Olivier Plé

A mechanical approach for fibre-reinforced clay in landfill caps cover application

ABSTRACT Solid waste landfills come with a cover barrier which includes a compacted clay liner essential to safety on site. However this barrier encounters numerous problems, in particular those related to in situ implementation and to mechanical loading which may cause stress in the clay layer leading to the development of cracks. Tensile stress damage is replaced by shearing when the thickness of the cap cover increases. Thus to characterise the behaviour of the clay, compression tests under confinement were carried out in the laboratory. It was proposed to improve the mechanical performance of the clay layer by adding fibre-reinforcement. This relatively new technique is adopted in geotechnical projects involving stabilization of soil veneers. An analysis of different types of fibre-reinforcement of clay was carried out. An improvement in soil resistance and in the brittleness index of fibre-reinforced soil was characterised.

Redistribution of Moment at Beam-Column Joints in RC Structures: Comparison Between an Experimental Study and Eurocode 2

The current approaches authorized by Eurocode 2 for the design of reinforced concrete (RC) structures at the structural scale are: linear-elastic analysis, linear-elastic with limited redistribution of moments, plastic and non-linear analysis. Numerous experiments on RC beams were performed which provided interesting information for the models proposed in the standards. However, at the RC frame level where the beam-column are present, although several experiments have already been carried out in the literature, this topic is still to be investigated, due the important number of parameters: interior joint, exterior joint, stiffness ratio between beam and column, steel reinforcement type, … This paper presents firstly an experimental study on an H-form RC frame structure (two vertical columns and one horizontal beam). Several loading-unloading cycles were carried out. Displacement fields during the test were measured by the image correlation technique. Then, experimental results were compared with that of the analytical models proposed in Eurocode 2 and an advance FE code (CAST3M). The comparisons show that analytical methods presented in Eurocode 2 underestimated about 20–30% of the ultimate capacity of the structure; while the non-linear analysis with CASTEM code – when material characteristics and boundary conditions are correctly identified - could provide good results comparing to the experiments.

Shear Parameters of Rammed Earth Material: Results from Different Approaches

Rammed earth (RE) is a construction material which is manufactured by compacting the soil in a formwork, in different layers. Several recent studies have investigated this material. The seismic performance of RE buildings is an important topic which needs to be carefully investigated. The complex numerical model seems a performant approach to investigate the seismic performance of a whole building. To correctly establish the model, the shear parameters of the material, which are the cohesion and the friction angle, should be identified. This paper first presents experimental studies on the shear parameters of RE through the direct shear tests, at two different scales. The differences of the results at different scales are analyzed. Then, the obtained experimental values are used in a numerical model to simulate the shear behavior of RE walls which are loaded by a constant vertical stress and pushed horizontally on the top. From the obtained results, the values for numerical models are recommended.

Statistical Size Effects on Compressive Strength and Mechanical Behavior of Concrete

The size effect on strength of concrete has been studied for a long time from various approaches. In particular, the weakest-link theory remains nowadays the basic tool to interpret statistical size effects, i.e. how the probability of failure under a given stress depends on external size. The main shortcoming of Weibull’s theory is that the activation of fracture from the weakest flaw is assumed to set the final strength, i.e. possible interactions between microcracks and defects during progressive damage are implicitly neglected. The objective of this study is to determine experimentally the influence of “external” (sample) size and “internal” (microstructural) size on compressive strength of concrete. For this purpose, more than 250 uniaxial compression tests were conducted on concrete specimens with two different cylindrical sizes (110×220 mm and 160×320 mm) and prepared from three different compositions (mean aggregate size and proportion). The relationships between compressive strength, dissipated energy up to failure in one hand, and microstructural as well as specimen sizes on the other hand, were analyzed statistically. This demonstrated the failure of the weakest-link approach to describe size effects on compressive strength of concrete.

3-Dimensional Digital Image Correlation for Strains Determination in Clayey Soil

Landfills come with a cover barrier which includes a compacted clay liner essential to safety on site. Unfortunately differential settlement may cause stress in the clay layer leading to the development of cracks. This is why tensile stress damage and shearing are observed on the cap cover. Due to difficulties of strains determination in clayey soil during direct tensile tests a 3-dimensional digital image correlation is used. Two digital cameras are used at a constant distance from the sample tested. Pictures taken at preset intervals are post-processed to extract specimen strain and strain localization. The suitability of this method is discussed in regards of experimental results. This technique is particularly well adapted to the clayey soils when conventional measurements are impossible.

Experimental Study of Suffusion in Granular Soils

Internal erosion is one of the main causes of instabilities within hydraulic earth structures. The occurrence of breach in new structures underlines the emergency to quantify phenomena induced. Over the years, several methods have been developed to characterize and model internal erosion. The intention of this paper is to present a new experimental device named Cross Erosion Test (CET) and devoted to the characterisation of internal erosion. This test consists of the injection, in a first drilling, of clear water and the recovery, in another drilling, of water charged with particles. With this technique, main processes occurred can be identified. Moreover, preliminary experimental results show the possibility to characterize suffusion into a specific soil.

Landfill Clay Barrier: Fibre Reinforcement Technique

Landfills come with a cover barrier which includes a compacted silty clay liner essential to safety on site. However this barrier encounters problems, especially those related to the differential settlement, which may cause stress in the clay layer leading to the development of cracks. Generally speaking, tensile stress damage and shearing are observed on the cap cover. Due to the weak mechanical performance of the clay layer it was proposed to add polypropylene fibre reinforcement. Direct tensile tests and compression tests under low confinement were carried out on unreinforced and reinforced soils. An improvement in soil resistance and in the brittleness index of fibre-reinforced clay was characterised. The proposed solution, technically feasible, enabled an optimization of the thickness of the mineral barrier.