The method could be used only during quiet periods when there were no construction operations in the vicinity. Repeated observations were made at permanent listening points (short drill holes) around the main excavations. The periods of observation were not regular, and observations could not always be made during critical periods.

Noises of two types were observed. The first consisted of individual sharp clicks. The type of activity producing these noises appeared to be present on a small scale at most times but was usually greater in areas where excavation was taking place. The rates of activity usually ranged from 0.1 to 4 clicks per minute. The second type of noise consisted of intervals of continuous noise usually lasting from 1 to 45 seconds, as if due to sliding of one rock surface on another, This type of activity was much less common and irregular in its incidence that the first type.

In several instances an increase in the rate of rock noise up to about 10 times the normal rate was observed in localized areas and appeared to be associated with the movement and loosening of joint blocks. This occurred, for instance, when the main body of the machine-hall excavation was approaching the already partly excavated portal of penstock No. 4. Opening of joints between blocks was observed, but falls of the loosened rock were prevented by previously installed rock bolts. For most of the periods of observation, however, rock noise was at a low level and was interpreted to indicate relatively stable rock conditions.

The method is considered to be a useful supplement to other visual and instrumental means of assessing rock stability.

CONCLUSION

The experience with the construction of T. 1 power station showed that the conventional methods of geological mapping and careful diamond drilling were adequate to avoid the gross weakness of the region, associated with major faulting. These methods were inadequate to determine sufficiently characteristics of the jointing in the rock mass deep underground or to identify very narrow faults and fractures that, because of their persistence and orientation, had an important influence on the behavior of the openings. Direct exploration by tunneling is necessary to determine such details. The experience demonstrated also the desirability for design and construction arrangements to be as flexible as possible to permit minor adjustments of location that could take full advantage of conditions revealed by the preliminary main excavations.

The initial natural state of stress has a major and pervading influence on the behavior of the rock around underground openings. The probability of the existence of high horizontal stresses of tectonic origin, in addition to the more or less predictable effects of depth and topography, makes it essential to measure the state of stress at the site. It is desirable for this to be done at an early stage in exploratory tunnels of simple shape to avoid complications caused by large complex openings as well as to enable full application of the information in the designs. It would be extremely valuable for methods to be developed for making determinations of rock stress in deep diamond-drill holes, for instance by the use of cylindrical pressure cells along the lines of those mentioned by M. E. Tincelin and P. Sinou (1958).