Landslides in the Morkill River Valley, British Columbia

Operational and construction problems in forestry developments in a valley in the Canadian Rockies are associated with landslides in weakly cemented lake sediments. The landslides have been inventoried. The undisturbed sediments contain up to 11% calcium carbonate and are susceptible to softening by mechanical and chemical weathering processes.;Chemical weathering by leaching and dissolution of the calcite cement is documented in the field by obtaining density profiles and observations of relative effervescence and quantified in the laboratory program. There is a linear relation in the increase in density with increase in carbonate content in the field profiles. Unconfined Compression Tests on field samples subjected to cycles of freeze and thaw showed a 50% decrease in strength after only one cycle.;The softening process leads to a decrease in shear strength and surficial landslides, composite earth slide-earth flows, in the silty soils and earth slides in the sandy soils.