THE CREEPY MANCOS SHALE; INCLINOMETER DATA FROM CRESTED BUTTE, COLORADO

Crested Butte Mountain Resort, Colorado, began monitoring the slopes of adjacent Snodgrass Mountain in 1995, in anticipation of developing it as a ski area. We now have 15 years of surface stake data and 3.5 years of inclinometer measurements (summer 2007 to present), and observe a correlation between slope movement on Snodgrass and the volume/rate of spring snowmelt. Snowmelt data come from the “Butte” SNOTEL station on Mt. Crested Butte.

The surface stakes are 0.5 m-long rebar, installed in 1995 and surveyed annually. In normal and wet years the stakes move down the local fall line, in proportion to the local slope angle. The largest cumulative movement since 1995 is about 0.3 m.

Inclinometers installed in summer 2007 show three types of deformation. First, every casing bends downslope in the upper 1.2 m, as a result of shallow soil creep (wetting/drying and freezing/thawing of shale). Second, five of the seven inclinometers show discrete shear at the base of landslide masses, at depths ranging from 8m to 16 m. Third, the only inclinometer installed in an earthflow shows distributed deformation over its 20-m length.

The basal shear amounts are small and correlate well with the surface water equivalent (SWE) of the yearly snowpack at the beginning of the melt season (May), over 3 years (2008-2009-2010). In a normal-SWE year, basal shear ranges from 0.45”/year at the toe of the East Slide, to 0.2”/year in the youngest-looking slide on the mountain, to 0.04”/year in the middle of the East Slide, to 0.025”/year in the small slump at Ken’s Crux.

Opponents of the project claim that inclinometers do not capture all the sliding, because there may be deeper shear planes below the inclinometers. If this were true, then shear measured in our inclinometers would be less than downslope movement of surface stakes in the same period. However, those two values are essentially identical. Using the relationship between SWE and basal shear, and assuming that ski area development mimics a permanent 10% increase in SWE, we can predict the cumulative basal slip over the 50 year life of the project. In all but one inclinometer the cumulative slip will be less than 25 cm, and can easily be mitigated by engineering design.