Shear Strength of Soils

Laboratory Tests

1. Direct Shear Test

2. Triaxial Compression Test

3. Unconfined Compression Test

4. Laboratory Vane Shear Test

5. Torsion Test

6. Ring Shear Tests

Field Tests

1. Vane Shear Test

2. Penetration Test

Triaxial Tests Unconsolidated Undrained Test

Drainage is not permitted at any stage of the test, that is, either during

confined compression or during the test when the shear stress is applied.

Hence no time is allowed for dissipation of pore water pressure and

consequent consolidation of the soil; also, no significant volume changes

are expected.

Since a relatively small time is allowed for the testing till failure, it is also

called the ‘Quick test.’ It is designated UU.

Consolidated Undrained Test

Drainage is permitted fully in this type of test during the application of

confined pressure and no drainage is permitted during the application of

the shear stress. Thus volume changes do not take place during shear and

excess pore pressure develops. Usually, after the soil is consolidated

under the applied normal stress to the desired degree, 5 to 10 minutes may

be adequate for the test.

This test is also called ‘consolidated quick test’ and is designated CU test

Consolidated Drained Test

Drainage is permitted fully before and during the test, at every stage. The

soil is consolidated under the applied confined compression and is tested

for shear by applying the shear stress also very slowly while drainage is

permitted at every stage. Practically no excess pore pressure develops at

any stage and volume changes take place. It may require 4 weeks to

complete a single test of this kind in the case of cohesive soils, although

not so much time is required in the case of cohesionless soils as the latter

drain off quickly.

This test is seldom conducted on cohesive soils except for purposes of

research. It is also called the ‘Slow Test’ or ‘consolidated slow test’ and

is designated CD test.

The shear parameters c and φ vary with the type of test or drainage

conditions. The suffixes u, cu, and d are used for the parameters obtained

from the UU-, CU- and CD-tests respectively.

For problems of short-term stability of foundations, excavations and earth

dams UU-tests are appropriate. For problems of long-term stability, either

CU-test or CD tests are appropriate, depending upon the drainage

conditions in the field.

Two identical samples of soil which are subjected to the same changes in

total stress but under different drainage conditions (i.e. at different

velocities) will have different internal effective stresses and therefore

different strengths according to the Mohr–Coulomb criterion. Rather than

have to determine the pore pressures and effective stresses under

undrained conditions, the undrained strength can be expressed in terms of

total stress, as an alternative description of the strength of the soil in these

conditions.

The failure envelope will still be linear, but will have a different gradient

and intercept; a Mohr– Coulomb model can therefore still be used, but the

shear strength parameters are different and, denoted by cu and ϕu =0, with

the subscripts denoting undrained behavior.

The drained strength is expressed directly in terms of the effective stress

parameters c′ and ϕ′ described previously.

In deciding whether to use drained or undrained strength parameters to

subsequently analyze geotechnical constructions in practice, the principal

consideration is the rate at which the changes in total stress (due to

construction operations) are applied in relation to the rate of dissipation

of excess pore water pressure (consolidation), which in turn is related to

the permeability of the soil.

In fine-grained soils of low permeability (e.g. clay, silt), loading in the

short term (e.g. of the order of weeks or less) will likely be undrained,

while in the long-term, conditions will ultimately be drained. In coarse-

grained soils (e.g. sand, gravel) both short- and long-term loading will

result in drained conditions due to the higher permeability, which allows

consolidation to take place rapidly. Under dynamic loading (e.g.

earthquakes), loading may be fast enough to generate an undrained

response in coarse-grained material. ‘Short-term’ is often taken to be

synonymous with ‘during construction’, while ‘long-term’ usually relates

to the design life of the construction (usually many tens of years).

Testing under back pressure involves raising the pore water pressure

within the sample artificially. In a drained test this connection remains

open throughout the test, drainage taking place against the back pressure;

the back pressure is then the datum for excess pore water pressure

measurement.

The object of applying a back pressure is to ensure full saturation of the

specimen or to simulate in-situ pore water pressure conditions. During

sampling, the degree of saturation of a fine‑grained soil may fall below

100% owing to swelling on the release of in-situ stresses. Compacted

specimens will also have a degree of saturation below 100%. In both

cases, a back pressure is applied which is high enough to drive the pore

air into solution in the pore water.

It is essential to ensure that the back pressure does not by itself change the

effective stresses in the specimen. It is necessary, therefore, to raise the

cell pressure simultaneously with.

Unconfined Compression test

The confining pressure σ3 = 0. The reported result from such a test is the

unconfined compressive strength (UCS), which is the major principal

(axial) stress at failure (which, because σ3 = 0, is also the deviatoric stress

at failure). As only one test is conducted it is not possible to define the

Mohr–Coulomb shear strength envelope without conducting further

triaxial tests. The test is not suitable for cohesionless soils (c′ ≈ 0), which

would fail immediately without the application of confining pressure. It is

usually used with fine-grained soils, and is particularly popular for testing

rock.

The direct shear test At failure within an element of soil under principal stresses σ1and σ3 a slip

plane will form within the element at an angle θ as shown in Figure 5.6.

The shear box is designed to represent the stress conditions along this slip

plane. Porous plates are placed below and on top of the specimen if it is

fully or partially saturated to allow free drainage: if the specimen is dry,

solid metal plates may be used.

Vane Shear Test

Example A boring log reveals that a thin layer of silty clay exists at a depth of 15

m below the natural ground surface. The soil above this layer is a silt

having γd = 15.5 kN/m3 γsat=19.84kN/m3 and w = 28%. The groundwater

table is found to exist approximately near the ground surface.

Triaxial shear tests on the undisturbed silty clay samples give the

following results:

ccu = 48.3 kN/m2, φcu =13° and cd′ = 41.4 kN/m2, φd′ = 23°

Estimate the shearing resistance of the silty clay on a horizontal plane (i)

when the shear stress builds up rapidly and (ii) when the shear stress

builds up very slowly.

Total unit weight of silt Submerged unit weight

γ′ = γsat – γw = 19.8 – 9.807 = 10.03 kN/m3

Effective pressure at a depth of 15 m

σ′n = 15×10.03 = 150.45 kN/m2

Total pressure at the depth of 15 m = 15×19.84 = 297.6 kN/m2

For a rapid build-up of stresses there is no time for dissipation of pore

water pressure, and the total stress parameters are used. Therefore,

Shear strength τf = ccu + σn tan φcu

= 48.3 + 297.6 tan 13° = 117.0 kN/m2

For a slow build-up of stresses, there is no excess pore water pressure,

and the effective stress

parameters are used. Therefore,

Shear strength τf = c′d + σ′n tan φ′d

= 41.4+150.45 tan 23°=105.3 kN/m2

Example

Given:

A direct shear test is run on a medium dense sandy silt, with the normal

stress σn = 65 kPa; K0 = 0.5. At failure, the normal stress is still 65 kPa

and the shear stress is 41 kPa.

Required:

Draw the Mohr circles for the initial conditions and at failure and

determine:

a-The principal stresses at failure. ·

b- The orientation of the failure plane.

c. The orientation of the major principal plane at failure.

d. The orientation of the plane of maximum shear stress at failure.

Pore Pressure Parameters

Δu = Δσ3 + A(Δσ1 - Δσ3)