The Håkåneset rockslide, Tinnsjø Stability analysis of a potentially rock slope instability. Inger Lise Sollie Geotechnology Submission date: June 2014 Supervisor: Bjørn Nilsen, IGB Norwegian University of Science and Technology Department of Geology and Mineral Resources Engineering I II ABSTRACT The Håkåneset rockslide is located on the west shore of Lake Tinnsjø (191 m.a.s.l), a fjordlake stretching 32 km with a SSE-NNW orientation in Telemark, southern Norway. The instability extends from 550 m.a.s.l. and down to approximately 300 m depth in the lake, making up a surface area of 0.54 km2 under water and 0.50 km2 on land. The rockslide comprises an anisotropic metavolcanic rock that is strongly fractured. Five discontinuity sets are identified with systematic field mapping supported by structural analysis of terrestrial laser scan (TLS) data. These are interpreted as gravitationally reactivated inherited tectonic structures. At the northern end the instability is limited by a steep south-east dipping joint (JF3 (~133/77)) that is one direction of a conjugate strike slip fault set (JF3, JF2 (~358/65)). Towards the south the limit to the stable bedrock is transitional. A back scarp is defined by a north-east dipping J1 (~074/59) surface that is mapped out at 550 m.a.s.l. Kinematic analysis indicates that planar sliding, wedge sliding and toppling are feasible. However, because the joint sets are steeply dipping these failure mechanisms can only occur for small rock volumes and are limited to steep slope sections only. Large scale rock slope deformation can only be justified by assuming deformation along a combination of several anisotropies. This assumption is based on the presence of the ~50-65 degrees NE dipping J1 and the up to 19 degrees NNE dipping foliation, making bi-planar sliding a feasible mechanism in case of a massive failure of the entire slope instability. Numerical modelling using Phase2 support assumption of bi-planar failure and indicate that significant rock damage by retrogressive failure mechanism is most likely for a stepped development of a basal sliding surface. The modeling results indicate that this sliding surface may daylight at a depth of ~100 m in the lake. By sensitivity tests for groundwater and different joint- and rock mass properties it is assumed that the instability is, besides the main structures, controlled principally by topography and rock strength conditions. III IV SAMMENDRAG Det ustabile fjellpartiet ved Håkåneset er lokalisert på vestsiden av Tinnsjø (191 moh.) i Telemark i Sør-Norge. Innsjøen strekker seg 32 km i SSØ-NNV retning. Ustabiliteten starter på 550 moh. og går ned til omtrent 300m dybde i Tinnsjø, og tilsvarer 0,54 km2 under vann og 0,50 km2 på land. Bergmassen i området består av en en anisotropisk metavulkansk bergart som er sterkt oppsprukket. Fem sprekkesett er identifisert ved systematisk feltkartlegging, og har blitt bekreftet med strukturell analyse av "terrestrial laser scan" (TLS). Disse er tolket som gravitativt reaktiverte tektoniske strukturer. Den nordlige avgrensingen av ustabiliteten er definert av et bratt sør-øst fallende sprekksett (JF3 (~133/77)) som er tolket som en retning av et konjugert "strike-slip" forkastningssystem (JF3, JF2(~358/65)). Mot sør er det anslått å være en gradvis overgang til stabilt fjell. I bakkant er ustabiliteten avgrenset av en bakvegg med samme orientering som sprekkesettet J1 (~074/59), og reiser seg fra ca. 550 m.o.h.. Kinematisk analyse av diskontinuitetene antyder at planær utgliding, kileformet utgliding og blokktopling er mulig. På grunn av at det bratte fallet på samtlige sprekkesett er disse bruddmekanismene mulig kun for små volum og kan kun forekomme i de bratteste delene av skråningen. For å forklare en massive utglidning av hele fjellpartiet må det antas at deformasjonen skjer langs en kombinasjon av flere anisotropier. I dette tilfellet er J1 som faller ~50-65 grader i NØ retning og det opp til 19 grader bratte NNØ fallende foliasjonen to sprekkesett som gjør bi-planar utgliding av et stort volum av fjell til en mulig bruddmekanisme. Numerisk modeliering i Phase2 støtter antakelsen om bi-planær utglidning og indikerer at utviklingen av et nedre bruddplan vil involvere betydeling ødeleggelser av intakt berg ved en retrogressiv bruddmekanisme. Resultatet fra modeleringen antyder at en sannsynlig lokalisering av et bruddplanet vil være på et nivå på ca. 100 m dybde i Tinnsjø. Sensitivitetstest av grunnvann og ulike sprekke- og bergmassestyrkeegenskaper antyder at stabiliteten av fjellsiden er kontrollert av diskontinuitetene, topografien og bergmassestyrke paramtre. V VI PREFACE AND ACKNOWLEDGEMENT This thesis is the final work of my master degree at the Department of Geology and Mineral Resource Engineering at the Norwegian University of Science and Technology (NTNU), Trondheim. The master thesis is written in collaboration with the Norwegian Geological Survey (NGU). Reginald Hermanns (head of the landslide department at NGU) and Bjørn Nilsen (professor at the Department of Geology and Mineral Resource Engineering, NTNU) have been my supervisors. I would like to thank Bjørn Nilsen and Reginald Hermanns for guiding me through the work with this master thesis. Thank you for always being so friendly set time for a meeting, to answer my questions and for all interesting discussions about geology - I have really learned a lot! I am also very grateful for the help I have got from Gro Sandøy and Thierry Oppikofer at NGU and Nghia Quoc Trinh at SINTEF when applying the different software techniques. Finally, I would like to speak to my classmates at NTNU. Thank you for early mornings and late night at "Lesesalen", ice cream in the sun, dancing in T-Town, good memories from field excursions and the years we have had together in Trondheim. You rock! Trondheim, 10.06.2014 Inger Lise Sollie VII VIII TABLE OF CONTENTS Abstract ................................................................................................................................... III Sammendrag ............................................................................................................................. V Preface and acknowledgement ............................................................................................. VII Table of Contents ..................................................................................................................... IX 1 Introduction ....................................................................................................................... 1 1.1 Systematic mapping approach of large unstable rock slopes in Norway ........................ 1 1.2 Background .......................................................................................................................... 3 1.3 Aim and restrictions of the study........................................................................................ 3 1.4 Available data and site specific literature .......................................................................... 6 1.5 Previous work ....................................................................................................................... 7 1.5.1 Gvålviknatten rock fall monitoring project, Norwegian Public Roads Administration (Statens Vegvesen) ...................................................................................................................................................... 7 1.5.2 Periodic monitoring with terrestrial laser scan (TLS), NGU ........................................................... 7 1.5.3 dGNSS displacement measurements (NGU, UiO) .......................................................................... 8 1.5.4 Student project assignment: Geological investigation of Håkåneset ............................................... 8 2 Site information: Regional and geological settings ......................................................... 9 2.1 Location and topography .................................................................................................... 9 2.2 Climate and hydrogeological conditions .......................................................................... 11 2.3 Geology ............................................................................................................................... 14 2.4 Historical events ................................................................................................................. 15 3 General background about large natural rock slope instabilities ................................. 17 3.1 Development and definition of rockslide ......................................................................... 17 3.2 Causes and controlling factors of rock slope instability ................................................. 19 3.3 A case study of a subaerial-subaquatic rockslide: The Hochmais–Atemkopf rockslide system, Austria ................................................................................................................................ 19 4 Methods used of stability assessment of the Håkåneset rockslide ................................. 21 4.1 Digital elevation model (DEM) analysis ........................................................................... 21 IX 4.2 Terrestrial Laser scan (TLS) analyses ............................................................................. 21 4.2.1 Structural analysis in Coltop-3D ................................................................................................... 22 4.2.2 Displacement