The use of early-works embankments in soft soil areas to optimise detailed design: Gateway Motorway Case Study

This paper presents a case study on the use of early-works preload1 embankments in soft soils areas to provide information to optimise detailed design. The Gateway Upgrade North (GUN) project involved the widening of the existing Gateway motorway from four to six lanes with some areas of re-alignment. Early-works for the motorway upgrade involved construction of sections of embankment located in areas of soft soils. From a geotechnical perspective, the early-works were essentially instrumented trial embankments constructed 9 to 12 months ahead of the main package and therefore provided an opportunity to observe embankment and wick drain performance and back-analyse soft soil consolidation parameters used for the detailed design for the final motorway construction.

Data from settlement plates, vibrating wire piezometers and inclinometers was used in conjunction with site investigation and laboratory data to assess consolidation parameters of highly compressible Holocene-age alluvial clays. Asaoka’s method and Terzaghi’s theory of one-dimensional consolidation were used in the back analysis of primary consolidation parameters. Secondary settlement was also observed allowing back analysis of secondary compression parameters. Using consolidation parameters derived from the back analysis, design parameters were allocated to relevant geological units which were then applied in settlement modelling for critical sections in the detailed design.

Assessment of the early-works embankment monitoring data enabled a more robust prediction of embankment behaviour during and post-construction. This resulted in a more cost-effective and optimised embankment design with higher confidence in predicted post-construction settlements.