COMMON SLOPE STABILITY PROBLEMS AT COLORADO SKI AREAS

Most Colorado ski areas contain prehistoric (and historic) landslides. Slope stability problems at ski areas can be viewed in two ways: (1) what hazards do future mass-movement processes pose to facilities? and (2) will proposed actions initiate or aggravate mass movements? Lift towers are the most sensitive facility, because movement of only a few inches perpendicular to the cable direction can cause the cable to derail. At Powderhorn Ski Area, a landslide in May 2000 moved one lift tower 0.9 m downslope, forcing replacement. Another landslide grazed a tower foundation but the foundation apparently extended deeper than the failure plane, so acted as a giant soil nail. At Tiehack Ski Area five towers on one lift lie on a creeping landslide. Remediation included installation of interceptor trenches and drains, plus fabricating an adjustable-track base between the tower and the concrete foundation. On many trails mitigation options are limited because heavy granular backfill cannot be trucked to the site. Mass movements on trails themselves can be viewed as a nuisance, exemplified by debris flows at Arapaho Basin in 1999. However, debris flows exiting the ski area can cause damage off-site, such as the Aspen Music School debris flow of 1996. In that example, as in many others, the trigger appears to be runoff from ski area diversion ditches that is released on steep sideslopes that have never carried natural concentrated flow. The infiltrating water at the end of the ditch causes a debris slide, known in the ski industry as a “blowout.” Trail clearing is thought to decrease slope stability by removing root support and increasing soil moisture, as has been documented on logged slopes. This decrease is partly offset by trail smoothing in hummocky terrain, which theoretically speeds up runoff and decreases infiltration. Unfortunately, there have been no rigorous experiments on the effect of ski trail cutting, or artificial snowmaking, on slope stability based on before-and-after surface water and groundwater monitoring. Until such work is done, predicting the effects of trail clearing and snowmaking on slope stability will have to rely on qualitative estimates and “professional judgment.”