GEOTECHNICAL CHARACTERIZATION OF EXCAVATIONS FOR THE DUSEL CONSTRUCTION

The Deep Underground Science and Engineering Laboratory (DUSEL) being planned at the site of the former Homestake Gold Mine in Lead, SD, will host physics experiments which require shielding from cosmogenic radiation. Therefore, current plans include the construction of rooms for these experiments at depths of 1.48 km (4850 ft) and 2.25 km (7400 ft) below the surface. The standard laboratory modules will consist of excavations between 50 and 100 m in length and 20 m in width. The long baseline experiment, however, will involve sending a beam of neutrinos through a chord of the Earth from northern Illinois to intercept a large detector at the 4850 level of the laboratory in Lead, SD. If it is determined that this will be a water detector, it will require one or more large cavities to be excavated that may be in the form of a right cylinder with a diameter of 55m and a height of 60m. These detectors would constitute the largest previous excavations at this depth by far. Construction of the cavities required the development of a geotechnical characterization program including preliminary drilling and coring, site mapping, in-situ stress measurements, and laboratory characterization of rock mechanics properties. The goals of the geotechnical work include delineation of prospective sites for the cavities. The Yates Member of the Poorman Formation, a metabasalt which is the preferred rock unit, has good rock qualities for the construction although the site may be complicated by the presence of Tertiary-age rhyolites. The rock quality at the currently identified site is excellent and the plans for the excavations are proceeding on schedule.