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

66 ENGINEERING PROBLEMS OF PANAMA CANAL. duced, and shortly after largo landslides began to occur along the railway line. Investigation showed that under the influence of mois- ture the hard clay became soft and lost its cohesion. A test was made in which some of the dry clay was placed upon a plate and water dropped upon it. It absorbed 50 per cent of its own weight of water without any change, but when 60 per cent of water had been absorbed it became almost fluid and completely collapsed. Sandy clays weighing 113 pounds per cubic foot when, dry are said to have been found by some English experimenters to readily absorb water until they became a sludge with an angle of repose of 16° and less. Argillaceous silt absorbed 53.5 per cent of its volume without altering in form, but after absorbing 78.5 it disintegrated and became a slurry. Certain blue clays deformed at slopes of 1 on 5 when saturated with water, and other more plastic clays showed flowage or deformation at slopes as flat as 1 on 7. SLOPES TO MINIMIZE SLOUGHING AND DEFORMATION. The character of material cut by excavations varies over a wide range, from granite to mud. Theoretically, then, the slopes should vary from perpendicular to almost horizontal. Because -of varying conditions it is nearly impossible to set down the exact figures to which slopes should bo made to conform for security and minimum excavation. The figures given herein are based largely on observa- tion of railway cuttings and mountain and canyon slopes in Central America and in the United States. The figures are not by any moans final, and any discussion that they may cause will be welcomed as the first stop in the best way to gather more information on the subject. It is hoped that the engineer and the geologist may thus bo aided in estimating the distance from any excavation to which deformations are likely to extend, and especially that more data for estimating the cost and yardage of excavations may bo available. SLOPES WHERE ROCKS MAY SLOUGH BUT WHERE THEY WILL NOT DEFORM BY FLOWAGELIKE MOVEMENT. Different type conditions under which rocks will slough but will not deform by flowageliko motion are postulated below: 1. Given solid rock of relatively high crushing and tensile strength, with a minimum of jointing, fissuring, and bedding. Such rock would include granitic and trap rocks, quartzites, solid sandstone and shale rocks, and, in fact, most of the hard, relatively tough rocks that have little jointing, fissuring, and bedding. An excavation in such mate- rial should have a slope of about 10 on 1, or 10 units up and 1 unit