SLOPE STABILITY EVALUATION: MORE OBSERVATION AND LESS CALCULATION

Transcription

1 SLOPE STABILITY EVALUATION: MORE OBSERVATION AND LESS CALCULATION Guy Lefebvre Université de Sherbroke, Sherbrooke, Québec, Canada, Denis Demers Ministère des Transports, Quebec City, Quebec, Canada Serge Leroueil Université Laval, Quebec City, Quebec, Canada Denis Robitaille, Catherine Thibault Ministère des Transports, Quebec City, Quebec, Canada RÉSUMÉ Les difficultés reliées au calcul du coefficient de sécurité pour statuer sur l'état de stabilité d'un talus argileux sont bien connues. L expérience québécoise des trente dernières années montre la grande variabilité dans les études géotechniques réalisées et les conclusions sur la stabilité, dépendamment des individus impliqués. Afin d'uniformiser les procédures d'étude, le gouvernement du Québec est à préparer un manuel pour l évaluation de la stabilité des talus dans les argiles marines sensibles post-glaciaires. Lorsque les conditions géologiques sont relativement uniformes à travers un territoire, comme c'est le cas pour les dépôts argileux du Québec, les observations qui peuvent être recueillies sur le terrain et au moyen de documents existants permettent de définir un ensemble d'indices qui permet une classification des talus en termes de stabilité. Le manuel recommande donc une évaluation qualitative basée sur un certain nombre de critères qui permettent de statuer sur l'état de stabilité, sans qu il soit nécessaire, dans la plupart des cas, de calculer un coefficient de sécurité. ABSTRACT The difficulties associated with the calculation of a factor of safety aiming at assessing stability conditions of slopes in clay are well known. More than thirty years of experience in the Province of Quebec have shown an important variability in the geotechnical investigations performed and in their conclusions concerning stability, depending on the people involved. With the objective to standardize these investigations, the Quebec Government is preparing a manual for evaluating of the stability of slopes in post-glacial marine sensitive clays. When geological conditions are relatively uniform, as it is the case in most clayey deposits in Quebec, the observations which can be gathered in situ and from existing documents allow the determination of a number of indices that can be used for classifying the slopes in terms of stability. The manual first recommends a qualitative evaluation of stability conditions based on some criteria without, in most cases, calculating a factor of safety. 1. INTRODUCTION A large portion of the developed territories in the Province of Québec is covered by soft sensitive clay deposits, notorious for frequent and sometime catastrophic landslides (Lebuis et al., 1983, Tavenas, 1984, Lefebvre, 1996). In the St. Lawrence Valley, these soils were deposited in large postglacial seas between and years ago. The relative retreat of the sea has left very large plains whith deep valleys, rivers and gullies. The banks of these rivers and gullies were affected by landslides early in their development until now. Any land overlooking river valleys constitutes a preferred location for residences; even in clay area (fig. 1) susceptible to landslides. To protect the citizens from the risk of being affected by landslides, legislation should prohibit construction in zones where there are landslides hazards. On the other hand, legislation should not unduly restrict land use. A compromise has thus to be found to reduce the risk, i.e. the lost associated to a landslide multiplied by its probability of occurrence, to a sufficiently low level to be acceptable to the society. Since the seventies, the Government of Québec has been involved in developing zonation maps to identify areas susceptible to landsliding in order to help the municipalities in their development (Lebuis et al. 1983) A new generation of detailed zonation maps have been under way at the Québec Government since 2003, at the scale of 1/5000 (Demers et al., this conference). The new zonation maps (as well as the older maps) prohibit construction in zone where the susceptibility of landsliding has been evaluated based mainly on geometric conditions and on the existence of sensitive clays (Robitaille et al., 2002). For example, at the top of clayey river bank of a height (H) greater than 5 m, construction is prohibited in a 2 H band behind the crest of the slope. However, this legislation offers to the citizens the possibility of having a geotechnical engineer evaluate the stability of the slope in order to state if construction can be allowed or not. The purpose of this paper is to present the development of a manual prepared by the Québec Government in order to help the geotechnical engineer in his evaluation. An expert committee was formed to prepare this document with the goal to minimize the problems generally associated with slope stability evaluation. In : J. Locat, D. Perret, D. Turmel, D. Demers et S. Leroueil, (2008). Comptes rendus de la 4e Conférence canadienne sur les géorisques: des causes à la gestion. Proceedings of the 4th Canadian Conference on Geohazards : From Causes to Management. Presse de l Université Laval, Québec, 594 p.

2 G. Lefebvre et al. 2. CONTEXT AND OBJECTIVES OF THE GUIDE 2.1 Standardization of slope stability analyses In Eastern Canada, a large expertise has been developed over the years for the stability analysis of slopes in sensitive clay deposits. Landslides and stability conditions were studied early (Hurtubise et al., 1957) but more intensively at the end of the sixties and in the seventies. Many contributions were added in subsequent years, until now (ex.: Tavenas, 1984, Lefebvre, 1986, 1996; Demers et al., 1999a; Leroueil, 2001; Leroueil, 2004;) illustrating continuous efforts over time. Figure 1. Example of houses built near a river eroding a clay plain The manual will contain several chapters, including one on the design of remedial works to correct the deficient stability of a slope and one on the retrogression which may transform a first slide into a very large earthflow. This paper presents only the guidelines for the evaluation of the stability of a slope against a deep rotational failure, as generally observed in the relatively homogeneous soft clay deposits of Eastern Canada and as traditionally assumed in slope stability analyses (fig. 2). The possibility of an afterwards retrogression process isn t addressed in the present paper. The first part of the paper describes the context and the objectives while the second part summarizes the proposed methodology for slope stability evaluation. The proposed methodology is innovative in the sense that, in most situations, the stability is evaluated qualitatively, based on existing information and field observations, without the need of a factor of safety calculation. Fairly early in the development of this methodology, the committee realized that the subjectivity associated with a qualitative evaluation of the stability of a slope was smaller than the one associated to the selection of the different parameters necessary for a numeric evaluation, in particular the prediction of the most critical groundwater conditions. Moreover, by analyzing the accumulated expertise on Québec clayey deposits, it became clear that a qualitative approach could be more easily standardized to ensure a more reliable evaluation of the stability conditions of a site. The long term stability of a slope is generally evaluated by effective stress analysis using post peak shear strength parameters and pore pressure conditions corresponding to the worse scenario. A large data base on shear strength parameters has been developed (Lefebvre, 1981) and is used to determine or to validate the shear strength parameters used in the analyses. The approach has been tested with many back analysis and there is a consensus that it allows a realistic evaluation of clay slope stability for the conditions generally encountered in Eastern Canada. Using the same approach for the analysis does not prevent a certain variation in the results, depending of the individual who performs the analysis. Professional liability is indeed a concern and the degree of conservatism may significantly vary with individual and/or firm. One of the purposes of the Government guidelines is thus to standardize the different steps of the analysis, from geotechnical investigations up to the final interpretation, in order to reach a fairly uniform evaluation of a slope irrespective of the geotechnical engineer in charge of the evaluation. Accurate guidelines published by public authorities become like official standards and are susceptible to reduce the professional liability exposure when the guidelines are followed for the investigations and analyses. 2.2 Making use of accumulated expertise The geotechnical conditions of clay deposits of Eastern Canada are relatively homogeneous due to similar and recent formation processes. The stratigraphic features controlling the ground water regime as well as its seasonal variations have been well documented in numerous investigations. The role and mechanism of toe erosion has been repeatedly observed and emphasized in slope stability studies and, in particular, in the preparation of zoning maps where the link between erosion and landslide activity is obvious. While several case records of slope failure in soft clay deposits have been published, a large number of well documented slope stability investigations and analyses remains in the files of the government agency responsible for landslide activity. One of the objectives of the manual is to build on accumulated knowledge to establish detailed procedures for the slope stability evaluation. Figure 2. Deep rotational failure (Nicolet, 2006)

3 Slope stability evaluation : more observation and less calculation 2.3 Making use of field performance Slope stability analyses in Eastern Canada are typically conducted along river banks or river valleys, the deepening of river valleys being responsible for slope formation. The 1996 floods in the Saguenay region, associated with record precipitations, induced numerous landslides. Inflitration and bank erosions having left many slopes in precarious conditions, this event triggered an intense period of geotechnical investigations, slope stability evaluations and remedial works under the direction of the.government agency for landslides (Demers et al., 1999b). The agency has remained involved in the Saguenay region for several years to help the municipalities identifying the real estate at risk susceptible to be relocated, involving an assessment of the zones at risk and initiating the development of a new generation of zoning maps. The Saguenay event and the associated studies identified a type of landslide which had not been previously well documented. The intense heavy rain had triggered shallow landslides mobilizing only the clayey crust of relatively high slopes, in old abandonned terraces or bluffs now far from any river and therefore not submitted to erosion (Perret et Bégin, 1997). The study of aerial photographs, as well as field observations, did not reveal any trace of typical deep rotational slide activity in these slopes, except for few cases with particular stratigraphic conditions (Bouchard et al., this conference). Also, many bluffs were formed during the relative retreat of the post-glacial seas and their geometry has remained the same for several centuries or millenaries. Along rivers banks, some slopes may be protected by alluvial plains since several centuries too. Experience shows that this kind of slopes did not experienced recent deep rotational slide if the vegetation and the drainage pattern aren t modified. For deep rotational slide, occurring usually along river banks where erosion is active, the properties at risk are generally located at the top of the slopes (figure 2). The database of the Ministry of Transportation of Quebec (Fortin et al., this conference) shows that the first failure of a deep rotational landslide reaches a distance over the crest of the slope never greater than the height of the slope. The majority of them reaches a distance less than half of the height. For shallow landslides affecting only the crust, damages are essentially related to the slope debris and can threaten the houses built at the toe. Previous works (Perret and Bégin, 1997; Demers et al., 1999b) shows that the debris can reach a distance at the toe equal to two times the height of the slope. Except for shallow landslides which do not affect the intact clay but are limited to the weathered crust, the ancient terraces and bluffs are considered completely stable against a classical deep rotational failure. These slopes have indeed been standing as they are for thousands of years and been tested by the most severe groundwater conditions susceptible to develop over so a long time period. Standard slope stability analyses for deep rotational slip surface would however indicate a factor of safety significantly lower than normally required. For slopes characterized by a geometry which has remained constant over a period of time sufficient to have been tested by the most severe groundwater conditions, the observations and analyses following the Saguenay event suggest that a qualitative evaluation of the stability based on field performance appears more reliable than the usual calculations of a factor of safety. 2.4 Making use of existing information In addition, to show the area at risk, the new zoning maps provide the information used to prepare them like borings, observation of aerial photos, field observations including erosion qualification, in addition to topography sufficiently accurate for slope stability analysis (Demers et al., this conference). For the regions not yet covered by the new zoning maps, one can often locate existing boring close to the site being investigated. Aerial photographs generally provided important information for the evaluation of slope stability, namely the erosion and the slide activity in the sector. One of the objectives of the manual is to make sure that information, existing or easily obtained by a site visit, is collected and analyzed before any geotechnical investigation and analysis be performed. 3. METHODOLOGY FOR THE EVALUATION OF SLOPE STABILITY The methodology proposed in the slope stability manual is based on the steps described schematically in figure 3. The first two concerns the analyses of existing information and field observations in order to arrive at a slope classification. Using this classification, based on a set of indices, the stability conditions of a slope can be considered as stable, potentially unstable or unstable. In many cases, such a qualitative evaluation is sufficient, in the context of the legislation on hazards zonation, to conclude on the stability without the need of further investigation or analysis. In some cases, the qualitative evaluation is followed by geotechnical investigation and the calculation of a factor of safety by means of stability analyses. The manual describes the different steps to calculate a factor of safety in a traditional slope stability analysis. A mandate for the evaluation of stability thus always starts by the collection of existing information. 3.1 Existing information This first phase of the study includes the analysis of different documents and site observations listed below. The zonation maps and associated documents When available, the zonation maps will locate restricted zones and identify, by a symbol, the nature of the risk. The new generation of zonation maps comes with several layers of information: a documentation layer which provides a sufficiently accurate topography, a soil deposit map, the identification of pre-existing slides, the localization of pre-

4 G. Lefebvre et al. existing borings and soundings, and a qualification of the erosion if any. Some documents are associated to the documentation maps namely the description of existing borings and soundings. A second layer gives the susceptibility of the zone to a type of landslide, based on geological and geotechnical information. Aerial photographs Stereographic observations of aerial photographs are almost essential in slope stability evaluation. The manual contains standard forms which lists various aspects to be observed and allows the recording of observations for inclusions in the geotechnical report. Good quality aerial photographs from cover most of the inhabited regions of Québec. Observations of recent aerial photographs compared with those taken more than forty years ago allows an appreciation of changes in terms of erosion or of any condition susceptible to affect stability. Analysis of existing documents Field observations Slope classification Qualitative assessment The role of the forms provided in the manual is to orient the analysis of the documents and to ensure that it is included in the report. 3.2 Field observations The existing information discussed above is normally sufficient to have a good idea of the conditions at a site and of the various aspects which need to be observed on site. The elements to be documented during the site visit generally include the following: Description and history of the site, in particular, vegetal cover, surface drainage and indications of any modification from observation and discussions with owners or neighbors. A particular attention to detect any change in geometry by erosion, filling or excavation with an emphasis on the erosion activity in the sector. A detailed examination of previous soil movements at the site and in the vicinity. An effort to identify any indices associated with marginal stability such as fissures with vertical displacement at the top or heaving near the toe. Any observation susceptible to help define the stratigraphy, for example logging the scar of previous landslides or erosion at the toe. Any observation to help identify the geomaterial on the river bottom, for example identifying the material on the beach in the first 30 cm with a hand shovel. Assumed stable Potentially unstable Assumed unstable Documentation of any resurgence or humid area on the slope. 3.3 Evaluation of stability conditions Protection distance adapted to local conditions Quantitative analysis Existing borings and soundings Mandatory mitigation works and quantitative analysis Figure 3. Schema describing the proposed methodology for the assessment of the stability conditions of a slope In regions covered by the new zonation maps, the borings and soundings implemented for the zonation or for other purposes are listed on the documentation maps. In other regions, they have to be looked for at the municipality level, at different ministries or agencies. The manual provides a form to summarize the information obtained from available borings and soundings in a given area. Any evaluation of stability begins with the analyses of the data previously described in 3.1 and 3.2. In all cases, these data are used to qualitatively assess stability conditions. The main steps of this stage are described in the next section. 4. QUALITATIVE ASSESSMENT OF SLOPE STABILITY The large expertise accumulated over the years indicates that stable clayey slopes in Eastern Canada have certain characteristics. The first step in the qualitative evaluation of stability is thus to look for each of these characteristics based on the existing information previously described. These indices allow afterwards a classification of the slopes in terms of stability. 4.1 Stability indices The set of stability indices considered significant are described below:

5 Slope stability evaluation : more observation and less calculation Non evolutive geometry The slope with a geometry which has not evolved over a long period of time (at least some centuries), are considered to have been tested by the most severe atmospheric and groundwater conditions and can consequently be assumed stable. Sliding activity in the area The absence of previous landslides in the neighboring slopes indicates that the conditions in the area are not, in a general way, prone to landslides. Geometric conditions and neighboring slopes If the slope has a lower height or is flater than its neighboring slopes which are not affected by landslides, the slope under analysis has a stability reserve larger than its neighbors, assuming that other conditions are similar. Slope with a non evolutive geometry A slope with no sign of instability and with a geometry which has not evolved since a long period of time is assumed stable in the context of the slope stability manual. Such a slope has indeed been tested by the most severe atmospheric and groundwater conditions and the probability of a deep rotational failure affecting the top of the slope is considered extremely low. This category includes all slopes which cannot be submitted to erosion (figure 4). The manual identifies a few exceptions considered potentially unstable where the small probability of landslide is taken into account by keeping a certain distance between the home and the crest of the slope, based on accumulated local experience. Note that the risk against debris of shallow landslide for the residences at the toe of such slopes is considered differently in the manual. The state of valley formation A river which has cut through the clay deposit and is today flowing on the underlying till or bedrock has normally created conditions for the till and bedrock to discharge at the river level. Such conditions induce under drainage in the clay deposit resulting in a groundwater regime in the slope vicinity which is favorable to stability. The slope angle The different parameters which control slope stability vary from site to site. In spite of that, clayey slopes in Québec inclined at 3H:IV or less are generally considered stable (Fortin et al., this conference). Erosion prone conditions A riverside slope will be submitted or not to erosion depending of its location in relation to the water level at its toe and to the current direction. The geometry of a slope which is far from the river or above high water levels is not susceptible to evolve with time. Inversely, if the toe of the slope is subjected to erosion all the year long or for some periods of time each year, its stability is continuously decreasing and the bank will eventually be affected by a landslide after a certain period of time. Previous studies showed that sliding activity along rivers in clay deposit occurred preferentially in slopes located on the outside of meander bends (Locat et al., 1984, Hugenholtz and Lacelle, 2004). 4.2 Slope classification and qualitative assessment of stability The slope geometry, evolutive or not, is the basis here to classify the slope in terms of stability. The other indices are used as complements. Three main groups are defined: slope with a non evolutive geometry, slope with an evolutive geometry and finally slope with a modified geometry. Each group may conducts to more than one stability condition (figure 3). Figure 4 Slope in clay which cannot be submitted to erosion Slope with an evolutive geometry Slopes which are modified by erosion are susceptible to develop with time a geometry which may become critical in terms of stability. Slopes with an evolutive geometry are divided in subclasses depending if the erosion is well defined and is seen to have an effect on stability or not. Slopes with a well defined erosion (scour at the toe 1 m high or more) and in a sector with traces of landslides is considered unstable. This kind of slope (figure 5) cannot be considered stable on the long term without remedial works, irrespective of slope stability calculations. If the erosion is not well defined and the slope is in a sector with no trace of previous landslides, the slope is considered potentially unstable. The likelyhood of geometry deterioration can be taken into account by keeping a certain offset distance between the residence and the slope, again based on accumulated local experience. Otherwise, the

6 G. Lefebvre et al. stability of the slope has to be evaluated by a quantitative stability analysis. Figure 5. Slope in clay submitted to a very well defined erosion and in a sector with landslide scars Slope with a modified geometry For some slopes, the geometry has been modified by human activity. If the changes have resulted in a decrease of stability, like for example an excavation at the toe (figure 6) or filling at the top, the time laps since modifications is considered too short for the slope to have been through the most severe atmospheric and groundwater conditions. This kind of slope is considered potentially unstable. Except for a few cases, the slope has to be evaluated by quantitative analysis. Such analysis is not required if the slope is at 3H:1V or flatter or if the modified geometry is less severe than those of adjacent slopes with no sign of instability. In those cases, the relatively small risk can be taken into account by keeping a certain distance between the residence and the slope, based on accumulated local experience. cases and is used to plan the geotechnical investigation when a slope stability analysis is necessary. The slope stability manual details all stages of investigations and analyses. The manual aims at having a fewer but higher quality quantitative slope stability analyses based on a more complete geotechnical investigation. For example, groundwater conditions are determined using piezometers at specific depths at the crest of the slope and at the toe, if artesian conditions are likely to exist there. The observed piezometric levels are afterward adjusted to represent the most severe conditions using a correction detailed in the manual. This correction is based on the compilation of detailed piezometric records at many sites in the St.Lawrence lowlands over a period of 2 to 5 years. The procedure to transform piezometric levels into pore pressure is also detailed. The manual also contains detailed considerations about geometric and statigraphic conditions. On the other hand, it is recommended, for most current cases, that the shear strength parameters be obtained from empirical correlations (Lefebvre, 1981). As initial landslides are in majority rotational, due to the homogeneity of clay deposit, the analyses are carried out using the Modified Bishop Method and circular failure. Results must be compared to field observations, and corrected if necessary to reflect real field conditions. Finally, acceptable values of the factor of safety are given for many situations, depending on the slope classification and on the consequence of a potential landslide. 5. QUANTITATIVE EVALUATION BY SLOPE STABILITY ANALYSES As noted earlier, construction can proceed in the zones restricted by the zonation maps under the requirement that the stability be evaluated by an expert and found sufficient. As seen in the previous sections, the slope stability manual prepared in the context of the legislation recommends that the stability be first evaluated qualitatively based on observations and existing information. Evaluation of an appropriate factor of safety by slope stability analysis remains necessary only in a few instances and for the design of remedial work when requested. The appropriate factor of safety may vary with the type of land use, higher values being required depending on the importance of the construction. It may be noticed however that remedial works are often limited by environmental considerations. The qualitative evaluation of stability based on observation and existing information remains however necessary in all Figure 6. Example of slope with a modified geometry (excavation at the toe)

7 Slope stability evaluation : more observation and less calculation 6. CONCLUSION The Quebec Government has recently revised its risk management method for landslide prone areas. A new mapping program has been initiated and is accompanied with set of rules and regulations. The approach tries to better control land use to keep landslide risk at a level low enough to ensure the protection of the people. The regional mapping of landslide prone areas, even at 1/5000 scale, is conservative. In order to not unduly restrict land use, it is possible to request geotechnical expertise to re-assess the stability conditions at a local site. The main goal pursued in preparing the manual, has been described in this paper, and is to indentify and correct major problems currently related to slope stability analysis. An expert committee was formed to prepare a guidelines with the aim of minimizing the identified problems. Early in the process the committee realized that the subjectivity associated with qualitative evaluation of the stability of a slope was not larger than the one associated with the selection of the different parameters necessary for a numerical evaluation, in particular the prediction of the most critical groundwater conditions. Moreover, by analyzing the accumulated expertise in the Québec clayey deposits, the committee became convinced that a qualitative approach could be more easily standardized and would result in a more reliable evaluation, in the context where landslide prone areas were already identified on detailed regional maps. The authors hope this manual will optimize land use, giving at the same time a better use of the investments, a better evaluation of the real stability conditions and a better risk management and, ultimately, safer living conditions. 7. AKNOWLEDGEMENT The work of the committee has been performed under the responsibility of the Geotechnical and Geological department of Ministry of Transportation of Quebec. Public Safety Canada contributed financially to the preparation of the manual. The author wishes to thank Janelle Potvin, Ministry of Transportation of Quebec who has taken part in the work. Jacques Locat, from Laval University, and Pascal Locat, from Ministry of Transportation of Quebec, have reviewed the manuscript. The help from all is gratefully appreciated. 8. REFERENCES Bouchard R., Michaud V., Demers D., Le glissement de la rue McNicoll, 20 juillet 1996, Saguenay, Québec : causes et conséquences. Comptes-rendus de la 4ème Conférence canadienne sur les Géorisques, Québec, mai Demers, D., Robitaille, D., Potvin, J., Bilodeau, C., et Dupuis, C., La gestion des risques de glissements de terrain dans les sols argileux au Québec. Comptesrendus de la 4 ème Conférence canadienne sur les géorisques, Université Laval, mai 2008, Québec, Canada. Demers D., Leroueil S., d Astous J., 1999a. Investigation of a landslide in Maskinongé, Québec. Revue canadienne de géotechnique, vol. 36 (6), p Demers D., Potvin J., Robitaille D. 1999b. Gestion des risques de glissement de terrain liés aux pluies des 19 et 20 juillet 1996 au Saguenay Lac-Saint-Jean. Rapport soumis au Bureau de reconstruction et de relance du Saguenay Lac-Saint-Jean. Ministère des Transports du Québec. Fortin A., Ouellet D., Paradis S, Demers D., Développement d un portail informatique pour l accès à des bases de données géotechniques, Comptes-rendus de la 4ème Conférence canadienne sur les Géorisques, Québec, mai Hugenholtz C.H., Lacelle D., 2004.Geomorphic Controls on Landslide Activity in Champlain Sea Clays along Green s Creek, Eastern Ontario, Canada. Géographie physique et Quaternaire, Volume 58, numéro 1, p Hurtubise J.E., Gadd N., Meyerhof G.G., Les éboulements de terrain dans l est du Canada. Proceedings, 4th ICSMFE, London, vol. 2, p Lebuis J., Robert J.M. et Rissmann P., Regional mapping of landslide hazard in Québec. Symposium on slopes on soft clays, Linköping, Suède, rapport no17, Swedish Geotechnical Institute, p Lefebvre G., Soft sensitive clays. In Landslides: investigation and mitigation. A.K. Turner, R.L. Schuster, éditeurs. Transportation Research Board, National Reseach Council, Special report 247, p Lefebvre G., Slope instability and valley formation in Canadian soft clay deposits. Canadian Geotechnical Journal, vol. 23(3), p Lefebvre G Strength and Slope Stability in Canadian Soft Clay Deposits. Canadian Geotechnical Journal, vol. 18 (3), p Leroueil S., Natural slopes and cuts : movement and failure mechanisms. Géotechnique, vol. 51(3), p Leroueil S., Geotechnics of slopes before failure. Proceedings of the Ninth International Symposium on Landslides, Rio de Janeiro. Landslides : Evaluation and stabilization; Lacerda, Ehrlich, Fontoura & Sayao (eds), Taylor & Francis Group, London, vol. 2, p

8 G. Lefebvre et al. Locat J., Demers D., Lebuis J., Rissmann P., Prédiction des glissements de terrain Application aux argiles sensibles, rivière Chacoura, Québec, Canada. Comptes-rendus, IV Symposium international sur les glissements de terrains, Toronto, Vol. 2, p Perret et Bégin, Inventaire des glissements de terrain associés aux fortes pluies de la mi-juillet 1996 : Région du Saguenay/Lac Saint-Jean, 26 p. Rapport remis au Bureau régional de la reconstruction, ministère du Conseil exécutif du Québec L institut national de la recherche scientifique INRS-Géoressources. Robitaille D., Demers D., Potvin, J., Pellerin F., Mapping of landslide-prone areas in the Saguenay region, Quebec, Canada. Proceedings of the International Conference on Instability Planning and Management, Ventnor, Isle of Wight, UK, May Tavenas F., Landslides in Canadian Sensitive Clays a Sate-of-the-Art. Proceedings, IV International Symposium on Landslides, Toronto, Vol. 1, p