Various methods have been reported in the literature to measure the swelling pressure of clayey soils. These methods generally include experimental measurements of the swelling pressure of the soil using indirect methods by conventional oedometer apparatus; while the measurement of swelling pressure of compacted samples using direct method has been less considered. Direct methods involve constant volume swell method with initial vertical stress and without applying vertical stress. Indirect methods consist of swell-consolidation (Free-swell and Restricted-swell) and zero swell experiments. In the present study, in addition to using conventional oedometer, an experimental technique using a modified loading frame was employed to directly measure the swelling pressure. It is observed that the swelling pressure measured by direct method is greatly affected by the vertical stress at which the sample is inundated. The measured swelling pressures obtained from direct and indirect methods are compared with each other. The results indicate that zero swell method slightly overestimates swelling pressure compared to the direct method of constant volume swell without initial vertical stress. Furthermore, the methods of free swell-consolidation and restricted swell-consolidation excessively overestimate the swelling pressure. Regarding the actual conditions of the soil in practical works, it seems that the swelling pressure measured by the method of constant volume swell with initial vertical stress is closer to the true value. The effects of initial conditions such as dry unit weight and initial water content on the swelling pressure are also evaluated in all methods.
