This paper presents a novel method for planning and supporting excavations in hard, jointed rock. It is based on the principle that there exist a few joint blocks on the surface of an excavation that are critical to movements of any larger mass of rock; these are termed key blocks, after the keystones of a masonry arch (Figure 1). Support of the keystone of an arch is fundamental; loss of the keystone means deterioration of stability and collapse if that deterioration is not speedily countered. The same is true in the case of the hypothetical underground cavern shown in Figure lb. The key blocks are numbered 1. Their removal permits movement of blocks 2 into the new space voided by 1 and movement of 2 FIGURE 1.(Available in full paper) Visiting Research Engineer, California Mining and Mineral Resources Research Institute, on leave from the Scientific Research Institute for Water Conservation and Hydroelectric Power, Peking, China permits successive displacement or rotation of 3, 4, and 5. In the end, either the entire opening will fill with debris or the collapse will progress to a stable arch, depending upon initial horizontal stresses in the roof rock, and other rock and environmental factors. In three dimensions, the idea of a key block is just as valid as in these two dimensional examples and since the key blocks are fewer in number, their identification is even more significant.

The progressive nature of rock movement and the potential magnitudes of such failure are known to many constructors of rock tunnels. One important example occurred in the Kemano headrace tunnel, where a void about 20,000 m 3 in volume extended 42 meters above the crown of an unlined tunnel 7.5 m in diameter in granitic rocks(1) Lang, Kendorsky, and Chawla(2) reported a void 10,000 m 3 in volume excending perhaps 60 meters above the 8 meter diameter, unlined New Colgate tunnel, California, where the amphibolite was crossed by a sheared zone. In China, a railroad tunnel in limestone collapsed progressively to a height 60 meters above the original roof (3) In all these examples, movement began small and enlarged subsequently.

A worker constructing an underground opening in hard, jointed rock who discovers an open joint in the wall or roof has a responsibility like that of a surface miner discovering a tension crack in the side or top of a high pit. The situation is potentially dangerous and an immediate response is demanded. One extreme possibility is to evacuate the area. But this will permit the rock to move, with possible progressive block movements as outlined above. A second response is to attempt to support the rock, as in the case of the Morrow Point Power House(4) where no other measure was possible. However, it is dangerous to construct supports in rock with opening joints. It is better to prevent the joints from opening in the first place. As in the New Austrian Tunnelling Method, we hold as axiomatic that rock loosening is to be avoided by timely and effective initial support.

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