SLIDES
Slides of earth and rock materials, both of the creeping and sudden types, have often been regarded as acts of Providence,—but studies of the geologic factors have in many cases disclosed preventable causes. A considerable geologic literature has sprung up with reference to rock slides, which is of practical use in excavation work of many kinds.
The cause of such movements is gravity. The softer, unconsolidated rock materials yield of course more readily than the harder ones, but even strong rocks are often unable to withstand the pull of gravity. The relative weakness of rock masses on a large scale was graphically shown by Chamberlin and Salisbury,[66] in a calculation indicating that a mass of average hard rock a mile thick, domed to the curvature of the earth, can support a layer of only about ten feet of its own material. The structural geologist, through his study of folds, faults, and rock flowage, comes to regard rocks essentially as failing structures.
Disturbances of equilibrium, resulting in rock movements under gravity, may be caused by local loading, either natural or artificial. Natural loading may be due to unusual rainfall, or raising of water level, or increased barometric pressure. Artificial loading may come from construction of heavy buildings or dams. Movement may also result from excavation, which takes away lateral support—and such excavation again may be caused by natural processes of erosion or by artificial processes involved in construction. Movement may be caused by mere change in the moisture content of rocks, or by alterations of their mineral and chemical character, affecting their resistance to gravity. In still other cases, earthquakes are the initiating cause of movement.
In unconsolidated rocks, a frequent cause of movement is the presence of wet and slippery clay layers. The identification and draining of these clay layers may eliminate this cause. In certain sands, on the other hand, water may actually act as a cement and tend to increase the strength of the rock. Planes of weakness in the rock, such as bedding, joints, and cleavage, are also likely to localize movement.
Earth materials, and even fairly hard rocks, may creep under gravity at an astonishingly low angle. The angle from the horizontal at which loose material will stand on a horizontal base without sliding is called the angle of rest or repose. It is often between 30° and 35°, but there is wide variation from this figure, depending on the shapes and sizes of the particles and on other conditions. It has been suggested that even the slight differences in elevation of continents and sea bottoms may, during long geologic eras, have caused a creep of continental masses in a seaward direction.
In problems relating to slides, the geologist is concerned in determining the kinds of rocks, their space relations, their structures and textures, their metamorphic changes, their water content and the nature of the water movement, their strength, both under tension and compression, and other factors.
In the digging of the Panama Canal, a geological staff was employed in the study of the rock and earth formations to be met. However, had more attention been paid to geologic questions in the planning stages, this great undertaking, so thoroughly worked out from a purely engineering standpoint, would have avoided certain mistakes due to lack of understanding of the geological conditions. It is a curious fact that in these early stages no strength tests of rocks were made, and that no thorough detailed study was made of the geologic factors affecting slides and their prevention. It was only after the slides had become serious that the geological aspects of the subject were intensively considered. The results of the geologic study, therefore, are useful only for preventive measures for the future and for other undertakings. One of the interesting features of this investigation was the discovery that certain soft rock formations were rendered weaker rather than stronger by the draining off of the water. It had been more or less assumed that the water had acted as a lubricant rather than as a cement.