A Modified Pressure Plate Device for SWCC Testing Under Anisotropic Stress States: Fid-Bau Portal

A Modified Pressure Plate Device for SWCC Testing Under Anisotropic Stress States

Hoyos, L. R. / Takkabutr, P. / Puppala, A. J.

The soil-water characteristic curve SWCC has become one of the most readily available experimental means for estimating fundamental engineering properties of unsaturated soils for a wide range of matric suction states, including permeability, volume change, and shear strength parameters. Numerous laboratory techniques have been developed for accurately assessing the SWCC of unsaturated soils, from filter paper technique to the more sophisticated pressure plate extractor devices. However, the majority of these techniques and devices allow for the testing of unsaturated soils under unknown or zero-confinement conditions, resulting in SWCC data that do not correspond to realistic in-situ stress states in the unsaturated soil mass. Moreover, recent advances in SWCC testing using oedometer and triaxial setups may prove costly and time consuming. This paper describes an attempt to develop a modified pressure plate extractor (MPPE) device for assessing the SWCC of unsaturated soils under anisotropic stress sates, including details of its main components and the step-by-step assembling process. The MPPE device features independent control of the net radial confinement and net vertical pressure. Results from a series of SWCC tests on poorly-graded sand (SP) and low-plasticity clay (CL), for different values of K_o ratio, that is, the net radial confinement to net vertical pressure ratio, are presented. Results show a paramount influence of the net radial confinement, and hence the initial K_o value, on soil's air-entry value and residual volumetric water content. Additional data and further refinement of the developed MPPE device, which builds upon a well known technique, will prove very useful in assessing key engineering properties of unsaturated soils under different K_o stress states.