Lewana Road landslide – Nannup, Western Australia

Philip Mather

On the 27th September 2005 a small landslide was noticed across Lewana Road, located approximately 20 km northeast of Nannup, Western Australia. When first observed the landslide was approximately 20 m long. Within a few days the landslide had removed a section of Lewana Road and increased to 150 m in length to be potentially threatening the Nannup–Balingup Road and Blackwood River located another 70 m down slope.

Routine monitoring of the landslide was initiated and continued with works to improve local drainage and divert surface water away from the immediate landslide area. A geotechnical consultant was engaged to carry out a site visit as part of an assessment of the landslide occurrence and to provide advice on potential risks for the major road and Blackwood River located down slope.

The potential causes of the Lewana Road landslide included a wide range of naturally occurring features including relatively steep topography, above average rainfall and shallow soil overlying rock with unfavourably orientated geological structures concentrating subsurface water flows. In conjunction with these naturally occurring features were a range of man made influences arising from recent removal of vegetation through clear felling of tree plantations, localised over steepening of slopes associated with cut to fill earthworks to construct skyline pads for tree harvesting and the concentration of surface water runoff associated with road drainage.

Although the Lewana Road Landslide was a relatively minor feature and did not progress to become a significant occurrence, it provides a useful insight into the range of naturally occurring features and man made influences that are commonly associated with landslides. In addition it provides a useful example of the types of management and remediation responses that can be adopted to limit and contain potential damage to existing infrastructure and environmentally sensitive areas that may be threatened by landslides.