Geotechnical analysis of large volcanic landslides: The La Orotava events on Tenerife, Canary Islands. A dissertation submitted to the TECHNICAL UNIVERSITY OF CATALONIA for the degree of Doctor of Sciences (Geologic) presented by Marcel Hürlimann Supervisors: Dr. A. Ledesma Dep. of Geotechnical Engineering and Geosciences, Technical University of Catalonia Dr. J. Martí Institute of Earth Sciences 'Jaume Almera', Spanish Research Council Barcelona October, 1999 Geotechnical analysis of large volcanic landslides: The La Orotava events on Tenerife, Canary Islands. Marcel Hürlimann "Getöse, Krachen und Geprassel erfüllt wie tief brüllender Donner die Luft – erschüttert jedes lebende Ohr und Herz, und tönt im Wiederhall von tausend Bergesklüften noch grässlicher." Schuttbuch des Goldauer Bergsturzes (Dr. Karl Zay, 1807) ACKNOWLEDGEMENTS This study was supervised by Alberto Ledesma from the Department of Geotechnical Engineering and Geosciences, School of Civil Engineering, 'Universitat Politècnica de Catalunya' (UPC) and Joan Martí from the Institute of Earth Sciences 'Jaume Almera', Spanish Research Council (CSIC). I owe many thanks to them and my Ph.D. would not have been the same without their continuous support and encouragement. To combine the ideas of an engineer and a volcanologist was not always easy, but the inter-disciplinary structure certainly improved the results of this work. I am very grateful to many people at the Department of Geotechnical Engineering and Geosciences (UPC) and the Institute of Earth Sciences 'Jaume Almera' (CSIC) who helped me in various ways. Special thanks to Antonio Lloret, Tomás Pérez and José Álvarez who introduced me into the design and handling of the apparatuses of the geotechnical laboratory and to Ignasi Queralt who carried out the X-ray analyses. I want to extend my thanks to the many colleagues who joined me during the Ph.D. including Silvia Zafrilla, Arnau Folch, Carles Soriano, Elisenda Turon, Dioni Cendon, Giray Ablay, Fernando Resta and François Legros at the CSIC and Joan Rius, Mauricio Barrera, Luciano Costa and José Moya at the UPC. Special thanks are due to Josep-Oriol García for his contribution to the mobility analysis and laboratory work. José-Manuel Navarro and the staff of the 'Consejo Insular de Aguas del CABILDO Tenerife' are appreciated for facilitating the visits of some water tunnels on the island and giving many useful hints on the underground geology around the La Orotava valley. Luis García-Cacho is thanked for many helpful discussions on volcanic landslides and making the bathymetric and topographic data available. Francisca Gómez is acknowledged the preparation of some GIS data. This study was supported by several financial sources. In the first place, I acknowledge to the Swiss National Science Foundation and also to the Spanish Research Council (CSIC) / the Swiss Federal Institute of Technology Zurich for paying my salary and travelling expenses. Additional help came from the Werner Steiger Foundation and the EC contract EV5V-CT-0283 / the CICYT project AMB96- 0498-C04. I would also like to express gratitude to my parents Karl and Ida and to my brothers Stefan and Guido who motivated my university education and assisted me generously. To my beloved Nieves, all my thankfulness for her faith in me and unconditioned support. i ii TABLE OF CONTENTS ABSTRACT vii LIST OF FIGURES viii LIST OF TABLES xii 1 INTRODUCTION 1 1.1 Hazards from large volcanic landslides 1 1.2 Problem 1 1.3 Test site 2 1.4 Goals 2 1.5 Thesis structure 3 2 STATE OF THE ART 5 2.1 Terminology of mass movements 5 2.2 Examples of large landslides 6 2.3 Causes of large landslides 11 2.3.1 Non-volcanic factors 13 Morphologic factors 13 Geologic factors 13 Hydrologic and hydrogeological factors 14 Sea level changes 14 Tectonic seismicity 14 2.3.2 Volcanic factors 16 Explosive eruptions 17 Dike intrusion 17 Caldera collapse episode 18 Volcanic seismicity 19 Hydrothermal activity 21 Stress regime 22 2.3.3 Geotechnical considerations 23 Progressive failure 23 Undrained loading 24 3 SITE INVESTIGATION 25 3.1 Introduction 25 3.2 The Canary Islands 26 3.3 Tenerife Island 31 3.3.1 Geological evolution 32 3.3.2 Morphologic features 34 3.3.3 Volcanic activity 36 3.4 Geomorphologic analysis 38 3.4.1 Tenerife Island 39 3.4.2 Northern flank of Tenerife 41 3.4.3 The La Orotava valley area 47 3.5 Geologic analysis 65 3.6 Assumed preslide conditions 77 3.7 Conclusions 79 iii 4 LABORATORY ANALYSIS 83 4.1 Introduction 83 4.2 Methodology 86 4.2.1 Sampling 86 4.2.2 Analysis of mineralogy, chemistry and microstructure 94 4.2.3 Geotechnical analysis 94 Soil characterisation 95 Consolidation tests 95 Direct shear test 96 Ring shear test 99 Triaxial test 99 4.3 Results 102 4.3.1 Analysis of mineralogy, chemistry and microstructure 102 Mineralogical and chemical analysis 102 Microstructure 108 4.3.2 Geotechnical tests 113 Soil characterisation 113 Consolidation tests 115 Direct shear tests 120 Ring shear test 131 Triaxial tests 132 4.4 Conclusions 148 5 STABILITY ANALYSIS 151 5.1 Introduction 151 5.2 Mechanisms applied 152 5.2.1 Seismic shocks 152 5.2.2 Dike intrusion 154 5.2.3 Caldera collapse event 155 5.3 Approaches 155 5.3.1 Global limit analysis 155 Limit Equilibrium Method 156 Upper Bound Method 159 5.3.2 Continuum analysis 161 Finite Difference Method 162 Finite Element Method 163 5.4 Model description 164 5.4.1 Global limit analysis 165 5.4.2 Continuum analysis 167 5.5 Results 169 5.5.1 Global limit analysis 170 Limit Equilibrium Method 170 Upper Bound Method 177 5.5.2 Continuum analysis 182 Finite Difference Method 182 Finite Element Method 183 5.6 Conclusions 190 iv 6 MOBILITY ANALYSIS 193 6.1 Introduction 193 6.2 Model description 195 6.3 Results 197 6.4 Conclusions 200 7 CONCLUSIONS 201 8 REFERENCES 205 APPENDIX 1: PUBLICATIONS ARISING FROM THIS THESIS 219 v vi ABSTRACT Large volcanic landslides are one of the most hazardous of geological processes. They have occurred about once every 25 years during the last 500 years, and are a serious risk for the population due to their great volume and mobility. In spite of their destructive potential there are few comprehensive studies analysing large landslides on volcano flanks, and the mechanisms of such mass movements are not yet resolved. Within the last few years, several hypotheses concerning the potential causes of volcanic landslides have been proposed including processes such as dike intrusion, volcanic spreading, hydrothermal alteration, seismic shocks and caldera collapse events. Tenerife exhibits three large subaerial valleys originated by giant flank failures with ages ranging from Upper Pliocene to Middle Pleistocene. The northern submarine flank of the island is characterised by a voluminous apron of landslide debris. The La Orotava valley has been selected for analysis due to the amount of available data concerning its structure and evolution, and has been used as a test site to validate new assumptions that could be applied to other volcanic areas. The site investigation has revealed that the present morphology of the La Orotava valley was formed by two different failures: one in the western sector and the other in the eastern sector. The mechanical stability of the preslide volcano flank was strongly reduced by geologic, morphologic, climatic and volcanological factors which play a fundamental role in the initiation of the landslides. Widespread residual soils (paleosols) might have acted as potential slip surfaces, while deep erosive canyons probably evolved into the lateral limits of the failures. A high coastal cliff and a humid climate have also contributed to the critical stability conditions. The location of the landslide amphitheatre is perpendicular to the active Dorsal rift zone and adjacent to the Las Cañadas caldera, both important influences on the stability of the volcano slopes. On Tenerife, the relationship between large volcanic landslides and vertical caldera collapses is supported by a temporal coincidence of at least two failures with caldera collapse events. The mechanical behaviour of a residual soil sampled in the La Orotava valley has been analysed. Red coloured residual soils are generally located at the top of phonolitic pyroclastic deposits and are proposed as potential slip surfaces due to their very weak behaviour and their flat, homogeneous characteristics. They represent the only planar surface within the succession making up the volcano slopes. Their weak mechanical behaviour is characterised by volumetric collapse during shearing, a substantial reduction of shear strength for high normal stresses, and a significant increase of pore water pressure during undrained loading. The last feature is fundamental to the stability of volcano flanks since it strongly reduces the soil strength. Earthquakes, common processes in active volcanic areas, and saturated conditions can generate high excess pore pressures indicating the importance of regional climate and seismicity. The stability analysis has considered three different mechanisms: 1) ground acceleration due to seismicity - including both tectonic earthquakes and volcano-tectonic seismic shocks produced by caldera collapse; 2) horizontal stress due to dike intrusion, and; 3) vertical shear stress due to caldera collapse. The results indicate that ground acceleration principally decreases the mechanical stability of volcano flanks, enabling failure. Horizontal stresses due to dike intrusion can also influence slope stability, but preferably act as a preparing factor destabilising the slope, and not as a final triggering mechanism. The 3D numerical simulations show the significant effect of deeply incised canyons creating high shear stress at their base. Applying the results to the La Orotava events, the following scenario is assumed: First, deep narrow canyons, weak residual soils, humid climate, coastal cliff and persistent dike intrusion have significantly reduced the mechanical stability of the volcano slope and determined the limits of the failing mass.