Yannick Fargier

Detection of Leakage Areas in an Earth Embankment from GPR Measurements and Permeability Logging

Ground penetrating radar (GPR) is a nondestructive method allowing the improvement of our knowledge of civil engineering structures. In particular, this method may be a nondestructive efficient tool for dike diagnosis and complete classical geotechnical methods. In this paper, we present GPR observations obtained on an earth embankment (crest and sloped paved revetment) in bad condition and located on the lateral canal of the Loire river (Saint Firmin, 80 km South East of Orleans). These measurements are combined with corings, visual inspection, and permeability logging performed with an updated drilling system, the Permeafor. This survey leads (i) to the detection of decompressed zones associated with leakage areas visible at the foot of the downstream slope and (ii) to the location of potentials voids underneath the paved revetment. This multidisciplinary approach complied with the dike inspection methodology proves its efficiency for the assessment of earth embankments.

Methodology Applied to the Diagnosis and Monitoring of Dikes and Dams

The recent and dramatic floods of the last years in Europe (Windstorm Xynthia, February 2010) and United-States (Hurricane Katrina, August 2005) showed the vulnerability of flood defence systems. The first key point for avoiding these dramatic damages and the high cost of a failure and its consequences lies in the conception and construction of the dams and dikes, taking into account the past flooding events. A well-designed dike with the correct height avoids failure and overtopping.

Application of Belief Functions to Levee Assessment.

We propose the use of Smets and PCR5 rules to merge artificial geophysical and geotechnical data, as part of fluvial levee assessment. It highlights the ability to characterize the presence of interfaces and a geological anomaly.

A cost-effective 3D electrical resistivity imaging approach applied to dike investigation

Although DC electrical resistivity imaging is widely applied to dike investigation, either rapid 2D or high-resolution 3D approaches fail to address actual needs. An intermediate electrical resistivity imaging approach referred to as “3D–” is introduced in this paper. The methodology is based on existing tools, and it offers useful and sufficiently reliable 3D images of the investigated structure within a cost-effective and flexible procedure. The survey design, the model discretisation, and the thorough integration of a priori information are the main phases of this procedure. To demonstrate the benefits and limitations of this approach, it is applied to an existing stretch of embankment levee along the Loire River. A numerical study was carried out both on synthetic and real data to assess the 3D imaging capability of the approach and the influence of prior information on the inversion outputs. The important role of a priori information is shown to be even more essential here. The results demonstrate the efficiency and versatility of the 3D– approach for reliable and cost-effective investigations of long dikes.

Contribution of SFM and ERI Methods to Assess an Underground Quarry Pillar

Natural degradation of underground quarries is a major issue from an economic and public safety point of view. Electrical Resistivity Imaging (ERI) method can assess efficiently the electrical resistivity distribution inside a medium and its evolution. However, conventional 3D ERI methods are not well suited for quarry pillar assessment. Moreover, a complete 3D ERI necessitates an accurate 3D geometry. In this paper, we propose the use of cost effective SFM techniques to generate a three-dimensional medium. A new assessment methodology based on the combination of SFM and Electrical Resistivity methods is proposed. This methodology is applied to a real limestone quarry pillar. Inversion results show the key role of the accuracy of the 3D model.

A Cost-effective 3D Electrical Resistivity Imaging Approach for Embankment Dike Assessment

Although DC-electrical resistivity imaging techniques are efficient tools for embankment dike and levee investigation and monitoring, the complexity of such structures often leads to misinterpretation when conventional 2D procedures are implemented. Nevertheless, fully 3D procedures can be very slow and costly, and bring results and resolution that may exceed the needs. This paper introduces the ‘3D– inversion’ approach that allows a useful and reliable 3D vision of the investigated structure while maintaining an acceptable number of electrodes and data. To compensate the lack of dense 3D data, the approach is mainly based on i) designing data acquisition based on a reasonable number of electrodes and measurements, ii) designing a well-adapted model discretization and iii) inserting as much a priori information as possible. The building process of the numerical model requires attention, particularly on the model discretization. To exemplify this new approach, synthetic results are presented on a model that is a simplification of a real scale experimentation. Preliminary outcomes show the cost-effective potential of the 3D– approach which is to be tested on various case studies in the near future.