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Before the popularization of electric traction, which gave rise to the use of tunnel boring machines, subway and underground roadway projects typically employed the cut-and-cover construction method. As opposed to the in-situ construction of a bored tunnel, cut-and-cover involves the excavation of a trench and the subsequent roofing over to recreate the surface. Used to construct shallow tunnels, cut-and-cover generally creates more disruption at the surface compared to the deeper excavation process involved in tunnel boring.
There are two basic types of cut-and-cover. In the conventional bottom-up method, workers dig a trench from the surface, the tunnel is completed, and the trench is then back-filled to restore the ground. With top-down construction, side support walls are built first, usually through slurry walls or bored piling. A shallow excavation follows, allowing the tunnel roof and surface to be constructed. Tunnel excavation then proceeds under the permanent roof. The top-down method permits the early reinstatement of ground-level services like roadways.
As the original sections of the vast London Underground network predated electric traction, the lines were constructed using the cut-and-cover approach. The technique was particularly practical in terms of ventilation, as the tunnel's close proximity to the surface ensured the easy escape of steam and smoke. The advancement of technology towards the end of the 19th century brought about a shift to bored tunnels, which precluded the disruptive removal of the ground.
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