Depressurization of a multi-layered artesian system for water and grout control during deep mine-shaft development

Abstract

Uranium in the Grants Mineral Belt of northwestern New Mexico is explored from progressively greater depths. As mining depths increase, more aquifers and higher hydrostatic heads are encountered. Water control during mine shaft sinking is critical to successful shaft development. Field testing procedures were designed to produce data on aquifer coefficients, grout-ability and rock strength appropriate to the short-term construction period involved in shaft sinking. Design coefficients were selected on the basis of field and laboratory test results and regional geohydrology. The uranium ore is located between 915 and 1220 m (3000 and 4000 ft.) below the surface. The geology of the area studied comprises alternating marine and non-marine sandstone, siltstone and shale. The ore is located in the lowermost of six major aquifers identified. A depressurizing system was designed to reduce the hydrostatic pressure and shaft water inflow from each of the aquifers. The groundwater velocity across the shaft was maintained below 0.61 m/day (2 ft./day) and minimized movement of grout away from the shaft.