BEHAVIOR OF THE EXCAVATIONS DURING CONSTRUCTION

The behavior of the rock around the excavations during construction was studied by instrumental measurements and by visual observations

Visual Observations of the influence of Geological Structures

After completing the pilot drive through the machine hall, the contractor excavated the first 120 feet of the roof section of the machine hall to full width using only rock bolts for support.

In the early stages of this work, however, the workmen were apprehensive that the rock bolts would not be adequate protection in an excavation of this size, and the contractor deemed it prudent to change the method of roof excavation. For the remainder of the machine hall a tunnel was driven along each abutment of the roof, and slots 8–10 feet deep were cut at roof level across the machine hall; these were widened laterally until all the roof was exposed. The roof was systematically rock-bolted, together with supplementary steel ribs. When later these steel ribs were progressively removed in order to enable the concrete ribs to be placed, there were no rock falls or detectable rock movements. When the roof excavation first began to intersect fault A and also clay-coated joints associated with fault B a block of rock weighing about 40 tons bounded by these two structures fell from this part of the roof causing about 4-5 feet of overbreak (P1. 6a). Up to this time the presence at this specific location of these geological structures was not known. It then became apparent that, throughout much of the reminder of the machine hall, fault A and fault B, together with their associated joints, intersected above certain areas of the roof to form blocks in the form of an inverted V. In these areas bolts were installed in directions chosen to be as nearly perpendicular as possible to the directions of the faults and joints. (Fig. 11).

No other rock falls as large as this occurred in the power station, but overbreak to the extent of 1-3 feet was not uncommon. In the gneiss the rock broke under the effects of blasting, mainly along the natural joint planes, but in the granite breakage occurred along fresh fracture planes as well as along joints.

After the roof excavation was completed, the permanent concrete ribs were installed; following this, the main body of the machine hall was excavated by quarrying methods. The vertical walls were systematically rock-bolted as soon as they were exposed. On the upstream wall rather extensive loosening of the granite joint blocks occurred. This loosening apparently was influenced by the zone of close jointing along fault A in the middle of the wall being intersected by joints of set b dipping steeply toward the excavation. In addition to systematic bolting, steel mesh was used between the bolts. On the downstream wall, the jointed granite was not loosened to the same extent, and rock bolts alone were used.

The transformer hall, 72 feet in maximum width, 128 feet long, and 43 feet deep, was supported entirely with rock bolts.

Rock Failure by Rupturing

As distinct from movement along existing joints and shears, in two places sound granite failed by rupturing and the development of fresh fracture planes. In these places the rock became wholly or partly detached from the surface of the excavation in slabs from 1/2 inch to 3 inches thick, and up to several square feet in area along rough surface fracture. The rock behind was often “drummy”, indicating further fracturing, This type of failure usually is referred to as spalling or popping rock (Terzaghi, 1946,p. 49).