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Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face.Other key geotechnical factors: For water crossings, a tunnel is generally more costly to construct than a bridge.Rail station platforms may be connected by pedestrian tunnels or footbridges.

The definition of what constitutes a tunnel can vary widely from source to source.However, navigational considerations may limit the use of high bridges or drawbridge spans intersecting with shipping channels, necessitating a tunnel.Bridges usually require a larger footprint on each shore than tunnels.In planning the route, the horizontal and vertical alignments can be selected to make use of the best ground and water conditions.It is common practice to locate a tunnel deeper than otherwise would be required, in order to excavate through solid rock or other material that is easier to support during construction.

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