Geotech

The proposed development at 96 Mill Point Road, South Perth, WA comprises 5 basement and 22 above ground levels. Ground water was encountered some 2m below natural ground level (NGL). This combination of features presented an interesting case study.  ARQ Consulting Engineers was approached to conduct a preliminary and detailed design for the lateral support and foundation of the development.

The proposed lateral support and foundation system, shown in the figure to the right,  comprised:

• Installation of a secant pile wall, using 830mm/750mm piles to a depth of 27m,
• Installation of 2,3,5 and 6 strand (from top to bottom) low relaxation 15.2mm ground anchors,
• A 3m thick jet grouted raft constructed at the base of the piles, thereby impeding water flow into the excavation.

Due to the cut-off effect of the long piles into the underlying limestone formation and the impermeable jet grouted raft, the drawdown of water level outside the site was practically eliminated.  This ensured that minimal settlements were induced outside the site, with practically zero effect on services, roads and adjacent structures.

The detailed analysis comprised a finite element analyses conducted using Rocscience’s Phase2 software.  The staged construction of a typical cross section of the excavation was utilised to conduct the analysis. Initially, a seepage analysis was conducted on the model, followed by a stability analysis.

Phase² software is able to determine a Strength Reduction Factor (SRF) for a model, which is akin to a conventional Factor of Safety and was used as such for this analysis.  An acceptable Factor of Safety was achieved for the serviceability load case, according to the Lateral Support in Surface Excavations Code of Practice (1989). A Factor of Safety greater than 1.01 was achieved for the ultimate load case, such that the structure would just be able to withstand the applied loading.

Project details