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

SLIDES. 55 front part of the block behind. The third and last stage consisted in the dropping downward of the front block, owing to the failure and squashing out of its base. The whole block then disintegrated and sloughed down into the excavation. This last stage ran its course in a*few hours to a few days; the other stages required some months or more than a year to reach completion. The movements of the last and of the second stages were often accompanied by bulging of the bottom and of the lower slopes of the excavation (see figs. 7 and 8 and Pl. XIV). CAUSES. This type of deformation was due to a primal cause, the unstable geologic condition of the rocks, and an immediate cause, the over- steepness and height of the slopes, the blasting, and other work attributable only to man. The unstable condition of the rocks resulted from several geologic factors, chief among which were: (1) Ihe for- mations involved consist of soft, weak rocks, comprising massive, partly indurated volcanic clays, friable, bedded tuffs, and soft, brittle, and slippery lignitic shales, all with a relatively high ground water content. These rocks have low cohesion and crushing strength. They are so weak and unstable that locally the unbalanced pressure at the foot of the high slopes was often greater than the resisting strength of the rock. This condition induced deformation, with accompanying slow flowagelike movement, and at times a bulging in the bottom of the cut in front of the slide (Pl. XIV). (2) The rocks have been further weakened by faulting and by joint and bedding planes. Shearing zones, due to faulting, trend across the canal, as shown in Plates XV,“ XVI, XXV, and figure 1. The grinding movement of the rock masses against each other along the faults has been sufficient to shear and crush them for several feet on both sides of the planes of movement. These crushed-rock zones greatly weakened the slopes. (3) Deforming movements caused much rupturing of the weak rocks so that from a dense, fine-grained, relatively impervious condition they locally became much, sheared and subject to invasion by large quantities of ground water which tended to leach, oxidize, and dis- integrate them. (4) Lignitic-shale beds, especially those dipping toward the excavation, were planes of weakness along which there was a strong tendency for the overlying material to slip. (5) The presence, too, of a considerable proportion of chlorite particles in the volcanic clay rocks tended to lubricate the mass. The second or immediate cause of this type of slide was chiefly the oversteepness of slopes where the banks were high and the rocks weak and more or less saturated with ground water. In an excav a- a Formerly published by the author in an article on “Slides in the Culebra Cut at I anama, Eng. Record, vol. 66, August, 1912, pp. 228-233.