DEEP-SEATED LANDSLIDE REACTIVATION: AN INTERAGENCY APPROACH TO INVESTIGATING GEOLOGIC CONTROLS AND THE EFFECTS OF FOREST CANOPY REMOVAL

Within the last 15 years, 12 of 21 deep-seated landslides that have impacted state highways occurred down slope of or within areas logged in the preceding decade. Mitigation for the associated landslide damages totals in the tens of millions of dollars.

The Washington State Dept. of Transportation (WSDOT) collects and maintains a large quantity of data for these highway-impacting landslides. The Washington State Dept. of Natural Resources (WADNR) has limited funds and opportunity for drilling and borehole monitoring; however, is obligated under its Habitat Conservation Plan to monitor the effectiveness of slope hazard mitigation of forest practice activities. Therefore, both agencies have a strong practical interest in understanding the effects of forest canopy reduction on groundwater hydrology as it relates to the stability of deep-seated landslides. This creates opportunities for interagency collaboration in research, data collection, analysis, and mitigation.

Boreholes drilled by WSDOT after landslide reactivation are instrumented and monitored to characterize the landslide and develop slope stabilization designs. Precipitation, geologic, piezometer, and inclinometer data are collected at the site. Some sites are also monitored after stabilization work, to assure effective landslide stabilization. These data provide an opportunity to evaluate the sensitivity of deep-seated landslides to changes in groundwater hydrology, and indirectly, to assess the effects of forest canopy removal. Groundwater levels and porewater pressures in glacial deposits and in deeply weathered marine sedimentary rocks are known to vary widely (temporally and spatially) in response to precipitation. However, characterizing a particular landslide might provide a way to successfully extrapolate slope stability and sensitivity analyses to similar slide-prone stratigraphy elsewhere.