Fontenelle Dam - Partial Failure

Partial Failure

Problems with the embankment became apparent in May 1964, when part of the backfill slid into the stilling basin downstream, attributed to too-fast lowering of the reservoir level. When the reservoir had filled to 10% of capacity, seepage became apparent at the dam's base, with further seepage observed about 4,000 feet (1,200 m) below the dam from shale outcroppings. In July 1965 another slide occurred in the area of the stilling basin. A significant leak appeared on September 3, 1965 at the west abutment, starting as a wet spot that grew with time. The weakened downstream face lost 10,000 cubic yards (7,600 m3) of material that slid into the stilling basin, accompanied by continuing water flow, a condition known as "hydraulic piping." Work began the next day to lower the reservoir, with a 24-hour watch on the dam. The wet spot became a waterspout, flowing at a rate of between 10 and 12 million gallons per day. On September 6 a 15-foot (4.6 m) by 20-foot (6.1 m) sinkhole developed on the upstream face of the dam's crest, which workers immediately filled with riprap bulldozed from the nearby dam surface into the hole. The hole was 30 feet (9.1 m) deep, with the bottom 11 feet (3.4 m) below the level of the reservoir, with only 45 feet (14 m) of dam structure remaining between the sinkhole and the downstream face. Further collapse could have created a breach in the dam, leading to total failure. Leakage did not increase during the incident. The emergency release of water flooded areas along the Green River downstream, damaging ranches and homes. The unusually large outlet works allowed the reservoir to be drawn down by as much as 4 feet (1.2 m) per day, a measure not available eleven years later at Teton Dam.

The reservoir was drawn down further during the remainder of the year, and work began on repairs to the embankment, along with an intensive program of pressure-grouting at the abutment and down the centerline of the dam embankment. Work continued through 1966, with a complete replacement of the right abutment embankment. The reservoir was partly refilled in the spring of 1967 to check the efficacy of the grouting work, which consumed 200,000 cubic feet (5,700 m3) of grout. Water was released through the power penstock with the turbine and generator removed while the outlet works were repaired. 23 observation wells were drilled at this time. The reservoir was fully refilled in the summer of 1968. Subsequent evaluations described failure as "narrowly averted." The near-failure was not widely reported, but did cause organizations other than the USBR to change their design and construction practices for embankment dams. The Bureau of Reclamation concluded that water used in mixing concrete was contaminated with trona. The sodium carbonate in the trona accelerated setting of the grout in the original grout curtain, leaving it weak and fissured.