maximum allowable working load for the pile.

Ul timate bearing capacity = Qu = Q b + Q s

For Q b, ignore the loose sands and use qb

400NkN/m2. Therefore

40ND/B or

92

Dqb =40x35x-- =400x35 max

0.275

D = 400 x 35 x 0.275 = 2.75 m penetration into the dense s40x35

Therefore

Q b = 400 x 35 X 0.2752 = 1058 kN.

Q s in loose sands is discounted. Q s in dense sands =I s x A

Is= N = 35 kN/m2. ThereforeQ s = 35 x 2.75 x 4 x 0.275 = 105 kN

Qu= 105 + 1058 = 1163 kN

. 1163Allowable working load = -- =465 kN

2.50

In granular strata the end bearing component is much

than the skin friction component on the sides of the p

mobilize this skin friction a significant movement

occur at the pile toe. In dense granular strata this mo

is very small and because of this the factor of safet

driven pile can be 1.50 for skin friction and 3.0 f

bearing. Applying these factors to Example 4.6 the all

working load would be

105 + 1058 = 70 + 352 = 422 kN

1.50 3.0

4.6.5 Dynamic pile formula

The ultimate static resistance of a driven pile can b

dicted from the dynamics of the driving operation itse

kinetic energy imparted by the piling hammer is equ

the work done by the pile in penetrating into the g

Therefore

Net kinetic energy =Work done during penetration of pile

For a hammer of weight W tonnes falling a drop heig

and causing a penetration or set of s mrn, the pile res

load R, can be obtained from the formula

R, = Wh - energy losses

The energy losses are due to the pile and pile cap co

ssion, hammer rebound, and frictional losses

equipment.

Driving a pile into sands and gravel strata will incre

relative density of the sands and gravels and this has a

ficant effect on the predictions of load-carrying capaci

For concrete piles the modified Hiley formula is

applied but the Dutch formula is also often used. Theformula should only be applied to piles which obtai

support in sands and gravels, stiff-to-hard clays or roc

not applicable to frictional piles which obtain their sup

soft clays by adhesion along their length.

Specialist piling contractors who rely on piling to

their living generally put their trust in the simple

hammer. These hammers are often considered to be

and old-fashioned but they are very reliable and

effective as sophisticated hammers at less cost. Some



where

reater

e. To

as to

for a

r end

pre·

. The

d to

t of h

the

the

signi-

often

Hiley

their

It is

rt in

make

drop

crude

st as

piling

firms have developed their own equipment and use purpose-

built hammers not readily available on the open market.

The Hiley formula can also be adopted for steel bearing

piles and a factor of safety of 2.0 can be used, where

Working pile load = F R f l'actor 0 sa ety

For practical purposes the ultimate load on a pile can be

defined as that load which causes a settlement of one tenth of

the pile diameter or pile width (Terzaghi and Peck, 1968).

The accuracy of a given dynamic formula can be im-

proved by recalibrating it for a given site against the test load

data obtained from static load tests. The formula can then be

more confidently used as a guide for selecting final penetra-

tions of those piles which are neither near any tested piles or

near ay borehole locations.

Dynamic formulae can be grossly inaccurate: using the

Hiley formula, the actual ultimate load obtained by test load-

ing may be between 0.70 and 3.0 times the figure obtained

by applying the formula. Pile testing should always be

carried out to verify the dynamic formula.

4.6.6 Re-drtve tests

These should be carried out on one or more piles at a

reasonable frequency rate with a minimum time interval of

12 h. Only if the re-drive final set (mm/blow) is equal to or

less than the set of the initial drive can the dynamic formula

be adopted. If the re-drive set is greater than the set obtained

on the initial drive the formula does not apply, and it will be

essential to re-drive the piles until a tighter set is achieved, or

to carry out a static load test.

4.6.7 Base-driven steel tube piles

R = 290W(1.0+h)

u s+ 12.70

where Ru = ultimate driving resistance in tonnes; W = weightof internal drop hammer in tonnes; h = actual drop of

hammer at final set in metres; s = final set (mm/blow). This

formula is applicable for:

• drops between 1.20 m and 2.0 m;

• sets less than 5 mm/blow, i.e. 5 blows to 25 mm .

4.6.8 Top-driven steel piles

W 2 Hs = ( ) metres penetration per blow

RW+p

where W = weight of hammer in newtons;

p = weight of pile (unit weight x length) in newtons;

H = effective hammer drop in metres;

R = penetration resistance or ultimate load capacity in

newtons;

s = set (penetration per hammer blow) in mm.

Allowing for 30% loss of efficiency:

Piling into sands and gravel strat

where WH =kinetic energy. Therefore

W2H

sets= ( )RW+p

Example 4.7 Steel piles

Working load on pile = 295 kN with pile length = 9.0 m. Factorsafety = 2.50. Therefore

R = 2.50 x 295 = 737.50 kN

Mass of hammer, m = 3.06 kN;

weight of hammer, w = 30 kN;

weight of pile, p =0.648 L kN where L =pile length;

effective hammer drop, H = 0.35 m.

Velocity of hammer at impact = 2 g H = 2 x 9.81 x 0.35 = 2.62 m

K" . mv2 3.06 x 103 x2.622menc energy =2 2 10500.0 Nm

Reduce this value by 30% for losses =0.70 x 10500.0 =7351 Nm

Therefore

s = _ W . , .. ,( _ W _ H ' - ,- )

R(W +p)

30x 103(7351)8.34 mm/blow

103 x37.50(30+0.648x9)

Therefore, use three blows of 350 mm for a set of 25 mm.

Example 4.8 Driving precast concrete piles

A 275 mm x 275 mm precast concrete pile is to be used to carr

safe working loads of 350-500 kN' The pile is reinforced with eigh

12mm high tensile bars in pairs bundled in each comer. Concret

strength is 50 N/mm2. Fy = 590 N/mm2; Feu = 50; A's = 452

As=452.

Ultimate axial compression load:

N = 0.40 Fe u A e + 0.75 As c r;= 0.40 x 50 x 275 x 275 + 0.75 x 2 X 452 X 590

= (1512 500 + 400 020) X 10.3= 1912 kN

For working load of 350 kN, using modified Hiley formula

Ru =_E__ with factor of safety = 2.25 :s+c/2

R« = 350 X 2.25 = 787.50 kN

Transfer energy at pile top, E = 0.85 X 104kN m; temporary

compression of pile and ground, c = 10mm. Set per blow inmm, S

is given by:

8500 10s =-_ - - = 5.80 mm/blow

787.5 2

Ten blows of hammer give a 58 mm set; therefore use a 4.0 t banu

hammer with a 400 mm drop. For working load of 350-500 kN:

S = 8500 . ! . Q . =2.60 mm/blow500 x 2.25 2

Therefore ten 'blows of hammer give a 26 mm set; therefore use

4.0 t banut hammer with a 400 mm drop, see Table 4.9.

9




Table 4.9. Hammer transfer energy x 104 (Rig: hydraulic Banut

type)

Hammer weight

(tonnes)

1.50

3.00

4.00

5.0

Transfer energy (tonne metres)

Hammer drop

(mm)

300 400 500 600 700

0.25 0.35 OA5

0.55 0.70 0.90

0.85 1.10

1.05 lAO

D UT CH F OR M UL A

This formula provides an alternative method of determining a pile

set using a dynamic formula.

W2KHs=

Ru(W+ p )

where s = set in mm/blow;

W =weight of hammer = 35 kN;

K =hammer efficiency = 0.70;

H = hammer drop =450 mm;Ru =working load x 10,350 x 10=3500 kN;

p =weight of pile = 18kN.Therefore

35 x35x 0.70 x 450s = 2.27 rnm/blow

3500(35+18)

For ten blows this equals 22.70 mm set.

94

BIBLIOGRAPHY

Berezantsev,V.O. (1961) Load bearing capacity and deform

piled foundations. Proc. Fifth International Conference

Mechanics, Paris, Vol. 2, pp. 11-12.

Broms, B. (1966) Methods of calculating the ultimate

capacity of piles: a summary. Sols (Soils), 5(18/19), 21-3

BSI (1986) BS 8004: British Standard code of prac

foundations, British Standards Institution.

Carter, M. (1983) Geotechnical Engineering Handbook,Press, London.

De Beer, E .E . (1965) Bearing capacity and settlement of

foundations on sand. Proc. Symposium on Bearing Capa

Settlement of Foundations, Duke University, pp. 15-33.

Gibbs, H.I. and Holtz, W.O. (1957) Research on determi

density of sands by spoon penetration testing. Proc.

ICSMFE Conference, London, Vol. I,pp. 35-39.

Meyerhof, 0.0. (1952) The ultimate bearing capa

foundations. Geotechnique, 2 (4), 301-332.

Parry, R.H.O. (1971) A direct method of estimating settlem

sands from SPT values. Midlands SMFE Society.

Powell, M.J.V. (1979) House-Builder's Reference Book,

Butterworth, London.

Terzaghi, K. and Peck, R.B. (1968) Soil Mechanics in Eng

Practice, 2nd edn, John Wiley, New York.Vesic, A.S. (1966) Tests on instrumented piles. Ogeeche

site. Journal of Soil Mechanics and Foundation D

American Society of Civil Engineers, 96, SM 2.



ion of

Soil

for

tech

the

y of

in

nes-The following guidance is given for builders and engineers

involved with the planning and construction of housing on

sites previously undermined by mineral extraction and on

sites where future extraction of coal or other minerals will

take place after the development has been completed.

There are various techniques for investigating and

consolidating old mine workings, securing of old mine

shafts, adits etc., and various foundation design options are

available to cater for any ground movements likely to arise.

Past and current coalmining is the most common cause of

subsidence but there are other minerals, such as fireclay,

sandstone (EIland flags), chalk, ironstone, salt and gypsum,

which can give stability problems below a site. The effects

of subsidence from modern longwall extraction methods

now in use in the British coalfield can be predicted fairly

accurately whereas movements resulting from old shallow

mineral workings are not so easily defined and require

sound judgements by engineers and geologists experienced

in this field based on the available mining and geological

data collected.

Building houses on land which is underlain by known

shallow coal workings or other mineral workings can result

in very expensive development costs. The total costs are

difficult to quantify prior to consolidation being carried out

because of the lack of information on the volume ofmaterial

extracted. On some sites it may well be cheaper not to de-

velop certain areas of the site and place public open space

over the no-build zones. If shallow workings are discovered

in the final phases of a development there will be less

properties available to spread the costs.

In known mining areas it is prudent to consult British Coal

or other bodies such as the Brine Boards, mineral valuers,

and British Geological Survey, before purchasing any land

for development. In some localities, planning authorities may

lay down conditions in regard to old or future mineral extrac-

tion. This has become more frequent since the closure of

many mines has allowed mines to flood which results in a

diaphragm effect in pushing mine gases such as methane and

carbon dioxide to the ground surface. This alone could

render a site undevelopable.

River

Chapter 5

B u ild in g in m in in g

localities

Coal, lead, tin, ironstone, fireclay, sandstone, gypsum, salt

chalk; sand, anhydrite and other minerals have been ex

tracted by various methods over the years, but many of the

industries related to the minerals have gone into decline. A

present the minerals most frequently extracted are coal

gypsum, anhydrite and salt.

Gypsum and anhydrite mines are extensively worked i

Cumbria, Yorkshire, Nottinghamshire and Sussex, but the

nature of the workings results in very large pillars being lef

to provide support for the overlying strata. The seams ex

tracted are very thick; often up to 10m is removed from

30 m thick beds of material.

Chalk was mined in a similar fashion using pillar-and-stall

methods in areas of Kent, Bury St Edmonds, Suffolk and

Norwich. In some areas the overlying rock strata may have

collapsed into swallow holes in the chalk and this material i

usually a mass of loose voided material. In areas of swallow-

hole activity piling taken below the chalk base is generally

required, but if the holes are large and widespread then the

site may not be viable owing to the effects of renewed and

often unpredictable subsidence O I l the site infrastructure.

Salt mining can be carried out by mining or by brine

pumping and the design of foundations in such areas require

special considerations.

5.1 COAL MINING, PAST AND PRESENT

Originally used as a means of obtaining fireclay or ironstone

bell pits (Fig. 5.1) were in use from the thirteenth century up

to the early 18oos. They were usually found in areas where

the thickness of drift (superficial deposits) was relatively thin

and the seams were shallow and fairly level. The depth o

these pits rarely exceeded 12m. They consisted of a vertica

shaft taken down to the various minerals or coal seam; the

shaft was belled out at the bottom to maximize the coa

recovery. Quite often, bell pits sunk for coal extraction

would encounter bands of ironstone and this was often re

moved for use. Sometimes shafts were excavated in pairs

and interconnected at the bottom. To facilitate ventilation fire

95