ISSN: 1402-1544 ISBN 978-91-7439-XXX-X Se i listan och fyll i siffror där kryssen är DOCTORAL T H E SI S Department of Civil, Environmental and Natural Resources Engineering Division of Soil Mechanics and Foundation Engineering - Division of Structural Engineering Progressive Landslides in Long Natural Slopes ISSN: 1402-1544 ISBN 978-91-7439-283-8 Formation, Potential Extension and Configuration of Finished Stig Bernander: Progressive Landslides Stig in Bernander: Long Progressive Natural Slopes Luleå University of Technology 2011 Slides in Strain-Softening Soils Stig Bernander Doctoral Thesis Progressive Landslides in Long Natural Slopes Formation, Potential Extension and Configuration of Finished Slides in Strain-Softening Soils Stig Bernander Division of Soil Mechanics and Foundation Engineering Division of Structural Engineering Department of Civil, Environmental and Natural Resources Engineering Luleå University of Technology SE-971 87 Luleå Sweden Progressive Landslides in Long Natural Slopes Formation, Potential Extension and Configuration of Finished Slides in Strain-Softening Soils STIG BERNANDER Avdelningen för geoteknik i samverkan med Avdelningen för konstruktionsteknik Institutionen för samhällsbyggnad och naturresurser Luleå Tekniska universitet Akademisk avhandling som med tillstånd av Dekanus för Tekniska fakulteten vid Luleå tekniska universitet för avläggande av teknologie doktorsexamen kommer att offentligt försvaras: Tisdagen den 16 augusti 2011, kl 10.00 i Sal F1031, Luleå tekniska universiteet Opponent: Professor Steinar Nordal, Institutt for Bygg, anlegg og transport, NTNU, Trondheim, Norge Betygsnämnd: Dr Hans-Petter Jostad, Norges Geotekniske Institutt (NGI), Oslo, Norge Professor Stefan Larsson, Inst. för byggvetenskap, Avd. för jord- och bergmekanik, KTH, Stockholm Professor Claes Alén, Inst. för bygg- och miljöteknik, Avd för geologi och geoteknik, Chalmers tekniska högskola, Göteborg Professor Ola Dahlbom, Inst. för byggnadsmekanik, Lunds tekniska högskola, Lund Professor Thomas Olofsson, Inst. för samhällsbyggnad och naturresurser, Avd. för byggkonstruktion och byggproduktion, LTU Docent Lars Bernspång, Inst. för samhällsbyggnad och naturresurser, Avd. för byggkonstruktion och byggproduktion, LTU (Ersättare) Tryck: Universitetstryckeriet, Luleå 2011 ISBN: 978-91-7439-283-8 ISSN: 1402-1544 www.ltu.se The photo on the cover illustrates the Surte Landslide on September 29, 1950, in the valley of Göta River some 10 km north of Gothenburg, Sweden II Preface In May 2000, I presented a licentiate thesis “Progressive Landslides in Long Natural Slopes”, LTU 2000:16. Already at this time it was my intention to up-date this edition in various ways – primarily in respect of addressing also up-hill progressive (or retrogressive) slides. Yet, other commitments delayed the work on up-hill slides until mid 2005. During the years 1978 to 1989 the author conducted a research program focused on the possible effects of brittle failure mechanisms in natural slopes of highly strain-softening clay. The analytical approach, on which the LTU 2000:16 licentiate thesis was essentially based, had been briefly published on various international conferences among other Xth ICSMFE, Stockholm, (1981), NGM, Linköping, (1984), IVth ISL Toronto, (1984), XIth ICSMFE, San Fransisco, (1985), NGM, Oslo, (1988), XIIth ICSMFE, Rio de Janeiro, (1989). A relatively simple computer program addressing these issues was developed already in 1981. However, a more sophisticated 2-dimensional Finite Difference version, developed in the years 1984-1985, was first published in Oslo 1988. However, the engineering department of Skanska Väst AB – then a subsidiary of Skanska Ltd, (a leading Swedish contracting company) applied this computer software to a number of practical cases in the mid-eighties both on behalf of Skanska as well as of the Swedish Geotechnical Institute (SGI). Yet, although the principles of brittle failure in soft sensitive clays have neither been rejected nor considered inconceivable by most soil mechanics engineers, little R & D was conducted before the turn of the century. However, since about 2003, intensified R & D on the topic of progressive failure in landslide formation is ongoing in several countries, particularly in Norway, Canada, Italy, and Switzerland. Geotechnical analysis of slope failure has of course many traits in common with various types of progressive or brittle failures in other disciplines of structural mechanics. Yet, the analysis of stability of long natural slopes harbours some rather specific additional complications. The strength parameters required are for instance strongly dependant on conditions that, for a number of reasons, are often not easy to define with sufficient accuracy in natural soil deposits. Such conditions are for instance: - The crucial – but often difficult – task of establishing the in-situ state of stress in accordance with past geological history, erosion, hydrology and other contributing agents. - Time dependent strain-/ and deformation-softening that is strongly dependent on the rate of load application, as well as on drainage conditions in the potential failure zone. - Loss of available shear resistance in over-consolidated clay on account of past and ongoing deformations and due deformation-softening closely related to the degree of over- consolidation (OCR). III - Progressive failure, being time-dependent, tends to develop in distinctly different phases, in which the conditions governing landslide development may vary widely. This implies in effect that the risk related to progressive landslide failure cannot be clearly defined on the basis of just a singular static condition or event. The safety criteria and basic State-of-the-Art research related to slope stability has in practical soil mechanics engineering long been adapted to the principle of the perfectly plastic equilibrium failure condition. In the opinion of the author the complications listed above demand new definitions for safety criteria, modified procedures for soil investigations and laboratory testing as well as radically different appraisal of the possible impact of local additional load effects. Hence, even for the engineer who recognizes the phenomenon of brittle slope failure its implications for practical engineering is hardly a straight forward procedure, as the entire philosophy related to landslide hazard is significantly changed. The objective of the present document is to highlight the complexity of progressive slope failure development, hopefully leading to improved understanding of the issues involved and to recognized investigation procedures. So although the Finite Difference method (FDM) applied is basically the same as the one developed in the mid-eighties, the present document largely focuses on various phenomena, conditions and failure criteria that are closely related to landslide formation in soft sensitive or in highly deformation-softening over-consolidated clays. For instance, importantly, the FDM-approach not only expressively predicts the high vulnerability of some slopes to local additional loading, but also compellingly explains the massive spread of downhill progressive landslides over large areas of level ground to great depth - and that already in terms of static loading. Analysis of case records and theoretical exemplifications over the years have rendered experience of brittle slope failure that believably may be of interest to practicing engineers and to those responsible for on-going and future R & D. MSc and PhD courses in Soil Mechanics and Fracture Mechanics have been conducted at LTU. These courses have proved to be valuable for the understanding among students of the principles and the complexity of these issues. The methods have been applied with easy-to- use spread sheets. *************** As mentioned above, work on up-hill progressive slides was performed in mid 2005. In October 2005, I was invited by professor Serge Leroueil (at ‘Faculté des sciences et de génie’, Laval University, Québec) to hold a few lectures on the topic of progressive failure formation. On this occasion there was also time for personal communication on this subject and existing computer software on both downhill and uphill progressive failure analysis was made available to the faculty for the intended study of the Saint-Barnabè-Nord landslide (December, 2005). The results of the investigation of Saint-Barnabè-Nord slide were presented in 2007 as a master thesis by Ariane Locat (Etude d´un Étalement Latéral dans les Argiles de l´Ést du Canada et de la Rupture Progressive), where the slide was explained in terms of an uphill IV progressive (or retrogressive) landslide. For the part of this comprehensive study dealing retrogressive failure analysis, the Finite Difference approach presented in the current document was applied. In view of the good progress in this field of geotechnical engineering being made by young researchers, I personally decided not to focus my further studies on retrogressive failure formation thus leaving them in the state they had reached in mid 2005. Acknowledgements First of all, I am greatly indebted to emeritus professor Lennart Elfgren, Division of Structural Engineering, LTU, not only for having initiated the inception of this project but also for his constant and inspiring support. Without his dedicated commitment, this document would not have come into being. Furthermore, I want to thank professor Sven Knutsson, Division of Soil Mechanics and Foundation Engineering,