Three-dimensional pile-soil interaction in soldier-piled excavations

Strutted excavations, including soldier-piled excavations, are often analysed using two-dimensional (213) finite element (FE) analyses with properties which are averaged over a certain span of the wall. In this paper, the effects of "smearing" the stiffness of the soldier piles and timber laggings into an equivalent uniform stiffness are examined, based on comparison between the results of 2D and 3D analyses. The ability of the 3D analyses to model the flexural behaviour of the soldier piles and timber laggings is established by comparing the flexural behaviour of various FE beam representations to the corresponding theoretical solutions, followed by a reality check with an actual case study. Finally, the results of 2D and 3D analyses on an idealized soldier piled excavation are compared. The findings show that modelling errors can arise in several ways. Firstly, a 2D analysis tends to over-represent the coupling to pile to the soil below excavation level. Secondly, the deflection of the timber lagging, which is usually larger than that of the soldier piles, is often underestimated. For this reason, the overall volume of ground loss is, in reality, larger than that given by a 2D analysis. Thirdly, a 2D analysis cannot replicate the swelling, and therefore softening, of the soil face just behind the timber lagging. Increasing the inter-pile spacing will tend to accentuate the effects of these modelling errors. (C) 2002 Elsevier Science Ltd. All rights reserved.