1 INTRODUCTION
The Columbia River basalts underlying the Hanford Site are among the rock types being considered by the U.S. Department of Energy as a site for the first commercial high-level nuclear waste repository. The Basalt Waste Isolation Project (BWIP) has been carrying out a comprehensive investigation to assess the feasibility of constructing a repository in basalt and to identify the technology required for engineering design and site characterization. One component of this project is the development of a geomechanical testing program to support repository design, site characterization, and performance assessment requirements.
Geomechanics information necessary to support these activities includes characteristics of the in situ stress field and the mechanical properties of the rock mass. The nature of the basalts at the proposed repository depth (approximately 970 m) imposes some relatively unusual requirements on testing methods typically used to assess the stress state and rock mass deformability. Expected conditions at the testing sites include a maximum in situ stress on the order of 60 MPa (Kim et al. 1986), rock mass deformation moduli in the range of 35 to 60 GPa, and an average unconfined compressive strength of intact rock of approximately 300 MPa (Archer 1986). Additionally, the rock mass is closely jointed, with the spacing estimated from vertical boreholes to be in the range of 5 to 10 cm for both vertical and horizontal joint sets. Rock mass classification Q values are expected to range from approximately 0.5 to 1.9 (Archer 1986). The ambient temperature at depth is about 50° C. The rock mass is considered fully saturated.
Hydraulic fracturing
Overcoring
Flat jack testing
Borehole dilatometer testing.
The combination of these conditions has significant implications for the development of a geomechanical testing program and results in the need for modifications to typically available methods and equipment. The purpose of this presentation is to summarize the development effort currently under way to support site characterization testing to be conducted at the candidate repository horizon in the Exploratory Shaft Facility (ESF). Specifically, development activities will be discussed in four areas: Figure 1. Schematic of hydraulic fracturing with borehole deformation gage packer assembly. (available in full paper)
2 DEVELOPMENT ACTIVITIES DESCRIPTIONS
The following sections briefly describe the testing methods listed above and the equipment involved, and provide a summary of the testing and/or development program that has evolved to meet the special needs of the BWIP. Plans for future testing or development also are discussed.
2.1 Hydraulic fracturing stress measurements
The first hydraulic fracturing tests at the Hanford Site were conducted in connection with construction of the BWIP Near-Surface Test Facility (NSTF) in 1978. Subsequently, tests were conducted in deeper boreholes as part of the repository location siting studies {Kim et al. 1986). This latter series of tests established that hydraulic fracturing tests could be successfully conducted in deep boreholes in fractured basalt and has led to the development of in-house capabilities to conduct such tests.
