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

SLIDES. 51 is only one stage of disturbances that later end in slides, especially if the excavation toward which they are strained be deepened. If a plane such as ef (fig. 7) be imagined to extend from the toe of the slope, e, through the point of farthest deformation, this plane may not bo the final slope at which the material will ultimately come to rest, for such a flat slope might never be attained by sliding. On the other hand, it is not the limiting plane below which deformation or movement of the rocks will not take place, for in certain of the Culebra Cut slides movement well below such a plane has disturbed and weakened the slopes (figs. 7 and 8 and Pl. XIV). The term “ angle,” as applied to the measure or extent of excavation deforma- tions, is apt to be misleading, so the notation explained below is now much used by engineers. If an excavation 200 feet deep causes a deformation in the con- tiguous rocks to a distance of 500 feet in a horizontal direction from a vertical lino at the toe of the slope, e, shown in figure 7, the phenomenon may be designated as an excavation deformation of 200 Figure 8.—Ideal cross section to illustrate canalward deformation movements. on 500 feet. This designation indicates not only the depth of the excavation and the width of the zone of deformation, but also the angle of what may, in sand for instance, be the final slope or angle of repose. Engineers now often refer to slopes as being 1 on 1, 1 on 2, etc., meaning 1 unit up and 1 or 2 units over. The designation, however, is a measure only of the angle of slope unless the actual number of units of depth of excavation and width of deformation zone are given. If the rock has a low tensile and crushing strength, conditions would, in the absence of excessive mobility through ground water or other causes, approach those of a talus slope or an excavation in sand, where the slopes would be permanent when, they had reached the “ angle of repose” for the material involved. INFLUENCE OF PHYSICAL AND CHEMICAL CHARACTER OF ROCK UNITS. Excavation slopes in very soft rocks, especially very fine-grained and compact argillites and clays, may remain almost vertical until the excavation reaches a depth of 50 to 125 feet, or until the un- balanced pressure is great enough to cause them to deform. This