Abstr.-Two case studies are presented where GPR was used as the chief investigative tool to understand the underlying cause for road failures as they relate to fill slope stability in the Coast Mountains of British Columbia, Canada. 1, Bridge River Road: GPR was applied to map the depth of volcanic ash under a 800 in long section of the Bridge River forest service road (FSR) built in 1984. Both 50 and 100 MHz antennae reached sufficient depth penetration (8 and 16 in respectively) and provided good data quality. The thickness of the overlying volcanic ash was noted by a horizontally continuous reflection pattern. The results indicated a variable thickness of ash over rocky colluvium. Comparing interpreted ash thickness with landslide locations and available geological information it was apparent that ash thickness alone was not necessarily the cause of the landslides. Steep slopes, excessive side-cast and undercutting due to wave action at the base of the fill slope all appeared to contribute to instability. GPR data, once confirmed through drilling was successfully used to find the appropriate engineering solution. 2, Zeballos Canyon Road: GPR was used to delineate the contact between the road fill and the underlying bench on a section of the Zeballos Canyon FSR built in 1970. Recent fill slope failures suggested the potential for more extensive instability. The purpose of the study was to assess the extent of benching in the road prism and the thickness of fill material along a 1570 in long section of the road using GPR and field observations. Two profiles along the alignment and 53 cross sections were surveyed at 30 meter intervals with 100 MHz antennae. Radar interpretation was based on identifying characteristic reflection patterns. Bedrock depth was interpreted based on identifying 1) a hyperbolic and 2) a strong, discontinuous, surface parallel reflection pattern. By comparing the GPR profiles with detailed topographic cross-sections, we were able to make specific recommendations for follow up work. The effective use of GPR requires an interdisciplinary approach, combining geophysics, terrain analysis, and road engineering. Bringing these skill sets together, these case studies demonstrate that GPR is an efficient, non-destructive and cost effective tool in characterising the nature of slope stability problems as they relate to road construction in the forestry geotechnical sector.
