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Cut-and-cover Tunnel Method

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Construction Processes
Scene from London Transport Museum Showing Cut and Cover Methods Used to Costruct the London Underground
Cut-and-cover tunneling
is a simple tunneling construction method used to build shallow tunnels such as those commonly used by subways, railways, and metro systems.[1]

Contents

[edit] Type/Process

[edit] Conventional Method

In the conventional method, excavating a trench in the ground and then backfilling and restoring the original roadway or ground is the process used to construct a tunnel.[2] A support system of some sort is also necessary to carry the load of the material used to cover over the tunnel such as shotcrete.

[edit] Bottom-up Method

In the cut-and-cover bottom-up or caisson wall method, a drilling rig is used to install caisson walls down to the existing bedrock. Once the caisson walls are in place, soil between the walls is excavated to a depth below the tunnel floor. The tunnel floor, a slab, is poured, followed by the sidewalls of the tunnel from the bottom-up. After the walls of the tunnel are completed, the roof is constructed and the roadway or ground on top of the tunnel restored.[3] Materials used to provide the structure and support in the construction of the tunnel may include concrete, pre-cast concrete, pre-cast arches, or corrugated steel arches.

[edit] Top-down Method

In the cut-and-cover top-down or diaphragm wall method, the opposite process takes place in constructing the tunnel. A trencher or trench cutter is typically used to dig a trench out of the the ground first before concrete walls are built. This processes consists of using a slurry mixture to build a slurry wall. The slurry wall provides temporary support to the sides of the trench before concrete is poured for a permanent wall structure. Once the concrete walls of the tunnel are completed, the roof of the tunnel is constructed and the surface roadway restored. Excavation of the tunnel is then carried out through openings in the tunnel roof top-down to the tunnel floor. The tunnel floor slab is the last part of construction to be completed.[4]

[edit] Cast-in-place Method

Another type of cut-and-cover tunneling is called cast-in-place. In this method, a trench is excavated with forms being built directly inside the trench. Concrete is then poured or cast into the concrete. After the concrete cures the forms are removed. The trench is then backfilled and the roadway reinstated. A shoring system is supports the sides of the excavation to prevent the shifting of soil.[5]

[edit] References

  1. Cut-and-cover tunnels. Tunnels, 2008-09-29.
  2. Tunnelling. Detroit River International Crossing Study, August, 2007. (accessed: 2008-09-29)
  3. Tunnelling. Detroit River International Crossing Study, August, 2007. (accessed: 2008-09-29)
  4. Tunnelling. Detroit River International Crossing Study, August, 2007. (accessed: 2008-09-29)
  5. Cut-and-cover Tunnel Construction in Vancouver. CanadaLine.ca, 2008-09-29.