Earth Pressure Balance type Tunnel Boring Machines (TBM – EPB) were introduced in the tunneling industry about 30 years ago with the aim of facilitating mechanized excavation in ground containing a significant proportion of fine grained soils, which were problematic for the existing slurry shield machines (TBM – SS). Until recently, TBM-EPB applications have continued to be limited to ground containing fine grained soil and low water permeability because of the difficulty, in coarse grained soil, of controlling the earth pressure transferred to the tunnel face. However, the addition of fines in the excavation chamber has permitted efficient and economical TBM – EPB employment in coarse grained soils of mainly sand and gravel. Therefore, these machines are gradually increasing their market. One example of successful TBM – EPB exacavation in highly permeable coarse grained soils is Turin’s first subway line, which includes 9 km of tunnels, 15 stations, a train depot and control centre, building protection measures and other secondary structures. Construction began in November 2000 and the metro line shall be operational at the end of 2005. Tunneling was completed between October 2002 and March 2005 with three earth pressure balance tunnel boring machines (EPB-TBMs) in Turin’s fluvial and glacial deposits of gravel, sand and cobbles in a silty matrix, with a grain-size curve falling outside the typical normally recognised limits of application of EPB shields. Soil improvement by grouting was undertaken to limit risk of damage to pre-existing structures potentially influenced by tunnel excavation. Ground conditioning in the excavation chamber took place with injection of a mix of a calcium carbonate filler in water, to compensate the natural grain-size curve and integrate use of traditional ground conditioning foams. A monitoring system with over three thousand instruments, based on a Web-GIS architecture, was installed to control safety during construction. The Turin experience is a clear demonstration that, taking correct measures, it is possible, even in an urban context which requires careful control of ground settlements, to economically employ EPB shields in very coarse and highly permeable soils, traditionally considered unsuitable for these machines.
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