In 1999, after a period of extensive rainfall, two shallow slope failures developed in the right-of-way of Provincial Road 259 near Virden, Manitoba. The rainfall caused dissipation of soil suction in the near-surface soil, thereby reducing shear resistance and triggering failure. A research project was initiated between the Geotechnical Group at the University of Manitoba and the Manitoba Department of Highways and Transportation to assess the mechanism of failure. The project included a field investigation program, laboratory testing program, and advanced numerical modeling to identify the cause of failure. The results demonstrate that the rainfall resulted in dissipation of the suction in the soil slope, resulting in a reduction in the soil shear strength that triggered shallow failures. The dissipation of the soil suction has been modeled using a time-dependent seepage model that accounts for the flux boundary condition that existed at the ground surface.Key words: slope stability, unsaturated soils, laboratory tests, soil suction, seepage modeling, flux boundary.
Several bridge crossings proposed for the Red River Floodway expansion project were recently constructed using vertical sand drains to accelerate excess pore-water pressure dissipation and settlement caused by embankment fill loading. With limited local data regarding the performance of sand drains, the calibrated model presented here addresses the need to optimize the design of sand drain configurations, maximize cost savings, and minimize construction delays for future structures. This study presents a coupled finite element embankment consolidation model calibrated against measured pore-water pressure and settlement data from the Salter Street Bridge embankment fill construction, which used vertical sand drains to dissipate excess pore-water pressures. A hydraulic conductivity modification procedure was used to simulate the axisymmetric flow conditions with a plane-strain model neglecting well resistance but incorporating the effects of a smear zone. A sensitivity analysis was performed using the calibrated model by varying the smear zone radius and hydraulic conductivity and the undisturbed soil hydraulic conductivity. The sensitivity analysis predicted that the observed behaviour was predominantly sensitive to the hydraulic conductivity of the smear zone and the surrounding soil.
Landslides are a risk to buried gas pipeline infrastructure, but these risks are particularly difficult to assess given the complex nature of landslide movements. This paper presents of portion of research conducted at the University of Manitoba where gas pipelines within active landslide areas were monitored over a four-year period. Two locations were examined in Western Manitoba within the Assiniboine river valley where a shallow natural gas pipeline runs parallel to the valley slope. A field investigation and monitoring program was undertaken where surficial ground movements and soil and pipe temperatures and pipe strains through strain gauges were measured. Monitoring results identified soil near the pipeline does not freeze, and ground movements are <50 mm/year. The monitoring results also showed pipe stresses and behaviour were affected by backfilling, thermal changes, soil-pipe relaxation, and ground movements. An unexpected outcome of the research was the response of the pipeline to slight ground movements was easily captured by the strain gauges and these movements, slow or surges tended to occur at the same time between the two sites suggesting movements occur due to regional environmental affects.
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