The concept of ‘arching’ of granular soil over an area where there is partial loss of support from an underlying stratum has long been recognised in the study of soil mechanics (eg. Terzaghi, 1943). Terzaghi considered shear stress on the vertical interfaces originating from the rigid supports at either side of a ‘trapdoor’ where support was reduced. This approach has found some application in the analysis of piled embankments (Russell & Pierpoint, 1997), where the pile caps act as rigid supports, and the underlying soft subsoil as the ‘trapdoor’, Figure 1. Other approaches for the design of such structures include a semicircular arch in the granular fill, as initially proposed by Hewlett & Randolph (1988) (and developed by Low et. al., 1994; and Kempfert et. al., 2004), and analogy with backfill over a buried pipe (BS 8006, 1995). Notably there is not one generally accepted approach for design (Love & Milligan, 2003). The theories tend to initially
consider plane strain conditions, which are then modified to account for the 3-dimensional nature of most piled embankments which use a square (or triangular) layout of pile caps in plan. This paper will consider a plane strain analysis, as the logical starting point which can be later extended to 3-d. The possible inclusion of a single or multiple layers of geogrid reinforcement in the embankment is not considered at this stage.