Compression mechanisms of unsaturated clay under high stresses

Abstract

This paper investigates the compression behavior of unsaturated clay under mean stresses up to 160 MPa and different drainage conditions. A new isotropic pressure cell was developed that incorporates matric suction control using the axis-translation technique, and a high-pressure syringe pump operated in displacement-control mode was used to control the total stress and track specimen volume changes. In addition to presenting results from characterization tests on the cell, results from a series of isotropic compression tests performed on compacted clay specimens under drained and undrained conditions are presented. These results permit evaluation of the hardening mechanisms and transition points in the compression curve with increasing effective stress. As expected, specimens tested under undrained conditions were much stiffer than those tested under drained conditions. In the drained tests, the rate of compression was sufficient to permit steady-state dissipation of excess pore-water pressure except under the highest stress ranges. Suction-induced hardening was observed when comparing saturated and unsaturated specimens tested in the drained compression tests. In both the drained and undrained compression tests, the range of applied stresses was sufficient to cause collapse or dissolution of the air voids (pressurized saturation) and convergence of the virgin compression lines for unsaturated specimens with that measured for saturated specimens. A gradual transition to full-void closure was observed at high stresses when the compression curves were plotted on a natural scale, but the shapes of the compression curves at high stresses were not consistent with conventional soil mechanics models when plotted on a semilogarithmic scale. The results from this study provide insight into how constitutive models for unsaturated soils can be extended to high stress conditions for drained and undrained conditions.

Constitutive Model for the Undrained Compression of Unsaturated Clay

“…Hypothetical isotropic compression curves for unsaturated, compacted clay under undrained conditions are shown in Figure 1, based on the observations from preliminary tests performed by Mun and McCartney (2015). The initial compression of compacted, unsaturated clay follows an elastic recompression line until reaching a mean apparent preconsolidation stress pc.…”

“…The pore air is expected to dissolve into the water at the point of pressurized saturation (Point C). Although continued particle rearrangements and crushing could occur at very high stresses, continued deformation is primarily expected due to elastic compression of the water (Points C to D), after the point of pressurized 4 saturation (Skempton 1961;Mun and McCartney 2015). Mun and McCartney (2016) observed that particle breakage occurred in undrained compression tests on saturated sand to mean total stresses of 160 MPa, even though the shape of the compression curve was mainly dominated by the compression of the pore water.…”

“…The soil water retention curve (SWRC) for the Boulder clay specimen used in the compression tests were inferred using the Transient Water Release and Imbibition Method (TRIM) of Wayllace and Lu (2012). The initial suction values of each specimen were measured using a UMS T5 tensiometer applying procedures followed by Mun and McCartney (2015). The SWRC for Boulder clay is shown in Figure 3 along with the initial suctions from the tensiometer measurements, which were observed to fall onto the drainage path of the SWRC.…”

“…The experiments were performed in a high pressure isotropic loading apparatus that uses a highpressure syringe pump to control the total stress and track changes in specimen volume. This device was previously used by Mun and McCartney (2015), who presented detailed explanations of the different aspects of the device, system calibration, and testing procedures. After preparation of the compacted specimens and placement within the isotropic cell, mean total stresses were applied at a constant rate of 2%/hour using the syringe pump without permitting drainage from the specimen until reaching a mean total stress of 160 MPa.…”

“…Other aspects of the model were developed using the results from a series of isotropic, undrained compression tests on unsaturated, compacted clay specimens having initial degrees of saturation ranging from 0.6 to 1.0 but with the same initial void ratio. These tests were performed following the testing methodology described by Mun and McCartney (2015), with an extension to a wider range of initial conditions.…”