Abstract
Measuring the volume changes of unsaturated triaxial soil samples is one of the main challenges when performing unsaturated triaxial tests. Under fully undrained conditions, Boyle's Law can be used to calculate the sample's volume changes caused by the compression of air, as Boyle's Law relates changes in volume to changes in pore air pressures within the sample. This method has been used to calculate the volumetric strains and pore air pressures in unsaturated samples tested under cyclic loading conditions. However, the volume changes calculated from using Boyle's Law and the increases in pore air pressures, have not been compared with the actual measured volume changes of unsaturated samples. This study presents the pore pressures and compares the measured and predicted volumetric strains calculated from using Boyle's Law, in unsaturated triaxial samples that were tested cyclically. In some cases, using Boyle's Law was found to be similar to the volume changes estimated from direct measurement, but sometimes the method did not appear to work. Reasons for the discrepancies will also be discussed.
Highlights
Unsaturated soils contain 3 phases; solid soil, air and water
According to Bishop’s effective stress equation [1], the pore air and water pressures and the degree of the saturation are required to determine the effective stress within unsaturated soils
There are several methods that have been commonly used in unsaturated soil studies to measure the actual volume changes of samples in triaxial tests
Summary
Unsaturated soils contain 3 phases; solid soil, air and water. according to Bishop’s effective stress equation [1], the pore air and water pressures and the degree of the saturation are required to determine the effective stress within unsaturated soils. Law has been used to calculate the air pressures from the measured volume changes in samples of silty sand tested cyclically under unsaturated and undrained conditions in triaxial tests [2,3,4,5]. There are several methods that have been commonly used in unsaturated soil studies to measure the actual volume changes of samples in triaxial tests. In this study, unsaturated samples of an artificial material, similar in grading to the metallic ore cargoes have been tested under cyclic undrained conditions at degrees of saturation between 55 and 90% in a modified triaxial apparatus. The pore air pressures measured from testing the artificial materials will be used to verify Boyle’s Law through comparing the theoretical volume changes calculated from the air pressures within the sample, with the measured volume changes of the samples from changes in the cell volume
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