Booth Creek, East Vail, Colorado
An example of a rockfall hazard and high risk area affecting a neighborhood is in East Vail at Booth Creek. The north valley wall of Gore Creek is benched with two high rock cliffs. Above the cliffs, the 1,100-foot high valley rim is composed of an eroding slope of glacial till, which is also composed of very large rocky material. After several repeated, potentially lethal, rockfall events that damaged several homes in the early to mid 1980s, CGS was asked to provide assistance to the Town. The neighborhood created a special Geologic Hazards Abatement District (GHAD) affiliated with the Town of Vail. The GHAD funded a rockfall hazard study that included a mitigation design. The construction of a rockfall catchment ditch and berm above the homes on the valley slope was completed in 1990 (Figure 1).
Owners of adjacent condominiums elected to not participate in the GHAD, and that poor judgment was brought into sharp focus in March, 1997. Another large rockfall event fanned down the slope toward the residential areas at the property line between the homes and condominiums. The existing rockfall ditch and berm was 100% effective in catching the rocks, but several rocks impacted the unprotected condos (Figure 2).
After that incident, which luckily resulted in no fatalities, the condominium homeowners association petitioned the town for their own mitigation. In 2001, specially designed impact barriers (Mechanically Stabilized Earth wall) were constructed on the slope behind the condos to provide a similar level of protection (Figure 3).
Glenwood Canyon Thanksgiving Day Rockslide
On Thanksgiving Day in 2005, a very large rockfall event occurred in Glenwood Canyon affecting a portion of Interstate 70. A segment of rock over 1,200 feet high on the canyon wall and 2,000 cubic yards in volume, detached from the cliff face, broke into many large blocks that rolled down a rockfall chute, and slammed into the highway at the canyon bottom (Figure 1). Thankfully, the westbound lanes were temporarily closed at the time. No vehicles were hit, but there was severe damage done to Interstate 70 highway structures, requiring the westbound lanes to be closed for almost three months for repairs.
Figure 1. Thanksgiving 2005 rockslide area in Glenwood Canyon. Detachment location, shown by black arrow, is 1,200 vertical feet above Interstate 70. Note the prominent nonconformity where this 600-foot thick cliff of Sawatch Quartzite lies over Precambrian basement rock. The rockfall path is well defined by the show-filled chute in the underlying Precambrian rocks.
The rockslide occurred near the Shoshone Interchange, which is a tightly constrained section of highway structures in one of the narrowest sections of the canyon. A series of bridges and retaining walls enable the highway to cross the Colorado River to the Hanging Lake Tunnel portal while still providing road and bicycle access to the Hanging Lake Rest Area. The rockfall was caught on the closed circuit video cameras used to monitor Interstate 70 traffic in the canyon. The video showed many rocks, up to 12 feet in diameter, impacting the on-ramp retaining wall of the rest area, as well as the bridges to the tunnel portal. A dust cloud generated by the rockslide filled the canyon afterwards.
When the dust cleared, the highway was littered with boulders of all sizes (Figure 2). Upon closer inspection, the true nature of the damage became apparent as large holes were punched though the concrete deck and the westbound retaining wall (Figure 3), demolishing a section of the bicycle path below (Figure 4), as well as damage to the bridge girder of the adjacent eastbound bridge.
Figure 2. Huge blocks of Sawatch Quartzite litter both the eastbound and westbound lanes of I-70. Photo by Ty Ortiz, CDOT.
Figure 3. Westbound deck and retaining wall with extensive damage. Photo by Ty Ortiz, CDOT.
Figure 4. Damage to retaining wall and bike path. View is below bridge at hole location shown in Figure 3. Photo by Ty Ortiz, CDOT.
The town of Glenwood Springs in west-central Colorado lies at the confluence of the Roaring Fork and Colorado Rivers. The town is tightly constrained by the steep river valleys so land-development pressure is causing more residential growth to push into rockfall hazard areas. In West Glenwood, on the west side of the Roaring Fork River, the valley is rimmed with dipping sandstone outcrops of reddish Maroon Formation. The sandstone layers are being undercut by the erosion of underlying softer siltstone and shale so that large sandstone blocks are being actively undermined and destabilized. In this area, there have been several large rockfall events from the valley rim; some that have severely damaged homes on the valley floor, 1,100 vertical feet below. Fortunately, there have been no injuries or fatalities. While there has been rockfall mitigation in some locations, the threat remains in other areas.
Figure 1. Valley rim west of the Roaring Fork River in Glenwood Springs looking north towards the confluence with the Colorado River. Note slumped (tilted) sandstone blocks in the exposed rock layer. Some of the rock blocks shown in this picture from 1994 have now fallen/rolled to the valley floor. Photo by Jon White.
Figure 2. Hole in side of house from impact of two large boulders in 2004. Note smaller rock embedded in roof. Photo taken from Midland Avenue embankment above home. River shown is the Roaring Fork. Photo by Jon White.
Figure 3. The main rock that slammed into the house shown in Figure 2 rolled through home to come to rest against an easy chair in the living room. This rock came from the source area shown in Figure 1. The homeowner built a rockfall protection fence afterwards. Photo by Jon White.
Colorado Geological Survey