Deep excavations in Ashfield Shale in the Sydney region can be a risky operation with the presence of many fractures. A primary feature of this unit is the presence of moderately to steeply dipping continuous, planar joints, often associated with thrust faulting.
Such defects will sometimes form an unstable wedge when exposed during excavation, with these joints ‘daylighting’ on the cut face. If not restrained properly, the wedge of rock will become unstable and mobilise or slide into the excavation. This failure mechanism can occur irrespective of rock strength. Douglas Partners’ experience is that vertical cuts in Ashfield Shale are generally associated with a significant risk of wedge failure.
In this instance, the builder wisely chose to extend their piles to below the bulk excavation level for this reason. Bulk excavation indicated the presence of a laterally continuous 45 degree joint, striking parallel to the face and dipping into the excavation. If not for the retention system, which included temporary ‘tie-back’ ground anchors, the wedge formed by this joint could have easily slid out causing massive damage to the structure behind the top of the excavation face and a serious safety issue for any personnel above or beneath the wedge.
On this project, Douglas Partners’ engineer outlined the risks of wedge failure to our client and the client agreed that the risk was too great to leave the face unsupported. In this instance, the additional effort and cost to extend the piles was well justified! The client was also very appreciative of Douglas Partners advice as it provided a safer work environment and enabled them to proceed with the project without the risk of a major incident!