In many construction projects, such as road building, earth retaining structures are frequently used. Large structures such as bridge abutments require substantial quantities of backfill and the cost of these structures contribute a significant percentage of the final costs of the whole scheme. In the majority of construction projects where earthworks are involved unsuitable clayey soil is removed and granular backfill imported. The ease of compaction, high strength and stiffness, as well as the free draining properties of these materials have made them ideal for such purposes. However, the practice of using high quality granular fill is becoming more costly as the availability of such fill reduces and the costs of transport increase. In the light of the increasing costs, interest is being directed towards the possible use of other materials such as the clayey soils that are currently considered unsuitable. A costing study (Naish, 1988, see IRRD 818045) indicated that substantial reductions in the cost of backfill could be made if some soils could be used which are at present classed as unsuitable. The aim of the present study is to investigate the possibility of extending the permitted limits of plasticity of backfill materials. Investigations into the swelling of compacted clayey soils have been undertaken using a variety of different laboratory techniques. The experiments were designed to measure the swelling pressure of compacted soils under conditions of zero lateral strain and having varying initial moisture contents and dry densities. This represents the worst conditions that may exist behind a stiff retaining wall. The apparatus used for the experiments were: 100mm stress path cell; oedometers; conventional triaxial cell; and geotechnical centrifuge. The report describes the problems associated with the use of clayey soils as backfill material adjacent to structures, the current standards governing the placement and compaction of clayey soils and the research that has been undertaken to investigate the factors that affect the build up of swelling pressure in such fills. (A)

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