ROCK SLOPE STABILITY ANALYSIS ALONG EAGLEVIEW ROAD ON RACEHORSE MOUNTAIN, NORTHWESTERN WASHINGTON STATE

Landslides and rockfall are common hazards in the Chuckanut Formation of northwestern Washington State, especially along transportation corridors in steep terrain. The Chuckanut Formation consists of Eocene age fluvial sedimentary deposits, and is composed of massive conglomerate, coarse arkose, medium to fine grained sandstone, siltstone and mudstone with local coal beds. Tectonic uplift has caused anticlinal folding, fault block rotation and other structural deformations in the Chuckanut Formation forming discontinuity patterns susceptible to failure. Our objective is to characterize the rock mass failure potential of steep roadcuts in the Chuckanut Formation along Eagleview road on Racehorse Mountain in Whatcom County, WA.

Kinematic analyses were conducted with 195 surface discontinuity orientations measured on seven outcrop sectors (about 0.5 miles) along Eagleview road to determine rock slide and toppling potential. Slopes that were considered kinematically unstable were further analyzed with limited equilibrium analyses to establish a factor of safety, probability of failure, and sensitive parameters related to rock block failures. Unstable wedges were modeled with the mean plane of joint sets for each outcrop sector and the assumption that discontinuities were continuous and had no cohesive strength. The kinematic and limited equilibrium analyses were conducted in the Rocscience software program suite using DIPS© and SWEDGE© and a range of literature values for sliding friction, unit weight, and seismic ground shaking magnitudes.

Modeling results indicate that four out of five kinematically unstable roadcuts have a probability of failure greater than 50% under static loading conditions. This is contradictory to field observations, where only one major failure along Eagleview was observed. We found that major discontinuity sets are dependent on the rock quality; with poor rock quality favoring bedding discontinuities and stronger rock quality favoring tectonic tensile fractures perpendicular to bedding. The scope of our study was limited to surface orientation data; therefore, bore-hole data, rock strength measurements, and a more thorough evaluation of discontinuity spacing and conditions would improve the rock-mass failure assessment.