Test: Shear Strength Of Cohesive Soil - Civil Engineering (CE) MCQ

The un-drained test is carried out on undisturbed sample of clay, silt and peat to determine the strength of the natural ground and also carried out on remoulded samples of clay to measure it its sensitivity.

In an un-drained test on saturated clays (B=1), both the major principal effective stress σ1’ and the minor principal effective stress σ3’ are independent of magnitude of cell pressure applied.

The consolidated-undrained tests are performed by two methods: i) the remoulded specimens are sheared under a cell pressure ii) the moulded specimens are consolidated under the same cell pressure and sheared with different cell pressure.

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.

 The change in the pore pressure due to change in the applied stress, during an un-drained, any be explained in terms of empirical coefficients called pore pressure parameters.

Type of shear, sample disturbance, and environment during shear such as temperature and nature of the fluid are the factors that affect pore pressure parameter.

The approximate value of pore pressure parameter at failure for very loose, fine saturated sand, saturated clays is 2 to 3.

A negative pore pressure is developed when we apply load on a sample of clay or sand because both sand and clay tends to expand on loading.

Both effective parameters C’ and φ’ can be expressed when pore pressure is measured during un-drained stage of the test.

The equation for consolidated undrained strength of the preconsolidated clay in terms of total stress can be approximately expressed as τf = ccu + σ tan φcu where, σ is the normal pressure.