Frédéric Jamin

Collapse phenomena during wetting in granular media

We present an experimental study of identification and analysis of the collapse phenomenon in granular media, using a sample made of glass beads. The collapse potential of a soil depends on several parameters such as liquid limit, matric suction, dry apparent density, initial water content and the amount of small particles. In this study, we investigate the liquid effect during the wetting process, without external mechanical stress. Experimental tests at different initial water contents were made for different grain sizes. The equipment used is a triaxial device for unsaturated soils, instrumented with local axial and radial displacement sensors. The experimental results determined a critical water content of collapse, as well as the influence of initial water content, matric suction and grain size.

Water flow in soil at small water contents: A simple approach to estimate the relative hydraulic conductivity in sandy soil

Unsaturated soil hydraulic properties are key properties in the fields of soil science and civil engineering. Because of their strong dependence on water content, there are considerable experimental and numerical difficulties in their determination, specifically in the dry range. This situation is encountered regularly in arid and semi-arid regions. The models commonly used for predicting the unsaturated hydraulic conductivity function rely on pore bundle concepts that account for capillary flow only and neglect film flow. Furthermore, the assumption of a local equilibrium between liquid water and its vapour is no longer valid at small water contents. Thus, with classical approaches, the experimental identification of hydraulic characteristics can fail at small water contents. To emphasize the weakness of capillary models, soil column experiments have been carried out with two sandy soils from Burkina Faso. Special care was taken to prevent any transport processes that are not directly related to liquid transport. Data from profiles of water content were used in an inverse numerical procedure to identify the coefficients of a new relative hydraulic conductivity function. Our results show that this simple approach is suitable for the analysis of flow processes at small water contents. It provides a simple, robust and inexpensive method to identify the properties of the unsaturated conductivity function that account for capillary and film flows. Highlights A new kind of soil column experiment is proposed to focus on the dry range. A new relative hydraulic conductivity function accounting for film flow is introduced. The weaknesses of classical relative hydraulic conductivity functions are emphasized. It provides a simple, cheap and robust method to identify the relative hydraulic conductivity.

A cohesive zone model for the characterisation of the interfacial transition zone (ITS) between cement paste and aggregates

Characterization of concrete behavior needs to know mechanical properties of the two phases constituting them : mortar and aggregates. Nevertheless this bi-phasic approach reaches its limits when concrete leaves the elastic domain. At that stage and according to several studies, phenomena which occur at the interface between mortar and aggregates, or in mortar between cement paste and aggregates, must be taken into account. If the occurence of an Interfacial Transition Zone (ITZ), with weak mechanical properties in regard to the two others surrounding them is well knwon, the modeling of this third phase is not settled yet. This study focus on the characterization of the adhesion at interface between cement paste and aggregates. A mortar compounded by two limestone aggregates binded by a cement paste is considered, and tensile tests are performed on a sample of this composite. Based on these experimental results, a numerical study is developed in order to see influence of interface quality in tensile strength. For thar mortar is modeled by finite elements with a cohesive zone model at the interface, substituting ITZ. With the cohesive zone model used, coupling friction and adhesion at the interface, three parameters have to be fitted : normal and tangential stiffness, and decohesion energy. A strong correlation is found between these parameters and tensile strength, but numerical results show also low values of stiffness and decohesion energy at interface. This result could be explained by a partial adhesion between mortar and cement paste in the sample.

Thermo-mechanical Behaviour of a Soil. Yield Surface Evolution

This paper presents a study of temperature influence on the yield surface for saturated and unsaturated soils. It lies within the framework of characterization and modelling of soil behaviour on thermo-hydro-mechanical loading. In this study, clayey silty sand samples are subjected to mechanical consolidation tests at different temperatures and fixed suctions. These experimental tests make it possible to determine the yield surface evolution according to temperature ranging between 20°C and 60°C. For the studied soil, the experimental study shows a thermo-extensible nature of the yield surface. An explanation of this result is proposed while gone by microscopic considerations in touch with capillary meniscuses evolution according to temperature and suction.