CONVENTIONAL TUNNELLING | TECHNICAL
GUIDELINE SPOTLIGHT ON CASE STUDIES
The ITA WG19’s guideline on conventional tunnelling in urban settings presents 14 case studies from across North America, Japan and Europe. Below, its numbered case studies are grouped by region:
North America ● Case Study 1 – The Dulles Corridor Metrorail Project, Virginia, US: planned tunnel layout and geology are briefed, and the challenge of shallow cover and the use of a pre-support pipe arch canopy.
● Case Study 2 – East Side Access, Northern Boulevard Crossing, New York, US: briefing covers site geology and geotechnical challenges, robust design, use of artificial ground freezing (AGF), multiple drifts and I&M.
● Case Study 3 – Chinatown Station, Central Subway Ph2, San Francisco, US: below a heavily built-up area, following passage of twin TBMs through the project site, the construction work then opened a mined cavern by sequential excavation. The briefing discusses geology, ground improvement, pre-support, excavation and risk mitigation.
● Case Study 4 – Regional Connector Crossover Cavern in Los Angeles, US: the focus of the briefing was on the sequentially excavated crossover cavern in the downtown location, although the LA metro regional Connector project included many different types of tunnelling.
● Case Study 13 – Highway 401/409 Rail Crossing, Greater Toronto Area, Canada: here, the description is of challenges in constructing two closely spaced tunnels under a live busy highway with low cover, and running close to an existing tunnel. The new works were performed by sequential excavation, using pre-support pipe umbrella arches in the poor ground, which was mostly backfill, sand and gravel. The project was procured under a Design-Build-Finance model.
Japan ● Case Study 5 – Yokohama Municipal Subway’s ‘Green Line’: the project, the ITA report says, is Japan’s largest conventional tunnelling endeavour, the method having been chosen to minimise effects at the surface, where there is a university campus. Geology is mainly sandy soil and mudstone, and a gravel aquifer is present in the crown area. Use of side drifts, pipejacking for the roof and ultra-long face bolts are discussed, along with I&M.
● Case Study 6 – Construction of a Large-Section Tunnel with Small Overburden by the Central Diaphragm Method, Sendai: the briefing focuses on the challenge of construction of the Tozai Line’s Yagiyama Tunnel, which included weak/soft rock and addressing asymmetrical overburden due to a retaining wall beside a live road, also over the tunnel. Tunnelling was performed by the Centre Diaphragm method to help face stabilisation, and multi-stage bench excavation.
● Case Study 7 – Large-Sectional Conventional Tunnelling as Part of the Mountain Tunnel in Fukuoka City, Fukuoka: the project description covers conventional tunnelling for the majority (approx 196m) of the Hakata subway station in geology of alternating mudstone and sandstone, with
weathered and altered features. I&M was an important part of project execution, especially with important infrastructure on the ground surface.
● Case Study 8 – Shin Kobe Tunnel, Kobe: the briefing describes conventional excavation through uncemented soft ground with 4–10m of overburden for a 270m-long city centre tunnel. Extensive grouting was employed, including for long steel fore-poling for the roof prior to excavation. Packing with grout-filled textile bags (between support and steel pipes) was also employed. The foot region was also jet grouted and the tunnel lining was shotcrete.
Europe ● Case Study 9 – Porto Light Metro System, Combatentes station, Portugal: on the D Line of the metro, in geology of fill and granite of varying quality [very poor, as saprolites, which can reach total thickness of about 8m, and weathered rock mass], conventional tunnelling was used to construct two connected caverns for the subway station. The method was flexible, allowing both design and construction adjustments depending on local conditions encountered during the tunnelling works.
● Case Study 10 – Bolhao station, Oporto Metro, Portugal: to be built as a single subway cavern and connected tunnels below the city centre, which has numerous historic and multi-storey buildings, the tunnelling works heavily used I&M. Geology is granite but there was weaker ground, of fill, alluvial soils and also severely weather granite (saprolite) up to about 13m in thickness. Pre-support to the short advances includes self-boring forepoling, and then excavation support with lattice girders, mesh and shotcrete.
● Case Study 11 – Jet grouting soil stabilisation method for a road tunnel at Evouettes Hill, Switzerland: at the almost 660m-long single tube project site, geology is a cohesionless and loose soil (till and scree), and boulders are occasionally present. The geology is therefore highly variable, lightly compacted, contains voids and is water-sensitive. The added complication is the tunnel’s shallow depth profile in an urban setting. Pre-support significantly called upon jet grouting of various ways, which was also involved in face support during excavation. Other support included lattice girders and shotcrete.
● Case Study 12 – Madrid Metro Extensions – Mined Tunnels Constructed with the Madrid Traditional Method (MTM), Madrid, Spain: the discussion here is more of the particular construction method as applied across a few subway projects in the city, the MTM being a locally developed approach within conventional tunnelling systems. It comes from adapting traditional mining methods, where the focus is to start by developing and stabilising the final tunnel arch, and only then excavating below.
● Case Study 14 – Tunnelling at the Appian Way in Rome, Italy: the geology is of volcanic origin and highly varied. The road tunnel project involved building 600m-long twin tunnels as part of a capacity expansion but also taking the opportunity to pass below an ancient roadway and so allow uninterrupted walking along the historic route.
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