Groundwater flow patterns in the vicinity of underground openings in unsaturated rock

Abstract

A two-dimensional modeling scheme is utilized herein to analyze the effects of mined out underground openings on groundwater flow patterns in unsaturated rocks. A series of modeling results is presented using the finite element computer program UNSAT2 to simulate steady groundwater flow that initially is vertical and downward. Model output is analyzed for: (1) variations in the direction of groundwater flow caused by the mined out openings; (2) the distribution of pressure head as produced by the openings; (3) variables that affect the rate of predicted groundwater inflow into the openings. Modeling results indicate that: (1) mined out openings will alter the pattern of groundwater flow in the unsaturated zone; (2) the hydraulic effects are dependent on the hydrogeology of the rocks and on the geometry of the mined out openings; (3) inflow of groundwater into mined out openings is possible in the otherwise unsaturated zone, including mines developed in homogeneous rocks; (4) groundwater inflow into the mined out openings can occur at values of downward flux that are smaller in magnitude than the saturated hydraulic conductivity of the surrounding rock; (5) the temporal and spatial distribution of flux through the Topopah Spring Member of the Paintbrush Tuff is the most critical factor regarding the occurrence of mine water inflow in the otherwise unsaturated tuff at Yucca Mountain, ND, U.S.A.