The design involved undercutting the low-strength slip debris and constructing two counterfort drains, a subsoil drain running along the base of the cut batter, three rows of 600 mm diameter CFA piles to a depth with 1.0 m embedment into the underlying completely weathered rock, and construction of a geosynthetic confined soil (GCS) wall to 500mm below road level.

A layer of A29 bidim cloth was placed across the subgrade undercut and backfilled with an AP65 compacted hardfill to average thickness of 500mm.

CFA columns were constructed using GSI’s FD45 CFA piling rig. Columns were spaced at 1.8 m centres and constructed between 6.2m and 11m depth. The depth varied depending on the underlying to highly weathered ECBF rock. Embedment of 1.0m into the weathered ECBF rock was determined by observing the increasing drilling torque pressure as indicated on the onboard drilling rig computer and visually assessing the drilling spoil as it comes out of the flight auger. The highly weathered ECBF typically was a light bluish-grey silty clay with extremely weak sandstone fragments and generally shallowed towards the southeastern portion of the site.

After completing the CFA piles, the drilling platform was cleared of excess spoil and carted off-site. An initial 150mm layer of AP65 was placed and compacted to provide a base layer

for the first layer of Stratagrid SGU80. The Cirtex Duramesh facing was placed at the edge of the outer row of CFA piles, and the geogrid was draped over the Duramesh with an approximate 100 mm wide void left to allow for placement of riprap rock once filling to the top of the panel was complete. This also allowed for the Geogrid to be wrapped over each layer of compacted hard fill in a ‘burrito’ like technique.

AP65 was placed in 200mm layers and compacted using a vibrating drum roller, and geogrid placement was repeated. The portions of hard fill adjacent to the edge of the wall facing were compacted using a plate compactor. This compaction sequence was repeated until the GCS wall met the finished height (approximately 500mm from road level).