A partial tunnel collapse has prompted a temporary closure of the Route of the Hiawatha bicycle trail.
U.S. Forest Service spokesman Dave O’Brien said a “large portion” of rock fell out of the ceiling of one of the tunnels near the upper end of the popular bicycle trail, which follows an abandoned rail bed along the Montana-Idaho border.
“It was very big,” O’Brien said. “It would have put a very big dent in a pickup, if not crushed it.”
The collapse happened Saturday afternoon in Tunnel 21, which is the second tunnel downhill from the Taft Tunnel trailhead. Between 10 and 15 cyclists were on the trail at the time – and at least one person heard the crashing rocks – but nobody was inside the tunnel, O’Brien said. The rocks have already been removed.
The Forest Service won’t know until Wednesday if the trail can be safely reopened for the remainder of the season. The trail has experienced record use this year, with more than 20,000 cyclists riding the 15-mile rails-to-trails route. The trail begins high in the Bitterroot Mountains and was scheduled to close for the season Oct. 2.
Engineers are now trying to determine what caused the collapse. A review of seismic records shows there were no earthquakes anywhere near the area at the time, O’Brien said. Another possible culprit is freezing temperatures causing water between rocks to expand.
Before the tunnel opens in late spring, workers commonly have to remove fallen rocks and small rockslides from the tunnel and trail. Although cooler weather descended on the region this weekend, it’s doubtful the recent slide was caused by a hard freeze, said Larry Shepherd, a Forest Service construction engineer.
“Minor rockfall is natural and expected during the freeze and thaw of the winter months, but the timing of this event was unusual and warrants further inspection,” he said.
The Forest Service will likely ask for expert advice on fixing the tunnel from one of the many mining engineers and experts in the Coeur d’Alene area, O’Brien said.
There are 10 tunnels on the Hiawatha. Before the route was opened for public use in 1997, the Forest Service inspected each of the tunnels to ensure their safety. Ceiling supports were installed in some tunnels. The agency spent at least $1.5 million to repair and strengthen the 1.8-mile-long Taft Tunnel.
The collapse also means Forest Road 506 is closed until repairs are completed. The road connects traffic from Interstate 90 to the Upper St. Joe River area. The Moon Pass road between I-90 and Avery remains open.