Soil-structure interaction for full-height piled bridge abutments constructed on soft clay

Edward Alan Ellis, Cambridge University
Geotechnical Engineering Group


Full-height piled bridge abutments constructed on soft clay are prone to lateral soil-structure interaction effects resulting from placement of the retained fill and consequent horizontal deformation of the foundation soil. The interaction increases lateral structural loading and displacement, and hence may result in unserviceable behaviour of the abutment or bridge deck. The aims of this research were to identify and quantify specific forms of lateral interaction for a structure of this type, and to investigate methods for analysis of these effects.

A series of 4 heavily instrumented plane strain geotechnical centrifuge tests were undertaken, examining the effect of clay layer depth and the rate of embankment construction, including the use of vertical model drains in the clay layer. The results confirmed significant lateral loading and displacement of the structure.

Twelve plane strain finite element method (FEM) analyses were undertaken to back-analyse the centrifuge tests, and also to examine potential differences in behaviour between centrifuge models and `equivalent field' full-scale constructions, which have different soil stress histories. Three-dimensional effects associated with the piles and vertical drains were incorporated in the plane strain representation using published techniques. A recently developed constitutive model, Strain Dependent Modified Cam Clay, was used to account for stress history and non-linear stress-strain response; and predicted inherently stiffer soil response in the equivalent field analyses. Results from the centrifuge model FEM analyses corresponded well with the centrifuge model test data, but lower structural displacement and loading were predicted in the equivalent field analyses, due to the increase in soil stiffness.

The centrifuge tests and numerical analyses confirmed the existence of established interaction effects such as `passive' pile loading due to lateral displacement of clay past the piles. However, additional effects associated with the retained embankment material were also identified. Lateral displacement of clay beneath the embankment and pile cap led to `shear transfer', which increased lateral loading on the structure due to the restraint imposed upon movement of the clay. The abutment structure also supported vertical stress re-distribution in the embankment, and the embankment material `arched' onto the structure, reducing vertical loading on the clay layer foundation nearby. The horizontal thrust associated with the arching action caused a significant additional component of lateral loading on the structure, which increased with time.

Although structural loading and displacement were reduced in the equivalent field analyses (when compared to the centrifuge model cases), the predicted displacements exceeded recommended values at the serviceability limit state. Bending moments in the piles due to interaction effects are also often of concern in these circumstances. It was illustrated that moment effects are most logically viewed as a consequence of the pile head loading and passive pile loading, rather than attempting to estimate moment directly.

The use of analytical methods to perform a fundamental analysis of the mechanisms of loading was investigated, contributing to discussion and understanding of the problem. However, the complexity of the interaction, and the increasing availability and use of FEM analyses in design suggest that FEM is likely to provide a more useful design tool for integrated prediction of all mechanisms of loading.

Keywords: full-height piled bridge abutment, embankment, soft clay, passive pile loading, shear transfer, arching, geotechnical centrifuge model tests, finite element method.