Rock mesh application in highly fractured basalt rock cutting in Western Ring Road widening project Melbourne – a case study

Widening of Melbourne’s M80 Western Ring Road carriageway required a significant vertical rock cut leading to a new fill embankment for the Moonee Ponds Creek crossing. The rock comprised highly fractured and variably weathered Newer Volcanics basalt. Excavation in other sections of the project in similar fractured basalt had led to significant overbreak. For a conventional concrete faced soil nail solution, similar overbreak in this rock cut was considered to create an appreciable budget overrun not only due to the additional volume of concrete required to fill in the overbreak, but also for the additional steel volume in the nails to support the weight of the thicker concrete facing. To overcome this, a combined rock nail and rock mesh retention system was adopted where a composite action provided restraint for both global and local face stability. Detailed assessment was necessary to determine the interaction between the local rock mesh facing support and the global support afforded by the nails and considerable effort was made to develop an installation procedure allowing construction of the system to be undertaken safely and efficiently. The final rock nail/rock mesh solution minimised the amount of steel and concrete required to support the rock giving a more sustainable solution than originally proposed.

The construction of the rock cut involved the installation of about 400 rock nails and was completed in 2 months at the end of 2011.