Anchored Sleeper Wall from Concrib

Concrib's anchored sleeper wall results in permanent soil mass stabilisation

Location: Nambour, QLD
Client: Lanskey Construction

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

Moving soil mass caused problems:

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.

These measures largely arrested further major failure, however 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 the stabilisation of the area.

Geotechnical investigations concluded the existing ground was a layer of fill and residual soil (friction angle 18 degrees, density 20kN/m with zero cohesion) approximately 3m deep overlaying a weathered sandstone rock. The solution offered by Concrib to address the long-term slope stability - without having to undertake the removal and replacement of a huge amount of unsuitable material - was a vertical cantilever sleeper wall system, with additional anchors through the steel columns to be founded into the underlying sandstone rock.

Features of the anchored sleeper wall:

Wall face area approx 130 square metres, with the wall stepping down the sloping embankment.
Custom-made heavily galvanized steel wall columns located at 2m centres.
750mm diameter pier hole with full spiral cage reinforcing penetrating 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 to 20metres from the rear face of the wall sleeper panels to be socketed into the sandstone layer.
Lower anchors have a capacity of 1000kn, with the upper anchors at 200kn capacity.
The entire void behind the wall was filled with free-draining clean gravel fill (friction angle 38 degrees, density 19kn/m3 with a clay plug over the surface at 300mm thickness).
Construction commenced late August and was completed by mid-November 2011.