Three-Dimensional Numerical Modeling of a Propped Cutter Soil Mix Retention System in Sand

Abstract

The construction of a multi-story building over two basements was proposed adjacent to Botany Bay in Sydney, Australia.  The basement excavation required excavations to depths of approximately 6.5 m.  The site was surrounded by open parkland on three sides, while on the fourth side a four-story building was present and was offset about 3 m from the site boundary.  A tidal creek flowed through a park along the eastern boundary.  The site investigation revealed that the subsurface conditions comprised a 16.5 m thick sandy profile overlying a stiff clay layer that in turn overlay better quality interbedded sandy clay and clayey sand.  Bedrock was encountered at depths ranging between 21.8 m and 27.32 m.  The relative density of sand was initially very loose, but at a depth of about 6 m it quickly increased from loose to dense to very dense.  The groundwater table rose to ground level during storm surges and heavy rainfall.  Due to the poor relative density of the upper soils and the presence of a high ground water table, a propped Cutter Soil Mix (CSM) wall was selected to support the excavation.  To limit ground movements, one row of hydraulic props was installed at the capping beam level and stressed by jacking.  One row of props, rather than two, was adopted to overcome constructability issues. This paper presents the geotechnical model used as the basis for the 3D numerical analyses of the shoring system which was completed using PLAXIS 3D.  It discusses the refinement of the prop loads and wall stiffness to maintain deflections of the walls to acceptable levels.  These results have then been compared with the monitoring data obtained during construction, which includes inclinometer monitoring, survey monitoring of the capping beam, and monitoring of prop loads.  The challenges and lessons learned during the design of this shoring system are discussed in this paper.