Spirit Lake Outlet Tunnel 2015 Interim Repair

Location: Mount St. Helens, Washington

On May 18, 1980, an earthquake struck below the north face of Mount St. Helens in Washington State, triggering the largest landslide in recorded history and a major volcanic eruption that scattered ash across a dozen states. The sudden lateral blast—heard hundreds of miles away—removed 1,300 feet off the top of the volcano, sending shock waves and pyroclastic flows across the surrounding landscape, flattening forests, melting snow and ice, and generating massive mud-flows. These mud-flows blocked the natural outlet of Spirit Lake, creating a bathtub effect with no natural drainage. In 1985, the US Army Corp of Engineers constructed an outlet tunnel to allow Spirit Lake to again flow into the Toutle River. Since its completion in 1985, the Corps has repaired the tunnel many times; the majority of which were small repairs to restore tunnel integrity. A more significant repair was performed in 1997, when tunnel movement due to pressure build-up in a shear zone caused a partial failure of some tunnel supports. Pressure build-up from that same shear zone recently caused substantial differential movement of over three feet and it was recognized that significant repair was required to prevent catastrophic failure. The resulting event having the potential to impact nearby cities, downstream of the lake. Catworks Construction was awarded the contract for emergency construction operations to repair the damaged portion of the tunnel. Scope of Work included: Temporary Stream Crossing – Fabrication of Steel Rib Sets – Care & Diversion of Water in the Tunnel – Tunnel Excavation – Temporary Support – Installation of Crown Drains – Installation of Steel Rib Sets – Installation of Channel lagging – Cast in Place Concrete & Shot Crete. Despite many challenges, most notably the remote location and inclement winter weather, the project was successfully completed on March 11, 2016.

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