Seepage into pipe can go unnoticed for years until sinkhole develops

---

Message: Take seriously any leaks into conduit.

On June 28, 1996, a large sinkhole, measuring twenty feet in depth, three to six feet in diameter at the surface, and about 18 feet in diameter at the bottom, was discovered in the crest of Willow Creek Dam. The sinkhole was located directly above the outlet works tunnel and about fifty feet downstream of the regulating gate.

The sinkhole may have developed gradually by internal erosion of glacial till core materials through open joints and fractures in the bedrock, progressive collapse or “stoping” of the bedrock into the void below, and piping of earth materials through open cracks and weep holes in the tunnel lining. Earth materials were observed to be piping periodically from a 1-1/2-inch-diameter weep hole in the right tunnel side wall.

• 

  Sinkhole in crest, above outlet conduit

• 

  Schematic of dam components and sink hole

• 

  Sinkhole during construction repair activities

• 

  18 foot diameter at bottom

• 

  Willow Creek Dam

Note: The content on this page was originally created as part of DamOutletWorks.org (DOWL, 2018). It has subsequently been updated and reformatted as part of the Dam Safety Toolbox.
Revision ID: 4683
Revision Date: 12/02/2022