The photoelastic tests made earlier covered a wide range of ratios of horizontal to vertical principal stresses, but for design purposes it was assumed most probable that the ratio would be about 0. 25, and therefore that there might be insufficient compressive stress in the roof to ensure stability. Accordingly, the arched concrete ribs proposed for permanent support for the roof were designed to carry loose rock which might become detached from the roof. As shown in Figure 12, the concrete ribs were installed after the roof section of the machine hall was excavated and before the main body of the hall was excavated. When the very deep excavation of the main body of the machine hall was constructed, the concrete ribs became subject to large horizontal compressive forces, and it can be assumed that the rock in the roof was in a similar state. This behavior of the rock in the roof is in accordance with that predicted from a natural state of stress with horizontal and vertical principal stresses about equal, as in the second case of the photoelastic tests.

Arrangement of Multiple Openings

In the original arrangement for the power station, the machines, transformers, and draf-tube gates were accommodated in three rather closely spaced separate parallel halls with connecting tunnels. Qualitative comparative photoelastic analyses on cross sections with the parallel halls showed high stress concentrations only in the comparatively narrow elongated pillars between openings. The joint pattern was superimposed on the stress pattern and showed that the direction of the joints of set b was close to the direction of high shear stresses (Pl. 4a). The interconnecting tunnels would have further increased the stresses. It was considered that these high stresses operating on the jointed rock would make construction very difficult and might even make the narrow pillars unstable.

--As a result the arrangement was changed to one similar to that which has been constructed, in which the transformer hall is set at right angles to the machine hall with a connecting tunnel. In this position the stress concentrations due to one opening have but little interaction with those due to the other.

Ground-Water Drainage

It was regarded as important to maintain the free-draining properties of the rock mass; this in effect meant keeping open especially the joints of set c and as many other joints as practicable. It was assumed that, if the free-draining nature of the rock mass were reduced the water table would tend to build up to its former natural level and thus increase ground-water pressures and ground-water pressure gradients around the openings. Penetration of high-pressure ground water along joints parallel and close to, but not communicating freely with, the surfaces of the openings could cause slabs of rock to be broken off by a hydraulic jacking action. There could be a similar damaging action on concrete structures poured against rock faces. Furthermore, in the design of the steel linings of the pressure shafts the external ground-water pressure was a critical factor in determination of the steel-plate thickness for the lowest parts of the shafts. The ground-water pressure adopted for these designs was based on the lowered water table.

The requirement to maintain the free-draining properties of the rock influenced a number of construction procedures and designs. No grouting for the purpose of reducing ground-water flows during construction was permitted, and localized grouting to consolidate rock foundations was kept to a minimum. The concrete ribs in the roof of the machine hall and transformer hall were designed to leave from half to two-thirds of the natural rock surface exposed for drainage purposes, and elsewhere in the walls of the machine hall considerable areas of rock face were left exposed.