foundation intro

Foundation Engineering

Introduction

Steps in Foundation Engineering

Understand project and site

Develop design criteria

Identify possible foundation alternatives

Conduct soil investigation

Characterize the site

Engineering analysis to evaluate alternatives

Develop recommendation and write report• Monitor design, construction and performance

• Project intent (from owner)• Assess general soil and site condition-

Previous borings, maps, reports etc• Previous experience in the area-Adjacent

structure (how they were constructed and how they were performed)

• Constraints-local building codes, neighboring facilities, access issues, economic limitations

• Allowable settlement and tilt• Acceptable factor of safety• Identify constructabilty issues• Obtain design loads• Building codes

Allowable settlement - total settlement, tilt and differential settlement

Type of structure Total allowable settlement (mm)

Office building 12 to 50 (25 is the most common value)

Heavy industrial building

25 to 75

Bridges 50

Tilt and differential settlement – Causes

• Non uniform site condition• Ratio of actual to design load

differs over structure• Ratio of dead load to live load

differs over structure• Built dimensions differ from plan

dimensions

Effects – differential settlement

• Stresses in structures• Cracking• Architectural and structural

damage

Effects – tilt

• Uncomfortable feeling

• Lack of stability

• Allowable settlement and tilt• Acceptable factor of safety• Identify constructabilty issues• Obtain design loads• Building codes

• Consider types of foundations • Assess benefits and potential

problems with each

Monitor design, construction, and performance:summary • Foundation design is just assessing all

your options through analysis and choosing the best one

• Requires understanding of site and project, ability to consider what might go wrong

• Soil mechanics is the tool that help you select, design and construct foundation elements and earth structures

Summary:Contd

• ‘Failure’ is an unacceptable difference between expected and observed behavior

• Key goal as foundation engineer-build economic foundation that works (safe and serviceable)

• ‘Build with confidence’- use field work, lab results, analysis and design but at the end, use what works

• Use rules of thumb when possible to check for reasonableness

Classification of foundation

Shallow foundation – less expensive, better for lighter structure on less problematic soils. Typical types – ‘spread’ footings and rafts or mats

Deep foundations – More expensive, typically used for heavy tall structures on more problematic soils. Typical types – ‘driven’ piles, ‘drilled’ piers

Design requirements

• Safety – adequate factor of safety against shear failure of soil, qsafe = qult/FS, where q is the bearing pressure

• Serviceability – acceptable magnitude of settlements (including immediate, consolidation and secondary compression)

• Maximum load that satisfies both is allowable bearing pressure, qa

Deep Foundations: Need

• Fill or poor soil conditions near to the surface, so that excavation for footing foundations would be difficult and expensive

• Ground permeabilities and ground water conditions likely to make it difficult and expensive to exclude water from excavations

Need: contd

• To get acceptable bearing capacity which may be difficult to achieve by footing foundations

• To keep the settlement within an acceptable limit which may be difficult to achieve in footing foundations

• Foundations to carry heavy column load

Need: Contd

• Foundations required to take large uplift forces when tension piles may be cheaper than providing the necessary footing size with the mass to resist uplift

• Foundations required to be stiffer than can be achieved with footings or raft

• Poor soil condition where ground improvement techniques may prove more expensive than piles

Deep Foundation Types

• Piles• Drilled piers• Caissons

Functional features are similar- each typically subjected to an axial load. Sometimes piles and caissons serve as anchors for special installations

Difference is primarily in their physical size and method of installation

Piles:

• Specially installed relatively slender columns used to transmit structural loads to a lower, firmer soil or rock formation

• Diameter is generally less than 750 mm

• The pile may be of concrete, steel, timber or a composite of steel and concrete

Drilled pier/Caissons:

• In general sense drilled piers and caissons are larger piles. Usually 750 mm or more in diameter

• The terms drilled piers and caissons, are frequently used interchangeably by engineer

• For drilled piers, typically a shaft is drilled into the soil which is then filled with concrete

• The shaft may be cased with a metal shell in order to maintain the shaft from collapsing

Drilled pier/Caissons:

• The casing may be left in the place as part of the pier or it may be gradually withdrawn as the shaft is filled concrete

• The lower part of the shaft may be uncut or belled out to develop a larger end bearing area, thereby increasing the capacity of the piers

• Typically, drilled piers and caissons are designed as end bearing members

Pile foundations: Classifications based on material

• Timber• Steel• Concrete• Composite

Material selection Criterion

• Corrosive properties of the stratum • Fluctuation in water table • Ease of installation • Length required • Availability of material • Installation equipment • Restrictions in driving noise and

vibrations and • Cost

Customary design loads for piles

Type of pile Load in tons (1 ton = 8.896 kN)

Wood 15-30

Composite 20-30

Cast in place concrete 30-50

Precast reinforced concrete 30-50

Steel pipe concrete filled

Steel H-section 30-60

Pile type Available maximum length

Steel H and pipe Unlimited length; short sections are driven and additional sections are field welded to obtain a desire length

Steel shell and cast in place

Typically between 100 to 125 ft

Precast concrete Solid small cross section up to 60 ft, large diameter cylinder pile can be up to 200 ft long

Cast in place concrete

50 - 75 ft depending on equipment

Bulb type cast in place concrete

Up to about 100 ft

Composite Depends on many factor maximum can be up to 150 ft

Timber Depends on wood type. Usual 50-60 ft, limited quantity 75 ft, up to 100 ft possible but very limited

Classification: Based on function

• End bearing• Friction• Tension or uplift• Compaction pile• Anchor• Batter pile• Laterally loaded pile• Sheet pile

Classification: Based on installation

• Driven pile

• Cast in situ pile

Pile capacity:

• Single pile• Piles in group

Single Pile

• Friction pile• End bearing pile• Both friction and end bearing

The load is transmitted to the soil surrounding the pile by friction or adhesion between the soil and the sides of the pile, and/or the load is transmitted directly to the soil just below the pile’s tip. This can be expressed in equation form as follows:

tipfrictionultimate QQQ

surfacefriction AfQ .

tiptip AqQ .

Qultimate = ultimate capacity of the pile

Qfriction = pile capacity furnished by friction or adhesion between the soil and the sides of the pile

Qtip = pile capacity furnished by the soil just below the pile’s tip

f = unit skin friction or adhesion between the soil and the sides of the pile

Asurface = Vertical surface area of the pile (for a circular pile of diameter D and length L, Asurface = D L)

Atip = area at the pile’s tip (for a circular pile of diameter D, Atip = D2/4)

q = ultimate bearing capacity of soil at the pile’s tip

• End bearing pile – Qtip is predominant

• Friction pile – Qfriction is predominant

Pile driven in sand:

Total capacity is summation of friction capacity and end bearing capacity

tipfrictionultimate QQQ

Estimation of Qfriction

surfacefriction AfQ .

can be evaluated by multiplying the coefficient of friction between sand and pile surface (tan ) by the total horizontal soil pressure acting on the pile

Material tan

Concrete 0.45

Wood 0.40

Steel (smooth) 0.20

Steel (Rough) 0.40

Steel (corrugated)

tan of sand

Horizontal soil pressure: The total horizontal soil pressure acting on the pile is function of effective vertical pressure of soil adjacent to the pile. Soil pressure normally increases as depth increases. However, it has been determined that the effective vertical pressure of soil adjacent to a pile does not increase without limit as depth increases. Instead, effective vertical pressure increases as depth increases until a certain depth of penetration is reached. Below this depth, which is called the critical depth and denoted as Dc, effective vertical pressure remains more or less constant

Eff. Ver. Pressure= v

v = constant

v , max

Depth, Z

Dc is the critical depth = 10 D for loose soil and 20 D for dense soil

DKDDLD

DKDDLZdzQ

friction

tan])()([

L is the length of the pileD is the diameter of the pileK is the coefficient of active earth pressure

Estimation of Qtip

tiptip AqQ .

DNNDq qf 3.0

BNNDq qf 4.0

for circular piles

for square piles

Where q is the bearing capacity at the pile tip, is the unit weight of soil, Df is the embedded length of the pile, Nq and N are bearing capacity factors, D is the diameter of the circular pile, and B is the width of the square pile

It can be noted that these equations have the same general form as the bearing capacity equations of shallow foundations. However, the magnitude of effective vertical pressure of soil adjacent to a pile is more or less constant below the critical depth. Thus, for design purposes, the term DfNq should be replaced by (v

)tipNq, where (v)tip is the effective

vertical pressure adjacent to the pile at the pile tip. Further, in most cases, driven piles are relatively small in cross section; therefore, the terms involving D and B are small compared to the other terms in the equation.

Thus, for practical work, it may be approximated as

qtipv Nq )( '

The soil just below the pile tip is displaced outward and upward, causing shearing stresses to be induced in the soil above the tip. These stresses alter shear patterns below the tip as compared with those for shallow footings. Hence the value of Nq for shallow foundations should be increased to Nq

*, the bearing capacity factor for piles driven in sand *'

qtipv Nq

is generally equals to Dc tipv'

Total capacity of a circular pile driven in sand

2 DNDDKDDL

DQ qccc

Pile Driven in Clay

surfacesurfacefriction AcAfQ .

f is the unit friction, c is the cohesion and is the adhesion factor. For soft clay = 1, and for stiff clays <1.0

tiptip qAQ

cfc cNDcNq

(for =0.0, Nq=1.0 and N = 0.0)

Hence total capacity of a circular pile driven in clay

4/2DcNDLcNQ cc

Test pile

One (sometime more than one for bigger project) of the designed piles on which load tests are carried out. Normally piles are designed initially by analytic or other methods, based on estimated load and soil characteristics. Pile load tests are performed on test pile during the design stage to check the design capacity

Results from the pile tests could be the source of most useful information in at least two general aspects:

• In determining the ultimate bearing capacity of the pile

• In evaluating the deflection characteristics of the pile

With regard to the ultimate bearing capacity or expected settlement, one may reflect on several additional and tangential benefits. For example, if the capacity of the pile is different or settlement is excessive from that desired, the pile length, diameter, and details of installation can be adjusted prior to the installation of rest of the piles

A side, but equally important benefit from the installation and testing of a test pile, prior to the installation of the rest, is general information about the site conditions with regard to potential problems which may or may not be fully reflected in prior soil subsurface investigations

Though the pile load test is the most reliable means of the determining the load capacity of a pile or estimating settlement, one should not assume that load tests are unquestionably accurate. The test results may be misleading if the relevant factors are overlooked:

1. Time lapse should be provided between the time of installation and the time of test loading (minimum three to four days for granular stratum and about a month for clayey soils). This is the time normally required for the respective soils to regain the strength lost during the driving operation. In the case of concrete piles a minimum time is also required to develop the material strength

2. The specific location for the installation and subsequent testing of the pile must be representative of the overall site if the test results are to be representative of the rest of the piles. It is common practice on the part of the engineers to select the most unfavorable conditions of the site

3. The pile characteristics, such as length, diameter, installation method, must closely resemble those of the piles to be installed

Compressible layer

Negative skin Friction

Pile In group: The behavior of a group of piles is different from that

of individual piles in number of rows

• In general, bearing capacity of a group of piles is less than the sum of the capacity of individual pile

• The settlement of a pile group is larger than that of individual pile for corresponding level of loading

• The efficiency of the pile group is less than that of s single pile

In spite of these shortcomings, a pile group is a much more common occurrence than a single pile. Single pile lack the overall stability against overturning, a deficiency that is easily overcome by a cluster of piles

For column – a minimum of two piles, but more commonly three or more piles, are clustered in a group and connected via a concrete cap to form a unit

For walls – a line of single piles is common

Efficiency of a pile group:

Where Qg is the group capacity and Qi is the individual capacity of pile and n is the number of piles in the group

• End bearing piles – an efficiency of 1.0 may be assumed

• Friction pile driven in cohesionless soil – an efficiency of 1.0 may also be assumed

• For a pile group composed of friction pile driven in cohesive soil, an efficiency of less than 1.0 is to be expected

Efficiency by the Converse-Labarre equation:

nmmng 90

where = tan-1(d/s), degn = number piles in a rowm = number of rows of pilesd = diameter of the piles = spacing of the piles, center to center

For friction piles driven in cohesive soil, Coyle and Sulaiman suggested that pile-group efficiency may be assumed vary linearly from a value of 0.7 at a pile spacing of three times the diameter to a value of 1.0 at a pile spacing of eight times the pile diameter.

For pile spacing less than three times the pile diameter, group capacity may be considered as block capacity, and total capacity can be estimated by treating the group as a pier.

For pile spacing less than three times the pile diameter, the group capacity can be obtained applying the following equation:

WLcNfLWDQ cg 3.12

where D is the depth of pile groupW is the width of pile groupL is the length of the pile groupf is the unit adhesion developed between cohesive soil and pile surface (equal to c)c is the cohesionNc is the bearing capacity factor for a shallow rectangular footing

Problem 1: A pile group consists of nine friction piles in clay soils (Fig. Q.4). The diameter of each pile is 0.4 m and the embedded length is 9 m. Center to center pile spacing is 1.2 m. Soil conditions are shown in Fig. Q.4. Determine (i) the block capacity of pile group using a factor of safety of 3, (ii) allowable group capacity based on individual pile failure (use a factor of safety of 2, along with the Converse-Labarre equation for pile group efficiency), and (iii) design capacity of the pile group

1.21.2

QDesign= ?

ClayUnconfined Compressive strength, qu = 100 kN/m2 γ = 18.0 kN/m3

Dimensions are in meter

c = qu/2 = 120/2 = 60 kN/m2

4.00.600.94.06075.0

cNDLcQ cult

(Assuming length of the pile is L)

Given Qult = 2350=700 kN (2)

Equating (1) and (2), L = 11.2 m

Solution:-

deg43.181200400tantan 11 sd

(n = 3, m = 3)

kNQnQ ig 23003500.973.0

Capacity of the group based on the individual pile failure

Based on block failure

Size of the block – W = L = 2S + d = 2800 mm = 2.8 m

D = 11.2 m

LWcNcDLWQ cultg

)8.28.2(14.50.603.10.6075.02.11)8.28.2(2

QQ ultg

allg 29303

8788,,

Qdesign = 2300 kN

Problem 2: A 0.5-m diameter steel pile is driven into dense sand. The pile is driven with the tip closed by a flat plate. The closed end, steel pipe pile is filled with concrete after driving. The embedded length of the pile is 20.0-m and water table is at 4 m depth from ground surface. Soils unit weight, γ = 20.5 kN/m3, φ = 370, K = 0.9 and tanδ = 0.4. Determine the design capacity of the pile , using a factor of safety of 2.0

Solution:-

Pile is driven in Dense sandHence, Dc = 20D = 10.0 m

2' /0.820.45.20 mkNv at 4.0 m depth

2' /0.1450.630.820.6)105.20(0.45.20 mkNv

…at 10.0 m depth i.e., at critical depth

Distribution of effective vertical stress over depth along the length of the pile is shown in the Figure.

2/6.7374.09.0]0.1450.100.60.630.822

10.820.4

1[ mkNqs

62.0 63.0

2*' /130500.900.145)( mkNNq qtipvt

kNAqAqQ ttssult 37194/5.00.130505.06.737 2

Qall = Qult/FS = 3719/2 = 1860 kN

foundation intro

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