In February 2003, earthworks were undertaken at a property at Lara Street Woy Woy, which involved an excavation of a building platform for a proposed new house at the site. The earthworks included an excavation up to 3m deep towards the rear of the land, immediately behind an old cottage residence and below some very large (i.e. about 100 tonne), precariously placed sandstone boulders on a steep pre-existing slope [viz: > 20°].
After requests from Gosford City Council and the property owners for advice on how best to manage and remedy the dangerous situation created by the excavations, Shirley Consulting Engineers designed a two stage stabilisation programme.
Stage 1 involved stabilising and securing the large sandstone boulders with rock bolts. This was vital to ensure the safety of construction personnel and that of the house, during the other site stabilisation works. The rock bolts were installed using specialist drilling contractors [MacDonald Contractors Australia Pty Ltd], who built a custom aerial work platform and rig to safely access the boulders and install the rock bolts.
Stage 2 involved constructing a large 2 part retaining wall at the base of the excavation, together with extensive site drainage.
The top of the retaining wall consisted of a ‘flexible’ Terraforce block wall (including about 860 blocks), which was positioned in front of the slope's colluvium material so as to be able to tolerate the down slope soil creep forces. If the top of the wall had been rigid, the slope material would have eventually gone over the top of the ‘immovable object’.
The lower section of the retaining wall consisted of steel soldier piles between reinforced concrete panels. This high strength wall has positioned in front of the exposed residual soil and weak bedrock material, and provided support for the Terraforce wall above.
The wall also consisted of complex surface and subsoil drainage systems that acted to prevent any destabilising hydrostatic water build-up behind the wall. The stabilisation works successfully reduced the risk to the house & building platform to acceptably low levels.