Kristi Ainslie, AECOM:
Determining Treatment Requirements for High-Permeability Faults in a Dam Raise Foundation

Kyle Kittle, Stantec:
Chimney Hollow Dam Foundation Grouting Program, Larimer County, Colorado

Abstracts:

Kristi Ainslie, AECOM Technical Services, Doug Yadon, AECOM; Felipe Garcia, Stantec; Paulo Virreira, Denver Water
Determining Treatment Requirements for High-Permeability Faults in a Dam Raise Foundation
Denver Water is expanding Gross Reservoir by raising the curved gravity dam by 131 feet. The raise will result in Gross Dam becoming the highest RCC dam raise in the world. During original dam construction in the 1950s, geologic mapping of the Precambrian granitic foundation (led by engineering geologist Ernie Wahlstrom) revealed a series of what became known as the “alphabet faults”. Among these faults the parallel “C” and “D” couplet form a 10- to 15-foot-wide zone of highly weathered and sheared rock with increased permeability in the mid right abutment. The faults were treated in the original foundation with additional excavation up to 25 feet. Higher grout takes and tighter hole spacing through the fault zone combined to achieve closure in the original grout curtain. Foundation investigations for the Gross Reservoir Expansion project (GRE) identified the “C”-“D” fault couplet as one of the high-priority targets in the right abutment. In addition to reconnaissance geologic mapping and an inclined 312-foot-long HQ3 core hole, existing foundation drains were televiewed and seismic refraction tomography (SRT) profiles crossing these features were scrutinized. Since GRE construction began in 2022, detailed mapping of the dam raise foundation has located these faults. Treatment of these features at the foundation surface has included additional excavation and replacement with treatment concrete. Televiewing and packer testing of existing foundation drains was performed as the basis for an ongoing assessment of the need for stitch grouting of the “C”- ”D” couplet to supplement the original grout curtain in this area.

Kyle Kittle, Stantec, and Don Riley, Stantec
Chimney Hollow Dam Foundation Grouting Program, Larimer County, Colorado
The Chimney Hollow Reservoir Project is located approximately 8 miles southwest of the city of Loveland in Larimer County, Colorado. The project is a component of the Windy Gap Firming Project, operated by the Municipal Subdistrict of the Northern Colorado Water Conservancy District. When completed the reservoir will impound 90,000 ac-ft of water and will help ensure a reliable water supply for over 800,000 people in Northern Colorado. The main dam will be a rockfill asphalt core dam with an embankment height of 355 ft and a crest length of about 3,674 ft. Construction began in August of 2021 and filling of the reservoir is planned to begin in 2025. Construction cost is estimated to be $510 million.

The project is located in a north-south trending valley along the Front Range of the Rocky Mountains. Foundation bedrock along the main dam alignment consists of a Paleozoic sedimentary unit (Fountain Formation) that is underlain by Precambrian metamorphic and igneous rock (gneiss, amphibolite, and pegmatite). The grouting program for the main dam is using the split-spacing procedure to install multiple blanket rows and a double-row grout curtain. Drilling and grouting are performed from a concrete plinth that is located along dam centerline. Rotary and percussion drilling methods are used for grout hole drilling and stable grout mixes are utilized for injection of all stages.