Melbourne’s Southbank interchange – A permanent excavation in compressible clay

The Southbank Interchange of the Melbourne City Link Project links major freeways and access roads. The permanent excavation covers an area of three hectares and extends to a maximum depth of 9 m, which is 6 m below the groundwater table.

Design and construction approaches were developed firstly to control seepage into the excavation to limit groundwater lowering beyond the site, and secondly to limit the effects of potentially damaging base heave during construction. Lateral flow of groundwater into the excavation was controlled by a cut-off wall extending through the compressible clay with a surrounding line of closely spaced recharge wells. Analyses showed the recharge wells in combination with the wall would be effective in limiting drawdown outside the site. Monitoring showed that pore water pressures were maintained within the design criteria, with the assistance of deep recharge of an underlying aquifer to control vertical seepage. Survey showed settlements outside the site were minimal.

Shallow pressure relief drains were installed to limit uplift pressures and control base instability in the deepest parts of the excavation. Monitoring of the groundwater levels in the underlying aquifer and comparisons with results from flow models were used successfully to control uplift pressures during construction.