SOIL MECHANICS II Shear Strength of Soil

INTRODUCTION

The shear strength of a soil mass is the internal resistance per unit area that the soil mass can offer to resist failure and sliding along any plane inside it.

The shear strength of a soil depends on condition where the deformation of soil taking place: Fully drained Undrained Both conditions

Different excess pore water pressure will caused different effective stress under different conditions, thus in result different strength.

In the context of soil stabilization, analyses are usually done to check for the long term condition (Undrained) and short term condition (Fully drained)

The soil strength decide the safety of the structure construct on it.

STRESS ON A POINT

If at any point on any plane within a soil mass, the shear stressbecomes equal to the shear strength of the soil then failure will occurat that point.

SHEAR STRENGTH IN SOILS

The shear strength of a soil is its resistance to shearing stresses.

It is measure of the soil resistance to deformation by continuousdisplacement of its individual particles.

Shear strength in soils depends primarily on interactions betweenparticles.

Shear failure occurs when the stresses between the particles are suchthat they slide or roll past each other.

SHEAR STRENGTH IN SOILS

Soil derives its shear strength from two sources:

Cohesion between particles (stress independent component)

Cementation between sand grains

Electrostatic attraction between clay particles

Frictional resistance between particles (stress dependent component)

SHEAR STRENGTH OF SOILS: COHESION

Cohesion (C), is a measure of the forces that cement particles of soils.

Generally there are four types of soils:

Dry sand with no cementation

Dry sand with some cementation

Soft clay

Stiff clay

Internal friction angle () is measure of the shear strength of soils dueto friction.

MOHR-COULOMB FAILURE CRITERIA

Mohr presented a theory for rupture in materials.

This theory states that a material fails because of a criticalcombination of normal stress and shear stress, and not from eithertheir maximum normal or shear stress alone.

For most soil mechanics problems, it is sufficient to approximate theshear stress on the failure plane as a linear function of the normalstress.

MOHR-COULOMB FAILURE CRITERIA

= + tanWhere c = cohesion and = angle of internal friction

In saturated soil, the total normal stress at a point is the

sum of the effective stress and the pore water pressure,

which is = + Thus, the Mohr-Coulomb Failure Criteria need to rewrite

as

= + tan = + tan

Where c = effective stress cohesion and = effective angle of friction

MOHR-COULOMB FAILURE CRITERIA

In saturated soil, the total normal stress at a point is the sum of theeffective stress and pore water pressure.

The effective stress is carried by the soil solids.

The value of c for sand and inorganic silt is 0.

For normally consolidated clays, c can be approximated at 0.

Overconsolidated clays have values of c that are greater than 0.

The angle of friction is referred to as the drained angle of frictionand some typical values are given in Table 7.1.

MOHR-COULOMB FAILURE CRITERIA

The parameter of is affected by various factors, which are Mineralogy soil contains various mineral. The existence of mineral could change the values. For instance montmorilonit have low which is 40

Shape Shape particle have higher friction angle compare to round particle.

Grade better graded soil have better degree of interlocking between particlesand thus have higher value.

Void ratio the reduction of void ratio in the consolidation process for instance willincrease the degree of interlocking and thus increase the value.

Organic materials the existence of organic materials may reduce the value.

DETERMINATION OF SHEAR STRENGTH PARAMETERS

The shear strength parameters of a soil are determined in the labprimarily with two types of tests:

Direct Shear Test

Triaxial Shear Test

DIRECT SHEAR TEST

The shear test can be either stress-controlled or strained controlled.

In stress-controlled tests, the shear force is applied in equal incrementsuntil the specimen fails.

In strain-controlled tests, a constant rate of shear displacement isapplied to one half of the box by a motor that acts through gears.

The advantage of the strain-controlled tests is that, in case of densesand, peak shear resistance and lesser shear resistance can beobserved and plotted.

In stress-controlled tests, only peak shear resistance can be observedand plotted.

DIRECT SHEAR TEST

Direct shear test is quick and inexpensive

Shortcoming is that it fails the soil on a designated plane which may not be the weakest one.

Used to determine the shear strength of both cohesive as well as non-cohesive soils

The test equipment consists of a metal box in which the soil specimen is placed

DIRECT SHEAR TESTThe box is split horizontally into two halves

Vertical force (normal stress) is applied through a metal platen

Shear force is applied by moving one half of the box relative to the other tocause failure in the soil specimen

DIRECT SHEAR TEST

DIRECT SHEAR TEST

DIRECT SHEAR TEST DATA

DIRECT SHEAR TEST

DIRECT SHEAR TEST

By refer to Fig 7.3, the following points can be drawn:-

In loose sand, the resisting shear stress increases with sheardisplacement until a failure shear stress f is reached. After that, theshear resistance remains approximately constant with any furtherincrease in the shear displacement.

In dense sand, the resisting shear stress increases with sheardisplacement until it reaches a failure stress of f , which called peakshear strength. After failure stress is attained, the resisting shearstress gradually decreases as shear displacement increases until itfinally reaches a constant value called the ultimate shear strength.

DIRECT SHEAR TEST

DIRECT SHEAR TEST

DIRECT SHEAR TEST

The direct shear test is rather simple to perform, but it has someinherent shortcomings. The reliability of the results may be questioned.This is due to the fact that the soil is not allowed to fail along theweakest plane but is forced to fail along the plane of split of the shearbox. The shear stress distribution over the shear surface of thespecimen is not uniform.

TRIAXIAL SHEAR TEST

The triaxial shear test is one of the most reliable methods in determinethe shear strength parameters.

The test is considered reliable for the following reasons:-

It provides information on the stress-strain behaviour of the soil that the direct sheartest does not.

It provides more uniform stress conditions than the direct shear test does with its stressconcentration along the failure plane.

It provides more flexibility in terms of loading path.

TRIAXIAL SHEAR TEST

THE EFFECT OF THE DRAINED CONDITIONS DURING THE SOIL TEST

The shear strength parameters will change according tothe water content and soil density.

These values not only depend on the initial condition butalso depend on the changes during the running of the test.

There are two stages take place in the triaxial shear test.

At the first stage a constant normal stress are applied onthe sample and the second stage the normal stressincrease slowly until the sample fail.

The different drained condition on each test stage willcause the change of water pressure and also the density.

TYPES OF TRIAXIAL SHEAR TEST

The three principal types of test are as follows:-

UU test

The specimen is subjected to a specified all-round pressure and then the principal stress difference is applied immediately, with no drainage being permitted at any stage of the test

CU test

Drainage is permitted until consolidation is complete; the principal stress difference is then applied with no drainage being permitted.

Drained test

Drainage of the specimen is permitted at all time and the excess pore water pressure is maintained at zero.

TYPES OF TRIAXIAL SHEAR TEST

TYPES OF TRIAXIAL SHEAR TEST

CONSOLIDATED DRAINED TEST

The specimen is first subjected to an all-around confining pressure.

The pore water pressure increase by uc.

When the connection to drainage is kept open, dissipation of theexcess pore water pressure and consolidation occur.

With time, uc will become 0.

The deviator stress, d on the specimen is increased at a very slowrate.

The drainage connection is kept open, and the slow rate of deviator-stress application allows complete dissipation of any pore waterpressure that developed as a result.

CONSOLIDATED DRAINED TEST

CONSOLIDATED DRAINED TEST

CONSOLIDATED DRAINED TEST

CONSOLIDATED DRAINED TEST

CONSOLIDATED DRAINED TEST

CONSOLIDATED-UNDRAINED TEST

It is the most common type of triaxial test.

The saturated soil specimen is first consolidated by an all-roundchamber fluid pressure

After the pore water pressure generated by the application ofconfining pressure is completely dissipated, the deviator stress on thespecimen is increased to cause shear failure.

During this phase of the test, the drainage line from the specimen iskept close.

Since drainage is not permitted, the pore water pressure will increase.

The general patterns of variation of d and ud with axial strain forsand and clay soils are shown in Figure 7.13d, e, f, and g.

CONSOLIDATED-UNDRAINED TESTIn loose sand and normally consolidated clay, the pore water pressure increases withstrain.

In dense sand and over consolidated clay, the pore water pressure increase with strainup to a certain limit, beyond which it decreases and become negative (with respect tothe atmospheric pressure).

This pattern is because of the soil has a tendency to dilate.

Consolidated-drained tests on clays soils take considerable time.

Thus, CU tests can be conducted on such soils with pore pressure measurements to obtainthe drained shear strength parameters.

Since drainage is not allowed in these tests during application of deviator stress, thetests can be performed rather quickly.

The and can be computed by (For sand and normally consolidated clay)

= 2 tan113

0.5

450

= 2 tan113

0.5

450

CONSOLIDATED-UNDRAINED TEST

CONSOLIDATED-UNDRAINED TEST

CONSOLIDATED-UNDRAINED TEST

UNCONSOLIDATED-UNDRAINED TEST

Drainage from the soil specimen is not permitted during theapplication of chamber pressure.

Because of the application of chamber confining pressure, the porewater pressure in the soil specimen will increase by uc.

There will be a further increase in the pore water pressure udbecause of the deviator stress application.

The total water pressure u is = +

The UU test is usually conducted on clay specimens and depends on avery important strength concept for a saturated cohesive soils.

The added axial stress at failure is practically the same regardless ofthe chamber confining pressure. (Figure 7.17)

UNCONSOLIDATED-UNDRAINED TEST

UNCONFINED COMPRESSION TEST

The unconfined compression test is a special type of UU test that is commonly used for clay specimens.

The confining pressure is 0.

An axial load is rapidly applied to the specimen to cause failure.

At failure, the total minor principal stress is 0 and the total major principal stress is 1.

=12=2=

Where qu is the unconfined compression strength.

UNCONFINED COMPRESSION TEST

UNCONFINED COMPRESSION TEST