Limit equilibrium analysis of the May 18, 1980 rockslide-avalanche at Mount St. Helens, Washington, utilizing geotechnical strength data

The largest historical rockslide-avalanche occurred on May 18 th, 1980 as part of the eruption of Mt. St. Helens. The retrogressive failure had a volume of approximately 2.8 km3 and decreased the elevation of the volcano by approximately 400 m. The flank collapse prompted a reappraisal of theories regarding volcano morphology, construction, and hazard assessment. Previous stability assessments of the volcano have been produced using small-scale shear tests on the reworked avalanche deposits. This study utilized strength data from (1) large and small-scale shear testing on reworked avalanche deposits and (2) uniaxial, triaxial and joint shear strength of collected in-situ samples of fractured domes and dykes. Laboratory strengths, a simplified cross section of the volcano, valid earthquake loadings, pore pressures, and different water tables were inputted into a limit equilibrium stability program to assess stability. The results show the importance of the afore mentioned factors on stability of the deformed May 1980 edifice.