Geologic history of the National Geotechnical Experimentation Sites, Texas A&M University Riverside Campus, Brazos County, Texas

Abstract

The National Geotechnical Experimentation Sites (NGES) TXAMSAND and TXAMCLAY research sites, located on Texas A&M Riverside Campus in Brazos County, Texas are part of a joint venture sponsored by the Federal Highway Administration and the National Science Foundation. The NGES sites were created to accelerate research in geotechnical and geo-environmental engineering by allowing multi-user access to well characterized sand and clay areas. The geologic history in this report will contribute to the background characterization of the Texas A&M University NGES sites. Three geological borings were drilled at each site and used in conjunction with existing geotechnical information to create a three dimensional view of the sediments at each site. Both the Sand Site and the Clay Site have similar geologic histories. The upper units at the sites are Pleistocene aged terrace deposits of the Brazos River, deposited as alluvium when river flow was at a higher level than the modem river valley. The terrace deposits lie unconformably on nearshore marine deposits of the Eocene aged Spiller Member of the Crockett Formation. The Pleistocene units at the Clay Site consist of a mottled red and grey colored Clay Unit. Sediments of this unit are overbank deposits which underwent a high degree of soil development. Below this is the Sand Unit, a thin quartz-rich channel sand that grades from fine sand to gravel. The third and fourth units at the Clay Site are part of the Spiller Member. The Upper Spiller Geotechnical Unit consists of dark grey clay shales which are defined geotechnically as a clay soil. The lowest unit, the Spiller Member, is an olive green, laminated shale with interbedded fine-grained sands. There are three Pleistocene units at the Sand Site: the Silty Sand consisting of mottled, red and tan colored, overbank deposits with varying degrees of silt/clay content; the Clean Sand Unit, a crossbedded fine grained sand containing little to no clay; the Mix Unit, deposited by a fluctuating energy stream which consists of a mix of clay, sand and clayey gravel layers. The deepest unit is the laminated shale of the Eocene aged Spiller Member of the Crockett Formation.