Design of Underpinning Piles to Reduce Settlements

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by H. G. Poulos, (F.ASCE), Coffey Partners Int Pty Ltd, North Ryde,

Document Type: Proceeding Paper

Part of: Vertical and Horizontal Deformations of Foundations and Embankments:

Abstract: Piles are frequently used to underpin structures which have suffered, or are expected to suffer, damage due to excessive settlement or differential settlement. However, existing methods of designing such piles are rather simplified and do not generally give proper consideration to the process of load transfer from the existing foundation to the underpinning elements. This paper describes an approach to analyzing such load transfer for the case where the loads remain constant but it is desired to limit future settlements and differential settlements. The approach involves the use of a soil-structure interaction analysis in which an existing shallow foundation is modelled as a strip or raft foundation, while the underpinning piles are treated as interacting spring elements of appropriate stiffness. The paper discusses the application of this approach to a large raft slab supporting a racecourse grandstand in Brisbane, Australia. It is demonstrated that, by a process of trial and error, a system of underpinning piles can be designed which reduces future settlements and minimizes future differential settlements.

Subject Headings: Pile settlement | Underpinning | Shallow foundations | Differential settlement | Soil settlement | Foundation design | Structural settlement | Australia

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