ABSTRACT:

Union Park Dam, a modern roller-compacted concrete (RCC) gravity dam, with its maximum height of 575 feet and crest length of 2,050 feet, will be the key feature of a major Colorado water supply and hydroelectric power project, the Central Colorado Project (CCP) also known as the Union Park-Aspinall Pool Project, proposed by Natural Energy Resources Company (NECO) in 1982. The dam site is located at an altitude of 10,000 feet on Lottis Creek in Union Canyon approximately 35 miles northeast of Gunnison, Colorado in the upper Taylor River drainage. When completed, Union Park Dam and Reservoir will store up to 1.2 million acre-feet of Colorado?s currently undeveloped Aspinall Pool and Colorado River Compact entitlements. This water can then be efficiently delivered by gravity tunnels and pipelines to urban and rural areas east and west of the Continental Divide. The project will be operated as a pumped-storage facility generating high value peaking power using the Bureau of Reclamation?s existing Taylor Park and Blue Mesa Reservoirs as lower pools. This paper presents the results of the preliminary geological and geotechnical investigations and engineering analyses thus far completed for Union Park Dam, to confirm the technical feasibility of the project.

The main objective of these investigations was to determine the location, size and type of dam required to safely and economically store up to 1.2 million acre-feet of water in Union Park Reservoir and develop recommendations for the more detailed field investigations and engineering analyses to follow. The reservoir water surface elevation of 10,120 feet and topography of Union Canyon were governing factors in locating the axis for the required 575-ft.-high dam. The excellent quality and strength of the rock in the foundation and abundant availability of construction material in close proximity to the site led to the selection of an RCC gravity dam. The final configuration of the dam was determined from finite element stress analyses performed on a number of two-dimensional models of the dam and foundation subjected to the governing seismic or extreme loading. To avoid cracking, the resulting tensile stresses in the dam body needed to be below the tensile strength of the concrete. The sliding stability of the dam was evaluated for static, hydrologic and seismic loading making use of strength and deformation properties of the rock mass determined from geological field mapping (discontinuity surveys), rock testing, and geophysical measurements (seismic refraction surveys). A simplified fault model was used to evaluate ground motion parameters (horizontal and vertical components of peak ground acceleration), acceleration response spectra, and anticipated seismic loads from a maximum credible earthquake (MCE).

A preliminary hydrologic evaluation of the drainage basin of Union Park Reservoir was completed to determine the probable maximum flood (PMF) and hydrologic loading of the dam. An opinion of the probable construction/project costs was prepared for the main 575-ft.-high RCC gravity dam and three smaller RCC (saddle) dams to be constructed at the north end of the reservoir. A core drilling/rock testing program and cost estimate was developed for implementation during the next phase of the feasibility study.

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