ABSTRACT:

Three series of triaxial tests have been conducted to investigate the behaviour of loosely compacted fill in Hong Kong. These series included consolidated undrained tests on saturated samples, constant water content tests on unsaturated samples with suction measurements and shear tests on unsaturated samples subjected to a constant total stress with a decreasing suction. The last series was conducted to simulate field stress paths during rainfall infiltration. The test results showed that the shear-induced volumetric behaviour of a loosely compacted fill seems to be stress path dependent. Dilative behaviour was observed from consolidated undrained tests conducted on saturated samples even though their initial states lie above the critical state line. On the other hand, compressive behaviour was observed from the constant water content tests on unsaturated samples. For the field stress path tests on unsaturated soil, contractive volumetric behaviour became dilative as the pre-shear consolidation pressure increased. These observed behaviour has some implications to the design of remedial work for loose fill slopes.

INTRODUCTION

There are many loose fill slopes formed before the establishment of the Geotechnical Control Office (GCO) in 1977 in Hong Kong. The upper part of these slopes is generally unsaturated in dry season. Many of the existing loose fill slopes are sub-standard and the stability of these marginal slopes is maintained by the in-situ matric suction (negative pore water pressure). Over the years, there have been a number of severe loose fill slope failures in Hong Kong during rainstorms, resulting in catastrophic flow slides and disasters (HKIE, 1998). To upgrade these slopes safely, quickly and economically is a major challenge to engineers in Hong Kong. Regarding the behaviour of unsaturated soil, most of the previous laboratory studies have been based on well-compacted soils (Alonso et al., 1990; Wheeler and Sivakumar, 1993; Gan and Fredlund, 1996).

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