Compressibility Of Auckland Residual Soil

This paper presents compressibility data for some of the residual soils in the Auckland area. These soils are unusual in two respects. First, presumably because of the residual origin of the soils, the water content is highly variable even within the confines of a block sample and yet the compressibility is not closely correlated with these variations. Second, if oedometer data is plotted using the conventional semilogarithmic axes, then an apparent preconsolidation pressure is evident. However, if the data is plotted using natural scales a yield point is not evident, and for many of these soils the plot is close to linear or even suggests stiffening behaviour. A feature of the work to be discussed is the range of laboratory testing methods used to measure the properties of the soil. These cover conventional oedometer tests, tests in a large Ko triaxial cell, both load increment ratio of unity incremental and constant rate of strain loading, and conventional drained triaxial tests. In the Ko cell the drainage path length of the back pressure saturated specimens may be up to 140 mm and on-specimen measurement of deformation is possible. An additional factor considered in the paper is the effect of sample disturbance on the compressibility. This is assessed by comparing results between samples obtained with push-tubes and with results from tests done on specimens trimmed from carefully cut block samples. A difficulty with these and other comparisons made in the paper is the variability in soil properties from specimen to specimen. Finally, since the compressibility was found to exhibit only modest changes with increasing effective stresss, an alternative to an oedometer test is to determine YoungÕs modulus and PoissonsÕs ratio from drained triaxial tests and then calculate the constrained modulus or mv.