Geotechnical Considerations for the Design of an Exploratory Shaft Facility for a Nuclear Waste Repository in Deaf Smith County, Texas

Abstract

Area-wide geologic studies have been conducted in six states to identify sites for detailed site characterization studies, and ultimately the construction of the nation9s first high level nuclear waste disposal facility. Host rock types in the respective states under consideration include basalts in Washington state, tuffs in Nevada, bedded evaporites (salts) in Utah and Texas, and diapiric salt in Mississippi and Louisiana. An exploratory shaft facility (ESF) is an integral part of the detailed site characterization activities mandated by the Nuclear Waste Policy Act of 1982 (U.S. Congress, 1983). One of the primary objectives of the exploratory shaft is to provide access to the target repository horizon so that detailed in situ testing can be performed to evaluate site suitability for repository construction, materials characterization, model validation, etc. In addition, the ESF will provide a means of evaluating construction conditions and applicable methods for the potential repository, and for characterizing the geologic and hydrologic conditions of the materials overlying the target horizon. Seven salt sites in four states have been considered for ESF preliminary designs (Parsons Brinckerhoff/PB-KBB, 1983). While salt has several advantages for waste isolation, such as its impermeability, thermal and visco-plastic properties, etc., it also presents some unusual design and construction problems. The solubility and visco-plastic nature of salt requires special lining designs and construction to isolate and control water inflow and to avoid excessive stress build-up due to the long term deformations (creep) of salt. Other major geotechnical factors involved in the design of the ESF, and of special interest to Texas are the protection of fresh water aquifers (particularly the Ogallala and Dockum formations in the Texas Panhandle), ground stabilization and ground-water control during shaft sinking (anticipated to require ground freezing methods through the upper 1,000 ft of overburden at the Texas sites), seismic stresses, in situ stresses, ground-water conditions in non-fresh water aquifers, etc. Exploratory borings will be drilled for each shaft at a salt site during ESF Final Design activities. The boring program will include comprehensive borehole and laboratory testing for the purpose of verifying or modifying design assumptions, as required.