1 Introduction

During the construction of three new entrances (portals) and inclined roadways (drifts) for an existing coal mine the face and roof of one drift collapsed, resulting in approximately 100 m3 of material within the roadway and piping extending to the slope in the hillside above resulting in sinkholes. This poster presents the methods used to assess the rock mass, the cause of the collapses and the design and methodology used to recover the portal and drift.

2 Background

Wongawilli Colliery is located approximately 80 km to the south of Sydney, midway up a coastal escarpment that is underlain by a sequence of interbedded sedimentary rocks that includes the Illawarra Coal Measures. Slope instability is well documented within the escarpment, which is generally overlain by colluvium. To construct the mine portal, a bench had been excavated into a steep to very steep hillside exposing the underlying weathered sandstones, siltstone and mudstone.

Support for the drift comprised 200 UC steel sets, supporting a span of approximately 6 m. Sets were installed at 1 m centres, blocked with timber in the roof. Sidewalls were poorly blocked with two 1.8 m long resin rockbolts at mid height to support each leg set. Initial instability was mainly due to roof collapse between the last set and the face, approximately 2 m wide. The face collapsed later on. The mine operator had safety concerns and stopped all work.

3 Inspection and site model

Detailed inspections were carried out to establish a site model and to characterize the rock mass and failures. Inspection of the site found cracking in the shotcrete on both sides of the portal, a number of steel sets had bent and/or settled under roof and lateral loads, significant water seepage from the collapsed rock mass within the drift and two collapses had occurred in the roof and wall of the drift (Fig. 1). These had resulted in piping and unravelling of the soil and rock profile extended to the ground surface in the hillside above the drift resulting in sinkholes, the larger sinkhole being approximately 3 m deep and 10 m wide. Sandstone exposed within the walls of the drift was of very low to low strength and very blocky. A fault with numerous associated joints, running parallel to the face, was exposed in the collapse directly behind the portal shotcrete. The model for the drift is shown in Fig. 2.

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