Simplified finite element modelling of unsaturated soil behaviour for design of retaining structures for the Torrens Rail Junction project, Adelaide

Roger Lim, Douglas Warne and Hugo Acosta-Martinez

The Torrens Rail Junction (TRJ) is where the interstate freight railway crosses the Outer Harbor passenger railway, located in the Park Lands, Adelaide. The original rail junction posed a productivity constraint to the strategically important Perth to Melbourne rail freight line, with freight trains forced to give way to Outer Harbor passenger trains at the junction. The TRJ project comprised a grade separation of the rail lines by lowering the Outer Harbor rail line below both the interstate rail line and the adjacent Inner Ring Route (Park Terrace). The lowered Outer Harbor rail line extends for a length of approximately 1.5 kilometres supported by a cantilever pile wall approximately 8 m deep.

The project is underlain by River Torrens alluvium. The River Torrens alluvial deposits transition to alluvial fan sediments of the Adelaide Lower Outwash Plain. These materials typically comprise stiff to hard clay soils with local interbedded sandier horizons. These clays are typically unsaturated above the groundwater table and are slightly to highly reactive.

The technical solution for the earth-retaining systems proposed the use of an economical, practical and compliant design. To achieve this, the retaining wall design approach which was proposed differed from normal design practice in that the beneficial effects of suction and unsaturated soil mechanics principles have been incorporated.

A simplified approach based on design standard from the Department of Planning, Transport and Infrastructure (DPTI), Government of South Australia, was adopted. The numerical methods which had been implemented via finite element modelling were employed to analyse soil-structure interaction. When assessing lateral earth-pressures against retaining structures and local/global instability, the analysis considered: staged construction; effects of volume change in clay soils due to unsaturated condition including variations from an initial equilibrium condition to a new equilibrium suction (wetting); effects of swell pressures; and the beneficial effects of suction in the shear strength of clays.

As this design approach was novel, the proposed methodology was compared to a full-scale pile wall trial, undertaken by DPTI, to verify the design approach. The results using the proposed design methodology indicate the numerical models could reliably predict both deflection and structural reaction at all construction stages.