Behaviour of Clay and Sand

8/19/2019 Behaviour of Clay and Sand

1/48

LECTURE 1 REVIEWPREPARED BY T. ILYAS


2/48

REVIEW OF BEHAVIOUR OF CLAAND SAND


3/48

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)


4/48

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


5/48


6/48

FINE GRAIN SOILS


7/48

COURSE GRAIN SOIL


8/48

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


9/48

REVIEW OF EFFECTIVE STRESS CONCEPT


10/48

TOTAL STRESS BELOW A RIVER/LAKE

10


11/48

TOTAL STRESS IN MULTI-LAYERED SOIL

11


12/48

NEGATIVE PORE PRESSURE (SUCTION)

The pore pressure in a partially

saturated soil consists of two

components:

· porewater pressure = uw· pore-air pressure = ua

12


13/48

γsoil

Incompressible

Stresses Within Soil Mass

H

σvertical = γsoil · H

σhorizontal = K · σve

∆σvertical

∆σvertical

Note: No External Shear S

Dry Soil


14/48

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


15/48

STRESSES WHEN NO FLOW TAKES PLACETHROUGH THE SATURATED SOIL MASS

15


16/48

REVIEW OF SHEAR STRENGTH

Shear strength:

• Mohr-coulomb


17/48

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


18/48

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


19/48

Mohr Diagram (3)

τ

σσvσh

K < 1

σvertical = γsoil ·

σhorizontal = K

∆σvertical

σv + ∆σv


20/48

20

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


21/48

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’


22/48

Mohr circles in terms of σ & σ’

σv

σh

σv’

σh’

u

X X X= +

total stresseseffective stresses

σvσhσv’σh’

u


23/48

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


24/48

REVIEW SOIL COMPRESSIBILTY

• Settlement : ST = Si+S+ Ss

ST = total settlement

Si = immediate settlment

S = primary consolidation settlement

Ss = secondary consolidation setllement


25/48

FAILURE MODE OF SHALLOW FOUNDATIONModes of bearing failure

a) General shear, b) local shear, c) punching shear (after Vesic )


26/48

FAILURE PATTERN OF PILE FOUNDATION


27/48

REVIEW OF FIELD TEST


28/48

V Sh T t


29/48

• 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


30/48

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


31/48

St d d P t ti T t


32/48

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


33/48

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.


34/48

Effect of relative density, based on field data. (After Skempton, 1986.)


35/48

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


36/48

PRESSURE METER TEST

SELF BORING PRESSURE METER TEST


37/48

SELF BORING PRESSURE METER TEST


38/48

PLATE BEARING TEST


39/48

LABORATORIUM TEST

DIRECT SHEAR TEST


40/48

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)


41/48


Shear force (T)

Vertical force (N)

T

∆l∆h

Boxes

Vertical force (N)

Loading plate

Boxes

specimen

Porous plates



42/48


∆ l

τ

Max.

Max.

Di t Sh T t (3)


43/48

Direct Shear Test (3)

τ

σ

Mohr Circle

Failure Envelope

TRIAXIAL TEST (1)


44/48

( )

σ1

σ1

σ3 σ3

σ1 : major princip

σ3 : minor principaσ1 – σ3 : deviator

Equalall-roundpressure

Axial stress

All-roundpressure supply

Types of Triaxial Test


45/48

• 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


46/48

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)


47/48

σ1 − σ3

εa

TRIAXIAL TEST (2)

Max.

Max.

Triaxial Test (3)


48/48

τ

Triaxial Test (3)

σ

Mohr Circle

q

p

Kf line

p-q Diagram

Documents

Behaviour of Clay and Sand