Geotechnical classification and characterisation of materials for stability analyses of large volcanic slopes

The main challenge faced by scientist studying large volcanic landslides is appropriate geomechanical characterisation of volcanic materials. Regardless of its significance, published data is scarce and does not present standard classifications or procedures. Herein a new global unifying geotechnical classification of volcanic materials encompassing geotechnical units potentially found in volcanic areas is proposed. Volcanic materials are divided into four main geotechnical units: lavas, autoclastic breccias, pyroclastic rocks and volcanic soils, which are further subdivided on the basis of hydrothermal alteration, welding and interlocking. A combination of new data from the slopes of Teide stratovolcano in Tenerife, Spain, and normalised reworked data from a very extensive literature review are presented for each geotechnical unit. Most rock matrix strength measurements in Tenerife were carried out with indirect methods (Schmidt hammer and point load tests) due to the remoteness of most outcrops and the abruptness of volcanic terrain. Rock mass quality was assessed in terms of the geological strength index (GSI). Lavas and strongly welded pyroclastic rock units have the highest uniaxial compression strengths (σci = 107 and 82 MPa, respectively) although these values are largely affected by hydrothermal alteration. Lavas have a worse rock mass quality (GSI = 55) than strongly welded pyroclastic rocks (GSI = 80). Special emphasis has been placed on studying autoclastic breccia and weakly welded and/or interlocked pyroclastic rock units, which despite having been classified as disintegrated granular units in the past, are here classified as rock masses due to the high degree of interlocking between clasts. They present relatively weak rock matrices (σci = 29 and 13 MPa, respectively) but good rock mass qualities (GSI = 60 and 80). Volcanic soils are the weakest materials found within edifices and therefore appear to be significant in the generation of large slope instabilities. The effects of high temperatures and pressures are discussed, as are the problems which arise when calculating equivalent linear Mohr–Coulomb geotechnical parameters for those rock masses which are characterised as Hoek and Brown materials.