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

54 ENGINEERING PROBLEMS OF PANAMA CANAL. an imperfect instrument improvised by C. M. Saville, one of the engineers. Its magnification was about 10, so that the amplitudes of the earth waves set up by the blasts were about 2 and 2.8 milli- meters, respectively, quite enough to damage seriously a steep slope of brittle rocks already heavily strained. Vibrations from railway trains may also damage slopes that arc already in danger of sliding. EFFECT OF OTHER FACTORS. Heavy structures contiguous to excavations greatly increase the tendency to slide or cave, as subway and foundation engineers well know. The wedging off of rock masses by the frsezing of water in joints, fissures, and interstices is familiar to all geologists who have studied talus slopes in temperate climates. Earth strains, originating from the variation of barometric pressure and the kneading of tidal pull, should not bo ignored in tho study of excavation deformations. One is apt to forget that the maximum variation of atmospheric pressure near sea level may bo more than 4,000,000 tons per square mile. So, if the adjustment of atmospheric pressure in a very large cave or mine lags behind any quick increase in atmospheric pressure at the surface, a considerable strain may be added to the roof of such an excavation. TYPES OF SLIDES OR DEFORMATIONS IN CULEBRA CUT. In Culebra Cut there wero four distinct types of slides, or excava- tion deformations, as follows: (1) Structural breaks and deformations; (2) normal or gravity slides; (3) fault-zone slides; and (4) surface erosion. STRUCTURAL BREAKS AND DEFORMATIONS. GENERAL DESCRIPTION. The largest and most important slides developed from structural breaks and deformations. Fortunately, they occurred only near Culebra in a section of the cut not much over a mile long. These deformations first manifested themselves by the appearance of one or of a set of cracks or fissures (fig. 4) parallel or somewhat oblique to the edge of the cut, and from a few yards to some hundreds of yards back from it and from each other. Some of them were traceable on the surface for several hundred yards and gradually developed into per- pendicular crevices up to one-third of a yard wide and many yards deep. The second stage of this phenomenon consisted in the settling or tilting of these big block masses that had been divided from each other by the fissures. This movement was a slight and almost even settling (0.1 to 1 yard) of the block or blocks, or a tilting of them toward the excavation, or both. Some of the blocks sank a little in front and tilted up in tho rear so that they were a yard above the