A $58,000 project to stabilize an above-ground sewer line that traverses a cracked slab on the granodiorite dome locals call The Rock, downstream from Twain Harte Dam and Twain Harte Lake, is on schedule to be completed by May 1.
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Most of the drilling for the project is done and contract workers are waiting for made-to-order rock bolts 3-feet to 15-feet long, Tom Trott, general manager for Twain Harte Community Services District, said Thursday.
“The rock is fractured and it has potential to move by gravity down the hill,” said Scott Lewis, principal engineering geologist with Condor Earth Technologies of Sonora, which prepared a project report for Twain Harte CSD in January 2015. “Our goal is to stabilize that rock to keep it from sliding.”
Sewer line connects to TUD
The sewer line serves about 500 homes, a third of Twain Harte CSD’s customers, Trott said. It flows around The Rock, across the face of Twain Harte Dam, below the lake parking lot, and then into lines operated by Tuolumne Utilities District, Trott said.
The slab area the sewer line rests on is about 150 feet long and 40 feet in slope-height, Lewis said. It’s 2-feet to 10-feet thick. Some of the rock surface has moss and lichen in places.
“We’re only bolting the area of the pipeline,” Lewis said. “We’re focusing on the foundation for the pipe to keep it from moving.”
Upstream from the work project, the private recreation reservoir called Twain Harte Lake is full, holding about 140 acre feet of water. One acre-foot can flood a typical football field 12 inches deep. Most of The Rock is fenced off and closed to the public right now.
Concerns about Twain Harte Dam’s abutment with The Rock, which cracked and prompted emergency draining of the lake and a flash flood warning in August 2014, have been resolved, said Dennis Wyckoff, general manager for the Twain Harte Lake Association.
Like other granodiorite domes in the Sierra Nevada, the one known as The Rock sheds layers or slabs from time to time in a natural process called exfoliation, due to uplift and erosion over millions of years.
Condor said four “natural rock-stress release events” happened on The Rock beneath the sewer line on Aug. 3, Aug. 6, Aug. 20 and Sept. 4, 2014.
“The stress release caused ground motion, the formation of new rock fractures at the ground surface, and buckling of surficial rock slabs,” Condor’s January 2015 engineering report states. The dates are based on information from Twain Harte Lake Association staff, visitors, residents and Condor staff on-site observations during the Aug. 6 event.
During inspections beginning Aug. 3, 2014, state Dams Safety Division staff observed cracking and offsets in the left abutment rock dome and stress cracks in the left arch barrel of the dam.
The reservoir was judged unsafe to store water, and the lake association was ordered to open the outlet and dewater the reservoir, Dams Safety engineer Param Dhillon said.
Satisfactory remedial work was completed at the dam April 29, 2015, and the owner was authorized for full storage behind the dam, Dhillon said.
Opening May 28 this year
Opening of the lake was delayed past Memorial Day last year, but Wyckoff and many Twain Harte business owners were pleased to have the lake open again by mid-June 2015.
This year, Wyckoff and Twain Harte Lake Association members are looking forward to having the reservoir open by the Memorial Day weekend.
“We’re happy they’re doing this,” Wyckoff said Thursday of the sewer line stabilization. “It’s a preventive measure. Exfoliation like this can happen. It happens more frequently in Yosemite where the cliffs are sheer. Here, this is a rounded dome.”
Wyckoff said Twain Harte Lake, which has about 800 association members, will open this year on May 28. National Dam Safety Awareness Day is May 31.