2024 Volume 11 Issue 8 Pages 302-307
As part of the Hobsons Bay Main Sewer - Yarra River Duplication project, there was a requirement for the existing sewer system on the eastern bank of the river to be redirected to a new siphon tunnel by a new Upstream Connection Structure (UCS) connected to a new Siphon Inlet Structure (SIS) by a short, mined tunnel. Deep mixing ground treatment was adopted to: mitigate the risk of hydrostatic ground fracture in the base of the UCS and SIS shafts and control ground water inflows during shaft excavation; act as an earth retention system between the toe of the secant-piled walls and the operating Hobsons Bay Main Sewer in the UCS; create suitably stable ground to enable mined excavation between the UCS and SIS shafts. The proposed ground treatment principally consisted of overlapping deep soil mixing columns forming a homogenous treated block installed from ground surface. Achieving this critical ground treatment required overcoming technical challenges, as detailed in this paper, which include but are not limited to (i) adapting the grouting parameters to accommodate for the significant geological & geotechnical variability, (ii) managing surface heave and spoil return in very soft soil conditions, (iii) working around existing 3.0m diameter sewer pipes running above and though the proposed ground treatment block, (iv) maintaining alignment for both vertical and inclined columns through pre-survey methods and drilling deviation measurements, (v) ensuring design grouted column diameters with sufficient overlap were constructed, and (vi) remediation of treated ground zone defects based on as-built 3D monitoring. To achieve this, the authors relied on a range of state-of-the-art trial-associated geotechnical testing, monitoring and mapping techniques. In addition, a comprehensive regime of in-situ testing was carried out from the ground surface confirming performance of the ground treatment to demonstrate that the project water tightness and mechanical requirements were consistently met. It includes full-length cored boreholes and the Cyljet electric cylinder method.
