A STRATIGRAPHIC AND MULTI-GEOPHYSICAL APPROACH TO LANDSLIDE INVESTIGATION: THE QUESNEL FORKS LANDSLIDE

A site investigation was conducted at a landslide located on the Quesnel River, British Columbia, Canada. The landslide and consequent damming of the river occurred on April 28th, 1996. Subsequently, the diverted river course resulted in bank-side erosion and is now impacting an important historical site. Stratigraphic and geophysical studies were conducted on the terrace and landslide to gain a better understanding of the triggering mechanisms and failure process and to better prepare for future failures in the area. This study represents the first successful integration of several multi-parameter geophysical techniques complementing sub-surface field observations at such a landslide.

The stratigraphy of the terrace primarily consists of advance-phase glacial sediment deposited during the last glaciation. The units include: interbedded silty sand and clay, pebble gravel, laminated sand and a thick (~20 m) massive clay. The surface of the terrace is composed of coarse recessional glaciofluvial gravel. The landslide is about 340 m wide and 400 m long and consists of two blocks representing approximately 1.7 x 106 m3 of displaced sediment.

Ground penetrating radar (GPR), direct current (DC) resistivity and seismic refraction and reflection surveys were conducted. Rupture surfaces and separation planes were identified in addition to the geometry of stratigraphic units. The rupture surfaces dip 40ºN and 65ºN for the upper and lower blocks, respectively. Both ruptures cut through all stratigraphic units except the lowest most unit. The toe of the surface of separation was 10 m deep and shallows away from the landslide.

The presence of a clay unit high in the stratigraphic succession creates the potential for failure. A combination of the removal of lateral support by toe erosion and loading of terrace sediment by a perched water table are thought to be the triggering mechanisms. As long as the landslide toe remains stable, further terrace failure is unlikely to occur.