ENGINEERING GEOLOGY OF THE HIGH SAVERY DAM SITE, CARBON COUNTY, WYOMING

The new High Savery Dam in south-central Wyoming is designed as an earthfill structure with a height of approximately 140 feet with a reservoir of a total capacity of 22,433 acre-feet. The engineering geology investigation identified several key seepage and stability issues. The geology of the site includes alluvial, fan, colluvial, terrace, and landslide deposits, Tertiary Browns Park Formation, and Cretaceous bedrock units (mudstone, sandstone, and low permeability clay layers). The nearly flat-lying Browns Park Formation was separated into three units: 1) upper fine-grained sediment unit, 2) basal conglomerate unit, and 3) fine-grained unit. The Cretaceous bedrock units are dipping approximately 12 degrees in the downstream direction. Important geologic features identified during the investigation include open fissures, high permeability fracture zones, weak bentonitic clay beds, and landslides.

Localized open fissures were encountered in the Browns Park Formation in the upper portion of both abutments. Two high permeability fracture zones were identified in the sandstone in the valley foundation and lower abutments. These fracture zones represent seepage pathways in the dam foundation. Low permeability bentonitic clay beds dip in the downstream direction and provide for potential slope/foundation failures and were a key issue in design of the embankment, primary spillway, and temporary cut slopes. The lower high-permeability fracture zone in the valley is also confined by one of the low permeability clay beds. This fracture zone would transmit high pore pressures to this weak, bentonitic clay bed. Landslide deposits were identified underlying the proposed inclined intake structure, and immediately adjacent to the primary spillway chute and stilling basin. The geotechnical design team used the results of the engineering geologic investigation to develop appropriate design elements to address each of these seepage and stability issues.