Application of Soil-Stratigraphic Techniques to Engineering Geology

Abstract

Soil-stratigraphic techniques are being increasingly applied to engineering-geological investigations for siting liquified natural gas (LNG) facilities, nuclear reactors, dams, and other critical structures. Soil (pedological) profiles in Quaternary sections are useful to ascertain the approximate age of site-area sediments, to reconstruct local geomorphic history, to date the last movement of faults, and in some cases to determine recurrence intervals of displacements associated with faults or large mass-movements. Soil stratigraphy includes the field of paleopedology, and generally employs the concepts and terminology of the soil scientist. Soil units particularly applicable to engineering geology are: the organic horizon of the modern solum and buried paleosol, frequently datable by radiocarbon assay; the albic horizon, sometimes indicative of local climatic and vegetative change; the argillic horizon, typical of strongly-developed profiles and useful as a regional stratigraphic marker; and the calcic horizon of arid and semi-arid regions, likewise often an indicator of climatic change. Exemplified in geo-technical investigations for a proposed LNG terminal near Point Conception (Little Cojo Bay), California, soil stratigraphy was employed to date last displacement of site-area faults, to estimate age of marine platforms, and to help reconstruct regional geomorphic history. Similarly, at the General Electric Test Reactor (GETR) site near Livermore (Vallecitos), California, soil stratigraphy was instrumental to date last displacement and recurrence of site-area slip surfaces engendered either by tectonic or by mass-wasting processes. Datable markers included four, strongly-developed buried paleosols, each of which marked epochs of regional landscape stability during the Quaternary. Soil-stratigraphic techniques are particularly effective when combined with geomorphic and other Quaternary studies. Many soils are also datable, either directly by radiometric assay, or indirectly by association with regional changes in climate, sea level, and landscape stability. These soils may thus well serve as important stratigraphic markers applicable to engineering-geological investigations in a wide variety of climatic and geomorphic settings.