Overview

Box Butte Dam and Reservoir lies entirely within the Miocene Age Monroe Creek Formation (Tmc). The Monroe Creek is underlain by the Gering, Brule, Chadron and Pierre Formations, which range from Miocene to Upper Cretaceous in age. Windblown silt and fine sand mantle the surface in the reservoir area. The Niobrara River incised a broad channel approximately 1,400 feet wide along the bottom of the valley. The channel was located near the left side of the valley at the time of construction. The left abutment and spillway are founded on weathered siltstone bedrock and alluvial deposits. The dam foundation to the right of station 19+30 is founded on surficial eolian and alluvial materials. The bedrock at the site is described as a tan to light brown, sandy siltstone containing numerous limy concretions, volcanic ash and bone fossils. Lenses of sandstone and conglomerate also occur. The beds are primarily cemented with calcium carbonate varying from weakly to non-cemented. Weathering of the siltstone decomposes to a soil-like sandy silt (ML) material. Bedding is nearly horizontal and usually massive. The Niobrara River Fault was mapped along the length of the river valley at the site of Box Butte Dam. The fault zone forms a graben along which the river traverses. The Nebraska Geological Survey indicated the last movement along the Niobrara River Fault occurred approximately 16 million years ago. Drilling for the CAS in 1988 along the left abutment and spillway encountered soil materials ranging from 1.1 to 15.0 ft. in thickness. The soil appears to be derived from the underlying siltstone of the Monroe Creek Formation. Dry density samples were tested and ranged from 63.48 to 103.45 pcf, averaging 84.95 pcf. The bedrock underlying the eolian deposits was described as uncemented to weakly cemented, soft siltstone. Thin discontinuous lenses of weakly to moderately cemented sandstone and fine conglomerate with light colored volcanic ash and a few scattered limy concretions were also encountered. The cutoff trench encountered siltstone and sandstone at approximate elevation 3942 between the service spillway and station 19+50. The remainder of the embankment material lies on alluvial material composed of silty sand with gravel up to 40 feet in thickness. The eolian and alluvial materials are considered to be liquefiable under seismic loading.

General

Dimensions

Hydraulics & Hydrology