G th tiGeosyntheticsand
Reinforced Soil Structures
K. Rajagopal, ProfessorDepartment of Civil Engineering
IIT M d Ch iIIT Madras, Chennaie-mail: [email protected]
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.
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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
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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
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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
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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
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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)
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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
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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
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PVDs for pre‐consolidationPVDs for pre consolidation
Corrugated plastic core for drainage
L
t
Geotextile filter
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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
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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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