Impacts Of Fracturing Caused By Valley Formation And Pillar Collapse On The Hunter Expressway Project

The Hunter Expressway is a new government funded dual carriageway motorway built to relieve congestion between Newcastle and Thornton, Maitland and Rutherford. The expressway is continuous over 40 km between the F3 at Seahampton and Branxton. The easternmost 13 km between the F3 and Kurri Kurri is being built by the Hunter Expressway Alliance (HEA), comprising constructors Thiess and designers Parsons Brinckerhoff and Hyder Consulting partnering with NSW Roads and Maritime Services (RMS). A solution was required to manage the risk of mine subsidence under the viaduct and bridge foundations. The solution used was to fill the mine voids out to a distance of half the seam depth.

The extent and depth of the mine voids and related subsidence fractures was required before treating the voids. First assessment was to complete a detailed geological long section of key existing boreholes. Following this, further boreholes with video camera footage were made to assess the mine workings. The footage, with GIS, was used to map and correlate pillar and roadway networks with existing mining surveys, to accurately determine the volume of mining space to grout. After grouting, 140 validation holes were made to assess if the mine voids had been filled successfully. The assessment of video footage also helped the geological and geotechnical assessment of abutment structures on the viaducts as there were numerous joints and fractures. With mapping, and from assessing the videos, the team could decide whether these fractures were formed by earlier tectonic events, stress relief due to valley formation, or mine subsidence.

The fractures are differentiated by their source — stress relief fractures related to valley formation, fractures caused by pillar collapse, and fractures related to tectonic joint/fault systems. The valley-forming fractures were found in an upper sandstone layer (Figure 3). These fractures are consistent in the three viaducts where bridge abutments have been exposed. The fractures caused by pillar collapse were found in a sandstone layer about 40 m above the mined horizon, between two coal bearing layers.

There were several outcomes from the work done. It has been demonstrated video camera monitoring can play a significant role in mining and geotechnical applications, and have a positive impact on time and budget-related issues.