Numerical Analysis of slopes ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (1) Numerical Analysis of slopes Simon Nelis Simon NELIS Formerly at GNS Science (Lower Hutt)

Numerical modelling and analysis ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (2) Numerical modelling and analysis What is numerical analysis? Why undertake numerical analysis? Provides insight into processes and mechanisms controlling stability Numerical analysis has become much more commonplace in the last 20 years Commercially available software is cheaply available and complex analyses can be run on desk top PC’s

Applicability to slope stability ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (3) Applicability to slope stability Ability to define Factor of Safety of a slope Back analysis to determine material strengths and/or triggering mechanisms Undertake sensitivity analyses Material properties Groundwater Analyse effects of changes to slope geometry, e.g. Ruapehu dam breach analysis Investigate the effect of changing static and dynamic loads on stability of the slope Determine the effectiveness of remedial measures on improving performance of the slope.

Software available Limit equilibrium Slide Slope/W ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (4) Software available Limit equilibrium Slide Slope/W Finite Element / difference Phase 2 FLAC2D and FLAC3D PLAXIS Discrete / Distinct elements UDEC PFC2D and PFC3D

Conventional analyses ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (5) Conventional analyses Analysis Method Critical Parameters Advantages Limitations Stereographic and Kinematic Critical slope and discontinuity geometry; representative shear strength characteristics. Relatively simple to use; gives initial indication of failure potential; may allow identification and analysis of critical key blocks using block theory; links are possible with limit equilibrium methods; can be combined with statistical techniques to indicate probability of failure. Only really suitable for preliminary design or design of non-critical slopes; critical discontinuities must be ascertained; must be used with representative discontinuity/joint shear strength data; primarily evaluates critical orientations; neglecting other important joint properties. Limit Equilibrium Representative geometry an material characteristics; soil or rock mass strength properties (cohesion and friction); discontinuity shear strength characteristics; groundwater conditions; support and reinforcement characteristics. Wide variety of commercially available software for different failure modes (plane, wedge, toppling etc.); can analyse Factor of Safety sensitivity to changes in slope geometry and material properties; more advanced codes allow for multiple materials, 3-D, reinforcement and/or groundwater profiles. Mostly deterministic producing a single Factor of Safety (but increased use of probabilistic analysis); Factor of Safety gives no indication of failure mechanisms; numerous techniques are available all with varying assumptions; strains and intact failure not considered; probabilistic analysis requires well defined input data to allow meaningful evaluation. Rockfall Simulators Slope geometry, rock block sizes and shapes; coefficient of restitution. Practical tool for siting structures; can use probabilistic analysis; 2-D and 3-D codes are available. Limited experience in use relative to empirical design charts.

Examples Conventional analyses ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (6) Examples Conventional analyses Rockfall simulation

Examples Conventional analyses ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (7) Examples Conventional analyses Limit Equilibrium

Advanced numerical analyses ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (8) Advanced numerical analyses Analysis Method Critical Parameters Advantages Limitations Continuum Modelling (e.g. finite element, finite difference). Representative slope geometry; constitutive criteria; (e.g. elastic, elasto-plastic, creep); groundwater characteristics; shear strength of surfaces; in situ stresses. Allows for material deformation and failure (Factor of Safety concepts incorporated); can model complex behaviour and mechanisms; 3-D capabilities; can model effects of pore water pressures, creep deformation and/or dynamic loading; able to assess effects of parameter variations; computer hardware allow complex models to be solved with reasonable run times. User must be well trained, experienced and observe good modelling practice; need to be aware of model and software limitations (e.g. boundary effects, meshing errors, hardware memory and time restrictions); availability of input data generally poor; required input parameters not routinely measured; inability to model effects of highly jointed rock; can be difficult to perform sensitivity analysis due to run time constraints. Discontinuum Modelling (e.g. distinct element, discrete - element). Representative slope and discontinuity geometry; intact constitutive criteria; discontinuity stiffness and shear strength; groundwater characteristics; in situ stress state. Allows for block deformation and movement of blocks relative to each other; can model complex behaviour and mechanisms (combined material and discontinuity behaviour coupled with hydro-mechanical and dynamic analysis); able to assess effects of parameter variations on instability. As above, user required to observe good modelling practice; general limitations similar to those listed above; need to be aware of scale effects; need to simulate representative discontinuity geometry (spacing, persistence, etc.); limited data on joint properties available (e.g. joint normal stiffness, joint shear stiffness). Hybrid/Coupled modelling Combination of input parameters listed above for stand-alone models. Coupled finite-/distinct-element models able to simulate intact fracture propagation and fragmentation of jointed and bedded rock. Complex problems require high memory capacity; comparatively little practical experience in use; requires ongoing calibration and constraints.

Examples advanced numerical analysis ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (9) Examples advanced numerical analysis Finite Element modelling

Examples advanced numerical analysis ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (10) Examples advanced numerical analysis Distinct Element East-west profile Equilibrium 13,000 iterations 20,000 iterations 25,000 iterations

Limitations and pitfalls numerical analysis ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (11) Limitations and pitfalls numerical analysis Numerical analyses are performed on the basis of given input, not on a through understanding of physics of the problem Errors arise from: Poor understanding of geological/geotechnical model Inadequate material testing to define material behaviour Inadequate zonation forcing the model to behave in a particular way Results using the same software and input parameters can be user dependant Models can be sensitive to constitutive models chosen to represent material behaviour Sensitive to sequence of calculation Results should be used to gain an insight into physical processes, rather than used as an absolute predictive tool

ICL Landslide Teaching Tools　 PPT-tool 3.064-1.1(b) (12) Conclusions Numerical analyses should be carried out, but the results should NEVER be considered an accurate representation of reality The ability of numerical models to predict mechanisms of behaviour is a major advantage Analyses can deal with both complex and simple problems Results from numerical models can be user dependant Limitations exist with constitutive models used to represent material behaviour in models An in depth knowledge of soil mechanics is required Familiarity of the software being used for the analysis is required