Bored reinforced piles for raisebore support: four case studies,and guidelines developed from lessons learnt

Abstract

Raiseboring is an attractive method of constructing shafts. However in the gravels and weathered near-surface rocks typical of the Western Australian goldfields, back reaming large diameter shafts through to the surface is risky, and conventional sinking from surface has normally been required.

An alternative construction approach has recently been used to stabilise the weak surficial materials and allow raiseboring through to the surface. The method is based on installing rings of non-contiguous reinforced bored piles around the shaft perimeter, for temporary support, then back reaming shafts at full diameter, and then supporting the weak exposed materials using remotely sprayed fibrecrete.

The method has been successfully applied to four shafts of 3 to 4·5 m diameter since July 2007. Bored pile layout designs have varied from 8 to 24 piles, of diameters 120–270 mm, in one or two rings. The practical implementation of the method was not always easy and many valuable lessons have been learnt.