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

56 ENGINEERING PROBLEMS OF PANAMA CANAL. tion through granite the slopes or walls might be made perpendicu- lar for depths of several thousand feet without their crushing in, but if such excavation were carried to a depth of several miles, perpendicular walls could not bo maintained even in granite, for the unbalanced pressure at the foot of such walls would exceed the crushing strength of the rock and collapse of the lower part of such excavation would result. The critical depth and steepness for the rocks involved have locally been exceeded in Culebra Cut. The earth vibrations set up by deep, heavy blasting, near slopes already under a severe strain and carrying much ground water, have had a strong tendency to develop slides. The geologic conditions wore not sufficiently con- sidered in the first plans for digging Culebra Cut. REMEDIES. For this type of slide tnero was oiJy one remedy that had utili- tarian value under tho conditions involved, and that was applied. It consisted in making tho slopes less steep by removing material from their upper parts until the unbalanced pressure at tho foot of the slope became less than tho crushing or deforming strength of the rock. To do this, steam shovels were put on tho banks to terrace them back on either side of the cut and relieve tho strained condition (see Pls. XVII and XVIII). At first sight it might seem preferable to have lot the slides come into the excavation and to have shoveled them out from the bottom until permanent slopes were reached, thus saving the expense of much blasting. But deformations of this kind weakened the rocks far below the bottom of tho excavation (see figs. 7 and 8), and this weakened material would stand only at a much flatter slope than was necessary before it became loosened and disintegrated by deformative movements. Further, as each block or mass crushed down it generally left behind not a gradual slope, but a steep face 12 to 25 yards or more high, which greatly assisted in the generation of other slides. The addi- tional expense and difficulty of shifting and adjusting the railroad tracks, the air pipe lines, the drainage, etc., were also involved. Timo figured as a causal factor in these large slides and deformations, for some of them ran their course in a few weeks or months, but others showed cracks for many months or even for years before sliding. Such fissures were sources of weakness which sooner or later, with tho deepening of the cut, became troublesome, unless remedied by tho reduction of tho slopes. SLIDES IN GOLD HILIj AND CONTRACTORS HILL NOT LIKELY. The highest and steepest parts of Culebra Cut, namely, Gold Hill and Contractors Hill, did not deform and crush, because they are composed of relatively strong rocks of volcanic origin. These more solid rocks extend down deep into the earth and thus constitute vast