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Importance of This Case Study
This Case Study is one example of the many uses of air photo interpretation in civil and geotechnical engineering.
Hazard Mapping
Historic air photos provide a record of landslide activity along Thompson Canyon, British Columbia that has plagued Canada’s national railways for over a century.
Location
Thompson River valley near Ashcroft, British Colombia, Canada
Background
Through much of mountainous British Columbia, railways, highways and pipelines are routed along river valleys that pose a number of engineering challenges related to geology and glacial history. Near the town of Ashcroft, Canada’s two national railway lines are routed along alternate banks of Thompson River (Fig. 1).
Thompson River valley is a broad rock valley approximately one-quarter-filled with sediments that were mostly derived during Quaternary-age glaciation about 100,000 to 10,000 years ago when more than 1 km thick of ice covered southern British Columbia. Deglaciation happened first in the highlands leaving masses of beheaded ice in the broad valley, profoundly altering the drainage and causing the impoundment of extensive, temporary glacial lakes.
Silt and clay deposits that accumulated in glacial lakes are problematic engineering soils. Silt is highly erodible with moderate drainage. Clay has low shear strength and is very poorly drained. Interbedded strata of silt and clay produce complex patterns of sediment-pore-water pressure, which lead to areas of ground instability. The combination of cohesive clays, unfavourable groundwater conditions and river erosion along the riverbank toe combine to generate ground instability in the area.
The Problem and Objectives
Some of the Quaternary sediment filling the lower Thompson River valley is prone to landsliding. Despite the large amount of background information available, there was no detailed map of landslide occurrences along the valley. This information was needed for models of hazard and risk management employed by railway companies to prevent and manage future landslides in the area.
Air Photo Interpretation
Air photo interpretation used photos taken at various scales in different years (Figs. 2, 3) to ensure all areas of instability were identified. This information was digitized and imported into a customized geographic information system (GIS). Location, size, volume of material, grain size and age were tabulated for each landslide occurrence and used in hazard and risk models.
Supplemental Information
Bedrock geology maps, surficial geology maps (Fig. 4), topographic maps (Fig. 5) university theses and published reports provided info on geology and history of the region. A number of engineering reports are available that describe mitigative measures taken for previous landslide activity.
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