Cross Passages Ground Treatment For Melbourne Metro Tunnel Project

As part of the Melbourne Metro Tunnel project, there was a requirement for ground treatment between tunnels, specifically at cross passages where the ground was identified as not being sufficiently competent to safely execute underground activities such as the safe removal of precast concrete segmental tunnel linings at cross passages and excavation of the cross passages. Cross passages CP02, CP03, CP04, CP05, and CP15 are located within soft or mixed ground conditions which required ground treatment to control groundwater ingress and/or provide suitable ground control to allow stability of mined excavation between the tunnel tubes.

The proposed ground treatment principally consisted of overlapping jet-grouting columns forming a homogenous treated block installed from ground surface to depth of 30m. Further, when the cross passages treatment area intersect rock, rock- grouting treatment was required to reduce the permeability of the rock mass and of the interface between jet & rock to reduce potential water inflow to the future excavation within allowable limits.

The realization of those critical ground treatment required to overcome technical challenges which are detailed in the paper and include but not limited to; adapting the grouting parameters to accommodate for the high geological & geotechnical variability at each site, working near various urban assets including under and around a live rail line, maintaining alignments for both vertical and inclined columns, ensuring proper grouted diameters with sufficient overlap and remediation based on as-built 3D monitoring. To achieve this, the authors relied on a range of state-of-the-art trial at each treatment location and associated geotechnical testing, monitoring and mapping techniques some of which were used for the first time in Australia. In addition, a comprehensive regime of in-situ testing was carried out from the surface and from within the tunnel following and confirming performance of ground treatment to demonstrate that the project’s water tightness and mechanical requirements were consistently met to mitigate the risks associated with excavation of the cross passages. The performance and quality of the treatment was ultimately confirmed during the excavation of the cross passages.