What is the difference between drained and undrained soil tests?

1 Drained and undrained tests

Drained and undrained tests are two types of triaxial tests that apply axial and confining pressures to a cylindrical soil sample and measure its deformation and strength. The main difference between them is the drainage condition, which controls the pore water pressure and the effective stress in the soil. In a drained test, the sample is allowed to drain water during the loading process, so the pore water pressure remains constant and the effective stress increases. In an undrained test, the sample is not allowed to drain water during the loading process, so the pore water pressure increases and the effective stress remains constant.

2 Drained test applications

Drained tests are used to simulate long-term loading conditions, where the soil has enough time to reach equilibrium and dissipate excess pore water pressure. Drained tests are suitable for coarse-grained soils, such as sands and gravels, that have high permeability and low compressibility. Drained tests can provide information about the shear strength, stiffness, and dilatancy of the soil, as well as the failure envelope and the friction angle.

3 Undrained test applications

Undrained tests are used to simulate short-term loading conditions, where the soil does not have enough time to reach equilibrium and retain excess pore water pressure. Undrained tests are suitable for fine-grained soils, such as clays and silts, that have low permeability and high compressibility. Undrained tests can provide information about the undrained shear strength, the pore pressure parameter, and the effective stress path of the soil, as well as the failure envelope and the cohesion.

4 Drained test interpretation

To interpret the results of a drained test, you need to plot the axial strain versus the axial stress and the deviator stress versus the effective confining stress. The axial strain is the change in length of the sample divided by its initial length. The axial stress is the difference between the axial pressure and the confining pressure. The deviator stress is the axial stress multiplied by the cross-sectional area of the sample. The effective confining stress is the confining pressure minus the pore water pressure. The slope of the axial strain versus axial stress curve is the secant modulus of elasticity. The slope of the deviator stress versus effective confining stress curve is the tangent modulus of elasticity. The peak point of the deviator stress versus effective confining stress curve is the failure point, where the shear strength is maximum. The angle between the failure envelope and the horizontal axis is the friction angle.

5 Undrained test interpretation

To interpret the results of an undrained test, you need to plot the axial strain versus the axial stress and the pore water pressure versus the axial strain. The axial strain and the axial stress are defined as in the drained test. The pore water pressure is the pressure of the water in the pores of the soil. The peak point of the axial strain versus axial stress curve is the failure point, where the undrained shear strength is maximum. The slope of the pore water pressure versus axial strain curve is the pore pressure parameter, which indicates the rate of pore water pressure generation. The effective stress path is the locus of points representing the effective confining stress and the deviator stress at different stages of loading. The angle between the failure envelope and the vertical axis is the cohesion.