Compressibility of foundation fills

The current emphasis on locating building developments on brownfield sites, many of which contain substantial depths of fill, means that the behaviour of fills which support the foundations of buildings is of increasing importance. This paper is concerned with the one-dimensional compression behaviour of these foundation fills. Shallow foundations normally rest on partially saturated ground where the major hazard is associated with settlement due to effects other than structural loading. Consequently consideration is given not only to compression induced by an increase in applied stress, but also to compression consequent on an increase in compressibility associated with an increase in moisture content. A programme of laboratory oedometer tests has been carried out on colliery spoil, an opencast mining mudstone backfill, and a boulder clay fill. The test results have given an improved understanding of fill behaviour in one-dimensional compression and, in particular, the effects of partial saturation on the relationship between applied stress and induced strain. The tests have confirmed that vulnerability to collapse compression on inundation will usually be the most significant hazard for buildings founded on fill. Most types of partially saturated fill are susceptible to collapse compression under a wide range of applied stress when first inundated if they have been placed in a sufficiently loose and/or dry condition. Although creep compression can be a problem in some loose fills, collapse will usually be the more serious hazard. For fills placed dry of Proctor optimum moisture content, collapse potential is principally a function of relative compaction.