by I F Symons, ground engineering division, TRL and C R I Clayton, civil engineering department, University of Surrey
High quality granular soils traditionally used as backfill to retaining structures are becoming increasingly scarce in some areas and the use of lower quality cohesive materials may result in significant savings in backfill costs.
This paper reviews the findings from a series of pilot scale retaining wall studies using both granular and cohesive soil as backfill It is concluded that for granular soils an upper bound assessment of the pressures produced by compaction can be obtained from established theories. For cohesive fills the horizontal total stresses on rigid walls after compaction are likely to be a function of the undrained strength and may depend on the rate of filling. The sign of the excess porewater pressures in the fill on completion of construction will then determine whether consolidation or swelling takes place subsequently. Consolidation will lead to a reduction in horizontal thrust on a retaining wall while swelling of cohesive backfill can result in high lateral stresses against rigid walls. These high stresses are likely to reduce in the longer term as softening of the fill takes place.