Sydney Metro will increase train service capacity across the city from about 120 services per hour up to 200 per hour after 2024 and has the potential to increase passenger numbers from 24,000 per hour to 40,000 passengers per hour.

BG&E worked with Arcadis in joint venture (JV) to design the temporary and permanent civil works, for Sydney Metro’s Tunnel Stations Excavation (TSE) project. This comprised roadworks, drainage, temporary and permanent storm water diversion, flood modelling, utility coordination, road safety audit, traffic modelling and traffic staging, and significant durability services.

The underground Sydney Metro Southwest line will extend the current Metro Northwest line (Stage 1) from its current terminus at Chatswood through the city to Sydenham (Stage 2) in the inner West. Stage 2 is expected to be completed by the end of 2021.

The project consists of 15.5-kilometre twin tunnels, additional stations at Crows Nest, Victoria Cross, Barangaroo, Martin Place, Pitt Street and Waterloo. It includes a 750-metre long twin running tunnel under Sydney Harbour. Entry and exit for the tunnels will be via dives at Chatswood and Marrickville, North of Sydenham Station.

The TSE involves construction of:

• tunnel boring machine (TBM) launch sites at either end of the alignment at Chatswood and Marrickville;
• access shafts at each of the three mined stations at Victoria Cross, Martin Place and Pitt Street to facilitate excavation of the station caverns;
• three cut-and-cover stations at Crows Nest, Barangaroo and Waterloo;
• a crossover cavern at Barangaroo;
• 57 cross passages at 240 metres intervals;
• a temporary TBM reception shaft at Blues Point, on the North side of the harbour to facilitate retrieval of the two TBMs that will complete the northern drives, as well as the TBM that will complete the parallel drives under the harbour;
• a grout batching plant and segment casting yard at the Marrickville site, plus storage for the 99,000-tunnel lining segments; and
• tunnel and station fit-outs.

Five TBMs will be utilised, two being driven from the northern access site at Chatswood through to Blues Point temporary site on the north bank of the harbour and two from the southern access structure at Marrickville that will drive through to Barangaroo Station on the southern side of the harbour.

The fifth TBM will be used for the twin 750-metre long harbour crossing drives. This machine will launch from Barangaroo Station with a slurry separation plant established at the Barangaroo Station site to service the drives. It will then be retrieved at Blues Point on the opposite side of the harbour for transport back to Barangaroo Station to complete the parallel drive.

The JV included, BG&E deploying a highly-skilled team of materials and structural engineers to undertake inspections of 900 precast box girder sections, cast insitu piers and parapet sections along the length of the viaduct. Assistance was also provided in the scheduling of precast element production, to help in the delivery of the station structures.

Prior to the preparing the design and undertaking the materials durability testing, the JV delivered hydrologic and hydraulic modelling, and flood modelling.

The hydrologic and hydraulic modelling included Australian Rainfall and Runoff (ARR) Guidelines 2016 assessment sensitivity, to assesses pre-development and post-development flood behaviour, impact assessment and flood immunity levels for the 12 station dive, shaft sites and stabling yard. The key challenge is preventing flood waters from entering the excavations for all events up to and including the probable maximum flood (PMF), into any potential openings into the tunnels.

Flood strategies were developed for each location to prevent inundation of excavations and shafts from PMF, typically through used of raised capping beams and/or amplifying the drainage and inlet system to capture additional flow to an upgraded stormwater system, as well as temporary realignment of the existing stormwater system to allow construction. At Marrickville, the overland flow path was diverted across the dive over a bridge structure. Alternative options for diverting the overland flowpath around the dive structure were not feasible based on the natural surface grade, available space for the flow path and potential impacts on the adjoining rail corridor.

The flood modelling package was completed within one year and included design of flood mitigation measures and modelling of construction staging.

Flood models from local Council flood studies were obtained and used for most locations. In some areas the models needed refinement, as although fit for purpose for a wider Council area flood studies, additional detail or modification were necessary to better represent the existing building footprints, drainage and local topographic features at the TSE locations. Models were updated using survey information and through site inspections of each location.

Interfaces with structure designers was important to determine flood immunity levels for critical infrastructure and flood mitigation measures which were achievable within the design. At Victoria Cross and Marrickville, interaction with the drainage engineers regarding the temporary drainage diversion of the Sydney Water assets was critical in developing a design suitable for both low and high flows which limited flood risks.