NPTEL Online IIT Guwahati

The difference between the total and effective stresses is simply the pore water pressure u. Consequently, the total and effective stress Mohr circles have the same diameter and are only separated along the s - axis by the magnitude of the pore water pressure.

It is easy to construct a series of total stress Mohr Circles but the inferred total stress parameters have no relevance to actual soil behaviour. In principle, the effective strength parameters are necessary to check the stability against failure for any soil construction in the field. To do this, the pore water pressure in the ground under the changed loading conditions must be known and in general they are not.

In an undrained triaxial test with pore pressure measurement, this is possible and the effective stresses can then be determined. Alternatively, in drained tests, the loading rate can be made sufficiently slow so as to allow the dissipation of all excess pore water pressure. For low permeability soils, the drainage will require longer times.

In undrained tests, the general expression relating total pore water pressure developed and changes in applied stresses for both the stages is:

where Du₁ = pore water pressure developed in the first stage during application of confining stress Ds₃,
Du₂ = pore water pressure developed in the second stage during application of deviator stress (Ds₁- Ds₃), and

B and A are Skempton's pore water pressure parameters.

Parameter B is a function of the degree of saturation of the soil (= 1 for saturated soils, and = 0 for dry soils). Parameter A is also not constant, and it varies with the over-consolidaton ratio of the soil and also with the magnitude of deviator stress. The value of A at failure is necessary in plotting the effective stress Mohr circles.

Consider the behaviour of saturated soil samples in undrained triaxial tests. In the first stage, increasing the cell pressure without allowing drainage has the effect of increasing the pore water pressure by the same amount.
Thus, there is no change in the effective stress. During the second shearing stage, the change in pore water pressure can be either positive or negative.

For UU tests on saturated soils, pore water pressure is not dissipated in both the stages (i.e., Du = Du₁+ Du₂).

For CU tests on saturated soils, pore water pressure is not dissipated in the second stage only (i.e., Du = Du₂).

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The difference between the total and effective stresses is simply the pore water pressure u. Consequently, the total and effective stress Mohr circles have the same diameter and are only separated along the s - axis by the magnitude of the pore water pressure. It is easy to construct a series of total stress Mohr Circles but the inferred total stress parameters have no relevance to actual soil behaviour. In principle, the effective strength parameters are necessary to check the stability against failure for any soil construction in the field. To do this, the pore water pressure in the ground under the changed loading conditions must be known and in general they are not. In an undrained triaxial test with pore pressure measurement, this is possible and the effective stresses can then be determined. Alternatively, in drained tests, the loading rate can be made sufficiently slow so as to allow the dissipation of all excess pore water pressure. For low permeability soils, the drainage will require longer times. In undrained tests, the general expression relating total pore water pressure developed and changes in applied stresses for both the stages is: Du = Du₁+ Du₂ = B.Ds₃+ B.A.(Ds₁- Ds₃) = B[Ds₃+ A(Ds₁- Ds₃)] where Du₁ = pore water pressure developed in the first stage during application of confining stress Ds₃, Du₂ = pore water pressure developed in the second stage during application of deviator stress (Ds₁- Ds₃), and B and A are Skempton's pore water pressure parameters. Parameter B is a function of the degree of saturation of the soil (= 1 for saturated soils, and = 0 for dry soils). Parameter A is also not constant, and it varies with the over-consolidaton ratio of the soil and also with the magnitude of deviator stress. The value of A at failure is necessary in plotting the effective stress Mohr circles. Consider the behaviour of saturated soil samples in undrained triaxial tests. In the first stage, increasing the cell pressure without allowing drainage has the effect of increasing the pore water pressure by the same amount. Thus, there is no change in the effective stress. During the second shearing stage, the change in pore water pressure can be either positive or negative. For UU tests on saturated soils, pore water pressure is not dissipated in both the stages (i.e., Du = Du₁+ Du₂). For CU tests on saturated soils, pore water pressure is not dissipated in the second stage only (i.e., Du = Du₂).
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