Rock mass strength assessment and significance to edifice stability, MountRainier and Mount Hood, Cascade Range volcanoes

Catastrophic edifice and sector failure occur commonly on stratovolcanoes w orldwide and in some cases leave telltale horseshoe-shaped calderas. Many o f these failures are now recognised as having resulted from large-scale lan dsliding. These slides often transform into debris avalanches and lahars th at can devastate populations downstream of the volcano. Research on these p henomena has been directed mainly at understanding avalanche mechanics and travel distances and related socioeconomic impacts. Few investigations have examined volcanic avalanche source characteristics. The focus of this pape r is to 1) describe a methodology for obtaining rock strengths that control initial failure and 2) report results of rock mass strength testing from M ount Rainier and Mount Hood. Rock mass and shear strength for fresh and hyd rothermally altered rocks were obtained by 1) utilizing rock strength and s tructural information obtained from field studies and 2) applying rock mech anics techniques common in mining and civil engineering to the edifice regi on. Rock mass and intact rock strength differences greatly in excess of one order of magnitude were obtained when comparing strength behavior of fresh and completely altered volcanic rock. The recognition and determination of marked strength differences existing on the volcano edifice and flank, whe n combined with detailed geologic mapping, can be used to quantify volcano stability assessment and improve hazard mitigation efforts.