The 1800 MW Dinorwic Pumped Storage Scheme for the Central Electricity Generating Board is located near Llanberis, North Wales. (Fig. 1).

The power station is sited underground in the Cambrian Slate Belt and comprises nine major caverns with associated hydraulic and access tunnels. The photograph depicts the largest of the caverns, the Machine Hall, viewed from the portal of an access tunnel at the lowest (17 m.O.D.) level of the excavation, looking in a south easterly direction along the main axis. The roof of the cavern is at 69 m.O.D, and can just be picked out above the floodlighting. The maximum span of 24.5 m is visible in the background and the overall length from the portal to the far gable end is about 180 m.

The principal rock parameters are given in Table 1. Water inflows were minimal and the excavation was effectively carried out in dry conditions. Deformations of the rock were recorded by borehole extensometers, convergence measurements and precise survey techniques. The maximum inward movement of the upstream wall of the Machine Hall (shown on the left of the photograph) was recorded as 48.4 mm at a level about 21 m above invert.

The upstream wall intersects a dolerite dyke on the left which, although obscured by sprayed concrete, is supported by a pattern of rock bolts and anchors.

In the left foreground striped purple slate is still exposed. Several discontinuity sets are visible—notably sub-horizontal ($1) joints near the invert and rather steeper (30-60°) ($2) joints which

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