The following paper summarizes the preliminary results of the numerical modeling effort performed in support of the Deep Underground Science and Engineering Laboratory (DUSEL). The investigation was performed to aid in the decisionmaking process for selection of the final size, shape, depth, and orientation of future DUSEL underground excavations at the Homestake Mine. Specific conclusions of the study reveal host rock may be moderately orthotropic in terms of: (1) in situ stress state, (2) deformational behavior, and (3) strength. Of the three cavity shapes considered (i.e., domed right-circular cylindrical cavity, triaxial ellipsoid, and horseshoe-shaped horizontal prism), the most stable configuration is the domed right-circular cylindrical cavity and the least stable is the horseshoe-shaped horizontal prism. A minimum cavern separation distance of two cavity diameters should be sufficient to limit structural interaction of multiple DUSEL cavities.
The National Science Foundation's (NSF) Deep Underground Science and Engineering Laboratory (DUSEL) is being sited at the former Homestake Mine in Lead, South Dakota (Figure 1). Current site characterization is being focused on the 4850 Level within the Yates Unit (lowest portion of the Poorman Formation). The proposed DUSEL cavities are large openings (up to 50 meters (165 feet) in diameter) with separation distances approaching 300 meters. Nine recent boreholes have been drilled and over 1.6 kilometers of core have been extracted. Laboratory tests have been performed to determine elastic and strength properties [1]. Data collected at the Homestake Mine (e.g., [2, 3]) suggest that the material strength may be strongly anisotropic (see Table 1). The strength parameters (unconfined compressive strength (Ci), tensile strength (Ti), and shear strength (Ri), where i = 1,3) indicate a wide variation but do not indicate a persistent trend for the Homestake, Ellison, and Poorman Formations; i.e., compressive strength ranges from 11 to 111 MPa, tensile strength ranges from 3 to 16 MPa, and shear strength ranges from 6 to 13 GPa. Therefore, as a conservative estimate, the strength of the Yates Unit was assumed to be orthotropic. Because orthotropic strength data are not currently available for the Yates Unit, the parameters given in Table 1 for the Homestake Formation were used for evaluating the orthotropic strength parameters in this study.
Candidate shapes that were evaluated in this study include: (1) domed right-circular cylindrical cavity, (2) triaxial ellipsoid, and (3) horseshoe-shaped horizontal prism. The primary goal of this analysis is the assessment of the three-dimensional structural stability of the DUSEL cavity based on the variation in the (1) cavity shape, (2) orthotropic strength, (3) excavation strategy, and (4) cavity spacing. Stress ratios were calculated for the host rock surrounding the cavity against potential rock damage using the threedimensional orthotropic strength model and associated strength properties given above. FLAC3D was used in the numerical simulations. FLAC3D is a three-dimensional explicit finite difference code and is designed to simulate the behavior of structures built of soil, rock, or other materials that may undergo plastic deformation when their yield limit is reached.
