Site investigation and geological challenges for a rail level crossing project in SEQ

An understanding of geology is very important to minimise design and construction uncertainties and challenges for any site. WSP was engaged to deliver a detailed design of an overpass bridge and associated structures to replace a very busy rail level crossing for a rail track in a metro area of Southeast Queensland (SEQ). The design services included design of an overpass bridge, earthworks, approach embankments, retaining walls, etc. Considering the design life of 100 years for these structures, a detailed geotechnical investigation (GI) and interpretation of the GI data were required to be fed into the design. As the site was located in an urban area, carrying out geotechnical investigation was not an easy task. It required dealing with various stakeholders, local community, and working night shifts to manage technical requirements and public expectations.

The project conceptual ground model benefited from some previous intrusive site investigations. However, WSP noted that the previous boreholes had limited rock coring and had relatively un-explained SPT refusals in soil material at depths of less than 10 metres below ground level. WSP scoped additional geotechnical investigation as per the Department of Transport and Main Roads (DTMR) geotechnical design standards and project specifications. When the geotechnical investigation was complete, the encountered materials were weakly and moderately cemented units, interlayered with poorly consolidated materials that are typically dominated by Residual (Sandy CLAY) to about 25 m below ground level overlying very low strength (UCS < 1.5 MPa) Sedimentary rock (Sandstone/Siltstone/Claystone) to the borehole termination depth of approximately 40 m bgl. This created additional challenges for pile foundation design and constructability issues.

This paper discusses how the challenges during geotechnical investigation were overcome and interpretation of the encountered strata for feeding into the design to manage geotechnical uncertainties considering constructability within a constrained environment.