The GPR (Ground Penetrating Radar) conference in Hong Kong year 2016 marked the 30th anniversary of the initial meeting in Tifton, Georgia, USA on 1986. The conference has been being a bi-annual event and has been hosted by sixteen cities from four continents. Throughout these 30 years, researchers and practitioners witnessed the analog paper printout to digital era that enables very efficient collection, processing and 3D imaging of large amount of data required in GPR imaging in infrastructure. GPR has systematically progressed forward from “Locating and Testing” to “Imaging and Diagnosis” with the Holy Grail of ’Seeing the unseen’ becoming a reality. This paper reviews the latest development of the GPR’s primary infrastructure applications, namely buildings, pavements, bridges, tunnel liners, geotechnical and buried utilities. We review both the ability to assess structure as built character and the ability to indicate the state of deterioration. Finally, we outline the path to a more rigorous development in terms of standardization, accreditation, and procurement policy.

A review of Ground Penetrating Radar application in civil engineering: A 30-year journey from Locating and Testing to Imaging and Diagnosis

Testing the mechanical response of coarse granular materials requires very large and expensive laboratory equipments. During the 1960s, pioneering experimental programs were carried out on several rockfill dam materials, and those results are still a reference for engineers and researchers. However, only few experimental works have been reported to this day, and due to the scarcity of empirical data, the role of the size effect caused by grain crushing is not well known. To improve understanding of this rarely studied issue and the influence of individual particle strength, this paper analyzes the size effect on rock aggregate crushing strength and its connection with the shear envelope of rockfills. The suitability of the 4-parameter Weibull equation to describe size effects on the crushing strength reported in the literature is discussed. Furthermore, a Weibull statistical analysis was carried out for a wide number of experimental results on rock aggregates, where it has been observed that strength decreases with particle size. In parallel, the results of large triaxial tests on homothetic scaled rockfill samples of 250 and 1,000 mm in diameter reveal that the coarser the material, the higher the amount of grain breakage and the lower the shear strength. The impact of size effects obtained from the experiments is analyzed and discussed in terms of the factor of safety of rockfill slope stability. Furthermore, the results are compared with the only existing theoretical method that links the rock aggregate with the strength of the granular assembly. Good agreement between the empirical results and this theoretical method has been confirmed.

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The effect of size on the strength of coarse rock aggregates and large rockfill samples through experimental data

Modeling the fragmentation of rock grains using computed tomography and combined FDEM

This paper presents simulations of true triaxial tests on crushable granular materials. Considering both the shear and tensile fracture modes of the particle, the Mohr–Coulomb model with tension cut-off is employed as the fracture criterion of single particle. When the particle stresses satisfy the fracture criterion, three new balls are generated to replace the broken particle. The new balls overlap enough within the bounding parent sphere to obey the mass conservation law. A polydisperse assembly consisting of 7935 spheres was isotropically compressed until the desired confining pressure was reached. Then the sample with no bias in initial fabric was subjected to axial compression at a constant minor principal stress under different intermediate principal stress ratios, b. The simulated stress–strain–dilation responses were in agreement with the experimental observations. The macro–micro responses including stress–strain characteristics, fabric tensors, anisotropies, the orientation tensor and force chain characteristics were discussed in details. It is shown that volumetric responses is affected by b. The anisotropies vary with b, but a unique macro–micro linkage exists between the anisotropies and stress ratio, q / p, irrespective of the value of b. A linear relationship is found between the obliquity of $${\sigma _{1} }/{\sigma _{3} }$$
 and the strength indicator (
 $$\beta =\ln ({\tau _{1} }/{\tau _{3} }))$$
 using the orientation tensor method.

Macro–micro responses of crushable granular materials in simulated true triaxial tests

In this paper, we consider the structure inspection problem using a miniature unmanned aerial vehicle (UAV). The influence of the wind on the UAV behavior and onboard energy limitations are important parameters that must be taken into account in the structure inspection problem. To tackle these problems, we derive three methods to inspect a structure. First, we develop a Zermelo-Traveling Salesman Problem (TSP) method to compute the optimal route to inspect a simple virtual structure. Second, we derive a method that combines meshing techniques with the Zermelo-TSP method. In this approach, the inspection coordinates for the interest points are obtained automatically by means of a meshing algorithm, then, the Zermelo-TSP method is used to compute the time-optimal route to inspect all the interest points in minimal time. Finally, we derive a method for structure inspection based on the Zermelo-Vehicle Routing Problem (VRP). These methods have been validated in a simulated environment.

UAV Path Planning for Structure Inspection in Windy Environments

Natural soils or coarse granular materials containing large particles such as rockfill or ballast are often an issue for geotechnical projects. However, since it is usually complicated or impossible to set up a big enough experimental apparatus to test these materials, their mechanical behavior remains poorly known. To circumvent this difficulty, we have developed a set of triaxial cells of various sizes to study the experimental behavior of granular soils containing particle sizes of up to 160 mm. This paper presents the first results of a two-part experimental study. The first part consists of triaxial tests on sand samples of different sizes in order to examine specimen scale effects. Pre-peak behavior is not affected by the specimen size, whereas post-peak behavior depends on the test conditions that control the development of strain localizations. In the second part, we present drained triaxial compression tests on large samples of calcareous rockfill materials that have parallel grain size distributions. The results show a nonstandard evolution of the friction angle in relationship to grain size if compared to similar studies on coarse granular materials. Although grain crushing occurs during compression, the strain-stress curves of the different materials with different grain sizes do remain close. We explain this phenomenon by the strength evolution of the individual grains with respect to their sizes.

https://hal.archives-ouvertes.fr/hal-00903824/document

Effect of Sample Size on the Behavior of Granular Materials

https://hal.archives-ouvertes.fr/hal-01592975/document

Utility detection and positioning on the urban site Sense-City using Ground-Penetrating Radar systems

A large scale equipment project, the Sense-City Equipex has received support from the French government in the framework of the 'Programme d’Investissement d’Avenir'. This project is led by the French Institute of Science and Technology for Transport, Development and Networks (IFSTTAR) with ESIEE-Paris, the Laboratory of Physics of Interfaces and Thin Films (LPICM) and the Scientific and Technical Centre for Building (CSTB) as consortium partners. Starting in 2012, Sense-City will offer a suite of high-quality facilities for the design, prototyping and performance assessment of innovative, micro- and nanotechnology based sensors devoted to urban instrumentation. Acknowledging the shortcomings of evaluating sensors performances in laboratory conditions only or in the ever-changing environment of our cities, Sense-City will provide a realistic urban test space in climatic conditions, far more complex than clean rooms and far less complex than actual cities. Sense-city revolves around the 'mini-city' concept, a large, fully customizable climatic hall able to host full- and reduced-scale models of essential urban components. The design of the models will allow for the simulation in climatic conditions of numerous scenarios of sustainable cities. The scenarios to be implemented will correspond to different research topics related to urban sustainability: energy performances in buildings, quality of air, water and soils, quality of fluid distribution networks (gas, sewage, drink water), control of waste disposal areas, durability and safety of infrastructures.

https://hal.archives-ouvertes.fr/hal-00860842/document

The Sense-City project

Necessaire pour garantir le niveau de service et la securite des usagers, l'inspection des ouvrages d'art est realisee tres majoritairement de facon visuelle sur l'ensemble de la structure et doit etre executee en assurant la securite des intervenants. C'est pour repondre a ces conditions que des passerelles et des nacelles motorisees ont ete developpees, en particulier pour observer les zones difficilement accessibles. Or ces equipements sont parfois difficiles, voire impossibles d'utilisation ; c'est pourquoi nous nous sommes interesses aux developpements de systemes de vision embarques sur des drones. Apres avoir valide le concept de drone par une experimentation sur un site reel au moyen d'un aeromodele d'helicoptere equipe d'une camera, nous avons acquis notre propre drone afin de disposer d'une plate-forme pour tester differents dispositifs de vision et d'instrumentation. L'enjeu est de rendre possible les prises d'images qui, une fois traitees, donneront des informations qualitatives et quantitatives des degradations observees. Apres un rappel sur l'organisation de la maintenance des ouvrages, cet article developpe l'interet d'utiliser des drones pour l'inspection des ouvrages et autres activites d'auscultation et de surveillance, montre le drone acquis par le LCPC et presente des resultats obtenus au cours de deux experimentations sur des sites reels. Il donne egalement un apercu des recherches en cours et de celles a entreprendre pour mener a bien ce projet.

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François Derkx

Papers

Effect of Sample Size on the Behavior of Granular Materials

Utility detection and positioning on the urban site Sense-City using Ground-Penetrating Radar systems

The Sense-City project

Inspection des ouvrages d'art par drone : Bilan et perspectives des travaux du LCPC

Experimental determination of the stress evolution in a prebent composite steel-VHPC beam by means of an active stressmeter