BOGACHIEL LANDSLIDE: MATERIAL-DEPENDENT CONTROL ON DEFORMATION BEHAVIOR

A recent investigation on the western Olympic Peninsula of Washington has been prompted by the reactivation of a portion of a large deep-seated landslide complex and its potential to sever highway US 101. The reactivated portion is about 700 feet in width and length. At this location, the highway is sited midslope on a steep, 300-foot-high valley wall adjacent to the Bogachiel River. Average annual precipitation is nearly 120 inches, about 75% of which falls between October-March; summers are typically dry. Landsliding is occurring within very weak, intensely sheared marine siltstones.

Oddly, initial activity in late August 2004 involved a rapid slump-earthflow within a small area of the reactivated landslide. Subsequent deformation monitoring of the larger reactivated landslide has identified three to four failure zones of either discrete shears or broad zones of creep-like movement to depths of 100 to 130 feet. Three inclinometers show nearly uniform deformation rates within all failure zones throughout the year. Proximal shallow and deep piezometers within this larger slide mass generally exhibit comparable response and water levels, supporting a relatively simple model of a single phreatic, mostly shallow surface within the slide mass; no pressurized conditions were encountered. Seasonally, groundwater levels vary from as much as 40 feet in the recharge area just above the headscarp to less than 6 feet within much of the slide mass. While groundwater levels within the slide mass are characteristically non-responsive to storm events, many of the horizontal drains installed in the upper half of the slide mass respond rapidly.

The drain response suggests that infiltration and recharge into the sheared siltstone bedrock occurs relatively rapidly; discharge, mostly in the form of seeps and springs, must then moderate fluctuation of groundwater levels within the slide mass. The year-round saturation of much of the slide and accelerated movement of the August 2004 slump-earthflow, suggests that the deformation behavior of the slide mass is more material-dependent than groundwater-dependent. That is, at some indeterminate period of material strain, a dramatic yield state seems to occur. This condition complicates the prediction of mid- to long-term behavior and urgency to enact costly remediation.