Over the past two decades soft ground and mixed face TBM tunnels have increased in diameter such that the term ‘large diameter TBM tunnel’ has become common usage. However, there is no actual definition of what constitutes ‘large.’ Therefore, in this article ‘large’ will refer to any tunnel of 30 ft, or greater, internal diameter.

Prior to the development of closed face TBM technologies large diameter soft ground tunnels were confined to favorable tunneling strata and constructed using compressed air. Examples include the Blackwall Tunnels (1897) under the Thames in the impermeable London Clay in London or the Hudson Tunnel (1927) and the Lincoln Tunnel (1938) in the United States.

Many deaths occurred during the construction of these tunnels under difficult ground conditions. In 1997 the Tokyo Bay Aqualine Tunnel opened to traffic. This tunnel was 23 years in planning and 9 years in construction. It comprises twin two-lane 39-ft ID highway tunnels 9.6 km long. The tunnels were bored through soft alluvial soils by eight Kawasaki slurry 46-ft OD TBMs.

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