Pore water pressures are important factors in determining soil strength. Dimensionless coefficients called ‘Pore pressure coefficients’ or ‘Skempton’s Pore Pressure Parameters A and B’ characterize the change in pore water pressure caused by a change in applied stress. Prof. A.W. Skempton proposed these parameters, which are now universally accepted (Skempton, 1954).
Pore water pressures develops in an undrained triaxial compression test when cell pressure or confining pressure is applied in the first stage, as well as when additional axial stress or deviator stress is applied in the second stage.
The ratio of established pore water pressure to the applied confining pressure is referred to as the B-parameter :
Since no drainage is allowed, the volume of the soil skeleton decreases at the same rate as the volume of pore water.
It can be shown using this and the concepts of elasticity theory that :
where Cv and Cc represent the volume compressibilities (change in volume per unit volume per unit pressure increase) of pore water and soil respectively and n is the porosity.
Cc is much greater than Cv in a saturated soil, and B is nearly unity; in a dry soil, Cv, the value for pore air is much greater than Cc. B is nearly zero or negligible.
The experimentally discovered variation of B with saturation degree is shown in Fig.
where,
∆ud = port pressure developed due to an increase of deviator stress
(∆σ1 – ∆σ3) and Ā = product of A and B.
The A-factor, also known as a parameter, is not a fixed value. It varies depending on the soil, its stress background, and the deviator stress applied. Its value may be set at any point during the test, including failure or maximum deviator stress. In the case of sands, the A-factor varies with the initial density index, while in the case of clays, it varies with the over-consolidation ratio. The difference in the over-consolidation ratio, as calculated by Bishop and Henkel (1962), is depicted in below figure.
The following is a general term for the developed pore water pressure and changes in applied stresses :
….. eqn 1
A can be shown to be 1/3 for a completely elastic material. This may also be published in the following format :
where Ā = A.B.
If ∆U is considered to be the sum of two components ∆Ud and ∆Uc,
For the conventional triaxial test at constant cell pressure, during the application of the deviator stress,
∆σ3 = 0 (zero) and ∆σ1 = (σ1 – σ3).
Taking B as unity for full saturation, eqn 1 for this case of UU-test will reduce to,
A and hence Ā can be easily determined from the conventional triaxial compression test of UU type.
For CU tests where drainage is permitted during the application of cell pressure, ∆Uc = 0, and the corresponding value of ∆u is given by
A-factor may be as high as 2 to 3 for saturated fine sand in loose condition, and as low as -0.5 for heavily pre-consolidated clay.
Also Read : Vane Shear Test [ IS 2720 (Part XXX) – 1980 ]
Also Read : Direct Shear Test for Shear Strength of Soil
Also Read : Unconfined Compression Test【IS 2720(Part 10):1991 PDF】
Uses and Application of Skempton’s Pore Pressure Parameters
- Skempton’s pore pressure parameters are extremely useful in field problems that require the prediction of pore pressures caused by known changes in total stress.
- The construction of an earth embankment or an earth dam over a soft clay deposit is a classic example.
- Undrained conditions prevail if the rate of construction is such that the pore water pressure induced in the foundation soil cannot be dissipated.
- If the pore pressure developed is too high, the shear strength of the foundation soil, which is dependent on the effective stress, decreases, putting the foundation’s stability at risk.
- The pore pressure parameters can be used to predict how the pore pressure changes as the total stresses increase as the height of the embankment/dam increases. As a result, the structure’s stability can be ensured.
- The construction engineer can recommend a reasonable construction rate in stages so that excess pore pressures can be controlled and stability can be maintained during and after construction.