Use of stiffness inserts in pile groups and piled rafts

In all but small symmetrical pile group configurations, the load distribution will be non-uniform, with a tendency for larger loads to be carried by piles near the perimeter of the pile group, while the inner piles may carry a significantly smaller load. If the design requires that a certain minimum factor of safety be applied to individual piles, the load that can be applied to a pile group may be limited by the most heavily loaded pile in that group. This may lead to an over-conservative design in many cases. This paper describes the use of stiffness inserts at the head of the more heavily loaded piles in a group. These inserts consist of a compressible material incorporated in the head of a pile, below the pile cap. By selecting appropriate stiffnesses for these inserts, it is possible to obtain an almost uniform distribution of axial load within the group, together with a specified settlement. In this way, the allowable load on a group can be increased, usually at the cost of a modest increase in the settlement. The paper sets out a relatively simple method for assessing the required stiffness of the inserts, which involves firstly an analysis of the pile group without inserts in order to compute the stiffness of each pile within the group environment. On the basis of this analysis, the required insert stiffness for each pile can be computed. The paper gives two examples of the difference in the load–settlement characteristics of a pile group with and without stiffness inserts where non-uniform load and/or settlement behaviour would occur without the inserts. A third case involves a piled raft, and demonstrates that improved settlement performance may be achieved without compromising the requirements for ultimate vertical capacity.