gthtigeosynthetics and reinforced soil structures · • slopes shallowershallower thanthan 7070...

G th tiGeosyntheticsand

Reinforced Soil Structures

K. Rajagopal, ProfessorDepartment of Civil Engineering

IIT M d Ch iIIT Madras, Chennaie-mail: gopalkr@iitm.ac.in

Embankment or Wall ?Embankment or Wall ?

• Any slope steeper than 70 is designed as aAny slope steeper than 70 is designed as a retaining wall.  All principles related to the geosynthetic reinforced soil retaining walls aregeosynthetic reinforced soil retaining walls are used for the designs.

• Slopes shallower than 70 are designed as• Slopes shallower than 70 are designed as slopes.

2Reinforced Soil Embankments - 1

Reinforced Soil Embankments - 1 3

Courtesy: Maccaferri India Ltd.

Surface Treatment of SlopesSurface Treatment of Slopes

• VegetationVegetation

• Erosion control mats like coir mat, jute mats, crimped mesh etccrimped mesh, etc.

• Stone pitching

• Spray concrete on surface

• Gabion facingsg

Reinforced Soil Embankments - 1 4

Geocells used to protect slope surface5Reinforced Soil Embankments - 1

Crimped mesh and a polymer textile for surface protection

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Use of coir and jute mats for erosion control on slopes

Geocells filled with cementGeocells filled with cement concrete for erosion control on canal slope

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Stabilisation of Existing Slopesg p

•Soil Nails•Grouted Anchors•Pre-stressed anchors

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Soil RequirementsSoil Requirements

Less stringent than for the retaining wallsg gParticle Size % passing20 mm 100%4.75 mm 100‐20%0.425 mm 0‐60%75  50%

Pl i i I d 20%Plasticity Index  20%Soil compacted to at least 95%MDD

Reinforced Soil Embankments - 1 10

DESIGN OF EMBANKMENTSSlopes on weak foundation soilsp

•Failure is in foundation soil

Steep slopes on strong foundation soilSteep slopes on strong foundation soil

•Planar wedge analysis

S•Slip circles through toe

•Bilinear wedge analysis

11Reinforced Soil Embankments - 1

Construction on Soft ClaysPROBLEMSLow bearing capacityLarge settlementsLarge settlementsLateral flow of soils/slip circle failureDifficult to move construction equipments.

b kembankment

failure wedge

soft Clay

Firm Soil

slip circle

12Reinforced Soil Embankments - 1

So

Construction on Soft ClaysSOLUTIONSSOLUTIONS

Replace the soil with good quality fillChemical or thermal treatment of foundation soilDeep mixing/jet groutingBasal reinforcementBasal reinforcementBasal mattressPre-consolidation with PVDsVacuum consolidationStone columns or Encased Stone columnsPiles and reinforced concrete slabPiles and reinforced concrete slabPiles with geosynthetic reinforced platform (piled

embankment)

13Reinforced Soil Embankments - 1

Shear Strength Properties to UseShear Strength Properties to Use

• Slopes may undergo large deformationsSlopes may undergo large deformations, especially those on soft foundation soils

• Constant volume strength parameters are• Constant volume strength parameters are more applicable than the peak strength parametersparameters

Reinforced Soil Embankments - 1 14

Partial material factors to applyPartial factors Ultimate limit ServiceabilityPartial factors Ultimate limit

stateServiceabilitylimit state

Load factors Embankment fill Ffs = 1.3 Ffs = 1.0D d l d F 1 2 F 1 0Dead loads, Line or point loads

Ff = 1.2 Ff = 1.0

Li l dLive loads Fq=1,3 Fq=1.0Soil material factors To tancv Fms = 1.0 Fms = 1,0

To c F = 1 6 F = 1 0To c Fms = 1.6 Fms = 1.0To cuu Fms = 1.0 Fms = 1.0

Reinforcement To Consistent with type of factors reinforcement

base strength

ypreinforcement and design life

Soil/reinforcement Sliding across Fs = 1.3 Fs = 1.0interaction factors

g s s

Pullout Fs = 1.3 Fs = 1.0Reinforced Soil Embankments - 1 15

In case of very weak foundation ilsoils

G d I t i i d t b iGround Improvement is required to bringup the foundation soil properties to

bl l l t hi i lreasonable levels to achieve economicalconstruction

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Stone columnsStone columns

• Rammed stone columnsRammed stone columns• Vibroflot stone columns by displacement

or replacement methodsor replacement methods

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Encased stone columns used at A380 factory in

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yHamburg for ground treatment

Sand Drains or PVDs for accelerated l dpre‐consolidation

surcharge fill

sand columnsdTt v2

cv

19Reinforced Soil Embankments - 1

PVDs for pre‐consolidationPVDs for pre consolidation

Corrugated plastic core for drainage

L

t

Geotextile filter

20Reinforced Soil Embankments - 1

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General view after installation of PVD’s at a site22Reinforced Soil Embankments - 1

Geosynthetic Pile Raft SystemGeosynthetic Pile Raft System

reinforcement

Pile cap

plain concrete

Soft clayconcrete

pilesy

Design principles are based on soil arching theories

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Design principles are based on soil arching theories

Stiff geocell mattress in case of thin l lclay soils

geocell layer

Thin soft foundation layer

Cq uuu14.5

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Use of large assembled geocells for

construction on soft claysconstruction on soft clays

Rigid foundation

D i l f T UKDesign manual of Tensar, UK

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• Increase in effective stress without increase in total stress

• Increase in effective stress constant depthconstant depth

Vacuum consolidation of soft clay soils

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Vacuum consolidation of soft clay soils

Basal reinforcement on soft clays

• Construction expedient: provided along • Construction expedient: provided along with a layer of granular soil to allow for free movement of construction equipment

• Provided to increase the factor of safety of the embankment

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d

Design of basal reinforcement based on uu=0

radius, r

Xhv

Wbasal reinforcement

XW

= Lc au rFSu

cu Le

XW FSu

XWforceent reinforcem toduemoment +

= Lc a u rFSr

La = r XW FSr

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radius, r

Xhv

W

basal reinforcementX

W

cuKink in reinforcement layer at slip surfacelayer at slip surface

Flexible reinforcement takes the shape of slip circle at

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junction with a kink

Stabilising Moment = reinforcement force x lever arm

Flexible reinforcement - reinforcement assumes the shape of slip surface

Lever arm = radius of slip circleLever arm = radius of slip circle

Rigid reinforcement – reinforcement remains horizontal

xw

Lever arm = vertical height

armlever xw)-( = FSFS urT d

Basal reinforcement that provides this much of force should be provided (least of rupture and pullout capacity)

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Safety against lateral sliding as a complete mass

H

Basal reinforcement

Length of the reinforcement should besufficient that the resistance against lateral

Basal reinforcement

sliding is adequate – shear resistancedeveloped only on one surface

21 qHKHKP aa 221

aK = sin ( - )sin + sin sin

2

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Safety against pullout failure of reinforcement

Le

Length of the reinforcement should besufficient to generate sufficient pulloutresistance – shear resistance developed onboth upper and lower surfaces

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Slip circle analysis for cohesive‐frictional soils

Rli = inclined R

yi

ilength at base of sliceWi = weight of ith

Wi

l

Ti = Wi sini

wi T1

Wi weight of isliceR = radius of slip circle

ii yTL ' )( in

iiia lu-coswRtan+Rc'

li

Ni = Wi cosi

iT2

circle

n

ii

ii

rwR

yTLFS

1

)(

i

i1=i

iiia

sin

lucoswRtan R c =

i i i

i 1

33Reinforced Soil Embankments - 1

Slip circle analysis – Bishop’s method

i

i SWbc r

/1

sectan)1(

ii

uiui

u WFSr

FS

sin/tantan1

)( =

i yWbcR r Tsectan)1(

ii

iri

ui

r WR

yFS

WbcR rFS

sin

T/tantan1

tan)1( =

i

ru = bishop’s pore pressure parameterb = width of slice

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FS = factor of safety

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Embankments constructed on competent foundation soilfoundation soil

•Foundation soil is very strong•Reinforcement provided to construct steep slopes•Reinforcement provided to construct steep slopes

Shallow unreinforced slope

Extra road space gained

Steep reinforced slope

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Planar wedge analysis of Steep SlopesAC = assumed ruptureAC assumed rupture

plane at angle = slope angle (>)R t t l ti l f

Assumed soil propertiesc=0, friction angle =

if h

Rv = total vertical force including weight of soil inwedge ADC and loads

uniform surcharge, q

CDon surface DC

N = normal force on AC= R cos

Rv

N

BTi

Rv cosS = shear force on AC

= Rv sinT f i f t

S

SRv

Ti = sum of reinforcement forces (horizontal)

N

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A

cos+tancosresistanceshear TR

sin cos+ tancos

= forceshear

resistanceshear = FSR

TRv

iv

tan + 1 = R

Tv

iRv

sin ( )2

aK = sin ( - )

sin + sin sin

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Reinforced Soil Embankments - 1 39