Measurements were made of the lateral soil stresses produced by the compaction and swelling of Lias Clay backfill against two pilot scale retaining walls. The plasticity of the clay was marginally beyond the upper limit for suitable cohesive fill given by the Specification for Highway Works (MCHWI, 1991). The study forms the corollary to studies on soils of lesser and greater plasticity undertaken by Carder, Pocock and Murray (1977); Carder, Murray and Krawczyk (1980); and Symons, Clayton and Darley (1989). The study was executed in three main stages. During Stage 1 the clay was deposited in and "as dug" condition and then compacted in layers behind the walls. Stage 2 was a 3 month period during which pressures were allowed to equilibrate before the installation of vertical sand drains to facilitate saturation. Stage 3 comprised a 14 month swelling period during which the water levels in the sand drains were maintained at surface level. Throughout the study, the lateral stresses developed against the walls were monitored regularly. The total lateral thrust reached about half its peak value after the first month of the swelling period, but the rate of increase slowed progressively thereafter, taking a further five months to reach a maximum of about 85 kN per metre run on the 2m high wall. This value was much lower than that obtained from the previous study on London Clay. (A)

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