Hollow Cylinder Simulation Experiments of Galleries in Boom Clay Formation

In the context of nuclear waste disposal in clay formations, laboratory experiments were performed to study at reduced scale the excavation damaged zone (EDZ) induced by the construction of galleries in the Boom clay formation. For this purpose, thick-walled hollow cylindrical samples were subjected (after recovery of in situ stress conditions) to a decrease in the inner confining pressure aiming at mimicking a gallery excavation. X-ray computed tomography (XRCT) scans of the specimens were carried out through the testing cell before and after the mechanical unloading and allowed to quantify the displacements undergone by the clay as a result of the mechanical unloading. The deformation of the hollow cylinders and the inferred extent of the damaged zone around the central hole are found to depend on the orientation of the specimen with respect to the bedding planes and show a great similarity with in situ observations around galleries and boreholes at Mol URL in the Boom clay formation. In the experiments performed on samples cored parallel to the bedding, the damaged zone is not symmetrical with respect to the hole axis and extends more in the direction parallel to the bedding. It is the same for the radial convergence of the hole walls which is larger in the direction parallel to bedding than in the perpendicular one. In contrast, a test on a sample cored perpendicularly to the bedding did not show any ovalisation of the central hole after the mechanical unloading. These observations confirm the significance of the pre-existing planes of weakness (bedding planes) in Boom clay and the need for a correct consideration of the related mechanical anisotropy

Modélisation physique et analyse numérique de la propagation d'une masse granulaire:contribution à l'étude des avalanches rocheuses

Rock avalanches are processes involving a huge amount of mobilised material, larger than 106 m3, and their propagation is characterised by a very high energy, such that no active mitigation measure is capable of protecting human lives and properties potentially exposed. Only land use planning, or emergency plans in case of occurrence of an event, can ensure safety of the communities threatened by these natural hazards. Consequently, it is necessary to predict the extent of the runout area of the process, which constitutes an objective still far from being achieved, due to the lack of knowledge about its mechanism of propagation. This PhD Thesis work mainly deals with the physical modelling of rock avalanches, which may allow for partially balancing the lack of site observations available for such processes. Laboratory tests were set-up using an unconfined inclined plane, representing the real slope, whose toe is constituted by a curved transition. A test consists in releasing a given amount of gravel, initially placed into a box (representing the source area), and measuring its propagation and final deposit features. The influence of volume, fall height, inclination of the plane and curvature radius of the transition was evaluated both on the mass propagation and on the morphology and characteristic parameters of the final deposit. In addition to the runout, width and length of the final deposit measured manually, a dedicated measurement system allowed for determining the morphology and the position of the centre of mass of the final deposit and of the mass in motion. The measurement system uses the fringe projection method, a non-destructive optical technique which allows for retrieving the shape of an object. The existing system, formerly used in a previous research work (Manzella 2008, Manzella and Labiouse 2008b), was advanced and further developed for measuring the features of the mass in motion. As the fringe projection is now performed during the whole test, the WINAnalyze code formerly used for detecting and tracking the mass front could not be used, and a new technique for measuring the front propagation was therefore developed. Numerical analyses of the laboratory tests were also carried out, using the DAN3D code (McDougall 2006, McDougall and Hungr 2004, 2005, Hungr and McDougall 2009), which assumes the mass in motion as an equivalent fluid. The code applies the depth-averaged form of St-Venant's equations, solved with a Lagrangian method base on smooth particles hydrodynamics (SPH). The basal shear resistance is modelled with simple rheological relationships. The reproduce rock avalanches, the frictional model (Coulomb’s law) and the Veollmy’s model are the most often used. Numerical modelling was performed for several tests carried out both in a previous and in this research work. The frictional rheological model is the most appropriate for reproducing laboratory tests. The range of values of the numerical parameters defined by McDougall (2006), and currently used for the back- analysis of real case studies, was modified in order to describe best the characteristics of the final deposit. The results obtained so far are promising. The DAN3D code does not present any major issue in reproducing laboratory tests characterised by different configurations of the source area and the topography

Hollow cylinder simulation experiments of galleries in Boom Clay formation

In the context of nuclear waste disposal in clay formations, laboratory experiments were performed to study at reduced scale the Excavation Damaged Zone (EDZ) induced by the construction of galleries in the Boom Clay formation. For this purpose, thick-walled hollow cylindrical samples were subjected (after recovery of in-situ stress conditions) to a decrease of the inner confining pressure aiming at mimicking a gallery excavation. X-Ray computed Tomography (XRCT) scans of the specimens were carried out through the testing cell before and after the mechanical unloading and allowed to quantify the displacements undergone by the clay as a result of the mechanical unloading. The deformation of the hollow cylinders and the inferred extent of the damaged zone around the central hole are found to depend on the orientation of the specimen with respect to the bedding planes and show a great similarity with in-situ observations around galleries and boreholes at Mol URL in the Boom Clay formation. In the experiments performed on samples cored parallel to the bedding, the damaged zone is not symmetrical with respect to the hole axis and extends more in the direction parallel to the bedding. So it is for the radial convergence of the hole walls which is larger in the direction parallel to bedding than in the perpendicular one. On the contrary, a test on a sample cored perpendicularly to the bedding did not show any ovalisation of the central hole after the mechanical unloading. These observations confirm the significance of the pre-existing planes of weakness (bedding planes) in Boom Clay and the need for a correct consideration of the related mechanical anisotropy

Considerations on Swiss methodologies for rock fall hazard mapping based on trajectory modelling

Rock fall hazard assessment and hazard mapping are essential for the risk management of vulnerable areas. This paper analyses some issues concerning fragmental rock fall hazard mapping methodologies. Two Swiss approaches based on rock fall trajectory simulations results are presented. An application to a site in Switzerland emphasises the differences in the results, uncertainties related to hazard zoning procedures and the influence of some factors on the mapping process. In particular, the influence of a change in the temporal rock fall frequency, of the longer propagation along the slope of only a few computed blocks (defined in this sense as “extreme blocks”) and of the number of runs performed in trajectory modelling have been studied. Results are discussed with the purpose of achieving a more reliable and objective hazard analysis. The presented considerations are based on the Swiss Federal Guidelines, but many of them could be extended to other countries that evaluate rock fall hazard using an intensity-frequency diagram

Considerations on rockfall hazard mapping based on trajectory modelling

The paper presents first considerations about methodologies for local hazard mapping related to rockfalls. This work is being currently carried out at the Swiss Federal Institute of Technology of Lausanne (EPFL), and it concerns the ESR8 research position within the framework of the Mountain Risks project (Marie Curie Research Training Network, funded by EC). The main aim of the research is to collect and use the most recent approaches and results in hazard analyses in order to develop an improved methodology for a detailed rockfall hazard mapping at local scale (and for risk mapping as well, likely at catchment scale). The resulting hazard maps will provide important information for land-use planning and for the design and location of protective measures (together with appropriate risk considerations). The basis of the hazard mapping procedure that is going to be used in this research project is constituted by the methodology CADANAV, developed at EPFL’s Rock Mechanics Laboratory for the Canton of Vaud in Switzerland. It takes into account the temporal frequency of rockfall events, the number of blocks released together with their intensity, to define three different degrees of danger (high, moderate and low), which will allow to obtain the rockfall related hazard zoning. To apply the method, a probability of failure of the unstable rock mass has to be known. Then, the probability of propagation of the detached blocks must be evaluated, that is the probability that a block reaches a selected area with a given intensity. By means of rockfall simulations, the energy profiles and the trajectories of the blocks can be analyzed to evaluate the magnitude of the rockfall, as well as the spatial extent of the process. By analyzing the trajectory results, probability curves for fixed energy thresholds can be drawn, knowing the cumulative frequency of the blocks that reach a particular abscissa along the profile with a given energy value. Then, considering assigned failure return periods, the frequency-intensity information can be crossed to obtain the extension (limits) of areas characterized by high, moderate and low danger. This step is completed by calculating the probability of propagation for each fixed “intensity-return period” couple (according to a diagram proposed by the Swiss Federal Guidelines) and by determining the abscissas (danger zone limits) beyond which the probability of propagation is lower than the calculated value. After the presentation of the CADANAV methodology, the paper will evaluate the influence on hazard mapping of three modelling scenarios: i) a change in the temporal probability of failure, ii) the longer propagation of an “extreme” block, iii) the number of runs performed in the trajectory modelling. (i) Since the CADANAV methodology takes into account the temporal probability of failure of a rock mass, it can describe the evolution of the hazard level due to changes in triggering factors. For instance, if the number of events per unit of time is increasing, the areas at higher hazard levels will be larger i.e. the limits of the danger zones move downslope. (ii) Compared to other methods, the CADANAV methodology is not very sensitive to eventual “extreme events” obtained from the computer simulations, i.e. the limits defined by the procedure are hardly influenced by the long propagation of an extreme block. (iii) Moreover, the hazard mapping obtained by CADANAV is also not highly dependent on the number of runs performed in rockfall simulations. If one simulation is constituted by (e.g.) 300 runs and another one by 10’000 runs, the limits of the high, moderate and low danger zones will not be affected by significant changes. So it is, if one compares the hazard mapping obtained by several simulations of (e.g.) 300 runs. The approach developed for the CADANAV methodology is consistent with the Swiss Federal Guidelines (intensity-frequency diagram characterized by its own energy and return period values), but it could be applied as well for different combinations of rockfall intensity thresholds and return period values, according to the laws and guidelines of other countries

Numerical simulation of gravel unconstrained flow experiments: A comparison between DAN-3D and RASH-3D codes

Four gravel unconstrained flow experiments were modelled with the DAN-3D and RASH-3D codes. Both the codes, based on continuum mechanics, were developed for the propagation of rapid landslides, like rock avalanches. The codes were at first run to back-calculate the dynamic basal friction angle (frictional rheology) in order to model the runout of one of the experiments. The best fit value was then compared with the dynamic basal friction angle measured with a tilting test and finally applied for the modelling of the three other experiments. The back-calculated frictional parameter is different for the two codes and higher than the measured dynamic basal friction angle. This value can then be used to model the runout of other experiments involving a change of volume or falling height. On the other hand, in case of a modification of the slope angle, the dynamic basal friction angle has to be redefined. It seems thus difficult to use a single value of the basal friction angle to model experiments on various topographical profiles

Numerical modelling of gravel unconstrained flow experiments with the DAN3D and RASH3D codes

Landslide continuum dynamic models have improved considerably in the last years, but a consensus on the best method of calibrating the input resistance parameter values for predictive analyses has not yet emerged. In the present paper, numerical simulations of a series of laboratory experiments performed at the Laboratory for Rock Mechanics of the EPF Lausanne were undertaken with the RASH3D and DAN3D numerical codes. They aimed at analysing the possibility to use calibrated ranges of parameters (1) in a code different from that they were obtained from and (2) to simulate potential-events made of a material with the same characteristics as back-analysed past-events, but involving a different volume and propagation path. For this purpose, one of the four benchmark laboratory tests was used as past-event to calibrate the dynamic basal friction angle assuming a Coulomb-type behaviour of the sliding mass, and this back-analysed value was then used to simulate the three other experiments, assumed as potential-events. The computational findings show good correspondence with experimental results in terms of characteristics of the final deposits (i.e., runout, length and width). Furthermore, the obtained best fit values of the dynamic basal friction angle for the two codes turn out to be close to each other and within the range of values measured with pseudo-dynamic tilting test