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

58 ENGINEERING PROBLEMS OF PANAMA CANAL. elevation there recorded. The presence of these rocks has a direct relation to the slide problem, for they are strong enough, except where faulted, to limit the downward depth of any deformative movement of the weak green clay rocks on top of them. Unfortunately, southward of station 1762 they are too deep to prevent present deforma- tive movements of the green clay rocks above them. Northward of that station they come to the surface for a considerable distance, and their presence is marked by sus- tained slopes between stations 1762 and 1749. Contractors /O.—Contractors Hill is a cap on top of a basalt plug which came up from a depth of probably much more than a thousand feet. It contains fragments of basalt, and consolidated at an elevation above the stability of its foundation. Gravity, cooling, and time were factors that caused a settling back to equilibrium, aa it has done. What is the evidence of this settling back? Plate XIX shows a part of a shear plane that extends around the canalward side of Contractors Hill, and must entirely surround it. On this plane is a hard fragment c, inclosed in a softer matrix and behind it (above) is a sloping little ridge of softer material. 1 his hard fragment, held fast in a softer matrix, had a part of this matrix scrubbed away from all sides of it except that on the upper side, the side opposite to the direction of its motion. This and other similar data absolutely establish the fact that the List movement of this mass was downward. The amount of this downward motion is indicated as follows: The zone of crushed rock d is about 10 feet wide. This wide zone of rock has been crushed and highly sheared by the downward motion, and the expe- rience of geologic observation of many different kinds of faults is that shear zones 10 feet wide, under conditions similar to these, result only where the differential move- ment of the rocks has been, say, more than 300 feet. Now, it is evident that if this great roughly circular mass of rock, after having come up, settled back through the weaker surrounding rocks a distance of 300 feet, its foundation must be at least a thousand feet below the surface. Therefore it seems certain that the unbalanced pressure caused by the excavation of a few hundred feet of the surface material will never result in any disturbance to such ;i deep foundation as that of Contractors Hill. Of course local masses of rock will crush down from its steep slopes occasionally, and some of these may be rather extensive, but the hill mass as a whole will remain per- fectly solid. Gold Hill, in its origin and composition, is somewhat similar to Con- tractors Hill. The top of it, however, is covered with a thick flow of hard basalt, and a massive basalt dike has come up around its periphery on at least two sides (see Pl. XV). The downward motion of Gold Hill is evidenced by the fact that the small lava breccia mass d', Plate XX, has come down from, and is a fragment of, the lava breccia d. With other sheared material, it has been dragged in by the downward frictional pull of the hill mass. The sheared zone c around the visible portion of the margin of the hill is as great as that around the margin of Contractors Hill, showing that the downward movement of both masses has been almost equally great, and that their foundations are undoubtedly several hundred feet below sea level. Gold Hill is also greatly strengthened by the massive lava dike that comes up on at least two sides of it, as shown in figure 1. The origin of this lava rock (basalt) has certainly been more than a thousand feet below sea level, and its strengthening influence is very great. Faults.—The extent, direction, and dip of the chief faults in the vicinity of Culebra are shown on Plate XV. Most of these diverge outward from the contact zone between the two solid masses, Contractors Hill and Zion Hill. These fault planes have greatly weakened the already weak, friable, greenish clay rock, and for this reason the “storm center” of “breaks” and slides for Culebra Cut will continue to be here until perma- nent slopes are finally established. Probable limits of sliding ground—Lines representing the probable limits of sliding ground are shown on Plate XV. Such lines must necessarily be more or less approxi- mate. In some places slides may reach beyond these approximate limits, and in other