Some Engineering Problems Of The Panama Canal In Their Relation To Geology And Topography

SLIDES. 67 over. Slopes in such material might be left perpendicular but for the fact that blasting leaves the rocks somewhat fissured, and a small slope greatly aids in preventing sloughing. Even if the slopes are channeled, weathering after a time opens small cracks, not recog- nizable in the fresh rock, and from these cracked places small masses aro liable to slough as time, goes on. 2. Given the same kind of rock as that included in class 1, but with jointing and fissuring increased to about the average of that commonly encountered in excavations, or the average common to the kind of rock described under class 1. The slope of an excava- tion under such conditions should be about 7 on 1. 3. Given the same rock as included in classes 1 and 2 but with, the jointing and fissuring increased to the maximum of that en- countered in nature in such rocks. For such the slope should be about 3 on 1. 4. Wherever an excavation parallels bedding or fault planes that dip toward it, the rock being the same as that mentioned in class 1 and the jointing corresponding to that described under 2 or 3, then the slope is likely to be controlled by such bedding or fault planes, as follows: (a) If the individual beds are a yard or more thick and if no clayey or slaty rock has formed along the bedding or fault planes, about 2 on 1 would be a safe slope. (b) If the rocks are thinly bedded, and if they are shaly, clayey, or slickensided along the bedding or fault planes, 2 on 3 would rep- resent about the maximum flatness necessary for the slope. If bed- ding is not rendered slippery by clay partings, a 1 on 1 slope would be quite safe. 5. For convenience, it should be mentioned here that the maxi- mum anglo of slope of sand dunes is about 1 on 2, which is practically the angle of repose for dry sand. The angle of repose for the waste dumped out of most lode mines is 37 degrees. I he above conditions pertain only to the rocks described, which will not crush or deform by flowage toward an excavation, but will only slough off fragments and masses loosened by fractures, joint- ing, bedding, faulting, or other causes. Under these conditions the slopes should all bo approximately plane surfaces, except in the upper part, where the material is much weathered and changed into. soil. Here erosion will remove some of the soft material and will tend to give a curved surface. The plane surface of the lower part of the excavation will then be tangent to the curved surface of the upper weathered material, and this latter surface will approach logarithmic curvature.