Test: Shear Strength of Soil - 1 - Civil Engineering (CE) MCQ

Unconfined compression test is a special case of tri-axial test in which the confining pressure is zero. This test may be conducted on undisturbed or remoulded cohesive soils. It cannot be conducted on coarse-grained soils such as sands and gravels as these cannot stand without lateral support.

For problems of short term-stability of foundations: excavations and earth dams UU (Unconsolidated, Undrained) tests are appropriate. For problems of long-term stability either UU test or CD test is appropriate, depending upon the drainage conditions of soil.

Effective minor principal stress.

Skempton’s B-parameter is given by,

C_v is volume compressibility of pore fluid under isotropic conditions,
C_s is the coefficient of compressibility of the soil skeleton;n is porosity
In a fully saturated soil, the compressibility of the pore water (C_v) is negligible compared with the compressibility of the soil mass (C_s). Therefore, the ratio (C_v/C_s) tends to zero and the coefficient B becomes equal to unity.
In a partially saturated soil, the compressibility of the air in the voids Is high. The ratio (C_v/C_s) has a value greater than unity, and, therefore, the pore pressure coefficient B has a value of less than unity.

As the sample is sheared, its length decreases and the diameter increases. The cross-sectional area A at any stage during shear can be determined assuming that the sample remains cylindrical in shape. Let ΔL₀ be the change in length and ΔV₀ be the change in volume. The volume of the specimen at any stage is given by V₀ ± ΔV₀.
Therefore, A(L₀ - ΔL₀) = V₀ ± ΔV₀
or

For an undrained test as the volumetric change
(ΔV₀) is zero therefore,

The shear strength of a soil is its maximum resistance to shear stress just before the failure. Shear stress develop when soils are subjected to compression.
Terzaghi established that the normal stresses which control the shear strength of a soil are the effective stresses and not the total stresses. In terms of effective stresses shear strength is written as

where c' and ϕ' are the cohesion intercept and the angle of shearing resistance in terms of the effective engineering stress.

Shear strength of soil (τ) is related with its angie of internal friction (ϕ) by,

c is cohesion; for sand c = 0
is effective normal stress; for dry sand = σ normal stress

Since only one Mohr circle in terms of effective stresses, is obtained from all un-drained tests, effective stress failure envelope cannot be obtained from this test.