CS6A - Concurrent Session 06A: 1964 FAILURE of LOWER HELL HOLE DAM – DFIC DECADE SERIES

Following a period of record rainfall, the partially completed Lower Hell Hole Dam failed on December 23, 1964. The failure released 30,000 acre-feet of water and sent 700,000 cubic yards of rock and debris surging 60 miles down the Rubicon River canyon. Located in the Sierra Nevada mountains approximately 100 miles east of Sacramento, California, the Lower Hell Hole Dam was designed to be a 410-foot-high zoned rockfill structure, scheduled to be constructed from 1964-65. The proposed contract required that the project be built for a guaranteed price with no provision for cost or schedule changes. Construction was phased so that the partially constructed embankment would serve as a cofferdam between construction seasons. However, during the first year of construction, fill placement was slower than anticipated and, as a result, the impervious core was not progressing as scheduled. Due to this slow progress, the design engineer approved a modified diversion plan that lowered the clay core requirement in the cofferdam design and shifted all risk to the contractor. In late December 1964, roughly 22-inches of rainfall fell over a period of five days, filling the reservoir to a depth of 100 feet above the clay core. The reservoir did not overtop the rock fill but flowed through the porous material in the downstream shell resulting in erosion and headcutting, which ultimately progressed to the point of dam failure. The resulting breach had a peak discharge of approximately 200,000 cfs. Fortunately, there was no loss of life resulting from the failure. An engineering consulting board was convened by the designer of the dam to investigate the design and feasibility of continuing construction of the dam. The board concluded that the remaining portions of the dam could be salvaged, with only minor adverse impacts observed to the core and filter zones, and construction could continue in accordance with the original design, provided there would be more oversight and no deviation from the new construction sequencing plan. Multiple lawsuits were filed and settled over the next few years. The dam was eventually completed in 1966. This case study will (1) identify the key physical factors contributing to the failure, (2) describe the incident, including the resulting downstream damage, (3) highlight the human factors involved in the failure, and (4) consider how the slower-than-anticipated construction and modified diversion plan may have been handled today using a construction potential failure mode analysis-type approach.