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

52 ENGINEERING PROBLEMS OF PANAMA CANAL. deformation destroys the stability of their clayey cementing material, loosens them for the invasion of quantities of ground water, which their fineness of grain had excluded previous to deformation, and induces muddiness and mobility, so that the slope, originally almost perpendicular, may break back to 1 on 10. Deformations closely approaching this have occurred in Culebra Cut in some of the volcanic clay rocks. Excavations that change the level of the water table may weaken the surrounding rocks by dissolving and loosening tlicir more soluble parts, especially in a region of abundant vegetation, where the ground water contains much carbon dioxide and organic acids. On the Canal Zone the limestones contain many caves, small caverns, en- larged joints, and pitted surfaces, evidences of comparatively rapid solution. The volcanic clay rocks, particularly those of the Cucu- racha formation, crumbled rapidly when they were dug from Culebra Cut and exposed to weathering action. Bowlders of this soft rock 2 or 3 feet in diameter have crumbled into clay in less than a year. The crumbling scorns to bo due to two causes: First, the outer layers of those clay rocks may absorb rain water and heavy dews, and subsequently be dried out by the hot sun. It is known that after having boon onco dried out most clay rocks have their absorbing capacity increased much beyond what it was before drying took place. By this process of absorption and drying out, soluble salts are leached out and are sometimes deposited on the outside of tho rock as barely noticeable white coatings or as scattered, minute, translucent crystals. These seem to be largely magnesium, iron, and calcium sulphates. This wetting, drying, and leaching loosens up the small individual grains and they drop off, thus removing layer after layer until tho whole rock is disintegrated. Second, oxidation, which gives an increase of volume and increases the tendency to dis- integrate, may occur. These rocks have a considerable percentage of ferrous iron (up to 3.7 per cent), which is somewhat oxidized to the ferric condition by contact with the atmosphere and with oxygen- bearing rain water. EFFECT OF GROUND WATER. Ground water in clay or in weak clayey rocks is an extremely im- portant slide-producing factor. Tho chief ways in which it aids in tho deformation of rock masses aro as follows: (1) By greatly in- creasing the mobility and slippery character of the rock material; (2) by adding weight to a rock mass, which may already bo strained toward an excavation; and (3) by weakening a rock through solution and softening. If a relatively porous material rests on top of rela- tively impervious rock, especially if tho contact between tho two slopes toward an excavation, ground water or rain will add weight