SEASONAL MOVEMENT OF THE SLUMGULLION LANDSLIDE DETERMINED FROM GLOBAL POSITIONING SYSTEM SURVEYS AND FIELD INSTRUMENTATION

Measurements of landslide movement made by Global Positioning System surveys and extensometers over a 3.5-year period (July 1998 – March 2002) show that the Slumgullion landslide in the San Juan Mountains of southwest Colorado moved throughout the monitoring period, but that daily velocities varied on a seasonal basis. Landslide velocities peaked in the early spring and summer in response to snowmelt and summer thunderstorms, respectively. Velocities were slowest in mid-winter when air and soil temperatures were coldest and precipitation was generally low and/or in the form of snow. We hypothesize that the seasonal variability in velocities is due to ground-water levels and corresponding pore pressures that decrease when surface water is unavailable or cannot infiltrate frozen landslide material, and increase when surface water from melting snow or rainfall infiltrates unfrozen landslide material. Field observations indicate that patches of bouldery debris and fractures (created by continuous movement of the landslide) act as conduits through which surface water can infiltrate, regardless of the frozen or unfrozen state of the landslide matrix material. Therefore, the availability of surface water is more important than landslide temperature in controlling the rate of landslide movement. This hypothesis is supported by field instrumentation data that show 1) landslide velocities closely coinciding with precipitation regardless of the depth of freezing of landslide material, 2) springtime and annual landslide velocities that were greatest when the depth of wintertime freezing was also the greatest, and 3) velocity and pore-pressure increases that followed rainfall within several weeks.

The persistent, but seasonally variable, movement of the landslide fits the bathtub model for landslide movement described by Baum and Reid (2000). In their model, weak, low-permeability clay layers at the basal and lateral boundaries isolate the landslide both mechanically and hydrologically from adjacent materials. These clays cause the landslide to retain water. Our data support this model by suggesting that pore pressures at the basal slip surface are always adequate to maintain landslide movement and that any infiltration of water at the ground surface of the landslide is adequate to increase landslide velocity.