Rail corridor mine remediation - from investigation to verification

The Goodna Colliery is one of Queensland's oldest underground coal mining operations and was operated from the mid 1860's through to the 1920's. An open cut mine was also in operation in the same area in the 1960's and 1970's. No original plans of the Goodna mine have ever been sourced and most information that was available was based on either estimates, anecdotal recollections or a combination of both. During the investigation and remediation of the Goodna Mine for the Dinmore to Goodna section of the adjacent Ipswich Motorway it became apparent that the Goodna coal mine extended beneath the current Queensland Rail Corridor, at shallow depth. This paper presents a case study of the work undertaken to remediate the mine workings and provide protection to the railway infrastructure from future mine induced subsidence. It covers the important geotechnical aspects of the work including historical information of the mine, local and regional geology, investigation methodologies (including video and sonar imagery) and results, risk assessment, remediation design, construction specifications and supervision and finally verification of the final product. Detailed geotechnical investigations were undertaken during a complete rail shutdown to determine the extent of the motorway corridor affected by previous mining activities. The investigations used a variety of techniques, such as conventional percussion and core drilling and sonar and video imaging to investigate the locations, dimensions and conditions of the abandoned mine. In all 65 holes were drilled and mapped in the 2 day investigation. The required measures to remediate the affected area was to completely fill the mine workings beneath the rail corridor with a low strength grout. This involved the construction of a barrier wall at the down-dip edge of filling boundary and then filling in an up-dip direction behind the barrier wall. During the filling operation a groundwater monitoring regime was undertaken to monitor groundwater levels and filling rates. To ensure the regional groundwater regime remained unaffected during the filling operation mine water was extracted from flooded workings at or below the rate of grout injection. The collected mine water was treated at a purpose built Reverse Osmosis treatment plant. Following the filling operation a series of verification works were undertaken to validate the completed works.