$24.6 Million Seismic Stability Retrofit For Lake Gregory Dam Completed

What started out as a relatively modest $4 million refurbishing of the now 82-year-old Lake Gregory Dam has been completed as a major $24.6 million upgrade.  The Lake Gregory Dam Rehabilitation Project is intended to maintain the safety and stability of what is a major edifice within the San Bernardino Mountain Community into the middle of the 21st Century and beyond.
After being deemed a “high hazard dam” by the California Division of Safety of Dams, the County of San Bernardino Special Districts division undertook an effort to retrofit the existing dam to meet state earthquake safety standards.
Dam rehabilitation included adding a 40-foot earthen buttress to the downstream side of the dam that now extends 62 feet beyond the toe of the embankment. The buttress is composed of 70,000 cubic yards of earthen material. To add the buttress, overgrown vegetation and sediment that formerly filled the dam was removed.
Work on the project began in July and truck hauling for the undertaking began in earnest in September. Hauling consisted of a daily transport of 80,000 cubic yards of material up Highway 18 through Crestline to the Lake Gregory Dam worksite. In October, a large piece of compaction equipment was delivered to the worksite.
As work on the west abutment of the dam was nearing completion in December, seasonal rains started to complicate the effort.  While the project was under construction, the county was legally required to keep the lake at a level roughly five feet below normal. After a thorough assessment of the Lake Gregory Dam buttress, the Division of Safety of Dams in early January gave the county permission to raise the lake level by 5 feet from the restricted level to an elevation of 4,520 feet, which is approximately three feet below the spillway.
A deluge during the third week of January resulted in the water level in Lake Gregory reaching the Division of Safety of Dams’ limit, requiring the county to open the outlet valve to drain water beginning the morning of January 17. The outlet valve was closed the morning of January 22 when the water level stabilized near the Division of Safety of Dams’ limit.
Due to another heavy storm that involved rain, sleet, hail and snow, the county from February 11 through February 24 halted work on the Lake Gregory Dam buttress to protect the integrity of the dam and ensure the safety of crews and residents.
Work on the project, which included a 40-foot-wide earthen buttress added to the downstream side and what is referred to as a rock or riprap layer over the slope of the dam, was essentially completed as of April 4. The project widened the dam to twice its previous span from the top to the bottom of the buttress. This width increase give the structure greater stability, as it was previously vulnerable to seismic mishap.  New valves and pipelines were layered into the structure to effectuate better and faster control of the lake level, including rapid emergency drainage.
The completion has allowed the reopening of Lake Gregory Drive, which had been closed to traffic since the initiation of construction last summer.
Originally completed in 1938, the dam is 97 feet high and has a 2.5:1 upstream slope and 2:1 downstream slope. Through the placement of the dam, Lake Gregory was created, as it collects water from Houston Creek.
Lake Gregory now forms the heart of the Crestline community, and features fishing opportunities, swimming, waterslides, boating, and a walking trail along the lake’s perimeter.
“This is a huge step for Lake Gregory Regional Park,”Beahta Davis, the county’s director of regional parks, said. “We are really excited about the completion of the dam.  If you look behind you, you can see how classic and beautiful the lake looks.”
-Mark Gutglueck

Leave a Reply