behaviour of clay and sand
TRANSCRIPT
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LECTURE 1 REVIEWPREPARED BY T. ILYAS
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REVIEW OF BEHAVIOUR OF CLAAND SAND
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THE “UNIFORMITY COEFFICIENT”
Cu = D60/D10 inverse of gradient of PSD!
If Cu ≤ ≈5* ⇒ uniform soil, P
If Cu > ≈5* ⇒ well-graded soil, W
* 4 for gravel and 6 for sand (AS1726)
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FINE-GRAINED SOILS
• Too fine for sieving
• Sedimentation and/or laser equipment?
Even then, sizes say nothing about clay mineralogy
and potential soil behaviour!
Fine-grained soils are defined by
how “ plastic” they are
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FINE GRAIN SOILS
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COURSE GRAIN SOIL
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THE PRINCIPLE OF EFFECTIVE STRESS
In soils, stresses are separated into :
Intergranular/effective stress (σ’)
resulting from particle-to-particle contact
Pore water/neutral pressure (u)
the pressure of the water filling the void space between the soild
particles
The sum of the effective stress and the pore pressure is called the total
stress (σ)
σ = σ’ + u
A soil mass consists of :
- solid particles- voids which contain water and air
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REVIEW OF EFFECTIVE STRESS CONCEPT
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TOTAL STRESS BELOW A RIVER/LAKE
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TOTAL STRESS IN MULTI-LAYERED SOIL
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NEGATIVE PORE PRESSURE (SUCTION)
The pore pressure in a partially
saturated soil consists of two
components:
· porewater pressure = uw· pore-air pressure = ua
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γsoil
Incompressible
Stresses Within Soil Mass
H
σvertical = γsoil · H
σhorizontal = K · σve
∆σvertical
∆σvertical
Note: No External Shear S
Dry Soil
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THE PORE WATER PRESSURE• is assumed not to change the volume of the particle
• doesn’t cause the particle to be pressed together • the pore water pressure at depth z, u = γw z
Total vertical stress at depth z = the weight of all material
(solid + water) per unit area
above that depth
σv = γsat z
The effective vertical stress at depth z
σv’ = σv - u
= (γsat - γw) z = γ’ z
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STRESSES WHEN NO FLOW TAKES PLACETHROUGH THE SATURATED SOIL MASS
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REVIEW OF SHEAR STRENGTH
Shear strength:
• Mohr-coulomb
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MOHR DIAGRAM (1)
• Principal Stress Planes: Orthogonal Planes on W
There Are Zero Shear Stresses• On These Planes: Normal Stresses Called Princ
Stresses
C.A.Coulomb
1736-1806
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Mohr Diagram (2)
τ
σ2θθ
A (σθ, τθ)
σ1 = σvσ3 = σh
K < 1
σθ = ½(σ1+σ3) + ½(σ1-σ3) cos2θ
τθ = ½(σ1-σ3) sin2θ
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Mohr Diagram (3)
τ
σσvσh
K < 1
σvertical = γsoil ·
σhorizontal = K
∆σvertical
σv + ∆σv
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Mohr-Coulomb Failure Criterion
φ σ τ tan f f c +=
Shear strength consists of two components: cohesive and
frictional.
σf
τf
φ
τ
σ
c
σf tan φ
c
frictionalcomponent
f r i c t i o n a n g l e
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SHEAR STRENGTH PARAMETER C & Φ
Mohr-Coulomb failure criterion :τf = c + σf tan φ
τf : shear strength
c : cohesion interceptσf : normal stress
φ : the angle of shearing resistance
In terms of effective stress :
τf = c’ + σf ’ tan φ’
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Mohr circles in terms of σ & σ’
σv
σh
σv’
σh’
u
X X X= +
total stresseseffective stresses
σvσhσv’σh’
u
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Envelopes in terms of σ & σ’Identical specimens
initially subjected to
different isotropic stresses
(σc) and then loaded
axially to failure σc
σc
σc
σc
∆σf
Initially… Failure
uf
At failure,
σ3 = σc; σ1 = σc+ σ f
σ3’ = σ3 – uf ; σ1’ = σ1 - uf
c, φ
c’, φ’
in terms of σ
in terms of σ
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REVIEW SOIL COMPRESSIBILTY
• Settlement : ST = Si+S+ Ss
ST = total settlement
Si = immediate settlment
S = primary consolidation settlement
Ss = secondary consolidation setllement
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FAILURE MODE OF SHALLOW FOUNDATIONModes of bearing failure
a) General shear, b) local shear, c) punching shear (after Vesic )
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FAILURE PATTERN OF PILE FOUNDATION
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REVIEW OF FIELD TEST
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V Sh T t
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• The test is used for the in-situ determination of the
undrained strength of fully saturated clays• The test is very suitable for soft clays (< 100 kN/m2)
• The torque strength : T = πcu d2h + d32 6
d
h
T
Vane Shear Test
D t h C T t
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Dutch Cone Test
The end resistance of the cone at anydepth is called cone penetration
resistance (qc)qc is the force required to advance thecone divided by the end area
Correlation between qc a
30º 40º35º 0
10
20
30
40
φ
qc
(MN/m2)
35.7 mm
60° cone
Union sleeve
Outer rod
Inner rod
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St d d P t ti T t
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Standard Penetration Test
Relative Density of Sands (Terzaghi and Peck)
N value Relative density0-4 very loose
4-10 loose10-30 medium dense30-50 dense
>50 very dense
It is used to assess the in-situ relative
density of a sand deposit using a splitbarrel sampler
The number of blows required to drive thesampler is called standard penetrationresistance (N) Due to excess pore water pressure, N valueshould be corrected by : N’ = 15 + ½(N - 15)
Boring rod
Sampler he
Air holes
Split tube
Shoe
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Correlation of cone resistance with SPT value. 1, Meigh and Nixon; 2, Meyerhof; 3
Rodin;4, Schmertman; 5, Shulze and Knausenberger; 6, Sutherland; 7, Thorburn an
McVicar.
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Effect of relative density, based on field data. (After Skempton, 1986.)
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PRESSURE METER TESTThe pressuremeter test is an in-situ testing method which is
commonly used to achieve a quick and easy measure of the in-situ stress-strain relationship of the soil which provides
parameters such as the elastic modulus
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PRESSURE METER TEST
SELF BORING PRESSURE METER TEST
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SELF BORING PRESSURE METER TEST
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PLATE BEARING TEST
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LABORATORIUM TEST
DIRECT SHEAR TEST
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DIRECT SHEAR TEST
• The disadvantages of the test :- the drainage conditions cannot be controlled
- only the total stress can be determined- shear stress on the failure plane is not uniform
- the area under the shear and vertical loads does not
remain constant
Shear force (T)
Vertical force (N)
T
∆l∆h
Boxes
• The advantage of the test :
- the test is simple
DIRECT SHEAR TEST (1)
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DIRECT SHEAR TEST (1)
Shear force (T)
Vertical force (N)
T
∆l∆h
Boxes
Vertical force (N)
Loading plate
Boxes
specimen
Porous plates
DIRECT SHEAR TEST (2)
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DIRECT SHEAR TEST (2)
∆ l
τ
Max.
Max.
Di t Sh T t (3)
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Direct Shear Test (3)
τ
σ
Mohr Circle
Failure Envelope
TRIAXIAL TEST (1)
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( )
σ1
σ1
σ3 σ3
σ1 : major princip
σ3 : minor principaσ1 – σ3 : deviator
Equalall-roundpressure
Axial stress
All-roundpressure supply
Types of Triaxial Test
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• Consolidated Drained Tests
- also known as S tests (S for Slow)
- drainage is permitted under a specified all-round pressure untconsolidation is complete
- deviator stress is then applied at a slow rate with drainage is
permitted
- the excess pore water pressure is maintained at zero
• Unconsolidated Undrained Tests
- also known as Q tests (Q for Quick)
- the all-round pressure is given initially, then the deviator stres
applied immediately until it reaches failure state
- no drainage is permitted at any stages of test• Consolidated Undrained Tests
- also known as R tests
- drainage is permitted under a specified all-round pressure untconsolidation is complete
- deviator stress is then applied with no drainage is permitted
- pore water pressure is measured during the undrained stage
APPLICATION OF THE TEST
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APPLICATION OF THE TEST
Drained strength
- can be expressed in terms of effective stress: c’ and ’
- applies in soils of low permeability after consolidation iscomplete and would represent the situation of a longtime after the completion of construction
• The shear strength in drained condition is different from
the strength in undrained condition
Undrained strength
- can be expressed in terms of total stress: cu and u- applies in soils of low permeability such as clays
immediately after the completion of construction
TRIAXIAL TEST (2)
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σ1 − σ3
εa
TRIAXIAL TEST (2)
Max.
Max.
Triaxial Test (3)
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τ
Triaxial Test (3)
σ
Mohr Circle
q
p
Kf line
p-q Diagram