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Dam Safety

Dam Safety

 

CSC Orchards Frost Protection Pond - Dam Breach

A Breached Section with Inlet/Outlet Conduit Equipped with Anti-Seep Collars at Bottom

Owner: CSC Orchards

Type of Dam: An off-stream, temporary storage facility supplying water for frost protection. The dam was a 15 foot high, earthfill embankment of fine to medium sands, founded on similar soils. The reservoir was filled and drained via a pressurized 36 inch steel conduit which passed beneath the maximum embankment section. The conduit was provided with two steel cutoffs presumably to minimize seepage along the pipe. Due to the pervious nature of the embankment and foundation soils, the reservoir was partially lined with a 40 mil, PVC geomembrane with a bodied solvent, field weld. The conventionally lined portions of the reservoir included the interior embankment face and the immediately abutting half of the reservoir floor. The remainder of the reservoir was covered with a thin 6 mil PVC plastic liner. The individual panels of this thin liner were not seamed together. Rather, the edges of liner sheets were overlapped, folded and buried to form a mechanical joint. This temporary joint reportedly was to serve only a few seasons until the owner would deepen this part of the reservoir. At that time the 40 mil liner would be extended over the full interior perimeter of the impoundment. All lining materials were covered with soil to protect them from the degrading effects of ultraviolet light.

Date of Failure: July 1995

Cause of Failure: The unbonded sheets of the 6 mil liner were not effective in cutting off leakage from the reservoir. Sink holes were noted at several locations on the reservoir floor as well as beneath the liner near the breach. This seepage passed beneath the liner and initiated an erosive piping failure of the foundation and then the overlying embankment. The failure was centered along the alignment of the inlet/outlet conduit. Presumably, the reportedly poorly compacted backfill around the pipe provided a preferential seepage path. With time this seepage began to force aside or remove soils by piping along the conduit to create a continuous "pipe". This "pipe" did not collapse as the adjacent conduit likely supported the weight of the overlying embankment.

No explorations have yet been made into the foundations beneath the main portion of the breached embankment. But, it is anticipated that basalt rock likely is present at a shallow depth. Typically, the rock in this area was created by a series of massive lava flows. The rock normally has high horizontal permeabilities through the highly weathered zones at the top and base of each lava flow. The weathered zones reflect the loss of impurities either trap at the base of the flow or those floating to the top. Furthermore, shrinkage cracks opened as the lava cooled leaving high vertical permeabilities through each rock flow. Presumably, such a rock layer was been passing seepage from the reservoir once it was placed in service. The failure likely resulted when the gradually enlarging "pipes" finally exceeded the capacity of the rock layer. Then seepage "short circuited" the rock outlet and exited out the nearby downstream toe. This explanation would account for the brittle nature of this failure where no one reportedly observed any seepage at the dam toe prior to the failure.

Downstream Damage: One house was damaged by the floodwaters and 3 acres of orchards were destroyed. We understand that to bring an acre of undeveloped land to a mature apple orchard represents years of work with an investment approaching $25,000.

Lessons Learned: A failure to properly bond individual sheets of a plastic liners together can introduce significant volumes of seepage into the pervious underlying soils.

It reinforces the importance of following good engineering practice regarding conduits through dams. All backfill around conduits should be well compacted and rigid cutoffs along pipes should be avoided in favor of filter diaphragms. The filter diaphragm scheme, developed by the National Resource Conservation Service and duplicated in Section 3.3.B of Part IV of our Guidelines, provides for the controlled interception and discharge of seepage along conduits and the associated lead out drain provides timely notice of an adverse seepage condition within the embankment.

Photographs

Panorama view looking downstream through the breach

Sink holes were noted at several locations on the reservoir floor as well as beneath the liner near the breach.

Location: Grant County, About 20 miles southwest of Moses Lake

View Looking Upstream Through Breach. (Inlet/Outlet Pipe at left, Spillway Pipe at right)

View of Reservoir Area. Note Inlet/Outlet Conduit Centered in Breach

View of Breach Sidewall. Small Piping Holes Were Noted on the Sidewall Just Beneath the Liner

The corner of a home damaged by the flooding when the dam broke