Innovative Integrated Jet Grout and Contiguous CFA Piled Solution for Complex Deep Shoring Wall in Soft Soils â€“ Retaining Structural Design and Analysis-Paper

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The paper presents the design, construction and performance assessment of a hybrid earth-retention system for a deep-basement development in Dee Why, New South Wales. The site had variable alluvial soils over sandstone, a high groundwater table and nearby buildings with existing basements. These constraints made conventional anchored walls impractical and created stringent requirements for excavation stability and water control.<br /><br />The selected system combined contiguous CFA piles with jet-grout infill columns. The grout filled gaps between piles to form a low-permeability water barrier. In areas where anchors could not be used, jet-grout columns below excavation level also provided passive resistance, enabling cantilever wall designs without excessively large piles. Additional CPTs and cored boreholes helped define the highly variable rock profile and refine the design.<br /><br />WALLAP was used primarily for design, while WALLAP and PLAXIS 2D and 3D were used to analyse monitored wall behaviour. Inclinometers and survey points tracked movements. Back-analyses generally matched observed performance when they accounted for soil stiffness increasing with depth and capping-beam restraint. The SW13 wall experienced much greater movement than predicted because its capping beam was not completed before excavation, contrary to a critical design assumption. This highlighted the need to carry design requirements through to construction.<br /><br />The excavation remained relatively dry, and the jet-grout infill stayed intact. The authors conclude that the hybrid system provided an effective, economical and faster alternative to diaphragm or secant-pile walls. They also emphasize the value of targeted ground investigation, appropriate numerical modelling, reliable monitoring and close adherence to construction sequencing.

Keywords

hybrid earth-retention system

deep-basement excavation

contiguous CFA piles

jet-grout infill columns

groundwater control

excavation stability

WALLAP

PLAXIS 2D and 3D

ground investigation

construction sequencing