Hebgen Dam, located near West Yellowstone, Mt, stands at over 6,500-ft in elevation tucked among the mountains. The nearest city of any size is 100 miles away. Hebgen Reservoir is in the environmentally sensitive headwaters of the Madison River with weather among the coldest and snowiest of any location in the lower 48 states.
The 700-ft earth embankment dam was constructed with a concrete core wall. A concretelined and gated spillway was constructed near the right abutment. The low level outlet for the facility is the intake structure, a 75-ft tall freestanding concrete tower at the upstream toe of the embankment on the left side. Flow through the intake tower was controlled with stoplogs in four bays for the full height of the structure.
On Aug. 17, 1959, a fault located a few hundred yards from the right abutment of the earth embankment dam ruptured during a 7.5 magnitude earthquake. The Madison River downstream of the dam was completely blocked by a massive rockslide, which also killed 28 people. The dam held but the dam and flow control structures experienced severe damage. The dam was repaired and operated for the next few decades.
In 2008, several stoplogs in the intake structure failed. Rehabilitating the intake was a top of the priority for PPL Montana to allow for reliable and safe operation of the intake. The outlet from the intake to the river downstream was a 12-ft dia. woodstave pipe through the dam and core wall downstream to the tailrace.
For a month, PPL Montana personnel worked feverishly to stop the uncontrolled release. More than 3,000 cubic feet per second (cfs) of water was being discharged at a time of year when only 1,000 cfs is normal. The main concern was for the integrity of the aged wood outlet pipe and subsequently the dam itself. After several ideas were attempted without success, banded packets of stoplogs were jacked into the opening 50 feet below the water level, and flow was mostly stopped.
URS Washington was retained for the intake rehabilitation design, and Pacific Pile and Marine was hired for the design-build contract for a cofferdam to isolate the intake. The intake rehabilitation design was complex. The seismicity was a complicated process, involving multiple experts and discourse with FERC, and a design event of a magnitude 7.3 earthquake at a distance of 100 meters, resulting in horizontal and vertical peak ground accelerations of 1.5 g, was selected. Substantial permanent anchorage and post tensioning of the structure was immediately identified as a necessary feature to combat the multiple load directions and reversals during an earthquake. Many factors were incorporated in the design just to allow access for construction and stability until the permanent features were in place. An array of temporary grouted rock anchors were specified for the back wall of the intake to stabilize the structure during dewatering until the permanent post tensioned multi-strand anchors and new concrete structure were complete.
The emergency closed cell cofferdam to isolate the deep intake structure was designed as a freestanding sheet pile structure with three cells containing fill material. The center cell was designed with a single vertical slide gate. Concrete tremie seal utilized to stabilize rock between existing intake. 1,350 cy tremie concrete poured in water depths up to 60-ft. Circular cells spaced 58-ft in dia. 70-ft centers using sheet pile between 80 and 85-ft. 36-in dia. shafts were installed 20-ft into bedrock at a water depth up to 35-ft. Construction on the intake had to be suspended at times while the Reservoir diverted the entire river flow through the area under construction creating challenges to schedule and sequence.