SHEAR STRENGTH by soilmechanicsecg426

ECG426 â&#x20AC;&#x201C; SOIL MECHANICS

SHEAR STRENGTH OF SOILS 1/3

LEARNING OUTCOMES Week 8 : Coverage ~ Introduction to shear strength of soils. The friction model. Laboratory determination of shear strength of soils

Learning outcomes: At the end of this lecture/week the students would be able to: i.

Concept of total and effective stress, pore pressure and excess pore water pressures.

ii.

Concept of friction model.

iii.

Discuss different laboratory testing methods to determine the shear strength parameters of soils.

SHEAR STRENGTH OF SOILS TOPIC 4 : SHEAR STRENGTH OF SOILS I INTRODUCTION & LAB TESTING

SHEAR STRENGTH OF SOILS

OUTLINE of PRESENTATION 4.1

Introduction & Overview

4.2

Concept of Total and Effective Stresses

4.3

Definition and the friction model

4.4

Laboratory determination of shear strength

SHEAR STRENGTH OF SOILS Introduction and overview

SINKHOLE AT ORAN ARMY CAMP, PERLIS

(14/10/2001)

SHEAR STRENGTH OF SOILS Introduction and overview Rock fall at Bukit Lanjan on N-S Expressway

Slope failure at Bukit Antarabangsa Condominium (15/5/1999)

SHEAR STRENGTH OF SOILS Introduction and overview

ROAD EMBANKMENT FAILURE Berita Harian (01/01/1994)

SHEAR STRENGTH OF SOILS Introduction and overview

Why does soil fail to perform its function ??  Is it because of soil erosion?  Degradation of soil materials ?  Loss in strength ?  Reminder from

God al-Mighty ?

SHEAR STRENGTH OF SOILS Total and Effective Stresses 4.2

CONCEPT OF TOTAL AND EFFECTIVE STRESSES

Consider an element of a saturated soil subjected to a normal stress,  as in the figure. Stress  is called total stress and for equilibrium (Newton’s third law) the stresses in the soil must be equal and opposite to .

SHEAR STRENGTH OF SOILS Total and Effective Stresses

Resistance to , is provided by a combination of the stresses from the solid called effective stress (’) and from pore water in the pores, called pore pressure u. Thus  = ’ + u ’ =  - u This equation is called the principle of effective stress – first recognized by Terzaghi (1883-1963) – research in soil consolidation.

Principle of effective stress

SHEAR STRENGTH OF SOILS Concept of effective stresses

SHEAR STRENGTH OF SOILS Total and Effective Stresses

`The principle of effective stress is the most important principle in soil mechanics. Definition of soils are a function of effective stresses not total stresses. The principles of effective stress applies only to normal stress and not to shear stressâ&#x20AC;&#x2DC; `Soil cannot sustain tension. Consequently the effective stress cannot be less than zero. The pore pressure can be positive or negative. The latter is sometimes called suction or suction pressure.â&#x20AC;&#x2DC;

SHEAR STRENGTH OF SOILS Total and Effective Stresses

For unsaturated soil: ’ =  – ua + (ua – uw) where ua = pore air pressure uw = pore water pressure  = factor depending on the degree of saturation: For dry soil  = 0, and for saturated soil  = 1

SHEAR STRENGTH OF SOILS Total and Effective Stresses Pore water pressure in soil mass can be determined using either (i) Porewater pressure transducer

(ii)

Piezometers

SHEAR STRENGTH OF SOILS Total and Effective Stresses Stress in the ground; For a case of total stress,  1) no surcharge

2) Ground below water level (lake/river/seabed)

3) With surcharge load, q

gwt

ground surface

o = γh The total stress in the ground in only due to overburden pressure only

ground surface

γsat

o = γsath + γwhw The total stress in the ground in only due to overburden pressure + hydrostatic pressure

o = γh + q The total stress in the ground in only due to overburden pressure + surcharge load

SHEAR STRENGTH OF SOILS Total and Effective Stresses

Stress in the ground; For a case with gwt occurance, ‘ 1) no gwt

2) gwt at surface

ground surface

γsat

gwt

3) gwt beneath surface ground surface

gwt

γsat o’ = o – u o’ = γh – 0 = γh

o’ = o – u = γsathw – γwhw = (γsat– γw)hw = γ’hw

o’ = o – u = γh + γ’hw

SHEAR STRENGTH OF SOILS Groundwater and pore pressure

suction

SHEAR STRENGTH OF SOILS Definition & the friction model

Shear Strength of Soil Shear strength between a soil mass is due to the development of frictional resistance between adjacent particles, and analyses are based primarily on the friction model. The force transmitted between 2 bodies in static contact can be resolved into 2 components:-

Normal component (N ) Shear component (T ) * both are perpendicular to each other.

SHEAR STRENGTH OF SOILS Definition & the friction model

SHEAR STRENGTH OF SOILS Cohesion

SHEAR STRENGTH OF SOILS Figure below shows a typical shear stress/displacement (strain) curve for the shearing of a prismatic element of soil under constant normal stress (ď ł n) The maximum value of the shear stress at the yield point is termed the peak stress and represents the limiting value corresponding to that value of normal stress.

Peak, ultimate and residual limiting stress

SHEAR STRENGTH OF SOILS The shear stress continue to fall until it levels off again at a lower value, known as the ultimate stress. The ultimate stress represents the shear strength of the material at its critical state and the volume is at critical volume. The ultimate stress value is usually be reached at strains of between 10% to 20%.

Peak, ultimate and residual limiting stress

SHEAR STRENGTH OF SOILS

At very much larger strains, e.g. on active landslip surfaces in clay soils, the limiting shear stress falls further, and at displacements of over a meter it may be reduced to values as low as 10 % of the peak stress. This very low large-strain value is referred as the residual stress.

Peak, ultimate and residual limiting stress

SHEAR STRENGTH OF SOILS Determination of shear properties

4.4 LABORATORY DETERMINATION OF SHEAR STRENGTH

 SHEAR BOX TEST  TRIAXIAL TEST i. Unconfined compression test (UCT) ii.

Unconsolidated undrained test (UU)

iii. Consolidated Isotropically Undrained Test (CIU) iv. Consolidated Isotropically Drained Test (CID)  LABORATORY SHEAR VANE TEST

SHEAR STRENGTH OF SOILS Shear Box Apparatus & Accessories 4.4.1 Shear box apparatus and its accessories

SHEAR STRENGTH OF SOILS Shear Box Apparatus & Accessories

SHEAR STRENGTH OF SOILS Shear box apparatus 4.4.1.1 Schematic diagram of a shear box apparatus

SHEAR STRENGTH OF SOILS Shear Box Apparatus & Accessories

SHEAR STRENGTH OF SOILS Data analysis of shear box test Shear stress,

From the prismatic element in Figure 7.7, the angle of dilation is given by;

tan = v/h   = tan -1(v/h)

dense or overconsolidated soil

loose or normally consolidated soil

At critical state

From ~ n plot in Figure 7.9(b), the angle of dilation is given by;

 = ’ p - ’ c v + v

Angle of dilation, 

h  10% Lateral deformations, h dense

- v loose

at peak strength

at critical state where strains,  = 10% ~ 20% (ultimate strength)

Whitlow, R., page 215-220

SHEAR STRENGTH OF SOILS Data analysis of shear box test

Figure 7.8

SHEAR STRENGTH OF SOILS Data analysis of shear box test 4.4.1.2 Results from a shear box test

 = ’ p - ’ c



Figure 7.9

SHEAR STRENGTH OF SOILS Data analysis of shear box test

Review work example 7.1, 7.2 and 7.3 in Whitlow, R. page 224-228 for data analysis for shear box tests.

SHEAR STRENGTH OF SOILS Advantages & Disadvantages of shear box test As a soil test the shear box is far from ideal. Disadvantages of the test includes:

• • •

Non-uniform deformations and stresses. The stresses determined may not be those acting on the shear plane. There are no facilities for measuring pore pressures in the shear box and so it is not possible to determine effective stresses from undrained tests. The shear box apparatus cannot give reliable undrained strengths because it is impossible to prevent localised drainage away from the shear plane.

SHEAR STRENGTH OF SOILS Advantages & Disadvantages of shear box test However, it has many apparent advantages: • It is easy to test sands and gravels. • Large deformations can be achieved by reversing the shear box. This involves pushing half of the box backwards and forwards several times, and is useful in finding the residual strength of a soil. • Large samples may be tested in large shear boxes. Small samples may give misleading results due to imperfections (fractures and fissures) or the lack of them. • Samples may be sheared along predetermined planes. This is useful when the shear strengths along fissures or other selected planes are required. • In practice the shear box is used to get quick and crude estimates of the failure parameters. It is sometimes used to obtain undrained strengths but this use should be discouraged.

SHEAR STRENGTH OF SOILS Triaxial apparatus and accessories 4.4.2 Types of Triaxial Tests  Unconfined compression test (UCT) No cell pressure (3) involved as this is unconfined condition. The load is applied on top of cylindrical sample until failure. The deviator stress (d) is equal to axial stress (1) since 3 = 0. Suitable for rock and stiff clay.  Unconsolidated undrained test (UU) Cell pressure (3) applied without allowing drainage. Then keeping cell pressure constant (3) increase deviator stress (d) to failure without drainage.  Consolidated isotropically undrained test (CIU or CU) Drainage allowed during cell pressure (3) application for consolidation process. Then without allowing further drainage increase deviator stress (d) while keeping cell pressure (3) constant until sample failure.  Consolidated isotropically drained test (CID or CD) Similar to CU test except that as deviator stress (d) is increased drainage is permitted (allowed). The rate of loading must be slow enough to ensure no excess pore pressures develop (u).

SHEAR STRENGTH OF SOILS Interpretation of test results 4.4.2.1 UNCONFINED COMPRESSION TEST (UCT)

SHEAR STRENGTH OF SOILS Triaxial apparatus and accessories 4.4.2.2 Triaxial Apparatus

SHEAR STRENGTH OF SOILS Triaxial apparatus and accessories (a) Triaxial accessories and sample set up

Photo of the standard triaxial cell and position of the sample.

1

(perspex)

3

CLOSE UP VIEW OF THE SAMPLE SET-UP

SHEAR STRENGTH OF SOILS Diagram of Triaxial cell Notes: B

3

Stages involve in conducting triaxial test are; 1.Sample preparation (cylindrical sample with ratio of size D:H1:2)

2.Saturation stage, B = u /3 (B  0.95 ~ where B is Skempton’s pore pressure parameter. Normally sample prepared in stage 1 is in unsaturated condition)

3.Consolidation stage (to investigate stress history and compression behaviour)

4.Shearing stage (to investigate failure behaviour) Schematic diagram of the triaxial cell and position of the sample

SHEAR STRENGTH OF SOILS Interpretation of test results (b) INTERPRETATION OF TRIAXIAL TEST RESULTS Strains and stresses in the triaxial test

(a) Principal strains

(b) Cell pressure only

a = 1 = major principal stress r = 2 = 3 = minor principal stress

SHEAR STRENGTH OF SOILS Interpretation of test results Universalsignage for deformations in triaxial test are; Compression +ve Swelling –ve

Expansion –ve Shrinking +ve Extension load applied

Compression load applied

- L

+ L compression Cylindrical sample

Expansion

Cylindrical sample

+ D shrink

- D swell D

SHEAR STRENGTH OF SOILS Failure modes (c) MODE OF SHEAR FAILURE IN A TRIAXIAL TEST

angle of failure surface

(a) Brittle shear failure

(b) Partial shear failure

SHEAR STRENGTH OF SOILS Failure modes



Example of brittle shear failure of sample under unconfined compression test (UCT).

SHEAR STRENGTH OF SOILS Advantages of Triaxial Test As a test for investigating the behaviour of soils the triaxial test has many advantages over the shear box test:

• • • • •

Specimens are subjected to uniform stresses and strains. The complete stress-strain behaviour can be investigated. Drained and undrained tests can be performed. Pore water pressures can be measured in undrained tests.

Different combinations of confining and axial stress can be applied.