Trenching as an exploratory method
The critical nature of siting nuclear power plants has led to increased emphasis on exploratory trenching. Trenching is the most definitive of all subsurface exploratory methods; it permits inspection of a continuous geologic section by both geologists and regulatory authorities and makes possible the preparation of a graphic log that delineates both obvious and subtle geologic features. About one of every two nuclear plant licensing efforts utilizes exploratory trenching. Many geologic hazards, such as "capable" faults, can be detected from trench exposures; they may otherwise remain undetected.
Trenches must be judiciously located, survey-controlled, excavated safely and adequately shored, logged in detail, and properly diagnosed. Useful techniques of trench logging include thorough cleaning of the trench walls, teamwork between geologist and recorder, logging against a carefully surveyed baseline and vertical reference grid, and panoramic photography. Soils, including paleosols, and glacial and glaciofluvial deposits present some of the most difficult media to log.
Trench logs must be thoroughly interpreted and correlated so that they document the geologic conditions governing suitability of the site. Age-determination techniques utilized in exploratory trenching include petrographic analyses, quartz inclusion studies, clay mineralogic analyses, and radiometric methods.
Figures & Tables
During the “great decade” of siting and construction of nuclear power plants that ended in 1975, the nuclear industry mustered the largest geologic task force in this country’s history, resulting in rapid advances in geologic technologies. Many of the advances are discussed in this volume, a major contribution to engineering geology. Subjects treated are the regulatory, siting, and licensing processes; seismicity of the central and western U.S., with a consumer’s guide to instrumental methods for determination of hypocenters; and techniques, such as remote-sensing, microfacies analysis, dating techniques in faults, trenching as an exploratory method, borehole geophysics, and ground-water studies. Includes a useful glossary.