„SEE Tunnel:Promoting Tunneling in SEE Region“ ITA WTC 2015 Congress and 41st General Assembly May 22-28, 2015, Lacroma Valamar Congress Center, Dubrovnik, Croatia Hydrostatic Pressure Control at Sanford Underground Research Facility Author: Roman POPIELAK, Golder Associates Inc., USA, Roman_Popielak@golder.com Co-author: Walter WEINIG, Montgomery & Associates Inc., USA, wweinig@elmontgomery.com Co-author: Larry STETLER, South Dakota School of Mines and Technology, USA, larry.stetler@sdsmt.edu Topic: Case study, water and other uses of underground space Keywords: groundwater, hydrostatic pressure, deep excavation Summary The former Homestake Gold Mine in Lead, South Dakota in the United States is being transformed into the Sanford Underground Research Facility (Sanford Lab) to host a variety of astrophysics experiments deep underground. While operating, Homestake was the deepest underground mine in North America with workings reaching over 2,439 meters deep. With the transformation to Sanford Lab, new laboratories and experiments are in operation and future facilities are planned at the 4850 Level, about 1,490 meters below ground surface. The laboratories are located at great depth to shield sensitive experiments from interference by cosmic radiation. Excavations of unparalleled size and span for such depths are under consideration at Sanford Lab to accommodate instrumentation and conduct long-term experiments. Geotechnical stability and ground control within strict tolerances are required to protect sensitive instruments and ensure personnel safety. One factor that bears on the stability of laboratory rooms is the hydrostatic pressure contained within the rock fractures. Although the water inflow rate at the 4850 Level is small, analyses of shut-in pressure measurements indicate that the likely maximum hydrostatic pressure will be about 8 MPa, consistent with the presence of mined-out workings about 870 meters above the planned excavations. Engineering measures to control hydrostatic pressure were proposed based on the future purposes of different excavations. Means of pressure control range from seepage interception and drainage layers fully enveloping large cavities to placement of geosynthetic drainage strips and weep holes along the less sensitive excavations. The fractures within the excavation blast damage zone may also help dissipate hydrostatic pressure. The intercepted seepage will be diverted to collection drains and eventually pumped to the surface. 1. Introduction The former Homestake Gold Mine in the Black Hills of South Dakota operated continuously from 1876 to 2001. In that time, approximately 40 million ounces (oz) of gold and 4 million oz of silver were produced. Water pumps were shut off and the mine closed in 2003 allowing water inflow to fill the workings to within 1,380 meters (m) of the surface. After the site was selected for the location of an underground research facility, pumping was resumed in early 2008 and the current water level is maintained roughly at the 1,750 m depth. The former mine is being transformed into the Sanford Lab to host a variety of astrophysics experiments deep underground. The experiments at Sanford Lab will explore some of the most challenging questions facing 21 st century physics, such as the origin of the universe, the nature of dark matter and the properties of neutrinos. The facility also hosts experiments in other disciplines including geology, biology and engineering. A Preliminary Design Report (DUSEL Project, 2011) describes the proposed facility layout at the time the work described in this paper was performed. With the transformation to Sanford Lab, new laboratories and experiments are in operation and future facilities are planned at the 4850 Level, about 1,490 m below ground surface. The