This report describes a pilot scale study of the compaction and swelling pressures developed by a clay backfill against two experimental retaining walls. The plasticity of the clay was well outside the upper limits for cohesive fill to structures given in the current specification (Department of Transport 1986). The investigation forms a sequel to studies using backfills of sand and silty clay (Carder, Pocock and Murray 1977, Carder, Murray and Krawczyk 1980) and was carried out in three main stages. During Stage 1 the clay was placed in an 'as dug' condition and compacted in layers to a depth of 3 metres. On completion of filling the measured total lateral pressures were considerably in excess of the calculated 'active' and 'at rest' values. Stage 2 was a four week rest period during which significant reductions in lateral pressure were measured on both experimental walls. Sand drains were then installed and the water level maintained constant at the surface of the fill during the 20 month swelling stage (Stage 3). The average total lateral pressures on both walls rose rapidly to reach maximum values about six months after the start of swelling and then decreased over the following seven months. On completion of Stage 3 the measured lateral pressure over the upper 1-1.5 m on both walls were substantially in excess of the calculated limiting passive values. (A)

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