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  A Finnish method for draining in drill and blast tunnelling

  Drainage of rock tunnel surfaces has shown to have a significant impact on project cost, time and on tunnel quality. Requirements might be set high and work can be extensive. According to research notes a threefold structure of pre-grout, rock drains and shotcrete is required to get dry conditions in the tunnel spaces. Thus, the objective is to drain the dripping or leaking water or even moisture from the pre-grouted rock to the crushed stone layer in the tunnel. According to the Finnish General Quality Specification the rock drains can be placed against rock surface. Narrow drains of 0,5 m width do not interfere with the tunnel rock support structure. The systematical average spacing is 3-5 m. The quantity of drain branches is estimated and specified on site as well as shorter drains. The drain includes cellular plastic and perforated PVC pipe. The construction enables maintenance. The installation of the drain in D&B tunnels is a profession requiring experience. The systematical draining functions well and the time needed for completing work is minimal. Total thickness of structure is small. The experience predicts long term use. Due to time and material savings the method improves total efficiency in tunnelling.

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