These sections are adjacent to intakes and control gates, and also where rock cover over the tunnel is insufficient to guarantee that internal water pressure will not cause excessive water loss or a failure by blowout. The Authority's practice is to concrete line also those sections which during excavation, are supported by steel ribs. In general, planning studies show that if more than one third of the length of the tunnel requires steel rib supports, it is economic to adopt a lined alternative irrespective of tunnel size (Dann, Hartwig and Hunter, 1963 (Fig, 2).

Estimates of the rock condition and need for lining and other treatment are made on the basis of geologcal studies including mapping, airphoto interpretation, drilling, trenching, seismic refraction, and experience from completed works.

During construction the worst areas for rock instability become apparent when the ground is first opened up and have to be adequately supported immediately to enable the heading to advance safely. However, ultimately the tunnels function as conduits for flowing water, in most cases under high pressure (up to 520 ft head) which may be subject to sudden changes. Under these operating conditions areas of rock which are sufficiently stable to stand up safely during construction may become unstable. All such areas of potential instability have to be recognized and treated appropriately before the tunnels are put into service.

The rocks cover a wide range of stability. The best ground does not require any support during excavation or any treatment other than

barring-down loose rock before the tunnel is put into service as a water-conduit (Fig. 3).

The worst ground obviously requires steel rib supports during excavation (Fig. 4), and concrete lining before the tunnel is put into service. Between these extremes is a wide range of rock conditions in which it is a problem to decide the extent and nature of the support required during excavation, and the final treatment required before the tunnel is put into service.

A conservative but costly approach would be to install steel rib supports during excavation wherever there was any doubt about the stability of the rock, and subsequently to line these sections with concrete; in other places where the rock was stable during excavation but contained weak seams which might deteriorate during service as a water tunnel, to line completely the sections of tunnel containing these defects with concrete. Thus during excavation there would be abrupt transitions from sections of tunnel rock fully supported with steel ribs to unsuported rock, and likewise in the final tunnel from fully concrete lined sections to bare rock tunnel.

However, by the use of rock bolts as rock support and reinforcement during excavation, and protective layers of plain or reinforced pneumatically applied mortar, combined with grouted rock bolts as final treatment, a more flexible, more rational aud more economical approach can be adopted in which the treatment of the tunnel can be closely adjusted to suit many variations of local conditions within the zone of intermediate rock quality.