Strength degradation and progressive failure in massive rock slopes

Transcription

1 29 th Canadian Geotechnical Colloquium: Strength degradation and progressive failure in massive rock slopes (the role of advanced numerical methods and geotechnical field measurements in understanding complex mechanisms) Erik Eberhardt Geological Engineering/EOS, University of British Columbia, Vancouver, Canada 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 1of 54 Despite improvements in recognition, prediction and mitigative measures, rock slope failures still exact a heavy social, economic and environmental toll in mountainous regions around the world. the 1915 Jane Camp rockslide ranks as B.C. s worst natural disaster with 56 deaths. Two days prior to the event, the inspected mountainside was deemed solid. rain triggered landslides in the Alps resulted in 37 deaths and $600 million in damage during Oct Population and economic growth has demanded expansion of habitat, lifelines and natural resources. However, the short history of human development in many regions makes the evaluation of potential landslide hazards and appropriate countermeasures very difficult. 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 2of 54 1

2 The difficulty - rock masses are complex systems! Often, field data (e.g. geology, geological structure, rock mass properties, groundwater, etc.) is limited to surface observations and/or limited by inaccessibility, and can never be known completely. 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 3of 54 Key Rock Slope Stability Issues to be Discussed: Complexity & Uncertainty Phenomenological vs- Mechanistic Approaches Temporal Prediction Spatial Prediction Where we are: Integration of Geotechnical Data Sets and Advanced Analyses Where we need to go: Progressive Failure in Rock Slopes 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 4of 54 2

3 Acknowledgements: Simon Loew (ETH Zurich, Switzerland) Doug Stead (SFU, Canada) Keith Evans (ETH Zurich, Switzerland) Mark Diederichs (Queens, Canada) Hansruedi Maurer (ETH Zurich, Switzerland) Peter Kaiser (Mirarco, Canada) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 5of 45 Acknowledgements Grad Students: Luca Bonzanigo (Eng. Geol., ETH Zurich) Heike Willenberg (Eng Geol., ETH Zurich) Florian Ladner (Eng. Geol., ETH Zurich) Tom Spillmann (Geophysics, ETH Zurich) Benoit Valley (Eng Geol., ETH Zurich) Björn Heinke (Geophysics, ETH Zurich) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 6of 45 3

4 Complexity & Uncertainty: Morgenstern s Casagrande Lecture (1995): Parameter Uncertainty: concerned with spatial variations, e.g. rock mass strength, and the lack of data for key parameters. Model Uncertainty: arises from gaps in the scientific theory that is required to make predictions on the basis of causal inference. Human Uncertainty: can range from simple human error to corruption. correct answer 20 MHA prediction competition collapse height of slope in soft clay. A substantial amount of shear strength data was provided to the 31 participants. 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 7of within ±50% ±15-25% ±15-25% within ±50-100% Rock Slope Toolbox: Mapping geological, geotechnical, geomorphological,, hydrogeological Hungr et al. (2005) Willenberg et al. (2004) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 8of 54 4

5 Rock Slope Toolbox: Empirical Design Run out susceptibility Randa Rockslide (April 18 th & May 9 th, 1991 events) Heim (1932): 1st use, defines travel angle Scheidegger (1973): empirical prediction Hsü (1975): links centres of gravity Evans & Hungr (1993): shadow angle Ayala et al. (2003): susceptibility mapping Schindler et al. (1993) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 9of 54 Rock Slope Toolbox: Monitoring geodetic, extensometers, crack meters, tilt meters after Terzaghi (1950) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 10 of 54 5

6 Temporal Prediction of Failure: after Fukuzono (1990) after Terzaghi (1950) Saito (1965): creep rupture life Kennedy & Niermeyer (1970): Chuquicamata Voight (1989): creep velocity & acceleration Fukuzono (1990): inverse velocity Salt (1988): empirical alarm levels Crosta & Agliardi (2003): alert thresholds 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 11 of 54 Temporal Prediction of Failure: road closed Grimselstrasse,, CH (2000) water injection down tension crack (~ 9000 l/min) blast 19 tonnes of explosives (for 150,000 m 3 of rock) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 12 of 54 6

7 Prediction of Landslide Behaviour road closed water injection down tension crack (~ 9000 l/min) blast 19 tonnes of explosives (for 150,000 m 3 of rock) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 13 of 54 after Gruner (2003) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 14 of 54 7

8 Rock Slope Toolbox - Empirical Prediction: after Fukuzono (1990) after Terzaghi (1950) Limiting Factors: Focuses on surface measurements, ignoring changes in behaviour with depth. Technique applied in the same way regardless of failure mode (translational slide, topple, etc.) and/or data source (crack meter, geodetic monuments, etc.). 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 15 of 54 Rock Slope Toolbox: Phenomenological approaches - are holistic as they disregard details of the underlying mechanisms while concentrating on the overall performance of a system. Mechanistic approaches - on the other hand, try to break the problem/system down into its constituent parts to understand the cause and effect relationships (and their evolution), which govern the behaviour of the system. 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 16 of 54 8

9 Honest look at our toolbox: Mapping Rock Mass Characterization Hoek et al. (1995) Instrumentation 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 17 of 54 Prediction Understanding of Landslide Behaviour Crash 8% 29% Impossible 4% 58% Compute Crilly (1993) Error Warning Survey of nine commonly used geotechnical modelling programs and their response to input data. Implausible 23% Error 69% Compute 8% Warning Eberhardt et al. (2002) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 18 of 54 9

10 Rock Slope Toolbox: Continuum Methods Rock/soil mass behaviour represented as a continuum. Procedure exploits approximations to the connectivity of elements, and continuity of displacements and stresses between elements. Discontinuum Methods Rock mass represented as a assemblage of distinct interacting blocks or bodies. Blocks are subdivided into a deformable finite- difference mesh which follows linear or non-linear stress-strain strain laws. Hybrid Methods Rock mass represented as an initial continuum or discontinuum. Procedure allows modelling of intact rock behaviour, discontinuity interactions, and the generation of new fractures through brittle fracture and adaptive re- meshing algorithms. 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 19 of 54 Slope Failure Initiation Temporal prediction: Almost exclusively empirical. after Fukuzono (1990) Constitutive Models: after Terzaghi (1950) Rarely include time. 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 20 of 54 10

11 Spatial Prediction Forward Analysis 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 21 of 54 Static Analysis Linked with Runout Analysis DAN 3D DAN 3D -Ph.D. Thesis:Scott McDougall Supervisor: Prof. Oldrich Hungr 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 22 of 54 11

12 Key Rock Slope Stability Issues to be Discussed: Complexity & Uncertainty Phenomenological vs- Mechanistic Approaches Temporal Prediction Spatial Prediction Where we are: Integration of Geotechnical Data Sets and Advanced Analyses Where we need to go: Progressive Failure in Rock Slopes 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 23 of 54 Integrating Data Sets Campo Vallemaggia, CH Geology - metamorphic gneisses & schists Mechanism translational slide (30 SSE) Surface Area -~6 km 2 Total Volume - ~ 800,000,000 m 3 Average Velocity -~5 cm/year Maximum Depth - ~ 300 m 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 24 of 54 12

13 Integrating Data Sets Campo Vallemaggia Bonzanigo et al. (2005) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 25 of 54 Integrating Data Sets Campo Vallemaggia Bonzanigo et al. (2005) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 26 of 54 13

14 Campo Vallemaggia Block Kinematics Bonzanigo et al. (2005) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 27 of 54 Integrating Data Sets Campo Vallemaggia Geodetic Movement (m) drainage adit opened Velocity (mm/day) Borehole Head (m) critical threshold at 1390 m Bonzanigo et al. (2001) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 28 of 54 14

15 Discontinuum: H-M H M Coupled Analysis Eberhardt et al. (2005) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 29 of 54 Discontinuum: H-M H M Coupled Analysis Pore Pressure (MPa) adit level 20 m above adit 40 m above adit 60 m above adit drainage adit opened Time Steps Eberhardt et al. (2005) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 30 of 54 15

16 Discontinuum: H-M H M Coupled Analysis without drainage adit X - Displacements (m) with drainage 1.00 drainage adit adit Campo Vallemaggia: opened 0.10 Distinct-element models suggest that very little drainage is required (approximately 10 without pore pressures (i.e. dry slope) l/s) to significantly reduce pore pressures and 0.01 to stabilize the slope Time Steps Deep Drainage: Fracture permeability corresponds to low storativities, therefore large water inflows through drainage are not necessary to achieve significant reductions in head. 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 31 of 54 Key Rock Slope Stability Issues to be Discussed: Complexity & Uncertainty Phenomenological vs- Mechanistic Approaches Temporal Prediction Spatial Prediction Where we are: Integration of Geotechnical Data Sets and Advanced Analyses Where we need to go: Progressive Failure in Rock Slopes 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 32 of 54 16

17 And as a rock falls precipitate from some mountain-crest, torn thence by the wind, or washed forth by the swollen rains, or loosened by the stealthy lapse of years; under mighty impulse the destroying cliff crashes in abrupt descent and bounds over earth, involving in its train forests and herds and men Virgil (20 BCE), The Aeneid 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 33 of 54 Understanding Rock Slope Behaviour photo by H. Willenberg 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 34 of 54 17

18 1991 Randa Rockslide current instability Schindler et al. (1993) photo by H. Willenberg data from SwissMeteo 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 35 of 54 Cumulative Damage, w ω AE cohesion damage Relative Cohesion Normalized Stress (σ/σ cd ) Bjerrum (1967): progressive failure in clay slopes Zienkiewicz et al. (1975): strength reduction Stacey (1981): extension strain Martin (1997): cohesion loss & stress path Cooper et al. (1998): Selborne cutting experiment Leroueil (2001): mechanisms of progressive failure in soil Hajiabdolmajid & Kaiser (2002): strain sensitivity Diederichs et al. (2005): damage & heterogeneity shear strain current instability horizontal displacements Eberhardt et al. (2004) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 36 of 54 18

19 Progressive Failure in Rock Slopes Natural stress distributions Rock bridges & brittle fracture processes 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 37 of 54 Progressive Failure Hybrid FEM/DEM intra-element fracture Eberhardt et al. (2004) inter-element fracture 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 38 of 54 19

20 Slope Failure Initiation Hybrid FEM/DEM Project rankine5 ELFEN May 9 th, 1991 Rankine tensile fracture constitutive model To = MPa E = 30 GPa G IC = 200 N/m Please Wait.. April 18 th, 1991 Eberhardt et al. (2004) inter-element fracture intra-element fracture 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 39 of 54 Internal Shearing & Strength Degradation tensile failure yield ε p c (MPa) φ ( ) To (MPa) Eberhardt et al. (2004) distinct-element strain-softening model showing development of Prandtl-type yield zone at base of slide surface and propagation of tensile damage upwards through intact slide mass. 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 40 of 54 20

21 current instability Mohr-Coulomb constitutive model with Rankine tensile fracture April 18th s 3 s 1 Extension Strain e p 3 + e p 1 current instability May 9th April 18th Stead et al. (2005) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 41 of 54 The Randa Rockslide Laboratory???? Surface/Subsurface Displacements Fracture Initiation Existing Fracture Pore Pressures Microseismic Emissions 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 42 of 54 21

22 Randa Rockslide Laboratory 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 43 of 54 Randa Rockslide Laboratory Geological investigations Geophysical investigations 3-D geological model Rockslide processes Willenberg (2004) Heincke (2005) Spillmann (2005) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 44 of 54 22

23 Randa Rockslide Laboratory Geological investigations Geophysical investigations toppling sliding 3-D geological model Rockslide processes Kinematics of the rockslide rotating Geotechnical monitoring? Numerical modelling Microseismic monitoring 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 45 of 54 Known active geological structures Addition of assumed basal sliding surface and auxiliary fractures Rock mass properties and constitutive models Block geometry discontinuum modelling (UDEC) Calculated displacement patterns resulting from unstable situation Comparison of modelled & measured displacements, including: Surface displacements Block displacements/rotations at depth derived from inclinometer & extensometers Willenberg (2004) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 46 of 54 23

24 Randa Rockslide Laboratory Modelled Measured in SB120 sliding rotating Willenberg (2004) toppling sliding toppling 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 47 of 54 Randa Rockslide Laboratory Geometries where the mode of measured displacements could be partially reproduced by numerical modelling: planar sliding surface (no agreement with geological model) step-path sliding surface (agreement with geological model) Willenberg et al. (2004) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 48 of 54 24

25 The Randa Rockslide Laboratory Event: 9. April, 2002, 04:19 B4- E B4- N B4- V A5- N A5- E A5- V A4- E A4- N A4- V Time [s] Spillmann (2005) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 49 of 54 raw event signal Hz bandpass filter increasing distance from source increasing distance from source Spillmann (2005) 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 50 of 54 25

26 Long-term Vision: To better integrate detailed geotechnical field measurements with state-of of-the-art numerical modelling to better understand the spatial and temporal evolution of rockslide processes and failure mechanisms. Long-term Vision: In doing so, our goals are to be able to follow coupled rock mass stability problems from their initial stages to catastrophic failure, thus using numerical modelling to model the complete failure process from initiation, through transportation to deposition. 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 51 of 54 Conclusions As demonstrated through the example of Campo Vallemaggia, field and instrumentation data both provide an important means to constrain numerical models (in this case, to better understand the performance of rock slope mitigation measures). At the same time, as shown in the case of Randa, it must be recognized that most in situ measurements are affected by the same issues of rock mass complexity and variability as the numerical analyses they are used to constrain, and therefore require a similarly large degree of interpretation. As such, if advances are to be made in the spatial and temporal understanding/prediction of natural slope behaviour, more emphasis needs to be placed on the integration of complex data sets. Iterative approaches should be taken where, for example, rock slope deformation data is used to constrain numerical analyses, but equally so, numerical analyses are used to constrain the interpretation of complex rock slope deformation data. Geological & Geotechnical Data Numerical Models 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 52 of 54 26

27 Conclusions & Further Work Numerical techniques have demonstrated significant potential for furthering our understanding of rock slope deformation and failure mechanisms/processes and the associated risk. Stead et al. (2005) However, this primarily applies to spatial prediction. To move closer towards better understanding the temporal evolution of massive rock slope failures, subsurface processes involving rock mass strength degradation and progressive failure must be considered. Continued advances in computing power requirements are much welcome as problems move into 3-D and increase in complexity (e.g. inclusion of brittle fracturing, hydro-mechanical coupling, etc.). At the same time, much needed improvements are likewise required with respect to engineering geological/geotechnical data collection methodologies. 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 53 of 54 Rock Slope Toolbox: Aiding the Judgment Process The more complex the model, the more input parameters it requires and the harder it becomes to determine these parameters without extensive, high quality (and of course, expensive) laboratory testing; As such, we should always begin by using the simplest model that can represent the key behaviour of the problem, and increase the complexity as required. Everything should be made as simple as possible but not simpler. Einstein Numerical modelling should not be used as a substitute for thinking, but as an aid to thought and engineering judgment 29 th Canadian Geotechnical Colloquium (Saskatoon, Sept. 20 th, 2005) 54 of 54 27