IN MIXED FACE CONDITIONS, any combination of rock and soil may be present in the tunnel cross section, but the classic scenario is the top half of the tunnel in soil and the bottom half in rock. In decades past such tunnels would be conventionally mined using timbers to stop material from moving, while breaking out the sections of rock. When the tunnel alignment cannot be raised or lowered, the process for EPB TBM tunnelling has generally been to adjust boring parameters and use additives to condition the soil.
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