INFLUENCE OF GEOLOGIC STRUCTURE AND ALTERATION STRENGTH ON EDIFICE FAILURE MODE AT MT. BAKER, WASHINGTON

Data and rock samples obtained from Sherman Crater in August 2006 combined with laboratory testing and computer modeling has constrained past and future failure geometries at Mt. Baker. Analysis of discontinuity mapping from the upper edifice indicates that shallow dipping and vertical to sub-vertical major weakness “planes” are in abundance. The distribution of hydrothermal alteration localized on these surfaces may have enabled the subsurface discontinuity geometries to control edifice failure in past and potential future collapses.

A range of fresh to highly altered rock and rock discontinuity data at three sites at and near Sherman Crater and from the vicinity of the Roman Wall was collected for strength testing and analyses. Extensive laboratory work including point load and Schmidt hammer testing, direct shear testing of rock and soil samples, soil gradation, and XRD analysis was completed. The data enabled the construction of slope stability models of the upper portion of Mt. Baker. The stability modeling was constrained by a) the volumes of past Holocene failures, b) the shear strength calculated from sample strength testing, c) seismic activity and d) subsurface failure geometry imposed by geologic structure. On-going modeling of potential slope failures under different geologic uncertainties including fluid pressure and seismic loading is underway.