LESSONS LEARNT FROM FIELD TESTS IN SOME POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND SARAH M. SPRINGMAN, AMIN ASKARINEJAD, FRANCESCA CASINI, SVEN PRIEDEL, PETER KIENZLER, PHILIPP TEYSSEIRE and ANDREA THIELEN about the authors Sarah M. Springman Institute for Geotechnical Engineering ETH Zurich, Suisse Amin Askarinejad Institute for Geotechnical Engineering ETH Zurich, Suisse Francesca Casini Institute for Geotechnical Engineering ETH Zurich, Suisse Sven Friedel Comsol Multiphysics GmbH Zürich, Suisse Peter Kienzler Institute for Geotechnical Engineering ETH Zurich, Suisse Philipp Teysseire Teysseire & Candolfi, Visp Suisse Andrea Thielen Friedlipartner AG, Zurich, Suisse Abstract Rain-induced slope instability is a significant natural hazard in Switzerland, Slovenia and elsewhere in Europe. This contribution was prepared especially for the 12th Suklje Symposium, and recognises that landslides occur both in mountain regions as well as in lowland regions during and following extreme-rainfall conditions. The Institute (and Professorship) for Geotechnical Engineering at the Swiss Federal Institute of Technology (ETH Zürich) has been engaged over several years in projects concerned with the characterisation, monitoring and modelling behaviour of slopes in mainly granular porous media across the full range of altitudes in Switzerland. A link is made to the doyen of the Suklje day and then three case histories are presented and discussed to demonstrate the principal reactions to seasonal rainfall. A small slip was released in two of these cases and the "triggering" factors have been investigated and are discussed in this contribution. It transpires that the mode of inslope drainage influences the way in which the ground saturates and hence the volume of the potentially unstable ground. Simple stability analyses using limit equilibrium and soil parameters that have been amended to account for unsaturated soil behaviour were found to function well for slopes in largely granular media. Keywords rain-induced landslides, slope stability, case histories, monitoring, characterisation, modelling 1 prof. dr. lujo suklje! on landslides and his contributions Academician Professor Dr Lujo Suklje (Fig. 1; [1]) was the pioneer of Slovenian Soil Mechanics. He was appointed to a full Professorship at the University of Ljubljana in the year of the first author's birth and died in the year that she came to ETH Zurich at the same professorial grade. He was reputed to have been a strict, yet caring teacher, who left his mark on the development of conceptual soil mechanics, particularly on the rheological behaviour of soils [2], including creep, anisotropy, Figure 1. Academician Professor Dr. Lujo Suklje [1]. ACTA GEOTECHNICA SLOVENICA, 2012/l 5. S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND PROCEEDINGS OF THE THÎRES INTERHÏTJON6L CONFERENCE OH SOIL MECHANICS AND FOUNDATION ENGINEERING 3WITZËSLAND 18S3 Figure 2. Proceedings of the third International Conference of Soil Mechanics and Foundation Engineering, Switzerland 1953 [3]. viscoplasticity and consolidation. Slope stability is treated too, in his highly regarded text book 'Rheological Aspects of Soil Mechanics' [2], with 4 chapters in Section 5. He insists that the laboratory testing of specimens should be conducted in the direct shear and the triaxial apparatuses following consolidation. This theme is taken up later in this paper, which is dedicated to celebrating 101 years after his birth in Jelsa, Hvar Island, in September 1910. Interestingly, records of the discussion on Session 4 concerning 'the foundations of buildings and dams, bearing capacity, settlement observations, regional subsidences' held at the 3rd International Conference of Soil Mechanics and Foundation Engineering (Fig. 2; [4a]) that was held in Zürich in 1953, report written discussions from Mr L. Suklje, which were presented in French. In the first, he took issue with a Mr S.J. Button concerning his treatment of foundation stability with a 0=0 assumption, when shear resistance changes with depth. He mentioned that the position of the most unfavourable slip zones could also include thin bands of lower shear resistance and were also affected by the presence of rigid or flexible foundations. Interestingly, both themes were investigated in recent doctoral theses in the first author's group [5,6]. He also makes some fascinat- ing comments on using either a grease or slimy mud to represent soft soils subjected to rigid foundation loads in physical models [7], initially at the Building and Public Works Laboratory in Paris and later at the Soil Mechanics Laboratory at the ETS of Ljubljana. These resulted in satisfactory analytical solutions by applying circular failure surfaces, again an area of interest to the authors. The second written discussion [4a] presented tests demonstrating the effect of secondary settlement and a means for calculating the duration thereof, by using laws of similarity to apply test results to full-scale experiments. Professor Suklje considered the micromechanics by explaining that the grains slid into more stable positions during this process and he either agreed or took issue with the General Reporter (Mr M. Buisson) and the ISSMFE-Harvard-MIT hierarchy (Profs. Terzaghi, Casagrande, Taylor). The last discussion [4b] was an oral one in the session on 'stability and deformations of slopes and earth dams, research on pore-pressure measurements, groundwater problems'. He again referred to the adoption of a 0=0 approach by a former doctoral researcher at ETH Zürich, who was then Director at the NGI in Norway, Dr Laurits Bjerrum. He was probably thinking of the Zalesina landslide, which is discussed below, and pointed out that this approach could not be used in Yugoslavia since most of the landslides occurred in heterogeneous clay soils and that effective stresses should be used at all times. It was also of great importance to include the influence of water pressures due to standing and flowing water. These last two are central tenets, which will form the backbone to this paper. 'En général, les expériences acquises montrent que l'eau souterraine peut en réalité réduire à rien la stabilité des talus, non seulement par la dissolution des forces de cohésion (adhésion), mais aussi par l'effet mécanique de la sous-pression ou de la pression du courant.......Le calcule montrerait une sécurité qui n'existe pas.......Il faudra, sous le même point de vue, reconsidérer également l'application de la méthode 0=0.' [4b]. Fifth Technical Session; Influence of Ground- Figure 3. Extract from the Fifth Technical Session of the Conference on Stability of Earth Slopes hosted in Stockholm, with photograph of the Zalesina landslide [8]. 6. ACTA GeOïeCHNICA SLOVENICA, 2012/l S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND Professor Suklje's contribution to the understanding of the cause of landslides, and his proposals for remediation, bear further mention and consideration in the context of this invited lecture. In conjunction with his former student Ervin Nonveiller (Fig. 3; [8]), he contributed to the Fifth Technical Session of the Conference on Stability of Earth Slopes hosted in Stockholm. The paper was entitled 'Influence of Groundwater on Slope Stability' and concerned the Zalesina Landslide, then within Yugoslavia and now in the Republic of Croatia, due south of Obcima Kostel in southern Slovenia. Figure 4. Mapping the Zalesina Landslide, Croatia, (Republika Hrvatska) [8]. ACTA GeûTeCHNICA SLOVENICA, 2012/1 J. S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND A slope above a railway line demonstrated 'a spontaneous natural phenomenon' (Fig. 4; [8]), with a maximum length of the front being 500 m, and an average of 300 m. This was caused by a combination of 'tectonic processes (dinaric and transversal faults), long-term erosive creek action and internal erosion', while newly developed springs emerging from the slope played a major role as well, with one delivering 200 litres/min [8]. After a drought year in 1950, in which a continuous crack was observed running along a fault line over the railway line down to the creek, 2 m of net downslope movement was accompanied by an average of 6-mm/day rainfall over 6 months at the end of 1951 into early 1952. This comparison between the rainfall and the movement also showed that a cumulative total of more than 1000 mm of rain fell before the masses came to a more stable position [8]. Initiated at the contact between two strata between the Upper Raibl formation (siliceous or dolomitic sandstone with various forms of schisty shale, slate and limestone) and the overlying Noric Dolomite, cracks developed mainly perpendicular to the main direction of movement and played a crucial role in enhancing macro-permeability. A combination of large, deeper slides, older slides and secondary, comparatively shallow landslides in the weathered upper layers and steep slopes were probably caused by uplift and seepage pressures, through exfiltration from the bedrock, which could be treated effectively by remedial drainage. Some exploration tunnels were excavated to reveal the extent of the location of the shear zone. The authors concluded a number of points that remain wholly relevant to today and were, at this time, still somewhat visionary. 1. An effective stress analysis must be conducted based on parameters derived from consolidated specimens in a ring-shear apparatus and from consolidated, undrained (quick and slow) triaxial tests. 2. Friction is more important than cohesion in tecto-nically disturbed beds, which govern the sliding conditions; a statement that would be "music to the ears" of one of the first author's mentors, Professor Andrew Schofield FRS FREng [9]. 3. Uplift and seepage pressures play a major role in slopes during heavy rainfall and must be accounted for in the stability calculations. 4. The sliding may occur over a broad zone, here approximately 7 m, but the whole mass slides mainly as a solid body. 5. Monitoring of the groundwater level should be achieved using piezometers and geoelectrics, the latter being a method that is now finding considerable favour. 6. The method of potential slip surfaces can still be applied (assumed here to be limit equilibrium calculations), notwithstanding the approximations made, likewise an approach dwelt on briefly in this contribution. 7. Partial natural stabilisation was achieved, which could be improved by: a. removal of mass from the top to the foot of the slide; b. draining the sliding masses with a drainage system; c. a combination of a) and b). SURFICIAL RAINFALL INDUCED LANDSLIDES Surficial rainfall-induced instabilities triggered in unsaturated slopes have been reported over many decades (e.g., [10-15]). Systematic studies of the most important triggering effects have been pursued as Pan-European multidisciplinary projects in recent years (e.g., MOUNTAIN RISK, TRAMM, SAFELAND). Lateltin et al. [16] commented that periods of extreme rainfall, combined with a long humid winter and a cool late spring, as well as rising groundwater due to infiltration from snowmelt, have proved to be highly significant in causing landslides in Switzerland. This is supported by evidence from historical, Swiss meteorological data calibrated against landslide events derived from dendrochronology [17]. Colleagues in SE Asia report other leading studies (e.g., [18-22]), which have contributed data to earlier opinions on whether 2-3 weeks of steady rainfall is more dangerous [e.g., 11,12] than localised short, sharp, intense rainfall events [13,23]. Sometimes, there is a critical combination of antecedent rainfall over a specific duration followed by a shorter and more intense storm [24]. However, no one has been able to achieve a universal link between the intensity and duration of rainfall despite the best efforts of a generation of researchers from [25] through to [26] (see Fig. 5). It is clear from Fig. 5 that rainfall-induced landslides present a significant natural hazard. Consequently, the field monitoring of slopes has been carried out by numerous researchers [e.g., 18-22, 31-39] and the statistics of rain-induced landslides have been presented for a series of storms in Switzerland in 2005 [40] and in neighbouring Austria [41] from 1950 to 2000. However, uniform conclusions have not emerged, largely due to the wide variety of sites, instrumentation as well as different levels of sophistication and investment in the characterisation, ACTA GEOTECHNICA SLOVENICA, 2012/1 8. S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND 10° 101 102 103 Duration (h) Figure 5. Global Intensity-Duration thresholds based on Guzzetti et al., [26] with published global (worldwide) Intensity-Duration rainfall thresholds proposed by: 1 [25]; 2 [27]; 3 [28]; 4 [29]; 5 [30]; 6 Inferred from this data; 7 thresholds inferred rom the probability estimates of the rainfall conditions, for two different rainfall periods (D < 48 h, D > 48 h). The horizontal line shows a 0.25-mm/h rainfall intensity. monitoring and modelling. This contribution attempts to add to the database obtained from past field tests and endeavours to provide a structure with which to accord a typology. It focuses on slopes that are predominantly in granular media and is accompanied by a very simple mode of analysis that would have common roots with Professor Suklje's work. Key factors affecting the likelihood of a mass movement being triggered are largely due to the location and origin of the slope (geological, geomorphological, anthropogenic factors), which are more or less constant with time in comparison with the more variable environmental effects, largely due to meteorology, hydrological, altitude and specifically precipitation [42] affecting infiltration [43] and water regimes, vegetation and temperature. Some impacts and outcomes that bear consideration are listed below, although they may not be the sole arbiters of whether a slope will remain stable or not: 1. topographical-soil engineering influences, particularly the slope angle related to the friction angle of the relevant strata derived from the original geology, in terms of deposition modes, anisotropy, macro-micro permeability with soil layering and underlying bedrock (in terms of the depth and the shape of bedrock surface); 2. precipitation-infiltration into the ground (soil) [44], groundwater flow processes, weathering at the soil-rock interface with attendant influence on permeability, exfiltration from the rock into the overlying soil layers and drainage into the underlying rock (e.g. [45-50]); 3. other factors affecting stability, such as the reinforcing effects of vegetation [51,52] and biological [51] or chemical [53] cementation; 4. triggering mode (extreme rainfall, changing groun-dwater and thermal regimes, seismicity and volcanic effects etc.; e.g. [54]; ACTA GEOTECHNICA SLOVENICA, 2012/1 9. S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND 5. surface area, volume, aspect and mobility of failing debris, which have been found to be due largely to the initial void ratio as well as the availability of water to fill, or enlarge, the voids to form debris flows. The locations of the three monitoring field sites (Table 1) discussed in this paper are given on a topographical map of Switzerland (based on [55]; Fig. 6;), showing the Alps (south) and Jura (north) mountains running approximately WSW to ENE, with the more highly populated areas (in pink) mainly located around the cities and in the 'Mittelland', between both mountain ranges. Also shown are cartoon pictures of the ground and a brief description of the main components. Some characteristics are extracted from each of the sites to enable subsequent analyses to be conducted and conclusions to be drawn. Two main mechanisms were observed. These were a function of the inhomogeneity and permeability of the soil and rock layers as well as possible drainage channels in the soil and rock forming the slope. These will be described as top-down or bottom-up saturation, with the attendant influence on the volume of debris mobilised in the failure mode and hence the risk entailed by failure. Infiltration into and exfiltration from the bedrock are found to be key factors as well. The interaction between the air and the pore water in the soil pores played a major role as did the cyclical response of the slope to rainfall and groundwater flow, as it became saturated or drained. For example, Petkovsek et al. [69] have measured this for unbound base and sub-grade layers for several roads in Slovenia and the case histories discussed here rely on the instrumentation listed in Table 1. Determining the soil state close to failure is dependent upon the stress path followed by the soil elements undergoing saturation. This is significant. Some laboratory stress path tests are presented and discussed for two of the sites to represent rainfall infiltration, together with data from a variety of direct shear box tests. GRUBEN The field data from the Gruben field test (Table 1) demonstrate that the fine-grained component of the matrix was significant at 15.9% < 63^m, and with < 2% at <2^m (from two representative surface samples Figure 6. Location of three field-monitoring and test sites on a topographical map of Switzerland [55], which are described more fully in Table 1. 10. ACTA G£OT£CHNICfl SLOVENICA, 2012/l S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND Table 1. Summary of field test sites in Switzerland. Test field & Canton Gruben Tössegg Rüdlingen Valais Zurich Schaffhausen Dates 1998-2000 2004-2007 2008-2009 Picture Geology scree slope, glacial debris /mo-raine, (Fletschhorn crystalline; Pretriassic Bernhard Plate of muscovite-rich gneiss / slate with some albite, chlorite, biotite) quarternary outwash gravels mixed with finer grained deposits overlying horizontally layered sandstone (upper salt-water marls) above the lower freshwater marls landslide debris (e.g. blocks & fine-grained soils) covering horizontally layered marls and sandstones in steep wooded areas Soil layers well graded angular granular soil particles with 10% fines < 60 |im grassy surface with clayey sand overlying silty sand until rock at 1-2 m depth roots in loose layer silty sand with %clay increasing with depth Preferential infiltration & drainage systems dense soil, voids e ~0.3, vertical & probably slope parallel flow with surficial run-off channels from roots & animals in clayey sand, sometimes slope-parallel flow in silty sand, with little run-off and some infiltration into bedrock primarily vertical drainage in pores (e=0.9) & along roots, infiltration into & exfiltration from bedrock Test area & masl§ 12 m (B) x 8 m (L)=100 m2 and 5.2 m (B) x 10.5 m (L)=55m2 2750 masl 15 m (B) x 13 m (L)=195 m2 367 masl 8 m (B) x 35 m (L)=280 m2 385 masl Slope a 31° (did not fail) & 42° (failed) 27° lower slope to 17° upper slope (did not fail) 38-42° (2008: did not fail; Mar. 2009 failed) Instrumentation* MS, TDR, MP, JFT, FO MS, TDR, MP, JFT, ST, UV, G-ERT MS, TDR, MP, JFT, EDZ, P, SG, INC, PG, ST, AS, G-ERT (UNIL) Friction angle f 39° (triaxial) & 41° (direct shear) 31° & 39° clayey & silty sand respectively in direct shear 30-32° (triaxial and direct shear) Sponsors Canton Wallis; BAFU HazNETH, CCES / TRAMM CCES / TRAMM Dissertations [56,57#] [58,59#] [60,61#] masl metres above sea level, B breadth, L length MS Meteorological data such as air temperature, rainfall, humidity etc., plus UV Ultraviolet radiation, Measurement of volumetric water content: TDR Time Domain Reflectometry, MP Moisture Point, Suction: JFT Jetfill Tensiometer, pore-water pressure: P Piezometer, Earth pressure: EDZ earth pressure cell with P, ST soil temperature, Deformations: FO fibre optic cables, SG strain gauges, INC inclinometer, PG photogrammetry (plus AS Acoustic Sensors (STEP/ETHZ), G - Geophysical monitoring through ERT - Electrical Resistance Tomography); UNIL - University of Lausanne, BAFU - Federal Office for Environment; HazNETH - Natural Hazards Network ETHZ, CCES - Competence Centre for Environmental Sustainability / TRAMM - Triggering of Rapid Mass Movements. These are doctoral dissertations. Additional semester or Masters' dissertations conducted on the Rudlingen landslide include [62-68]. § * # at up to 1 m depth, which were taken adjacent to Field 1; [70-72]. These fine grains and interstitial capillary space controlled the permeability and some aspects of the shear strength, in particular relating to the development of suctions. Furthermore, full saturation was not achieved and hence suctions were not dissipated completely, even after almost one week of intense rainfall in 1999 with a minimum baseline (after the first 2 days) of 10 mm/h. In total, 1.9 m of rainfall was applied artificially (averaging 13 mm/h over the week in 1999, Fig. 7). The Mois- ACTA GEOTECHNICA SLOVENICA, 2012/1 11. S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND ture Point readings (Fig. 7, [70]), which averaged the volumetric water content over a given length, indicated that 'full' saturation was being approached only over the top 15 cm, following a gradual increase in the second half of the week. Even then, Sr was only 92% at the end of the experiment in 1999. Interestingly, there was a strong response to the raised rainfall intensity to > 20 mm/h on the morning of the 12th of July, in which little change was observed in the top 15 cm, but there seems to have been a consistent 'breakthrough' suddenly within a couple of hours to all sensors monitoring 15-30 cm, 30-60 cm, 60-90 cm and 0.9-1.2 m. Recovery to lower degrees of saturation occurred somewhat variably as the rainfall intensity dropped to 10 mm/h for the ensuing 16 hours. Both test fields (2000/1 & 2000/2: slopes of 31° and 42° respectively) were subjected to a lower rainfall intensity in 2000, (8-22 mm/h: 0.76 m and 0.7 m of cumulative artificial rainfall on Fields 1 and 2 respectively). Fig. 8 shows the calibration of saturation data from 1999 to 2000 from using the same Field 1 (but with slightly less instrumentation), although this is only for ~2 days of rainfall, when there was a surficial failure in the steeper Field 2 (42°) and the tests were stopped. After failing to measure suctions in any instrument in 1999 due to the challenging environment [40], this was achieved in 2000 on Field 1 and the results were consistent with the measurement of water content, showing the initial and latest responses in terms of loss of suction at the shallowest and the deepest tensiometers, respectively. A summary of the net loss is shown in Fig. 8 [71]. saturation degree Sr [%] 0 20 40 60 80 100 0111111111111 suction [kPa] 10 20 30 40 11 i i 11 i i i 11 11 11 0.1 0.2 0.3 0.4 0.5 * TDR&IVP1999 □ T DR 2000 • Tensiometers Figure 8. Gruben: summary of immediate pre- and postrainfall event data showing increase in saturation combined with loss of suction as infiltration progresses (a) saturation degree determined from TDR and MP probes: Fields 1/1999 & 1/2000 (31° slope), (b) Suctions determined from tensiometers: Field 1/2000 (31° slope), Gruben [71]. T0SS6GG The Tossegg site is located in Canton Zurich on the banks of the river Rhine. It was selected due to a series of extreme events in May 2001 in which 42 surficial landslides occurred nearby following 10cm of rainfall in 00:00 12:00 00:00 12:00 00:00 12:00 00:00 1 200 00:00 1 200 00:00 1200 00:00 12:00 00:00 1200 00:00 06 July 07 08 July 09Jdy 10 July 11 July 12 July 13 July Figure 7. Gruben: Rainfall intensity and Moisture Point (MP) measurements in terms of saturation degree with time: Field 1/1999 (31° slope) [70]. 12. ACTA G£OT£CHNICfl SLOVENICA, 2012/l S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND 40 minutes [73]. The slope was investigated and characterised using geophysical, geological and geotechnical methods [58,74-76]. A clayey sand was revealed below the grassy humus layer, with a silty sand overlying sandstone-marl bedrock at between just less than 1 m to nearly 2 m depth. The electrical resistivity tomograms shown in Fig. 9 were developed for measurements made over the extent of the grassy slope in 8/2003 and 7/2004 [75], whereby resistivity depends on porosity, degree of saturation and mineral content. The rainfall differed significantly in the summer 2003 and summer 2004, in which 2003 was a very dry summer, reflected in the higher surface resistivities determined in Fig. 9. Variations in resistivity can be explained by changes in saturation, since other parameters remain more or less constant. The saturation of the layer below the topsoil, and up to about 1 m depth, on top of the underlying bedrock is greater in July 2004 in the lower part of the slope, where the instrumentation was ultimately installed for the monitoring experiment [74]. Table 2. Tossegg test field: layers at location of upper / lower / right / left instrument clusters (after [58]). Layer thicknesses (cm) Upper left Bottom left Upper right Bottom right Topsoil 25 20 30 25 Clayey sand 60 135 0 100 Silty sand 10 45 150 45 Depth to bedrock (cm) 95 200 180 170 The reactions to summer (mid-August 2005) and winter (April 2005) rainfall events in terms of volumetric water content 9 are shown with depth before, during and after the rainfall event for instruments installed at the four corners of the test field in Fig. 10, with soil conditions recorded in Table 2. The highest saturation was reached in winter (Fig. 10a) over the top 60 cm, mainly in the clayey sand, after 24 hours of rainfall with a greatest change in volumetric water content up to 0.075. There was some saturation of the underlying silty sand layer at about 1.2 m depth, on the right-hand side of the field, indicating percolation and a short-term increase in saturation in the more permeable layer. The saturation degree reverts to the original state four days after the rainfall event. The maximum saturation for the summer event (Fig. 10b) was increased over the top 45 cm only, from a relatively dry state by increments of 9 up to 0.15. The volumetric water content was generally lower prior to the Figure 9. Tossegg: Comparison of electrical resistivity tomograms taken in August 2003 and in July 2004 [75] combined with ground model and location of the test site [74]. summer event, than when fully saturated in winter [58]. Most of the rainwater had infiltrated with low runoff and there was little reduction in saturation over 48 hours after the rainfall stopped because both the temperatures and the evapotranspiration rate were low for this season during the days after the rainfall event. Small-scale sprinkling experiments, covering a circular area of 1 m2, were used after the monitoring period to ACTA GeOïeCHNICA SLOVENICA, 2012/1 13 . S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND a) b) 0 -30 e -so V a, -90 Q> ■o -120 -150 0 upper left upper rigl -30 I -60 f -90 50 m2) was large relative to the depth of failure (< 0.5 m). The minimum factor of safety (Eqn. 4) was calculated to be smaller than 1.0 without a contribution from an apparent cohesion. Although the failure in Gruben was only surficial, the value of c* < 1 kPa was deduced at depths z <0.5 m, which agreed well with the results from the direct shear tests in the field (40% < Sr < 100%), (Figs.18 & 19), 0.2 < c* < 0.5 kPa for 0.9 < Sr < 0.95 (z~0.2 m). Although some of these assumptions cannot represent the failure mechanism exactly, at least they help to explain why a failure surface at a depth of <0.5 m is possible for a 42° slope, when Sr = 0.95. Application to Tossegg Although the shape of the critical failure surface is transitional towards a slip circular form at Tossegg [58], the outcomes are briefly described here. Thielen [58] used coupled thermo-hydraulic (TH) finite-element analyses within the Geostudio code VADOSE/W [105] both to model the slope response during the monitoring period at Tossegg, using data from one year to calibrate the model and then to insert the parameters obtained in the second year of data to complete a validation. This was quite successful and then the relevant parameters and state in the ground were input to the parallel two-dimensional limit equilibrium SLOPE/W model [106] to find the critical failure mechanism. This was selected by the programme itself from the most critical case according to the approaches proposed by Morgenstern and Price [107], Spencer [108], Bishop and Morgenstern [109] or Janbu [110]. Calculations were carried out at monthly intervals for 2005 (Fig. 22). The lowest (2D) factor of safety was 1.74 on 1.2.2005, reflecting the saturated state of the clayey sand in the winter months and partial saturation in the underlying silty sand layer (see Figs. 11a&b; 15b). This is also conservative because side friction is ignored. It is unlikely that the factor of safety will approach unity given a lowest angle of friction of 31° and a steepest slope angle of 27° unless significant artesian pressures develop in the silty sand. Application to Rudlingen Fig. 23 summarises a simplified stability analysis using a channel of infinite length and the dimensions shown as a slope inclination a = 40°, width of d = 8 m (Figs. 23a-d). A void ratio e = 0.9, a lateral earth pressure of a'h = K(av + Sr (ua - uw)) and K = 1 - sinf, with the water table at depths of z = 0.8 m, 1.2 m, 4 m, and f'=31° were chosen as representative values for the field conditions in Rudlingen. The contribution from cr was varied with depth (Eqn. 7) showing, as was deduced from the first experiment in October 2008, that the presence of roots and a deep water table was essential for stability (Fig. 23d). This is entirely consistent with the position described due to bottom Figure 22. Tossegg: seasonal variation of factor of safety in 2005 against slope failure calculated using SLOPE/W after coupling with suctions and pore pressures obtained from TH Modelling with VADOSE/W (after [58]). 22. ACTA G£OT£CHNICfl SLOVENICA, 2012/l S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND up saturation to build a water table above the bedrock (e.g. Fig. 15c) as the SF drops and with the critical shear surface at the boundary with bedrock as the water table rises from 4 m (Fig. 15d) to 1.2 m (Fig. 15c) to 0.8 m (Fig. 15b). The failure body (Fig. 13b) bottomed out on rock even though it has not followed a channel form with the base shear surface parallel to the surface and vertical sides. A constant value of cr = 5kPa along the sides and the base contributes to the stability in comparison with the case in which there is no contribution from the roots, or merely a local effect close to the surface (two other models with cr constant at 5 kPa over the top 0.2 m of soil or with cr reducing from 3 kPa over the top 0.2 m, decreasing by 1 kPa each additional 0.2 m). a) b) c) d) Figure 23. Two-dimensional limit equilibrium stability calculations for a channel geometry, including suctions (when above the water table) and root reinforcement on the sides and base: (a) channel dimensions with parameters appropriate for Rudlingen; results for various values of cr and water tables at (b) z = 0.8 m (c) z = 1.2 m (d) z = 4 m. ACTA GEOTECHNICA SLOVENICA, 2012/1 23. S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND Fig. 23d shows the effect of suction combined with roots, in which the critical depth for such a failure mechanism with the lowest SF occurs between 1.2 to 1.4 m, depending upon the degree of root reinforcement. This was not too dissimilar from the values obtained, notwithstanding the differences between the geometry assumed in Fig. 23a and that observed in Fig. 13b. Subsequently, Bischof [66], Malecki [67] and Askar-inejad et al. [111] have investigated the development of failure mechanisms using more advanced coupled finite-element analyses, and further work is underway. A PRACTICAL APPLICATION! CONDITION INDICATORS 'Condition Indicators' were adopted for the use of the observational method (Peck, 1969) to determine safe access to the test site in Rudlingen (after Fig. 24) during the artificial rainfall experiments. The critical combination of Sr and suction s (= ua - uw) in an infinite slope stability analysis, as described in Equations 4 above with c*= Sr(ua - uw)tanip', was applied to the Ruedlingen slope. These provided the basis for an alarm system and were supported by observational markers: green (s > 20 kPa) - no restrictions on access or behaviour; orange (20 kPa > s > 7 kPa) - warning, within the uncertainty envelope for failure, restrictions to access on, below or within 10 m to the sides and above the slope; red (s < 7 kPa) - danger - whereas several activities could be permitted or limited, to reduce any risk to the participants and public. 0 10 20 30 40 50 60 70 75 Suction s (kPa) Figure 24. Relationship between saturation degree and suction of Rudlingen silty sand in the laboratory based on the wetting branch of the Water Retention Curve [80] superimposed with alarm levels [86]. SUMMARY Leroueil [113] observed that 'the failure of natural slopes constitutes an important geotechnical problem that involves a variety of geomaterials in a variety of geologi- cal and climatic contexts, and which has a major socioeconomic impact in many countries.' This paper reviews the state of knowledge for rainfall-induced landslides in primarily granular materials, examining the influence of partial saturation, infiltration and saturation regimes on slope failure. The infiltration of rainfall has led to surface instability in slopes steeper than the internal angle of friction, just prior to full saturation being reached in three field sites in Switzerland that have been well characterised and instrumented. Each slope has contributed significant learnings and a generic approach to categorising the saturation of slopes has been proposed. This has been accompanied by several calculation methods that have been presented in this short contribution for adoption in making a preliminary judgement of slope stability. Triaxial, field and laboratory shear box data have been used to determine the shear parameters for the mobilised shear strength at failure using a simple MohrCoulomb analysis and assuming the effect of suction is modelled by an apparent cohesion, which depends on the saturation degree. Alternatively, suction may be estimated from the saturation degree at various depths and included in a shear-strength envelope, which is dependent on suction and saturation degree. Prediction of this 'apparent cohesion' term has been made, by adopting a factor of safety equal to unity under simple limit equilibrium, infinite-slope stability analysis with an extension to a simplified three-dimensional case where necessary. An apparent cohesion, depending upon saturation degree, offers a simple option for modelling the significantly more complex unsaturated behaviour in a slope, whereas the modelling benefits greatly from understanding of the slope hydrology and the way in which the cyclical saturation and drainage processes develop with time. Despite the simplicity of the analyses and extreme heterogeneity of the moraine, it was found for the moraine slope in both cases that the factor of safety reduced almost to unity at depths of < 0.5 m for the 42° slope, as had been observed from the field test. Despite modern computational capacity and highly advanced, multi-parameter, constitutive models, a case is made that the use of simple models is still as valid today as they were in Professor Suklje's time. The presence of water in slopes, as discussed by Professor Suklje, will always challenge the calculation of stability. However, understanding the mechanisms and applying simple and robust models and good engineering judgement can often lead to the ability to make challenging decisions about slope safety. 24. ACTA G£OT£CHNICfl SLOVENICA, 2012/l S. M. SPRINGMAN ET AL.: LESSONS LEARNT FROM FIELD INVESTIGATIONS IN POTENTIALLY UNSTABLE SLOPES IN SWITZERLAND ACKNOWLEDGEMENTS While the 2011 Suklje lecturer and first author was never privileged enough to have met Professor Suklje herself, she acknowledges his signal contribution to soil mechanics with gratitude, remembering, with great pleasure, the most excellent Balkan Conference in Ljubljana in 2006, organised so well by his students and mentees [114]. The authors would like to thank Ernst Bleiker, Marco Sperl, Dr. Cornelia Bronnimann, Prof. Cristina Jommi, Dr. Massi Schwarz, Dr. Barbara Suski, Francesca Gamba-zzi, Gernot Michlmayr, Werner Attinger, Karl Steiner, Dr. Christian Rickli, Dr. Manfred Stahli, Dr. Andreas Schmid, Dr. Peter Lehmann, Prof. Dani Or, Dr. Mathieu Nuth, René Rohr for their various contributions to the fieldwork presented here. We would also like to express our warm gratitude to Ms Mengia Amberg and Mrs Gabriela Laios for their help with graphics and text, also to Dr.-Ing. Jan Laue, Dipl. Ing. ETH Ralf Herzog, Dipl. Ing. Pierre Mayor, Adrian Zweidler, Heinz Buschor, Fredy Ehrbar, Anita Meyer for their help and advice in the workshops and laboratory. Constructive discussions with Prof. Paolo Burlando, Dr. Peter Molnar, Dr. Les Davison, and Dr. Muni Budhu are also recognised. 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