Concrib’s anchored sleeper walls for permanent soil mass stabilisation

Concrib provided an effective and permanent solution to stabilise the soil mass along a steep slope on the Nambour Connection Road in Queensland.

Prolonged rain during late 2010 and early 2011 created the ideal conditions for landslips on the steep and wet slopes surrounding Nambour, as well as other regions in the Sunshine Coast hinterland in South East Queensland. 

One such location was a steep slope adjacent to the Shell service station at the top of the climb on the Nambour Connection Road. The slope runs away to the west, with several houses on the downside of the slope. Movement and tension cracks were the first problem to be noticed, so emergency temporary works to excavate the sliding material and replace with controlled fill were undertaken. 

Though these measures largely arrested further major failure, a permanent solution was required to stabilise the soil mass moving along the slip plane. Concrib was engaged to provide a complete turnkey design/construct/certify service for stabilisation of the area.

Geotechnical investigations concluded the existing ground was a layer of fill and residual soil approximately 3m deep overlaying a weathered sandstone rock. 

To ensure long-term slope stability without having to undertake the removal and replacement of a huge amount of unsuitable material, Concrib recommended a vertical cantilever sleeper wall system, with additional anchors through the steel columns to be founded into the underlying sandstone rock.

Concrib’s anchored sleeper walls covered a wall face area of 130m², with the wall stepping down the sloping embankment. Custom-made heavily galvanised steel wall columns were located at 2m centres, while a 750mm diameter pier hole with full spiral cage reinforcing penetrated 2.5 metres into the hard sandstone, making the total pier depth up to 6 metres. 

Rock anchor cables were located near the toe of the steel columns and toward the top third of the columns, and extended 18-20 metres from the rear face of the wall sleeper panels to be socketed into the sandstone layer. While the lower anchors have a capacity of 1000kn, the upper anchors have 200kn capacity.

The entire void behind the wall was filled with free-draining clean gravel fill. Construction commenced late August and was completed by mid-November 2011.