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

ROCK MATERIAL USED IN CANAL CONSTRUCTION. 45 unmixed with clay, occurs in the deposita except at or very near the bottom. The reason for the boring records specifying “sand and gravel” as they do, is that when samples are taken from time to time during the process of sinking the hole, only the coarsest material is collected, the finer clay being held in suspension and carried off with the water flowing from the hole. In certain of the holes temporary flows of water were encountered, but after a few hours the flow invariably ceased. Such flows are believed to occur when, in the course of making a boring, the casing is introduced into a lenticular deposit of sand or water-bearing gravel under pressure from the overlying beds. The sands, being surrounded by much less pervious mate- rial, are in the nature of reservoirs in which water is stored under pressure, and when this pressure is released at the point where the casing enters the sands water may rise to the surface, if under sufficient pressure, and flow from the top of the casing until the pressures are readjusted. Far from indicating porous materials underlying the site for the dam, the occurrence of such flows of water only proves the extremely impervious character of the materials lying between the surface and the water-bearing beds. As a foundation for an earth dam, the geological facts show that the alluvium filling the Pleistocene valley of the Chagres at Gatun will be entirely satisfactory. The rock in which excavation must be made for the locks is firm and hard, and only slightly permeable to water; it will stand in vertical walls without timbering and will support loads many times greater than those to which it will be subjected. Spillway sites 1 and 2 in the Trinidad drainage are at low divides between the Trinidad and streams flowing into the Carribbean. The rocks at these two points are essentially the same as those occurring at Gatun. They are found in a fresh condition a short distance below the surface and have about the same hardness as the Gatun rock. At spillway No. 3 the borings failed to discover fresh rock at the depth required for foundation, the material being a clay of decomposition derived from rocks probably the sains as those at the other two spillways. FOUNDATIONS OF THE PEDRO MIGUEL LOCKS. The Pedro Miguel Locks are built on material that seems slightly less resistant locally than tlie foundation rock of the Gatun Locks, and may show local differences in settling. It consists of the upper member of the Culebra beds described on pages 21 and 22 and the andesitic dikes that cut them. These dikes have two chief influences as follows: (1) They are harder than the rocks that they cut, so, locally, they will constitute areas of minimum settling, and if proper precautions had not been taken they might give rise to cracks where the concrete structures cross them. (2) The dikes are somewhat in the nature of walls that will tend to lessen the seepage through the slightly porous sediments that they cut. They would greatly lessen seepage were it not for the fact that they arc considerably jointed. DESCRIPTION OF LOCK SITES. Concerning these lock sites Howe® says: Although the Culebra beds are variable in texture, those in which excavations for the Pedro Miguel Locks will be made arc remarkably uniform. They consist of well-bedded sandy shales containing plant remains in a few places, and at some » Howe, Ernest, op. cit., p. 128.