Project phase Residual risk rating Category Potential event Construction stage excavations cause ground movement Damage to rail lines resulting in disruption of services Construction 2 – Western portal 1 – Tunnels (western portal to Arden station) Medium Construction stage excavations cause ground movement Damage to tram lines resulting in disruption to services Construction 6 – CBD South station Medium Construction stage excavations cause ground movement Damage to Telstra Tunnels resulting in disruption to key infrastructure Construction 6 – CBD South station Medium Tunnel construction encountering rock with greater rock mass strength than expected May necessitate a change in construction methods in a zone of geological conditions leading to increased ground movement or cause TBM to go off-line. Requirement to change construction method or repair/retool TBM could result in project delays Construction 1 – Tunnels (western portal to Arden station, Arden station to Parkville station, CBD South station to Domain station) 2 – Western portal 3 – Arden station Medium Ground heave as a result of excessive face pressure by the TBMs in shallow cover areas Unacceptable ground movement Construction 1 – Tunnels (All) 2 – Western portal Medium The assessment has been based upon a review of the ground conditions and preliminary modelling that provides a conservative estimate of the potential settlement values, their distribution and their effects on structures and civil infrastructure. The risk register was developed with the geotechnical risks associated with geological variability in mind and the potential for excavations to encounter unforeseen/unexpected conditions. The levels of confidence in the current interpreted geological model are reflected in the current register. Ongoing geotechnical and hydrogeological investigations would allow refinement of the model and increase levels of confidence in those areas identified as having medium residual risk ratings, potentially reducing them to low.
Moderate impact: North Yarra Main Sewer Condition survey of the sewer to determine extent of re-lining requirement. Investigate potential requirement to the sewer at detailed design stage Lloyd Street bridges Minor impact to abutment retaining walls on shallow foundations Assess settlement impact on the bridge structures and implement any strengthening works required. Conduct very close monitoring of ground movements as TBM excavations advance towards these bridges Essendon Rail Flyover West Melbourne Terminal Station (tower with raked pile foundations) Minor impact (Induced pile deformations, moments and shear forces all expected to be structurally acceptable) Subject to confirmation at detailed design stage CityLink Viaduct (pier with pile foundations) Parkland and waterways Moonee Ponds Creek and bike path Negligible to minor impact Impact Management: sites to be remediated on completion of works, if required Table Tunnel sector 2: Arden station to Parkville station Preliminary assessment findings Potential mitigations Buildings Typical for the area Negligible to minor impact Impact Management: fine cracks could be treated during normal decoration. Repointing may be required of external cracks to ensure weather-tightness Women’s Hospital Negligible damage is predicted for each structure, although serviceability damage to footpaths and road may occur Impact Management: footpaths and road could require resurfacing on completion of Melbourne Metro works in this area Victoria Comprehensive Cancer Centre Existing Grattan Street bridge foundations) Existing Grattan Street tunnel Minor – at junctions-tunnel connection to piled buildings (Potential for serviceability cracking at interface of piled building and tunnel) Impact Management: inspect and undertake repairs as required Royal Women’s Hospital Service tunnel
Road Moderate impact Ground improvement in shallow tunnel section, comprehensive real time monitoring strategy Rail Negligible impact: Flinders Street Station lines Impact Management: monitor and existing lines as required Trams lines Negligible impact: St. Kilda Road - Moderate impact: St. Kilda Road (near Wadey Street and CityLink crossing) Ground improvement in shallow tunnel sections Comprehensive real time monitoring strategy Utilities Negligible impact - Existing CityLink tunnels Preliminary assessment results found the change in stress in the existing concrete lining caused by Melbourne Metro excavation would be within the strength capacity of the lining There is a risk of aesthetic cracking in the unreinforced secondary lining. As the linings are unreinforced, cracks would not pose any structural capacity or durability risk Condition survey and monitoring of the CityLink tunnels prior to and during the excavation and construction of Melbourne Metro tunnels would be required. An appropriate management plan would be required that documents acceptable stakeholder criteria for potential cracking and a predetermined program for undertaking any potentially required minor repairs. After construction of Melbourne Metro tunnels is completed, the CityLink tunnels should be inspected and, if required, new cracks sealed during a regular maintenance closure. Detailed assessment of impact on CityLink tunnels Swanston Street Bridge (between Princes Bridge and Flinders Street) Negligible to minor impact - Princes Bridge Minor impact Completion of current site investigation works Strict control on TBM operation. Further analysis at detailed design stage Alexandra Avenue retaining walls Minor impact Strict control on TBM operation. Further analysis at detailed design with the benefit of pre-construction settlement monitoring data
Table Ground movement assessment: Precinct 4 Asset Preliminary assessment findings Potential mitigations Buildings Typical for the area (including tall buildings) Minor impact Impact Management: Cracks could be filled. Repointing may be required of external cracks to ensure weather-tightness Basement car park Minor impact Impact Management: Cracks could be filled. Repointing may be required of external cracks to ensure weather-tightness Heritage buildings Negligible impact Includes Main Entrance Gates, Vice Chancellor’s House and Gatekeeper’s Cottage, University of Melbourne Impact Management: Fine cracks could be treated during normal decoration infrastructure Road Negligible to minor impact - Tram lines Negligible to minor impact: Flemington Road and Grattan Street junction Royal Parade and Grattan Street junction - Utilities Negligible to minor impact - Precinct 5: CBD North Station would be the potential for ground movement in this precinct associated with the excavations for the station cavern and entrances. Table Ground movement assessment: Precinct 5 Asset Preliminary assessment findings Potential mitigations Buildings Typical for the area (including tall buildings) Steel and concrete buildings: impact Impact Management: Cracks could be filled. Repointing may be required of external cracks to ensure weather-tightness Masonry buildings: Minor-moderate impact Impact Management: Cracks could require some opening up and patching by a mason. Repointing of external brickwork and possibly a small amount of brickwork may need to be replaced
Heritage buildings Negligible to minor impact: Includes City State Library, and Storey Hall (RMIT) Impact Management: Fine cracks could be treated during normal decoration Approved developments Minor impact Impact Management: Cracks could be filled. Repointing may be required of external cracks to ensure weather-tightness Civil infrastructure Road Negligible to minor impact - City Loop Minor impact. There is a risk of serviceability cracking in the unreinforced secondary lining. Further assessment of the City Loop tunnels would be required in the detailed design stage, including procurement of more detailed information on the as-constructed linings and a detailed inspection and assessment of the linings condition. Following the condition survey, refinement of the modelling should be undertaken to account for the actual City Loop construction and condition. Subsequent to refinement of the modelling, a plan of monitoring and required, protective measures (strengthening works) should be established in consultation with the stakeholders. An appropriate management plan would be required that documents acceptable stakeholder criteria for potential cracking and a predetermined program for undertaking any potentially required minor repairs. After construction of Melbourne Metro is completed, the City Loop tunnels should be inspected and if required, new cracks sealed during a regular maintenance closure Tram lines Negligible to minor impact: Swanston Street and Victoria Street junction (north bound and west bound lines) Swanston Street and Victoria Street junction (north bound and west bound lines) Swanston Street – La Trobe Street junction - Moderate impact: Swanston Street – CBD station cavern excavation Comprehensive real time monitoring strategy
Table Environmental Performance Requirements for Ground movement and land stability Draft EES evaluation objective Environmental Performance Requirements Proposed mitigation measures Precinct Timing Risk No. Land Stability – To avoid or minimise adverse effects on land stability that might arise directly or indirectly from project works Develop and maintain geological and groundwater models (as per the Environmental Performance Requirements GW2) Use monitored ground movement and groundwater levels prior to construction to identify pre-existing movement Inform tunnel design and the construction techniques to be applied for the various geological and groundwater conditions Assess potential drawdown and identify trigger levels for implementing additional mitigation measures to minimise potential primary consolidation settlement Assess potential ground movement effects from excavation and identify trigger levels for implementing additional mitigation measures to minimise potential ground movement effects. While not specifying required recommended Environmental Requirements are framed to ensure mitigation and management adopted and implemented in the construction of Melbourne Metro. also to the potential impact measures identified in the ground assessment tables provided for each All Design/ Construction Design and construct the permanent structures and temporary works to limit ground movements to within appropriate acceptability criteria (to be determined in consultation with relevant stakeholders) for vertical, horizontal, and angular deformation as appropriate for project activities during the construction and operational phase. All Design