Interim geological investigations in the U12e.04 tunnel, Nevada Test Site, Nye County, Nevada

Abstract

The Ul2e.04 tunnel is a part of the Ul2e tunnel system, which has been driven southwestward beneath Rainier Mesa in the northern part of the Nevada Test Site. The Ul2e.04 tunnel was driven about S. 15? W. in zeolitic tuff of subunits E and F of Tunnel Bed W near the top of the lower member of the Indian Trail Formation (upper Miocene or lower Pliocene). Dolomite of Paleozoic age lies about 915 feet below the end of the tunnel; vertical cover over the end of the tunnel to the surface of Rainier Mesa is about 1,390 feet. The tuffs in the tunnel strike almost north-south and dip to the west. There are several normal northwest-trending faults with vertical to steep dips and small displacements. The predominant joint set strikes northwest and dips mostly vertically or steeply to the northeast. Petrographically and chemically the tuffs in the U12e.04 tunnel are similar to other tuffs of the Indian Trail Formation from the Nevada Test Site, The tuffs in the tunnel have an average porosity of 38.6 percent, dry bulk density of 1.46 g/cc (grams per cubic centimeter), grain density of 2.38 g/cc, water content of 20.7 percent by weight. Shore hardness of 25.1, and unconfined compressive strength of 4,400 psi (pounds per square inch). Separately, the tuffs from the chamber at the end of the tunnel have an average porosity of 36.8 percent, dry bulk density of 1.51 g/cc, grain density of 2.38 g/cc, water content of 20.0 percent by weight, Shore hardness of 27.2 and unconfined compressive strength of 4,500 psi. An NX-size cored hole was drilled from the surface of Rainier Mesa into the Ul2e.04 chamber.