Joëlle Riss

Influence of fracture geometry on shear behavior

Abstract Laboratory research during the past 10 years has explained many critical links between the geometrical characteristics of fractures and their hydraulic and mechanical behavior. One of the remaining research challenges is to directly link fracture geometry with shear behavior, including behavior in response to changes in normal stress and shear direction. This paper describes results from a series of shear tests performed on identical copies (replicas) of a natural granite fracture. Based on these tests, we developed a method using image processing techniques to identify and quantify damage that occurs during shearing. We find that there is a strong relationship between the fracture’s geometry and its mechanical behavior under shear stress and the resulting damage. Using a three-dimensional geostatistical model of the fracture surfaces, we analyze the dependence of the size and location of damage zones on the local geometry and propose an algorithm for predicting areas that are most likely to be damaged during shearing in a given direction.

Anisotropy of flow in a fracture undergoing shear and its relationship to the direction of shearing and injection pressure

Abstract The work presented deal with the effects of shear direction and injection flow rate on the directional anisotropy of the flow for a given normal stress. Also, is related to these effects the evolution of the intrinsic transmissivity of the fracture in relation to the shear direction and tangenital displacements, particularly for small displacements. Tests are performed on mortar replicas of a natural fracture so that the fracture roughness and void space geometry are the same for each test. Three shear directions are defined. These tests results show that the evolution of the intrinsic transmissivity of a fracture depends on the shear direction particularly for small displacements (less than 0.5 mm), and that shearing induces noticeable modifications in flow direction.

Sheared rock joints: Dependence of damage zones on morphological anisotropy

Abstract The objective of this work is to determine the 3D structural morphological factors such as position, elevation, extension, and dipping of asperities, and relative position of asperities that are essential to analyze the shear behavior of rock joints. A series of shear test was performed at three normal stresses (7, 14 and 21 Mpa) with identical cement mortar replicas of a natural fracture. For each normal stress, under the same shear rate, shear displacement was stopped at five different values. The shear tests were performed at four different directions. After each shear test, grey level images of both, upper and lower walls of the replicas, were acquired using a CCD camera. Then, using a segmentation method we have developed, the damaged areas are identified in such a way that damaged contours can be superposed on a topographic map. Results of the investigation include a set of images showing both the morphology of the joint and the specific position of each damaged asperity. Numerical data are also available.

The evolution of void spaces (permeability) in relation with rock joint shear behavior

Abstract A review is done on the geometrical description of the joint surface roughness morphology to define asperity angularities and spatial information on roughness structures. Characterization of damaged areas, in relation with shear displacement under three different constant normal stress magnitude is discussed within the more global contact area ratio. A synthetis of matched joint shear behavior offer the basis for a qualitative evaluation of the coupling complexity between the mechanical behavior and void space evolution between the joint walls during shear displacement on the basis of the morphological characteristics of the joint surface roughness and contact/damaged areas analyses sustained by joint shear test results from natural joint replicas.

Electrical resistivity monitoring with buried electrodes and cables: noise estimation with repeatability tests

Electrodes and cables are sometimes embedded or buried permanently into the ground in order to minimize electrode location errors during long electrical resistivity monitoring. This procedure is efficient and useful. In order to evaluate the feasibility and suitability of this technique for a long monitoring of water content in a rock mass, electrical resistivity data have been repeatedly acquired from a buried array. Forty-eight steel electrodes composed the array, connected to two multicore cables, which have been buried in a trench since January 2006. A fast resistivity-meter was used to carry out dipole-dipole electrical measurements, over a period of one year, starting in December 2006. Data acquisitions of repeated resistivity measurements have been realized in order to distinguish between variations in electrical resistivity due to noise and to changes in water content. Around once every month, successive dipole-dipole array measurements on the same day (between three and eleven arrays with around 10 minutes for one array) have been performed. Using all the pseudosections recorded the same day, the coefficients of variation have been calculated for each data point. Results show that the variability of the data is six times greater when the data from the first array of the day-series are taken into account. The data collected from this first array are therefore significantly noisier than the following measurements and must be removed for correct interpretation. We propose two main explanations for this effect: 1) polarization of the system array/clay at the time of the first acquisition and 2) damage encountered into the buried cables. Despite this damage, we have shown that the electrical data can be consistent and correctly exploited if the first acquisition of the day is not taken into account. In conclusion, on the basis of the results presented here, we recommend that further studies be made using buried equipment to systematically carry out several acquisitions before starting the long monitoring. We also recommend to remove data from the first acquisition whenever they are found to be significantly noisier than data of the following acquisitions.

3D NUMERICAL SIMULATIONS OF WAVES GENERATED BY SUBAERIAL MASS FAILURES: APPLICATION TO LA PALMA CASE

Three-dimensional (3D) waves generated by landslides are simulated using a three-fluid Navier-Stokes VOF model. With this approach, the interaction between slide and water is implicitly solved. The model capabilities are first tested for benchmark cases featuring rigid body motion. Results are good in two dimensions (2D) and encouraging in 3D. Wave generation by a potential collapse of the Cumbre Vieja Volcano, on La Palma island, is then studied. Stability analyses show that the Cumbre Vieja flank is currently highly stable and that potential slide volumes are likely to be closer to 100 km, rather than the 500 km predicted in earlier studies. Results of the Navier-Stokes model show that waves generated are highly dependent upon the details of slide mechanism and kinematics. In our worst 3D scenario (assuming an inviscid fluid), the initial wavelength is 20 km and the wave height decrease due to lateral spreading is high.

Contribution of hydrochemical and geoelectrical approaches to investigate salinization process and seawater intrusion in the coastal aquifers of Chaouia, Morocco

This study aims to identify groundwater salinization origin and to determine seawater intrusion extension toward the inland in Chaouia, Morocco. To reach these objectives, firstly, 46 groundwater samples were analyzed for major chemical elements during January 2012 and, secondly, 10 electrical resistivity tomography (ERT) profiles were performed perpendicularly to the coastal fringe. Statistical analysis provided the distinction between three Clusters reflecting different hydrochemical processes. Cluster I and Cluster II-a showed a high water electrical conductivity (EC) (from 2.3 to 11.2mS/cm) with the dominance of Na+ (668mg/L on average) and Cl- (1735mg/L on average) ions as a consequence of seawater intrusion. However, Cluster II-b presented low ECs (from 0.5 to 1.7mS/cm) and Ca2+ (99.6mg/L on average) and HCO32- (235.2mg/L on average) ions dominance. Water chemistry in these wells was controlled by water-rock interaction, cation exchange, and anthropogenic activities. The Hydrochemical Facies Evolution Diagram highlighted the succession of different water facies developed between intrusion and freshening phases. The formation of Na-HCO3 facies, which characterizes the last facies of freshening phase, followed the succession of Na-Cl, MixNa-MixCl, MixCa-MixCl, MixCa-MixHCO3, and Na-HCO3. In contrast, Na-Cl facies formation, which characterizes the last facies of intrusion phase, followed the evolution of Ca-HCO3, Ca-MixHCO3, Ca-MixCl, MixCa-MixCl, MixCa-Cl, and Na-Cl. Moreover, the obtained ERT results allowed determining the extent of different hydrochemical facies and provided more details about seawater intrusion extension. The conductive level assigned to seawater contamination showed a resistivity less than 36Ω.m, which remains limited to 3000m from the ocean, where Na-Cl water type dominates. The seawater intrusion depth varied between 5 and 40m from the surface. Overall, this original study in Chaouia region demonstrated the effectiveness of combining hydrochemical and ERT methods to investigate seawater intrusion, particularly in areas with restrictive water chemistry data.

Detection of landfill cover damage using geophysical methods

On closed landfills, impermeable covers are capping the waste in order to minimize water infiltration and accumulation of leachate inside the waste. In France, the cover composition depends notably on the kind of stored waste. In cases of hazardous waste, the cover must be composed of a drainage layer under the top soil and a geomembrane or a Geosynthetic Clay Liner (GCL) associated with an underlying 1.0 m thick low permeability material to ensure its tightness. However, this protection cover is sometimes damaged leading to an escape of landfill gazes and an unusual increase of leachate within the waste after rainy events. As leachate treatment is very expensive, it appears necessary to locate the weakness zones of the cover and assess their sizes in order to limit maintenance cost on landfills. In order to detect damages in the cover, the 2 following geophysical methods have been carried out on a French landfill: cartography with an Automatic Resistivity Profiling (ARP©), the Electrical Resistivity Tomography (ERT) and the Self Potential method (SP). The joint use of these methods has given us the opportunity to determine the contribution of each of them for covers characterization. The ARP survey has put in evidence the lateral heterogeneity of cover materials on the whole landfill. The ERT has confirmed the variability of the cover composition but also provided information about the cover thickness and the damaging of the GCL. SP measurements have revealed a negative anomaly at the top of the landfill, possibly linked with a thin and damaged cover and therefore a greater proximity of waste. Finally, manual augers holes have enabled to associate electrical resistivity properties with different materials used in the cover and the damaged GCL. This study on a hazardous waste landfill shows that geophysical methods associated with manual auger holes have allowed to improve the knowledge of the cover. Thus, the damaged areas detected thanks to measurements performed on site may be useful for the landfill manager who can optimize the drilling survey necessary to check the nature of defects and then choose a suited remediation of the cover.

Geophysical methods applied to characterize landfill covers with geocomposite

We attempt to characterize with geophysical methods the state of landfill covers to detect damages that can induce preferential water pathways and unusual increase of leachate within the waste mass. The geophysical methods used were the Electrical Resistivity Tomography (ERT), cartography with an Automatic Resistivity Profiling (ARP©), and the Self Potential method (SP). We worked on experimental parcels reproducing common defaults on landfill covers (clay material and geocomposite) and on a larger scale on a french landfill cover. The joint use of these methods gives us the opportunity to test their ability to detect defects. Results on the parcels have shown a good detection of the larger cracks (0.10 m) on the compacted clay cover but a less easy detection of defaults on the geocomposite. Results on the landfill have shown conductive zones correlated with important SP variations that could indicate a preferential infiltration zone in the cover.

Uncertainty analysis and probabilistic segmentation of electrical resistivity images: the 2D inverse problem

In this paper, we present the uncertainty analysis of the 2D electrical tomography inverse problem using model reduction and performing the sampling via an explorative member of the Particle Swarm Optimization (PSO) family, called the Regressive-Regressive PSO (RR-PSO). The procedure begins with a local inversion to find a good resistivity model located in the nonlinear equivalence region of the set of plausible solutions. The dimension of this geophysical model is then reduced using spectral decomposition, and the uncertainty space is explored via PSO. Using this approach, we show that it is possible to sample the uncertainty space of the electrical tomography inverse problem. We illustrate this methodology with the application to a synthetic and a real dataset coming from a karstic geological set-up. By computing the uncertainty of the inverse solution, it is possible to perform the segmentation of the resistivity images issued from inversion. This segmentation is based on the set of equivalent models that have been sampled, and makes it possible to answer geophysical questions in a probabilistic way, performing risk analysis. This article is protected by copyright. All rights reserved