North East Link Project, Melbourne, Australia Lower Plenty Road Interchange Diaphragm Wall Works-Paper

Paper

Pdf Summary

The Lower Plenty Road Interchange, part of Melbourne’s North East Link project, uses a 1.2 m-thick diaphragm wall to retain water and enable cut-and-cover tunnel construction. The wall comprises 471 panels, some up to 45 m deep, excavated using four Hydromill cutters—the first deployment of this scale in Australia. Bentonite slurry supports each trench during excavation and is cleaned before concrete is placed.<br /><br />Construction encountered highly variable Silurian rock, including steeply dipping bedding planes, joints and fault zones. These discontinuities could form unstable rock wedges, particularly where faults intersected the trench. Although initial stability analyses indicated acceptable safety factors, testing of fractured rock found much weaker parameters. Revised analyses showed that standard-length panels could be unstable.<br /><br />Several trenches collapsed in fractured areas, often immediately after working slurry was replaced with lighter casting slurry. This highlighted the importance of slurry density and hydrostatic pressure to trench stability. The team responded with measures including increasing casting-slurry density to the specification limit, pressure grouting voids, installing soil nails to secure concrete wedges, shortening panels in risk zones, and raising working platforms and guide walls to increase fluid head.<br /><br />The changes reduced the scale of subsequent collapses. Analysis showed that shorter panels improved stability, achieving an acceptable factor of safety in the most challenging ground. Despite significant rework, including modifications to reinforcement cages, panels in the affected areas were ultimately excavated and cast without further incident.<br /><br />The case demonstrates that conventional trench-stability assessments may not adequately account for adverse rock defects or temporary open-trench conditions. Careful monitoring, site-specific reassessment, and coordinated remedial measures were essential to protect workers and equipment and complete the diaphragm wall safely.

Keywords

Lower Plenty Road Interchange

North East Link

diaphragm wall

Hydromill cutters

bentonite slurry

cut-and-cover tunnel

Silurian rock

trench stability

soil nails

pressure grouting