A Case Study Describing the Use of a Top-Down Construction Sequence for a Three Level Basement Using a Diaphragm Wall and CFA Foundation Piles-Paper

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The case study describes construction of a three-level basement for a hotel and residential development at Elizabeth Quay, Perth. The project used a top-down sequence, enabling above-ground construction to proceed while basement excavation continued.<br /><br />Ground conditions comprised fill and river alluvium over interbedded Guildford Formation sands and clays, with Kings Park Formation siltstone bedrock at variable depths. Groundwater was near the surface, creating a major challenge: excavation extended up to about 9 m below the water table. Additional site investigations found that the low-permeability Lower Guildford clay was thinner and less continuous than expected.<br /><br />A diaphragm wall was selected to retain the ground and limit groundwater inflow. To improve the hydraulic cut-off, bentonite slurry cross-walls were added, and wall panels in the southern area were extended to bedrock. This reduced dewatering needs and avoided depressurising the lower aquifer.<br /><br />The foundation combined a hydrostatic raft with continuous-flight-auger (CFA) piles, designed as a “raft-enhanced pile group.” For heavily loaded columns, a single pile was insufficient, so primary and secondary piles were used. Steel plunge columns installed in the primary piles supported the structure during staged excavation; secondary piles and the raft were connected at the lowest basement level to share permanent loads. Three-dimensional finite-element analysis assessed load sharing and settlement, with predicted working-load settlement limited to 30 mm.<br /><br />The project addressed the installation tolerances of plunge columns, pile trimming, load transfer, construction sequencing, and waterproofing. Guide frames helped achieve the required column alignment. The combined diaphragm wall, slurry cut-offs, and innovative pile-and-raft system provided an effective, cost- and time-conscious solution to the site’s challenging groundwater and foundation conditions.

Keywords

Elizabeth Quay

Perth

three-level basement

top-down construction

diaphragm wall

groundwater control

bentonite slurry

CFA piles

raft-enhanced pile group

plunge columns