Engineering Geology of Dam Site and Spillway Areas for the Monroe Reservoir, Southern Indiana

Abstract

The Salt Creek drainage basin in Monroe, Brown, and Jackson Counties, Indiana was studied to provide basic information for the planning of the Monroe Reservoir. This report principally concerns the dam site and spillway areas of the proposed reservoir. Four units, defined on the basis of engineering characteristics, are present in the reservoir area. Impermeable siltstones of the Borden Group form the valley walls along Salt Creek and will confine the reservoir. Rocks of a second unit, the Harrodsburg and Salem Limestones, cap the ridges high above flood-pool level (556 feet). The two remaining units consist of unconsolidated materials in the valley of Salt Creek. Terrace deposits above the flood plain are sandy to gravelly silts. Valley-fill deposits as much as 70 feet feet thick are presentin the valley of Salt Creek. Clayey silts predominate in the valley fill and forma satisfactory foundation material of low permeability for the dam. Two spillway locations have been considered for the Monroe Reservoir. The first, a short distance from the dam site, would require a channel through about 90 feet of bedrock, the upper 50 feet of which would be the Harrodsburg Limestone. This site would be near the dam, and rock excavated from the spillway channel could be used as fill material in the dam. A second potential spillway site lies about 1 mile south of the dam. Here the spillway channel would cut through 40 feet of material, 20 to 30 feet of which are unconsolidated sediments and the remainder is siltstone belonging to the Borden Group. Field and laboratory investigations for the Salt Creek drainage basin in Monroe, Brown, and Jackson Counties, Indiana, were made to provide basic geologic information for the planning of the Monroe Reservoir. In addition, the engineering characteristics of geologic materials in the Salt Creek drainage basin at the proposed dam site and upstream from it have been interpreted from the geologic data. This report summarizes the more significant geologic factors that will influence the design, construction, and maintenance of the proposed Monroe Reservoir.