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Methods of tunneling vary with the nature of the material to be cut through. When soft earth is encountered, the excavation is timbered for support as the work advances; the timbers are sometimes left as a permanent lining for the tunnel. Another method is to cut two parallel excavations in which the side walls are constructed first. Arches connecting them are then built as the material between them is extracted. Portions of the unexcavated center, left temporarily for support, may be removed later. A tunnel cut through rock frequently requires no lining. Hard rock is removed by blasting.
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