SLOPE FAILURES IN NORTHERN VERMONT, USA

Rockfalls and debris avalanches from steep hillslopes in northern Verm ont are a continuing hazard for motorists, mountain climbers, and hike rs, Huge blocks of massive schist and gneiss can reach the valley floo r intact, whereas others may trigger debris avalanches on their downwa rd travel. Block movement is facilitated by major joints both parallel and perpendicular to the glacially over-steepened valley walls. The s lope failures occur most frequently in early spring, accompanying free ze/thaw cycles, and in the summer, following heavy rains. The study re ported here began in August 1986 and ended in June 1989. Manual and au tomated measurements of temperature and displacement were made at two locations on opposing valley walls. Both cyclic-reversible and permane nt displacements occurred during the 13-month monitoring period. The m easurements indicate that freeze/thaw mechanisms produce small irrever sible incremental movements, averaging 0.53 mm/yr, that displace massi ve blocks and produce rockfalls, The initial freeze/thaw weakening of the rock mass also makes slopes more susceptible to attrition by water , and heavy rains have triggered rockfalls and consequent debris flows and avalanches, Temperature changes on the rock surface produced time -dependent cyclic displacements of the rock blocks that were not insta ntaneous but lagged behind the temperature changes. Statistical analys es of the data were used to produce models of cyclic time-dependent ro ck block behavior. Predictions based solely on temperature changes gav e poor results. A model using time and temperature and incorporating t he lag effect predicts block displacement more accurately.