GROUTING OF AN ARTESIAN ZONE IN ADVANCE OF POWERHOUSE EXCAVATION FOR THE RED ROCK HYDROELECTRIC PROJECT-Paper

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The Red Rock Hydroelectric Project required a powerhouse excavation extending up to 40 feet into limestone at the toe of Red Rock Dam. Before excavation, engineers grouted around the perimeter to strengthen fractured rock and limit seepage. Most areas were relatively tight, but along a 160-foot section on the landside, drilling encountered a 2–5-foot-thick zone of partially infilled voids and artesian flows, generally between elevations 645 and 635 feet. Some holes flowed at 20–30 gallons per minute or more.<br /><br />The zone’s complex, connected pathways contained sand, clay, silt, and rock fragments. Initial cement grouting produced exceptionally high grout takes and limited pressure buildup. Treatment was adapted using increasingly thick, low-mobility cement mixes, closely spaced split-spaced holes down to 18 inches apart, and up to fifth-order holes. Sand-cement was tremied into a major void, and hydrophilic polyurethane grout was also used to address persistent flows. A second grout row, and some treatment on a third, provided additional coverage.<br /><br />Excavation later exposed a soil-filled seam beneath a thin limestone roof, confirming the conditions inferred from drilling and grouting. Drain holes were installed to relieve water pressure before excavation through the zone. Although some drains continued to flow, little to no seepage passed through the soil horizon during excavation. The combined grouting and drainage measures therefore controlled artesian water and improved the ground sufficiently for excavation.

Keywords

Red Rock Hydroelectric Project

powerhouse excavation

limestone

perimeter grouting

artesian flows

cement grouting

voids

hydrophilic polyurethane grout

drain holes

seepage control