This report presents the results of a study into road construction practices and the terrain attributes associated with road fill landslides on moderately steep to steep terrain in a portion of the Kalum Forest District, near Terrace, B.C. The influence of terrain, road construction methods, road maintenance, and drainage alteration were examined by comparing attributes at 40 road fill landslide sites to the same attributes at 89 randomly selected, non-landslide sites. The majority of the forest roads examined were constructed before implementation of the Forest Practices Code in 1995 and are about 10-40 years old. The older roads were constructed using bulldozers and more recently constructed roads used excavators. In general, roads selected for the study were not deactivated and the majority were inactive and not maintained.
Most of the road fill landslides observed likely occurred because of the loss of strength in the foundation soils, rather than in the fill soils. The most probable triggers of road fill landslides are related to poor control of surface and subsurface water flows including water concentration and diversion, a reflection of inadequate drainage construction, maintenance, and deactivation. A comparison of terrain and road attributes at road fill landslides sites and null sites or sites that have not experienced landslides is provided. Those road and terrain factors or attributes that were found statistically associated with road fill landslides on moderately steep to steep slopes in the study area include:
- terrain containing natural slope instability;
- gullied terrain and deep surficial materials located on an escarpment or straight slope;
- over-steepened fill greater than 2 m in height, supported by logs, trees, stumps, or woody debris (perched fill);
- poor ditch conditions and poor drainage control; and
- terrain classified as moderately or imperfectly drained.
Those factors or attributes that were not found statistically associated with road fill landslides include:
- the simple presence of wood in the fill;
- the presence of cracks in the road surface; and
- the width of the road.
Reinforced soil fills are discussed as an alternative to full bench and 3/4 bench cut and endhaul construction. These methods where applicable could involve the use of wood (logs) as reinforcement material in construction of the road fill slope. However, a geotechnical engineer must take responsibility for the terrain assessment, design, and construction of an engineered reinforced earth-filled structure. Road drainage control and maintenance of surface and subsurface water is paramount to prevent fill slope landslides.
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Updated May 11, 2007