A logging road was constructed in a back-country drainage on US Forest Service land in western Montana.  The contractor was unaware that the slope was highly unstable, and shortly after the toe of the slope was cut, the hillside catastrophically failed.  After several years of continued instability it was determined that dewatering structures should be designed and installed on the slide.

ECHOTECH collected 10 electrical resistivity profiles over the most critical areas of concern in an effort to image the structure of the bedrock and overburden material.

Liquefaction of the supersaturated overburden occurred easily (even by jumping on it) in many locations, which added to the difficulty of this deployment.  There was excellent contrast, however, in the conductivities of the saturated slide material and the bedrock schist.

The cross-section below was collected from Profile 8 in the center of the lower study area.  The image has been rotated 27 degrees to match the slope of the slide.  The model shows that the bedrock structural planes (red) are horizontal and that distinct steps, or benches, exist which are acting as water collection zones (blue-green).  The dewatering drain pipes were designed to be located within these zones which will allow water to be wicked from the slide face and channeled to the adjacent creek.