Cross Passages Ground Treatment for Melbourne Metro Tunnel Project-Paper

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The Melbourne Metro Tunnel project required ground improvement at cross passages where soil and rock conditions were inadequate for safe excavation between the twin tunnels. The paper describes treatment at CP03 and CP04, where groundwater inflow and ground instability could otherwise cause ground loss, tunnel damage, settlement, or surface subsidence.<br /><br />Treatment designs reflected differing geology. At CP03, overlapping jet-grout columns formed a reinforced, low-permeability block. At CP04, weak-concrete secant CFA pile walls formed a box, supplemented by jet-grout plugs and rock grouting where needed. Design targets included unconfined compressive strength of at least 1.5 MPa and permeability below specified limits.<br /><br />Construction took place in a constrained urban setting near live railways, roads, utilities, and buildings. Low headroom, overhead power lines, and variable ground required inclined drilling, careful spoil management, and close monitoring. Field trials used CYLJET geophysical measurements and core drilling to assess column diameter, continuity, strength, and permeability. During production, drilling deviation was measured and 3D as-built layouts were used to adjust column positions and sequences to reduce gaps.<br /><br />Across CP03 and CP04, 489 columns were installed, treating 11,638 m³ of ground. Testing confirmed the treatment’s physical and mechanical properties. Probe drilling and grouting from within the tunnels verified that water inflow met the required limit before excavation. Cross passages were then mined using remote-controlled equipment and temporary shotcrete support.<br /><br />Monitoring showed ground movement remained within the design limits, groundwater levels were generally stable, and tunnel lining loads and convergence stayed below trigger thresholds. Excavation exposed columns with diameters consistent with design, and the treated ground performed as intended, with only minor, manageable seepage.<br /><br />The authors conclude that location-specific trials, detailed quality control, 3D construction records, real-time monitoring, and pre-excavation probing were essential to managing geological uncertainty and reducing excavation risk.

Keywords

Melbourne Metro Tunnel

ground improvement

cross passages

jet grouting

secant CFA piles

groundwater inflow

urban tunnelling

quality control

geotechnical monitoring

excavation risk