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

SLIDES. 69 vature for the excavation. In all excavations, allowance must be made for the fact that the soft decayed rock and soil material near the top will tend to erode back from the excavation until the sur- face approaches logarithmic curvature. The maximum angles at which, certain classes of rock will stand, in open excavation at different depths, are given in the table which follows. The classes of rock mentioned in the table are designated in the paragraphs A to G. A. Certain fairly soft and weak sandstones, shales, and a few limestones; also a few soft tuffs, agglomerates, and clay rocks— rocks that will deform under great pressure but with relative slow- ness. Under this heading como most of the rocks that cause swelling ground in coal mines and other excavations, but which are stronger than the clay rocks and tuffs of Culebra Cut. B. The same rocks as included in class A, but with medium shear- ing and jointing. C. The same rocks as included in classes in A and B, but with jointing, fissuring, and shearing increased to the maximum for such rocks. Also those in which the bedding may dip toward the excavation. D. Soft volcanic-clay rocks, bedded friable tuffs, lignitic shales; rocks similar to those in the Culebra and Cucuraclia formations in Culebra Cut. These are the rocks in which most of the big slides have occurred. Such, rocks have a high water content and contain considerable cliloritic material. They are ordinarily very fine to medium grained and may contain thin beds of partly cemented gravel, ami some lenses of soft sandstone and brittle limestone. These recks are considered as having a minimum of jointing, fissuring, and bedding. E. The same rocks as included in class D, but with jointing, fissuring, and bedding increased to the average for such rocks. About equivalent to the conditions in Culebra Cut, except in a few localities where the Culebra Cut rocks have been excessively sheared by faulting. F. The same rocks as included in. classes D and E, but with, joint- ing, fissuring, and bedding increased to the maximum. Under this headins would come the local areas in Culebra Cut that are crushed by faulting. G. Any extremely soft rocks that are much crushed and rendered soft and slippery by ground water, talcose clays, etc. Most of the ground that is in motion toward Culebra Cut. Most sliding material already moving.