Soft Soil Case History: Shellharbour Sewage Treatment Plant upgrade

P.R.E Davies and A. Faulkner

This paper describes ground engineering challenges and solutions employed at a soft ground site near Shellharbour, New South Wales. Geotechnical issues encountered during the sewage treatment plant upgrade project included:

Deep peaty soils at the backfilled swamp site
High groundwater level and potentially high inflows through permeable fill
Large clarifier tank excavations (approx 80 m x 80 m x 5 m deep)
Flooding and acid sulphate soils
Potential settlement impacts on existing infrastructure.

Ground engineering risks were successfully managed through adequate scoping of investigations, numerical modelling of designs and involvement of geotechnical engineers during construction. A comparison of actual versus predicted behaviour for an anchored sheetpile wall is presented, enabling an evaluation of WALLAP and PLAXIS software. The value of geotechnical observations and monitoring during construction is also discussed.