Design of Railway Embankment over Soft Ground

This paper discusses the geotechnical design and performance monitoring of a 2.6 km long railway embankment constructed over soft ground at Hexham, located approximately 16 km northwest of Newcastle and 160 km north of Sydney. The rail alignment traverses over soft ground with soft soil thickness of more than 25 m on the southern end and about 12 m on the northern end of the project corridor. There were challenges in relation to the geotechnical design including (a) design of adequate formation to reduce the risk of soft subgrade failure under cyclic loading (b) excessive settlement of soft ground with time, impacting the performance of the railway tracks in the long term and (c) presence of waste material in the site, as between the 1960s and 1990s, the area was the site of a coal washery and loading facility. Hence, the site has already been preloaded with varying thickness of coal reject fill material placed in isolated areas along the proposed rail corridor, causing potential differential settlement of the proposed rail tracks. Therefore, geotechnical design of formation needs to be carried out by adopting appropriate subgrade modifications in order to reduce (a) the rail formation thickness above the existing ground surface and (b) post construction settlement. Selection of appropriate subgrade modifications requires careful consideration of number of factors including ground conditions, available construction time, performance requirements and cost.

A number of subgrade modification methods including remove and replace, rigid/semi-rigid columnar inclusions, and mass stabilisation using cement or lime have been considered to reduce the post construction settlement as well as to provide a stable formation for the railway embankment. Each modification method has its own advantages and limitations. After discussion with construction team and the Australian Rail Track Corporation (ARTC), “remove and replace” has been selected as a suitable method considering (a) relatively low cost and easy to construct; (b) reduction in primary settlement due to removal of material; and (c) significant testing regime and possible installation difficulties through coal reject fill associated with columnar/mass stabilisation methods. The assessed post construction settlement has been provided to ARTC for the development of maintenance regime. An approach of tamping the tracks to maintain the track geometry within ARTC Standards has been adopted.

The rail tracks have been constructed and in operation successfully from late 2014. Settlement monitoring has been carried out during and after construction. The observed settlements are reasonably consistent with the design prediction. In addition, dynamic deflection of the track has been monitored during the operation of tracks and compared with the design prediction. This paper summarises the geotechnical site investigation, development of design parameters, selection of subgrade modification method, formation design, and back calculation of settlement and dynamic deflection to compare with monitoring data.