soil shear strength
DESCRIPTION
civil deparmentTRANSCRIPT
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Lecture-8
Shear Strength of Soils
Dr. Attaullah Shah
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Strength of different materials
Steel
Tensile strength
Concrete
Compressive strength
Soil
Shear strength
Presence of pore waterComplexbehavior
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What is Shear Strength?What is Shear Strength?
• Shear strength in soils is the resistance Shear strength in soils is the resistance to movement between particles due to to movement between particles due to physical bonds from:physical bonds from:
a.a. Particle interlockingParticle interlocking
b.b. Atoms sharing electrons at surface contact Atoms sharing electrons at surface contact pointspoints
c.c. Chemical bonds (cementation) such as Chemical bonds (cementation) such as crystallized calcium carbonate crystallized calcium carbonate
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Influencing Factors on Shear StrengthInfluencing Factors on Shear Strength• The shearing strength, is affected by:The shearing strength, is affected by:
– soil compositionsoil composition: mineralogy, grain size and grain size : mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid. content, ions on grain and in pore fluid.
– Initial stateInitial state: State can be describe by terms such as: : State can be describe by terms such as: loose, dense, over-consolidated, normally consolidated, loose, dense, over-consolidated, normally consolidated, stiff, soft, etc. stiff, soft, etc.
– StructureStructure: Refers to the arrangement of particles within : Refers to the arrangement of particles within the soil mass; the manner in which the particles are the soil mass; the manner in which the particles are packed or distributed. Features such as layers, voids, packed or distributed. Features such as layers, voids, pockets, cementation, etc, are part of the structure. pockets, cementation, etc, are part of the structure.
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Shear Strength of SoilShear Strength of Soil• In reality, a complete shear strength formulation In reality, a complete shear strength formulation
would account for all previously stated factors.would account for all previously stated factors.
• Soil behavior is quite complex due to the Soil behavior is quite complex due to the possible variables stated.possible variables stated.
• Laboratory tests commonly used:Laboratory tests commonly used:– Direct Shear TestDirect Shear Test– Unconfined Compression Testing.Unconfined Compression Testing.
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Soil Failure and shear strengthSoil Failure and shear strength.
• Soil failure usually occurs in the form of Soil failure usually occurs in the form of “shearing” along internal surface within the “shearing” along internal surface within the soil.soil.
• Thus, structural strength is primarily a Thus, structural strength is primarily a function of shear strength.function of shear strength.
• Shear strength is a soils’ ability to resist Shear strength is a soils’ ability to resist sliding along internal surfaces within the sliding along internal surfaces within the soil mass.soil mass.
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Slope Stability: Failure is an Slope Stability: Failure is an Example of Shearing Along Example of Shearing Along
Internal SurfaceInternal Surface
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Mass Wasting: Shear FailureMass Wasting: Shear Failure
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Shear Failure: Earth DamShear Failure: Earth Dam
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Shear Failure Under Foundation Shear Failure Under Foundation LoadLoad
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Shear failureSoils generally fail in shear
strip footing
embankment
At failure, shear stress along the failure surface reaches the shear strength.
failure surface
mobilized shear resistance
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Shear failure
The soil grains slide over each other along the failure surface.
No crushing of individual grains.
failure surface
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Shear failure mechanism
At failure, shear stress along the failure surface () reaches the shear strength (f).
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Retaining wall
Shear failure of soilsSoils generally fail in shear
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Retaining wall
Shear failure of soils
At failure, shear stress along the failure surface (mobilized shear resistance) reaches the shear strength.
Failure surface
Mobilized shear resistance
Soils generally fail in shear
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Mohr-Coulomb Failure Criterion
tancf
c
failure envelope
cohesion
friction angle
f is the maximum shear stress the soil can take without failure, under normal stress of .
f
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Mohr-Coulomb Failure Criterion(in terms of total stresses)
f is the maximum shear stress the soil can take without failure, under normal stress of .
tancf
c
failure envelope
Cohesion
Friction anglef
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Mohr-Coulomb Failure Criterion(in terms of effective stresses)
f is the maximum shear stress the soil can take without failure, under normal effective stress of ’.
’
'tan'' cf
c’
’
failure envelope
Effective
cohesion
Effectivefriction angle
f
’
u '
u = pore water pressure
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Mohr-Coulomb Failure Criterion
'tan'' ff c
Shear strength consists of two components: cohesive and frictional.
’f
f
’
'
c’ c’cohesive
component
’f tan ’ frictional component
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Mohr-Coulomb Failure Criterion
tanff c
Shear strength consists of two components: cohesive and frictional.
f
f
c
f tan
c cohesive
component
frictional component
c and are measures of shear strength.
Higher the values, higher the shear strength.
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Other laboratory tests include,Direct simple shear test, torsional ring shear test, plane strain triaxial test, laboratory vane shear test, laboratory fall cone test
Determination of shear strength parameters of soils (c, orc’’
Laboratory tests on specimens taken from representative undisturbed samples
Field tests
Most common laboratory tests to determine the shear strength parameters are,
1.Direct shear test2.Triaxial shear test
1. Vane shear test2. Torvane3. Pocket penetrometer4. Fall cone5. Pressuremeter6. Static cone penetrometer7. Standard penetration test
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Laboratory tests
Field conditions
z vc
vc
hchc
Before construction
A representative soil sample
z vc +
hchc
After and during construction
vc +
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Laboratory tests
Simulating field conditions in the laboratory
Step 1
Set the specimen in the apparatus and apply the initial stress condition
vc
vc
hchc
Representative soil sample taken from the site
0
00
0
Step 2
Apply the corresponding field stress conditions
vc +
hchc
vc + Traxial t
est
vc
vc
Direct shear test
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Direct shear testSchematic diagram of the direct shear apparatus
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Direct shear test
Preparation of a sand specimen
Components of the shear box Preparation of a sand specimen
Porous plates
Direct shear test is most suitable for consolidated drained tests specially on granular soils (e.g.: sand) or stiff clays
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Direct shear test
Leveling the top surface of specimen
Preparation of a sand specimen
Specimen preparation completed
Pressure plate
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Direct shear test
Test procedure
Porous plates
Pressure plate
Steel ball
Step 1: Apply a vertical load to the specimen and wait for consolidation
P
Proving ring to measure shear force
S
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Direct shear test
Step 2: Lower box is subjected to a horizontal displacement at a constant rate
Step 1: Apply a vertical load to the specimen and wait for consolidation
PTest procedure
Pressure plate
Steel ball
Proving ring to measure shear force
S
Porous plates
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Direct shear test
Shear box
Loading frame to apply vertical load
Dial gauge to measure vertical displacement
Dial gauge to measure horizontal displacement
Proving ring to measure shear force
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Direct shear testAnalysis of test results
sample theofsection cross of Area
(P) force Normal stress Normal
sample theofsection cross of Area
(S) surface sliding at the developed resistanceShear stressShear
Note: Cross-sectional area of the sample changes with the horizontal displacement
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Direct shear tests on sands
Sh
ear
str
ess
,
Shear displacement
Dense sand/ OC clayfLoose sand/ NC clayf
Dense sand/OC Clay
Loose sand/NC Clay
Ch
ang
e in
hei
gh
t o
f th
e sa
mp
le Exp
ansi
on
Co
mp
ress
ion Shear displacement
Stress-strain relationship
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f1
Normal stress = 1
Direct shear tests on sandsHow to determine strength parameters c and
Sh
ear
stre
ss,
Shear displacement
f2
Normal stress = 2
f3
Normal stress = 3
Sh
ear
stre
ss a
t fa
ilu
re,
f
Normal stress,
Mohr – Coulomb failure envelope
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Direct shear tests on sandsSome important facts on strength parameters c and of sand
Sand is cohesionless hence c = 0
Direct shear tests are drained and pore water pressures are dissipated, hence u = 0
Therefore,
’ = and c’ = c = 0
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Direct shear tests on clays
Failure envelopes for clay from drained direct shear tests
Sh
ear
stre
ss a
t fa
ilu
re,
f
Normal force,
’
Normally consolidated clay (c’ = 0)
In case of clay, horizontal displacement should be applied at a very slow rate to allow dissipation of pore water pressure (therefore, one test would take several days to finish)
Overconsolidated clay (c’ ≠ 0)
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Interface tests on direct shear apparatusIn many foundation design problems and retaining wall problems, it is required to determine the angle of internal friction between soil and the structural material (concrete, steel or wood)
tan' af c Where,
ca = adhesion,
= angle of internal friction
Foundation material
Soil
P
S
Foundation material
Soil
P
S
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Triaxial Shear Test
Soil sample at failure
Failure plane
Porous stone
impervious membrane
Piston (to apply deviatoric stress)
O-ring
pedestal
Perspex cell
Cell pressureBack pressure Pore pressure or
volume change
Water
Soil sample
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Triaxial Shear Test
Specimen preparation (undisturbed sample)
Sampling tubes
Sample extruder
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Triaxial Shear Test
Specimen preparation (undisturbed sample)
Edges of the sample are carefully trimmed
Setting up the sample in the triaxial cell
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Triaxial Shear Test
Sample is covered with a rubber membrane and sealed
Cell is completely filled with water
Specimen preparation (undisturbed sample)
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Triaxial Shear Test
Specimen preparation (undisturbed sample)
Proving ring to measure the deviator load
Dial gauge to measure vertical displacement
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Unconfined Compression Test (UC Test)
1 = VC +
3 = 0
Confining pressure is zero in the UC test
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Unconfined Compression Test (UC Test)
1 = VC + f
3 = 0
Sh
ear
stre
ss,
Normal stress,
qu
τf = σ1/2 = qu/2 = cu
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The EndThe End