Pore water pressure behaviour and evolution in clays and its influence in the consolidation process

Abstract

Consolidation is linked to changes in effective stress, which result from changes in pore water pressure. Upon application of an external or internal load, there is an increase in pore water pressure throughout the sample known as excess pore water pressure. Accordingly, flow takes place due to the hydraulic gradient generated by the initial excess pore pressure distribution. At any stage of the consolidation process, the pore water pressures will vary within the soil layer. Terzaghi’s one-dimensional consolidation theory is commonly adopted to describe the dissipation of excess pore water pressure within a consolidating soil over time.

The tests performed in this study are a variant of the standard incrementally loaded oedometer test. With these tests, it is intended to observe precisely how pore water pressures develop in a soil sample, during the loading process. One of the objectives is to determine exactly what is happening during load steps: pore water pressure values, times required, and how its dissipation develops. In the same way, this allows to know better how settlements generate, as well as any other kind of effects. Another objective is to study the potentials of this testing method, and to point out possible disadvantages of the current testing equipment and method, and to suggest future modifications. The chosen material is a soft clay from Ossinlampi (Otaniemi), taken from the vicinity of Aalto-University’s laboratory of soil mechanics and foundation engineering in Espoo, and a total of twelve tests have been carried out.

The results show that the development of pore water pressures present a behavior somewhat different to that generally assumed. These differences are manifested mainly in the very early phase after the application of the load on the soil sample. The transfer of the total pressure to the water particles does not occur instantaneously but requires a period of time to be completed which is generally around 10 seconds. In addition, not all of this load is transferred to the sample, but values close to 90 % of the load.

The test is relatively new, and does not consist of a defined procedure, so it sometimes requires more time and preparation. The sample and cell preparations are the most problematic phases, which may cause some problems. However, it has many applications in this field of geotechnics.