Along with the U.S. Army Corps of Engineers, the agency awarded a contract repair the tunnel near Mount St. Helens on the Gifford Pinchot National Forest for USD 3M to Cat Works, LLC, last October. The Forest Service said "the repairs will address critical short-term needs while more sustainable long-term solutions are explored."
During the repair work contractors installed additional structural supports, restored the outlet tunnel’s diameter and excavated the material that had seeped into the outlet tunnel’s interior.
"Protecting downstream communities is our number one priority," said Jim Peña, regional forester. "These important repairs will reinforce the outlet tunnel’s integrity and give us time to work with our partners to find a more viable solution for this geologically active area."
According the public affairs office of the Army Corps of Engineers, when Mount St. Helens erupted on May 18, 1980, a massive avalanche of rock and debris completely blocked the natural outlet where Spirit Lake flowed into the North Fork Toutle River and on into the Toutle, Cowlitz and Columbia rivers. The avalanche created a dam of sediment, ash and fragmented rock, with heavier, less porous materials below and more erodible pumice and ash from pyroclastic flows above it. Snow and rain continued to feed Spirit Lake, but without an outlet, the water level rose closer to the more easily erodible material.
Concerns for floods lead to the construction of the Spirit Lake Outlet Tunnel, which was completed in 1985, and comprises a 40ft (12m) deep intake shaft connecting to an 8,465ft (2.6km) long, 11ft (3.4m) diameter tunnel. The Corps said since project completion, it has repaired the tunnel many times, mostly small repairs to "restore tunnel integrity."
In October 2014, a routine inspection revealed damage to the outlet tunnel lining, deposits of clay in the outlet tunnel’s interior, and a 2.5ft reduction in outlet tunnel diameter due to the encroachment of surrounding rock. A subsequent inspection this spring revealed further damage and an additional half-foot reduction in outlet tunnel diameter.
