Hervé Péron

An Improved Volume Measurement for Determining Soil Water Retention Curves

The complete determination of soil water retention curves requires the sample volume to be measured in order to calculate its void ratio and degree of saturation. During drying in the pressure plate apparatus, cracks often appear in the sample altering its deformation and evaporation patterns. Consequently, this causes a significant scatter in the volume measurement when using the volume displacement method. This paper proposes a simple method to avoid cracking, by limiting friction and adhesion boundary effects, to allow for unrestrained shrinkage of the sample. Such modification of the technique decreases the measurement error by a factor of three.

Geotechnical analysis of heat exchanger piles

There is currently a lack of established calculation method for the geotechnical design of heat exchanger piles, although the technology is experiencing a fast expansion. Instead of quantifying the effects of temperature changes on the static behavior of heat exchanger piles, the common geotechnical practice is to apply a large overall security factor. This is done in order to be on the side of safety with respect to thermal effects. The few existing in situ experiments show that applying a thermal load induces a significant change in the stress-strain state of a pile. This paper presents a geotechnical numerical analysis method, based on the load transfer approach, which assesses the main effects of temperature changes on pile behavior. The method is validated on the basis of two in situ measurements of the loads and deformations experienced by heat exchanger test piles. The occurrence of critical design situations is further discussed. Some conclusions are formulated on concrete failure and the full mobilization of the pile shaft friction and base resistance during the operation of the heat exchange system.

Desiccation cracking of soils

ABSTRACT The scope of this paper is to present the global mechanisms of soil desiccation, including drying shrinkage and cracking. The paper first reviews the basic processes that are beneath the word “desiccation”. Then the results of an experimental study of desiccation are presented, in which strains, suction, water content, degree of saturation and crack geometry are investigated. The results show that cracking initiates close to the onset of de-saturation. Insights into the micro-scale are proposed to explain this observation. A scenario for the processes leading to crack initiation is further established in terms of the macroscopic variables: an assessment of the stress building up is proposed, until a critical point at which the tensile strength is met. Desiccation crack pattern formation is finally discussed.

Experimental study of desiccation of soil

Note: Sols Reference LMS-CONF-2006-007 Record created on 2006-11-09, modified on 2016-08-08

A method for the geotechnical design of heat exchanger piles

There is currently a lack of established calculation method for the geotechnical design of heat exchanger piles. Thermo-mechanical effects are ignored and large overall security factors are therefore applied. This paper presents a new geotechnical numerical design method for heat exchanger piles, based on the load transfer approach. The method is validated on the basis of two in situ measurements. It is shown how the pile design could be adapted and optimized with respect to concrete resistance and the mobilization of the pile shaft friction during the operation of the heat exchange system.

Numerical and phenomenological study of desiccation of soil

Note: Sols Reference LMS-CONF-2006-002 Record created on 2006-11-09, modified on 2016-08-08

Drying shrinkage of deformable porous media: mechanisms induced by the fluid removal

Mechanisms of free shrinkage strains of desiccatin g deformable porous media are studied . The roles of surface evaporation rate, surface tension, and viscosity of pore fluid, soil compressibility and permeability are investigated. For drying tests on two geomaterials with three different pore fluids the evolution of shrinkage strain and fluid content is reported. Most of the strain s occur in the saturated phase of drying prior to cracking . Simulations of this phase include evaporative fluxes at the external surface s, a consequent water transport across the sample toward the surface producing pore pressure and stress gradients, local water content change, and deformation. Biot theory is used.

A multi-scale multi-physics model of soil drying

Drying shrinkage is modeled at a microscale as a two-stage process of deformation and evacuation of a two-tube vessel system, based on porosimetry evolution. In the first stage water flow is driven by an external evaporation flux. Flow is associated with a negative pressure (suction) gradient along the vessel. The amount of water evaporated in this stage equals to the vessel volume reduction through the deformation of its walls, as long as the suction constricting the vessel can be supported by water. At some point suction required to further deform the vessel reaches at the vessel exit the water tensile strength,. As a result liquid/gas interface penetrates the vessel via undistiguishable mechanisms of meniscus plunging or near-surface cavitation (isothermal evaporation). The subsequent process is modeled as a moving drying front, without much of deformation of the vessels. The resulting macroscopic effective stress is generally compressive due to large suction. However, locally around defects a relatively small tensile total stress is greatly amplified, and hence despite the large suction, the effective stress is tensile and may easily reach tensile strength. Thus, the condition for tensile failure involves microscopic characteristic size of the soil pore system, and its defects.

The Influence of the Pore Fluid on Desiccation of a Deformable Porous Material

This paper aims at elucidating the influence of pore fluid properties on the shrinkage strains of deformable materials subjected to drying. Results of isothermal drying tests of two kinds of silts saturated with three different pore fluids are presented. Results show different strain amounts during drying and the final void ratios, depending on pore fluid. The rates of drying are consistent with saturated vapor pressures, while their amount with the surface tension. The shrinkage limit appears also to be controlled by a limitation in skeleton compressibility.

Analysis of the swelling pressure development in opalinus clay – experimental and modelling aspects

In the context of nuclear waste geological storage, deep argillaceous formations are likely to be subjected to complex mechanical, hydraulic, and thermal loads. In particular, the argillaceous material can be firstly dried, and then re-wetted. During the latter process, the material experiences swelling and can develop swelling pressure if swelling deformations are constrained. In this contribution, the results of swelling pressure tests on shale performed in totally constrained conditions (isochoric tests) are presented. A constitutive model (ACMEG-S) is used to predict the value of the swelling pressure in such conditions. The model is made of two parts. The mechanical part addresses the stress-strain behaviour of the material, as a result of effective stress variation. An elasto-plastic approach is employed, and Bishops unsaturated effective stress, which is a function of the degree of saturation, the suction and the externally applied stress, is used as the mechanical stress. The water retention part of the model defines the relation between the degree of saturation and the suction within the material. The results put into light some factors that control the swelling pressure value, in particular the degree of saturation and the plastic behaviour of the material.