Olufson Dam - Sinkhole in the dam crest
The sinkhole in the dam crest the night of December 11, 1996
The sinkhole appeared to have resulted from a collapse in the top section of the cast-in-place box culvert that served as the principal reservoir outlet.
Location: Pierce County, Near Gig Harbor
Type of Dam: This privately owned dam was an 18 foot high, earthfill embankment with a storage capacity of 15 acre-feet and 21 acre-feet at the top of dam. The principal spillway consisted of a two foot square, concrete, drop inlet conduit. An open channel in the abutment served as an emergency spillway.
The dam was constructed in the 1960s without the benefit of engineering plans. The owner did the work himself, including placing fill and mixing his own concrete on site. Conditions exposed by the failure suggest that the elements of the construction that required skill were substandard. In particular, the concrete work suffered from inadequate cement content, poor overall mix gradation and improper reinforcing. Thick steel cable was substituted, in part, for conventional reinforcing steel. Likewise, these cables were improperly positioned in the conduit section thus, minimizing its enhancement of the tensile load capacity of the conduit. To limit concrete volumes, it appeared the owner had embedded bricks, rocks and concrete rubble into the walls as a filler during concrete pours. This practice, termed cyclopean concrete construction, has been successfully used in large gravity structures but is inappropriate for thin walled, concrete box culverts.
Date of Incident: December 11, 1996
Description of Failure: On December 11, 1996, a sinkhole 20 feet in diameter and 17 feet deep opened up in the crest of the dam. At the time the sinkhole developed, the property on which the dam sat was uninhabited due to the recent death of the property owner. The sinkhole was discovered by neighbors walking the streambed to investigate the cause of muddy streamflows. It was fortuitous that the hole was discovered before it lead to an embankment failure.
The sinkhole appeared to have resulted from a collapse in the top section of the cast-in-place box culvert that served as the principal reservoir outlet. The failed segment of the outlet allowed overlying masses of embankment soil, over time, to repeatedly drop into the pipe, where flows then flushed the soil downstream. This sequence of events is supported by the record of stream flows in a downstream gaging station. The gage record shows normal flows interrupted by a series of near zero creek flows immediately followed by short, abnormally high channel discharges. The zero flows are interpreted as incidences of soil masses falling into and plugging the outlet. The following anomalous high flows represent a blowing out of the plug and a release of back watered flows in the outlet pipe and inlet tower upstream of the plug.
As an immediate response to the threat of a dam breach, County maintenance staff filled the sinkhole with some 200 yards of angular cobbles and boulders. State dam safety staff saw no viable alternative to the county’s scheme to address the immediate crisis. Finer grained soils would likely have been sluiced through the top of the collapsed box culvert. This could have worsened the situation by plugging what limited outlet capacity remained after sediment had largely blocked the outlet culvert. Nonetheless, it was obvious that the rock fill was but an interim measure and immediate follow-up action was necessary to lower the reservoir and permanently resolve the public safety threat.
Three days of pumping were necessary to lower the reservoir to allow excavating a trapezoidal shaped breach of the dam. The floor of the breach was armored with a geotextile fabric and capped with much of the rock originally dumped into the void the night of the failure. To improve fish passage, an attempt was made to include a number of pools along the breach channel at the direction of the Washington State Department of Fish and Wildlife. Washington State Water Quality staff assisted in blanketing the disturbed sections of the dam with hay to minimize further sediments entering the water course.
Downstream Damage: Damage downstream was limited to the streambed. Primarily, it occurred in the form of stream habitat degradation from sediment deposition. It is presumed that much of the salmon eggs in this fish producing stream were smothered under sediments for several hundreds of yards downstream of the dam. As bad as it was, the emergency action prevented a likely failure of the dam. Thus, the possible threat of loss of life was averted along with extensive property damage abutting the streambed.
Lessons Learned: An examination of the failed outlet conduit through the dam revealed it to be of poor quality with minimal reinforcing. What reinforcing was provided, consisted of misplaced, steel cable rather than conventional deformed bars. It is remarkable, given the construction of the conduit, that it functioned for over 30 years.
This failure reinforces the notion that conduits have a definite service life, measured in decades. At the end of that service life, they require retrofitting for their continued satisfactory functioning. A failure to do so, risks a failure of the dam
Proper care taken in the design and construction can materially increase the conduit service life. Conversely, poor workmanship may reduce it.
Finally, it is important that periodic inspection of conduits be made to confirm that they are structurally sound, and to provide timely notice of a developing problem with age.
Downstream end of outlet conduit filled with sediment
View of reservoir after pumping.
View of upstream dam crest nearing completion of breach.
Section of failed outlet conduit in breach.
Excavating into reservoir to allow water to flow into new channel.
View looking down completed channel through breach
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