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

68 ENGINEERING PROBLEMS OF PANAMA CANAL. SLOPES WHERE ROCKS MAY DEFORM BY SWELLING OR FLOWAGELIKE MOTION. The most desirable slopes to adopt where excavations cut rocks that will deform by flowage as well as by fracture are next considered. In dealing with such rocks an entirely new set of conditions enters into the problem, because deformations or movements may extend to some depth below the excavation as well as to considerable dis- tances from it horizontally. The swelling of ground in tunnels, especially in certain coal mines, and the shattering of marble and sandstone slabs in quarries is due to a variety of rock deformation. As already explained in this paper, certain rocks may stand at a steep angle until the excavation that they overlook reaches a depth of say 20 to 30 yards or more; then they may begin to deform and later to slide, until a flat angle is reached. For such rocks the slope that will minimize the danger of deformation and give maximum steepness and utility will be a curved surface instead of the plane surface that suffices for the more stable rocks. The reasons why tho surface should be curved are as follows: (1) Earth, pressures, as against a retaining wall, for instance, vary in a general way as the square of the height of the wall; (2) with increase of depth, ground water becomes more active in penetrating and softening rocks and in dissolving cementing material from them; (3) gravity stresses, helped by ground water and the element of time, cause slight movements and swellings in the rocks, and every such movement is an added weakness to the slope. Hence, if for any cross section of an excavation, especially in soft rock, the different depths were used as abscissas, and the corresponding tendency to deformation at that depth as ordinates, the plotted result would be a curve. TABLE OF SLOPES. Tho following information and the accompanying table have been given by tho author in a paper read before the Twelfth Interna- tional Geological Congress at Toronto, Canada. The following are given as approximately the best curvatures and slopes to adopt for excavations in the materials described. Tho slopes for other materials, which fall between these, may be esti- mated by interpolation. A theoretical slope should first be deter- mined according to the depth of the excavation, character of rock, etc., from the tables that follow. Then a cross section of tho slope and bottom pianos of the excavation, as selected, should be plotted. A hyperbola tangent to these two, with its vertex in the projection of tho bottom plane, will represent about tho proper slope and cur-