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Al Heib M., Delmas P., Riot M., Emeriault F., Villard P.

Recommendations for the use of geosynthetic reinforcement to reduce the risks associated with a localised collapse

Road and urban infrastructures are sometimes confronted with damages caused by localised collapses. Mitigation of this damage can be achieved with instrumented geosynthetic reinforcement. This recommendation guide provides a presentation of the different types of cavities and the ground movements that can be induced by localised collapses, available treatment methods to reduce the vulnerability, a description of reinforcement method by geosynthetics for granular and cohesive soil, especially the "bi-modulus" instrumented geosynthetic developed in the REGIC project and a methodological recommendations for the use of instrumented geosynthetics.

Fourmont S., Pellez JC.

Innovative mechanically stabilized earth walls with geotextiles geocells

Structures reinforced with geosynthetics consist in increasing the mechanical performance of a soil (mainly shear resistance) by associating it with flexible geosynthetics inclusions. One of the important issues in the construction of geosynthetic reinforced walls is the supply of natural backfill materials with the required properties needed for the stability of the wall. Indeed, unlike geosynthetics that exhibit stable properties due to extensive quality controls during the manufacturing process, soil matrix will vary from a site to another and even from the beginning to the end of the excavation work. It influences the soil stability itself and also the soil-geosynthetic interface. As it minimizes the influence of soil characteristics on the stability of the reinforced structure, M3S geotextile geocells make possible, in addition to the construction of reinforced structures with complex shapes, to reuse the soil material excavated on-site to build the wall, including those with very poor geotechnical characteristics. This publication presents the M3S cellular system and its mechanical and functional characteristics. It also gives a case study on the construction in 2019 of two MSE walls as part of the A71 motorway bypass on the APRR network, France.

Khoueiry N., Briançon L., Daouadji A., Riot M.

Developed full-scale cyclic plate load and traffic load tests for unpaved roads on soft subgrade and reinforced with geosynthetics

With the expansion of urban areas, the construction on soft subgrade becomes a more often issue due to excessive settlement, especially for roads network. Nowadays, the tradition soft soil replacement solution is substituted by stabilization solutions to reduce the surface settlement. Geosynthetics (GSYs) are used to stabilize base course over soft subgrade under unpaved roads. GSYs improve this structure by the following mechanisms : lateral restraint and reinforcement of base course aggregates, tension membrane effect in rutted areas, and reduction of mixing between subgrade and base soils. With the reinforcement addition, the mechanisms developed at the interface become even more complex. It is important to identify and clarify these mechanisms in order to propose an efficient design method for this kind of structure. A large-scale laboratory test was designed and developed to characterize the GSYs effects and the reinforcement mechanisms in unpaved roads. An unpaved road platform was subjected to cyclic plate load. The platform consisted of a soft subgrade layer supporting a base course layer and placed in a box of 1.9 m of large, 1.8 m of length and 1.1 m of height. The composition of soft soil, the installation and the quality control procedure are detailed in this paper. The surface rutting, the subgrade settlement and the vertical stress distribution were monitored during the loading cycles. Moreover, the GSY strain was monitored using the fibre optic technology. Six tests were performed; two repeatability tests and four reinforced and unreinforced tests with different base course thicknesses. The tests performed proved the repeatability of the experimental protocol. Moreover, it is concluded that the used GSY has a negligible effect if the base course thickness is equal or higher than 350 mm. On the other hand, for a base course thickness of 220 mm, the geogrid reinforcement provides a surface rutting reduction of 22%, and a subgrade central vertical stress reduction of 30%. In comparison with the empirical and the analytical design methods from the literature, we conclude that these methods overestimate the base course thickness for unreinforced platform. These experiments consist in a preparation program to a full-scale experiment, with a cyclic Traffic load applied on the unpaved road surface, using the Simulator Accelerator of Traffic (SAT) machine developed at INSA Lyon.

Khoueiry N., Briançon L., Daouadji A., Riot M.

Large-scale test on geosynthetic reinforced unpaved roads on soft subgrade

Geosynthetics have been widely used since 1970 in unpaved roads. Various research studies showed the benefits of the reinforcement in facilitating the fill material compaction, improving the platform bearing capacity, which will allow the reduction of the fill material thickness, and the increase of the structure serviceability term. Different mechanisms take place between the aggregates platform and the reinforcement. Which affect the structural behavior: the aggregates platform confinement, the separation between the weak subgrade and the fill material, the membrane effect. The road structure becomes even more heterogeneous and the mechanisms more complex with the addition of the reinforcement layer and the underlying mechanics are still not completely understood. Therefore, it is important to provide more knowledge regarding these mechanisms, in order to propose an efficient design method for such structure. A full-scale laboratory test on unpaved roads has been designed and developed to characterize the effect of the reinforcement in this application. The platform tested is placed in a large box of 5 m in length, 1.9 m in width and 1.4 m in height. The tested platform is composed of 60 cm of weak subgrade supporting 22 cm or 35 cm of well-compacted fill material. A special attention has been given to the soil layers preparation, installation and quality control. The tested structure was subjected to a cyclic plate load and to a circulation traffic load using a large-scale apparatus SAT (Simulator Accelerator of Traffic). This apparatus was developed and adapted for this flexible structure. Indeed, it allows the application of a heavy traffic load on the unpaved road surface even for large surface displacement. During each test, the rut development, the vertical stress distribution and the settlement in the subgrade soil are monitored.

Khoueiry N., Briançon L., Daouadji A., Riot M.

Geosynthetics in unpaved roads on soft subgrade : Large-Scale Experiments

The geosynthetics were used in unpaved roads on soft subgrade since 1970. However, the developed mechanisms in unpaved reinforced roads are complex. In order to clarify and identify these mechanisms a full-scale laboratory test has been developed. An unpaved reinforced or unreinforced tested platform has been constituted in a laboratory large geotechnical box. The prepared platform was subjected to a cyclic plate load of a maximum magnitude of 40 kN resulting in a surface pressure of 560 kPa. The platform was subjected to 1,000 cycles. Two base course platform were tested (350 and 220 mm). A knitted geogrid was used in the reinforced platforms. A special attention was given to the soil layers composition, installation and compaction. The test repeatability was checked. The experimental results showed the reinforcement benefits in the platforms with a base course thickness of 220 mm. However, for a base course thickness of 350 mm the reinforcement was not effective. A numerical model was developed using the software FLAC 3D® to simulate the structure behavior under the first applied load. The results showed that the numerical model captures the structure behavior for the reinforced and unreinforced platforms.

Lesueur D., Le Guernevel G., Riot M.

On the performance of geogrids for asphalt pavement reinforcement : laboratory evaluation and selected case studies

The use of geogrids for pavement reinforcement has been continuously increasing over the past decades. Their main role remains to limit crack reflection and they are therefore typically used on cracked pavements before renovating the upper asphalt layers. Still, evaluating their performance is not a simple task because most available testing methods are still at an early development stage with sometimes limited field validation. As a consequence, no clear guidelines currently exists for selecting this type of products in preparation of a construction project. This paper presents the laboratory results gathered so far on fiber-glass reinforced geocomposites in view of their use in real jobsites. Fatigue testing and the potential to delay crack reflection as measured with the Cerema Autun device, are presented and discussed in the light of recent jobsites on Paris Charles de Gaulle Airport and French highway A7. In addition, the correct positioning of the geogrids on the asphalt support was further evaluated using the Leutner test. This clarifies the conditions under which the placement of these materials will be optimized. From this work, a clearer picture tends to emerge on how to specify these very specific products in order to maximize their benefit for extending pavement life time.



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