Cockatoo Island Seepage Barrier - Permeation Grouting, Jet Grouting and a Soil/Cement/Bentonite Mixed Slurry Capping Trench-Paper

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Cockatoo Island’s Stage 4 seawall was built to protect an expanding iron ore mine extending up to 50 metres below sea level. Because clay core material was scarce and earlier seawall stages had experienced failures, a grout curtain was selected to reduce seepage through the rockfill wall and underlying seabed sediments.<br /><br />The 261-metre barrier was installed through highly variable rockfill and permeable coral sediments, with tidal fluctuations exceeding 10 metres creating a serious risk of grout washout. Work combined three methods. First, top-hammer drilling and rapid-setting cement, bentonite and sodium silicate grout filled voids and stabilised holes through the seawall. Second, duplex jet grouting mixed grout into the underlying sediments and was extended upward into the rockfill to overlap the permeation grout. Finally, a soil-cement-bentonite slurry trench capped the upper section, where grout columns were difficult to make continuous.<br /><br />Construction required ongoing adjustments to drilling, grout mixes and procedures in response to variable ground conditions, obstructions and tidal flows. Remote-site logistics also constrained progress: approximately 4,000 tonnes of cement, 200 tonnes of bentonite and 240 tonnes of sodium silicate had to be transported to the island.<br /><br />Verification combined piezometer monitoring, grout and drilling records, core drilling and testing. Piezometers showed reduced coupling between groundwater levels behind the wall and the tides as the curtain closed. Performance during the first king tide after closure was judged satisfactory.<br /><br />The combined treatment significantly reduced and controlled seepage, enabling mining to proceed safely below sea level. The project demonstrated that permeation grouting, jet grouting and a mixed-slurry capping trench could form an effective seepage barrier through a rockfill seawall under challenging tidal and logistical conditions.

Keywords

Cockatoo Island

seawall

grout curtain

seepage control

rockfill wall

jet grouting

permeation grouting

bentonite slurry trench

tidal fluctuations

mine protection