Reviews in Engineering Geology
This is the second in a series of volumes prepared by the Geological Society of America Division on Engineering Geology designed to summarize the states of knowledge on various aspects of the application of geology to engineering problems. Through an unfortunate series of delays, publication of the book was delayed several years beyond the completion of the contained papers. The geologic principles are still sound, however, and are certain to be useful to the practicing engineering geologist. Nine papers review the following subjects: foundations for heavy structures; geology and pedology in highway soil engineering; clay as a canal sealant; Portland cement and concrete; pozzolan; geocryology and engineering; land subsidence due to withdrawal of fluids; land subsidence due to the application of water; and geologic settings of subsidence.
This paper reviews the role of geologic processes that contribute to subsidence in order to aid those starting investigations of ground-surface subsidence. Subsidence occurs, or at least is discovered, only infrequently, and little organized information has been available. In order to assess our present state of knowledge, the author gathered fragmentary bits of information from many sources widely scattered in the literature of geology and other earth sciences.
The author cites examples for each geologic process that is a potential contributor to ground-surface subsidence together with geologic evidence for diagnosing their causes. The geologic processes involved are: (1) solution of gypsum and salt and redistribution of transient fill materials through solution cavities in calcareous rocks; (2) underground erosion of uncemented or lightly cemented silt and sand through temporary underground passageways; (3) lateral plastic flow of salt, gypsum and anhydrite, shale, and clay under loading; (4) compaction of sediments by loading, drainage, vibration, and hydrocompaction; (5) tectonic movements including primary and secondary effects of earthquakes, folding, and warping; and (6) volcanic activity. Because the first four processes may be accelerated by various engineering activities, examples have been selected to illustrate subsidence both under natural conditions and under conditions modified by man's activities.
Although this extensive search for existing information on the role of geologic factors in subsidence indicates that much detailed work remains to be done, the future prospects for advancing our geologic knowledge are excellent. Methods of measuring ground-surface displacements are improving rapidly. Also, broadly based investigations of known areas of major subsidence throughout the world are developing new methods of diagnosis and treatment and are yielding quantitative data that will aid our evaluation of the rates and magnitude of present-day geologic processes.