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

16 ENGINEERING PROBLEMS OF P/ÏNAMA CANAL. must be so slow that a little dredging will easily take caro of it. Then, again, the rise may stop or sinking may begin at any time. Tidal currents tend to fill in the channels at many places in the bays, but dredging easily keeps them clear, especially as the silt brought in is very fine. GENERAL GEOLOGY OF THE CANAL ZONE IN ITS ENGI- NEERING RELATIONS. GENERAL ENGINEERING RELATIONS. Within the Canal Zone there are many different kinds of rock. Many of the rock units, however, occupy relatively small areas, so that any considerable excavation may pass through several of them. Most of the Canal Zone types can be included in 11 différent groups of bedded and noncrystalline formations and 6 different groups of igneous or crystalline rocks. Each rock unit, however, may vary considerably from place to place, especially in such physical char- acteristics as shearing, jointing, texture, and hardness. They all differ somewhat in age, in character of material, and in origin. Figure 1 shows a general geologic section across the Isthmus, showing the different formations between tho Atlantic and the Pacific shores. Figure 2 shows tho rock succession in the Canal Zone. All of the rock descriptions following have a bearing on the engi- neering work, especially on tho excavation methods used in Culebra Cut. Each description is followed by a discussion of tho practical considerations that particularly apply to that formation. However, the general phases of practical interest are summed up first. The cost of breaking any rock for excavation depends on throe chief factors, which are: (1) The hardness of the rock and its resist- ance to drilling; (2) tho minimum size to which it must be broken 'to give greatest efficiency of results in handling; and (3) the tenacity or resistance to breakage of largo masses of the material. If the rock is hard tho cost of drilling is increased not only by tho longer time required but also by the larger cost of sharpening drills, greater drill breakage, more frequent delay from drill bits sticking and from broken drill points obstructing and sometimes necessitating tho abandonment of tho holes, and easier deflection of tho drill hole if it crosses at an acute angle an open joint or fracture zone containing soft or crushed rock. Tho degree of smallness to which tho rock must be broken depends chiefly on tho size that gives greatest economy in handling consistent with tho capacity of tho excavating machinery and tho use, if any, to which tho material is to bo put. A steam shovel with a dipper of 4 or 5 cubic yards capacity can pick up, balance, and dump on a railway car a bowlder weighing several tons. Tho most economical