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Sub-project 3: Innovative approaches for landslide assessment
Numerous landslides have been triggered in past earthquakes causing a large number of fatalities. The dynamics of landslides is not straightforward since it involves widespread areas, may induce large displacements after triggering and is very much influenced by the soil strength parameters that may vary during the process of landsliding. The knowledge of the physical mechanisms governing flow-induced movements is poor and, consequently, prediction of the extent of the flow slides is more than uncertain. It is thus the objective of this sub-project to make an attempt to quantify the influence of the parameters that govern the mechanisms of landslides, to assess the consequences in terms of induced displacements and effects on the built environment. Both aspects related to the driving forces (pore pressure changes due to rain falls, temperature changes due to glaciation, rainfall earthquake induced forces taking into account the topographic amplification, the forward directivity and fling effects) and related to the soil constitutive behaviour (rigid plastic, viscous-rigid plastic (Bingham model) and elasto-plastic) will be examined. Various modelling techniques will be adopted: rigid block models, Lagrangian finite element models, arbitrary Lagrangian Eulerian finite element models. Rate effects on the behaviour of soils are also of paramount importance and will be experimentally investigated.
These theoretical studies will be completed by the compilation of a database of well-documented case histories of major landslides during a number of instrumented earthquakes. This will serve for the validation of the various numerical modelling that are proposed with the scope of this sub-project. The final goal will be to develop practical design tools for determining landslide movements and for designing defensive measures. The sub-project is coordinated by GDS and features also the participation of AUTH, BRGM, INPG, NTUA, SAA and UNIMIB.
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Deliverables Presentations Reports Publications Events Meetings
A summary of each of the tasks involved in this Sub-Project is provided below: Task 1.3.1: Constitutive relationships of soil behaviour to predict landslide movements Sub-task 1.3.1.1: Assessment and improvement of constitutive relationships
Assessment of appropriate soil constitutive relationship(s) for the prediction of landslides movements involves sophisticated elasto-plastic model with the capacity of predicting soil behaviour due to temperature and pore pressure changes, with soil softening. The development of more dedicated models is also foreseen which could be incorporated in specific numerical tools in relation with the prediction of large displacements associated with liquefaction (sliding-surface particle breakage, constitutive law for the liquefied soil). Once liquefaction has been triggered, the soil constitutive behaviour governing further movements is more than controversial. Some researchers assume that the liquefied soil behaves as a viscous fluid, others model the soil behaviour as a rigid plastic material with a residual shear strength and some others invoke the formation of a water film at the sliding surface to explain the large observed movements. Each of these assumptions will be reviewed and other new ones (viscous fluid with a threshold strength - Bingham model) will be investigated. The objective will be to develop a consensus, if possible, on the most appropriate constitutive modelling of soil during the progress of a landslide. In general, models for analysing slope stability, rockslides and debris flows, run-out distance, impact forces and tsunamis need to be improved or developed. With the improved understanding of the geological processes, an important aspect of the research is the development of reliable numerical models for simulating geo-hazards and modelling ground deformation. Further research tasks include: Improvement or development of new material models for unsaturated soils, gas-charged sediments and gas hydrates, weathered and residual soils, and strain-softening soil (e.g. sensitive clays), geo-hydrological regime to predict pore pressure response to rainfall and flooding events, slope stability and deformations for different geological settings and for different triggers, and run-out distance and forces on objects hit by a submarine slide, rock falls or rock slides. Improvement or development of analysis methods to predict soil liquefaction and slide development due to earthquakes; include soil response analysis from seismic source to site, impact of dynamic and static stresses, and long-term instability effects over a large region. Sub-task 1.3.1.2: Laboratory investigation of the mechanical behaviour of geomaterials involved in mass movements
Laboratory investigations by means of ring shear tests into the large shear strain behaviour and in particular residual strength of cohesive soils and the influence of rate of displacement on shear strength of both previously unsheared and presheared cohesive soils, which are involved in slope stability problems. The aim is to identify the type of rate effect on the behaviour of soils, as well as soils that may loose strength at fast rates of displacement. The results will lead to the formulation of a model of soil behaviour, linking shear strength with displacement on the shear surface, while the model will be used in both co-seismic and post-seismic kinematics of sliding mass. The soil mass performance will also be assessed using displacement and velocity as criteria rather than factors of safety used in conventional stability analysis. Task 1.3.2: Landslide mechanisms and triggering forces
Sub-task 1.3.2.1: Hydraulic-triggered ground motion mechanisms
The effects of local soil conditions leading to large deformations will be investigated, specifically the role of progressive accumulation of water at the interface between soil layers with high permeability contrast. This process is observed not only in earthquake-triggered landslides, but also under quasi-static conditions and may be initiated by heavy rainfalls or water flow (infiltration, etc.). Sub-task 1.3.2.2: Earthquake-triggered ground failure mechanisms
The role of some features related to the source mechanisms and propagation process of the seismic ground motion will be investigated in this sub-item, such as azimouthal, directivity and fling effects, spatial variability of ground motion and loss of coherency of seismic waves, apparent velocity. Improvement of our knowledge of the role of "topographic amplification" of the ground motion, and of the significance of ground shaking influenced by "forward directivity" and "fling" effects on the magnitude of the driving forces. This will be achieved with numerical parametric studies with a classical equivalent linear soil constitutive relationship. Sub-task 1.3.2.3: Advanced geomechanical modelling of localised and diffused failure
It is proposed to further improve geomechanical modelling of localised and diffused failure based resp. on Rice and Hill criteria. Failure mechanisms are investigated through various criteria. The analysis of localised failure is based on Rice's criterion (vanishing determinant of the acoustic tensor), while the diffuse failure is described by Hill's criterion (vanishing second order work, linked to vanishing determinant of the symmetric part of the constitutive tensor). The classical plastic analyses (limit analysis methods) are based on vanishing values of the determinant of the constitutive tensor itself, which occurs always after Hill's criterion. Finally the geometric instabilities are exhibited by bifurcation analyses. Two main domains can be distinguished in the proposed research approach:
landslides, where the hypothesis of continuous media can be assumed and a finite element method be applied. For simulation purposes, existing software will be used, together with special constitutive models for natural media, which have been properly validated at an international level in the past;
rockfalls, where the computations need to be developed inside the framework of the discrete mechanics. For simulation purposes, 2 codes are used either based on the "Contact Dynamics" method or on the "Molecular Dynamics" method.
The developed analytical procedures will be calibrated by back analysis of well-documented case histories of seismic landslides. Task 1.3.3: Deterministic tools to predict landslide displacements at a local scale Development of numerical tools to predict the displacements associated with landslides and their effect on the built environment. Classically, slopes displacements are calculated, since the pioneering work of Newmark, with uncoupled rigid blocks mechanisms. This method suffers serious limitations especially for the case of flow-induced liquefaction. It is proposed to further developed the method by taking into account the strength degradation, by better locating the initial position of the sliding mass for real complex slope profiles and geometries and by introducing some degree of coupling between the induced forces and the associated displacements. Other numerical tools will also be either developed or improved, based on more rigorous mechanical approaches: Eulerian finite element model, or arbitrary Lagrangian-Eulerian finite element model. Task 1.3.4: Stabilisation and mitigation techniques
This work package investigates the effectiveness of various stabilisation measures against earthquake and rainfall, using both observational data and numerical analyses with varying degree of detail and sophistication, ranging from elaborate elasto-plastic finite element methods to sliding-block models. The research effort will review a number of preventive measures, such as soil nailing, buttressing and stone columns. The choice of the stabilisation method depends primarily on the cost and implementation time. In addition to state-of-art methods, this work package will investigate the effectiveness of an innovative stabilisation method consisting of driving stiff inclusions near the toe of the slope. The use of stiff inclusions has recently been implemented to improve the seismic behaviour of the foundations of a large bridge. These inclusions will not only increase the average soil shear resistance due to their high shear and bending capacities, but will also “nail” the slope by pushing the potential failure surfaces towards layers of higher strengths. The method will be applied to actual slopes having suffered severe damages during past earthquakes. The new methodology will be validated with sophisticated calculation tools and the potential effectiveness of the improvement technique will be assessed.
Sub-task 1.3.5: Application of the deterministic tools to actual case histories
The above deterministic numerical tools, developed to assess the influence of ground motion parameters on sliding deformations, will be applied to the appropriate test sites documented in the database compiled under sub-task 1.1. |
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