Mt Eliza — Slope Stabilisation

Triggered by excessive water, a landslip occurred on the cliff face of this bayside suburb with the potential to repeatedly slip and undermine beachfront residences located at the top of the cliff. Stabilization was imperative.

The coastal cliff is almost 15 m high, at an angle of 40 degrees to 60 degrees to the horizontal. The landslip, which was of approximately 4 metres wide was located immediately above a foreshore area that is frequented by the public. The cliff geology comprises Tertiary age sedimentary sandy clays, clayey sands and gravels of the Baxter Sandstone formation. Unlike most of the landslip issues in the Frankston South / Mt Eliza area, which are associated with the Selwyn Fault or the unfavourable geological conditions of the Balcombe Clays, the landslip on this site was considered a direct result of human interaction with the slope.

For the remedial works ATC Williams considered several methods including staggered retaining walls, and retaining wall with soil nail combinations; but ultimately developed a soil-nail only solution. Design challenges included a high groundwater with preferential seepage zones within the cliff face. However, the prime challenges were during construction, with site access only available across the foreshore at low tide, and access up the slope for nail installation achievable by small plant only. The success of the project depended on developing an appropriate construction method which enabled design requirements to be achieved.

With no equipment access to the top of the cliff, various means of providing access from the foreshore were considered including earth bunds, platforms, scaffolding and long reach excavators.

It was concluded that the only viable option was to use a crane to support a drilling rig mast at each nail position.

This arrangement limited the drilling depth and soil nail installation length to a relatively shallow 6 m and required a design modification necessitating the re-design of the pattern of soil nails to match the depth capabilities of the rig, whilst still meeting the local and global slope stability requirements. In total 137 soil nails were installed and the cliff was adequately stabilised.