An elastoplastic model for unsaturated expansive soils based on shakedown concept

It is important to model the behaviour of unsaturated expansive soils subjected to hydromechanical loadings, because these wetting and drying cycles alter significantly their hydromechanical behaviour which may cause a huge differential settlement on the foundations of individual buildings, pavements, dams, etc. From experimental observations, these expansive soils can generally reach a final equilibrium state at the end of the suction cycles where the soil behaviour can be supposed elastic. In this context, this paper presents an analytical method based on shakedown concept for the hydromechanical behaviour of expansive soils. The required parameters of the shakedown-based model are calibrated by the experimental results obtained for bentonite/sand mixtures subjected to cyclic suction loadings in an oedometric test. The comparison between the experimental results and the modeling demonstrates the capacity of the proposed shakedown-based model to simulate the hydromechanical behaviour of unsaturated expansive soils

Swelling Soils Behaviour in Cyclic Suction-Controlled Drying and Wetting

Cyclic drying and wetting phenomena of the expansive clayey soils cause the progressive settlements which could affect principally the foundations of buildings, the drainage channels and the buffers in radioactive waste disposals. In order to better understand the coupling between these hydraulic cycles and the mechanical behaviour of the swelling soils, this article presents an experimental study performed on two different expansive soils (molded and natural) using oedometer tests by imposing suction variations with the osmotic technique. Several successive swelling and shrinking cycles were applied under different constant vertical net stresses. During the suction cycles, the compacted samples showed cumulative shrinkage strains. On the other hand, the natural samples presented cumulative swelling strains. At the end of the suction cycles, the volumetric strains reached an equilibrium stage which indicates an elastic behaviour of the samples. We can relate these elastic behaviours to the soil fabric and especially to the microstructural content of soil

Retrait-gonflement d'un matériau argileux exposé aux cycles d'humidification/séchage

Cet article présente les résultats d'études expérimentales effectuées au laboratoire sous différents chemins de contraintes hydrique et mécanique sur un matériau gonflant compacté. Plusieurs cycles de variation de potentiel hydrique compris entre 0 et 8 MPa ont été appliqués sur des éprouvettes sous différents chargements mécaniques constants (15, 30 et 60 kPa). Les éprouvettes manifestent un retrait cumulé pendant ces cycles, et les déformations convergeant vers un point d'équilibre. L'ensemble de ces résultats expérimentaux a été interprété par le modèle élastoplastique du sol gonflant BExM proposé par Alonso et al. (1999) qui prend en compte l'accumulation des déformations lors de l'application de différents chemins de chargement. La correspondance entre les résultats mesurés et calculés est satisfaisante pour chacun des chemins suivis

Influence of suction cycles on the soil fabric of compacted swelling soil

International audienceThe soilfabric plays an important role in complex hydromechanical behaviour of the expansive soils. This article addresses the influence of the wetting and drying paths on the soilfabric of compacted bentonite and silt mixtures at two different initial dry densities corresponding to loose and dense states. To obtain the hydric response of the soil, two suction imposition techniques were used: osmotic technique for the suction range less than 8.5 MPa and the vapour equilibrium or the salt solution technique for the suction range between 8.5 and 287.9 MPa. Additionally, the soilfabric analysis was performed using mercury intrusion porosimetry (MIP) and nitrogen gas adsorption (BET) techniques. The dense samples produced cumulative swelling strains during the suctioncycles, while shrinkage was observed for the loose samples. The suctioncycles induced an equilibrium state indicative of the elastic behaviour of the samples. The soilfabric analysis showed that regardless of the soil's initial state (loose or dense), the samples obtained the same soilfabric at the equilibrium state. The experimental results illustrated also the existence of an elastic void ratio (e0el) where the compactedsoils at this state present an elastic hydric behaviour during the successive suctioncycles

Unsaturated resilient behavior of a natural compacted sand

Granular materials are generally used in unbound layers of road pavement structures. The mechanical behavior of these materials is widely studied with repeated load triaxial tests (RLTT) in which the elastic response is defined as the resilient behavior. Usually observed under total stress conditions, the effect of pore pressure changes during loading are not usually included in design. Further, the unbound layers frequently exist under partially saturated conditions. The influence of the unsaturated state, i.e., the suction, on the mechanical behavior, of unbound granular materials for roads has not been sufficiently studied and is generally not taken into account in models used for these materials. This article presents an experimental study of the repeated load response of a compacted clayey natural sand, and describes a model for the response which includes the effects of soil suction. The response of the proposed model formulated in terms of effective stress is compared with a similar model formulated in terms of total stress. The results from both the effective stress model and the total stress model are compared with the measured volumetric and deviatoric response. It is suggested that since the model parameters for the effective stress formulation are relatively constant for all values of suction (water content), the resilient response can be best captured by an effective stress model

Three-dimensional numerical simulations of methane gas migration from decommissioned hydrocarbon production wells into shallow aquifers

Three-dimensional numerical simulations are used to provide insight into the behavior of methane as it migrates from a leaky decommissioned hydrocarbon well into a shallow aquifer. The conceptual model includes gas-phase migration from a leaky well, dissolution into groundwater, advective-dispersive transport and biodegradation of the dissolved methane plume. Gas-phase migration is simulated using the DuMux multiphase simulator, while transport and fate of the dissolved phase is simulated using the BIONAPL/3D reactive transport model. Methane behavior is simulated for two conceptual models: first in a shallow confined aquifer containing a decommissioned leaky well based on a monitored field site near Lindbergh, Alberta, Canada, and secondly on a representative unconfined aquifer based loosely on the Borden, Ontario, field site. The simulations show that the Lindbergh site confined aquifer data are generally consistent with a 2 year methane leak of 2–20 m3/d, assuming anaerobic (sulfate-reducing) methane oxidation and with maximum oxidation rates of 1 × 10−5 to 1 × 10−3 kg/m3/d. Under the highest oxidation rate, dissolved methane decreased from solubility (110 mg/L) to the threshold concentration of 10 mg/L within 5 years. In the unconfined case with the same leakage rate, including both aerobic and anaerobic methane oxidation, the methane plume was less extensive compared to the confined aquifer scenarios. Unconfined aquifers may therefore be less vulnerable to impacts from methane leaks along decommissioned wells. At other potential leakage sites, site-specific data on the natural background geochemistry would be necessary to make reliable predictions on the fate of methane in groundwater

Seasonal thermal energy storage in shallow geothermal systems: thermal equilibrium stage

This paper is dedicated to the study of seasonal heat storage in shallow geothermal installations in unsaturated soils for which hydrothermal properties such as degree of saturation and thermal conductivity vary with time throughout the profile. In the model, a semi-analytical model which estimates time-spatial thermal conductivity is coupled with a 2D cylindrical heat transfer modeling using finite difference method. The variation of temperature was obtained after 3 heating and cooling cycles for the different types of loads with maximum thermal load of qmax = 15 W.m−1 with variable angular frequency (8 months of heating and 4 months of cooling).and constant angular frequency (6 months of heating and 6 months of cooling) to estimate the necessary number of cycles to reach the thermal equilibrium stage. The results show that we approach a thermal equilibrium stage where the same variation of temperature can be observed in soils after several heating and cooling cycles. Based on these simulations, the necessary number of cycles can be related to the total applied energy on the system and the minimum number of cycles is for a system with the total applied energy of 1.9qmax

Comportement hydromécanique des matériaux granulaires compactés non-saturés

Ce travail présente l'effet de la non-saturation sur le comportement réversible mécanique des matériaux granulaires de chaussées à court terme. On commencera par caractériser le comportement hydrique des différents matériaux. A partir des matériaux précédents, on caractérisera à la boite de cisaillement direct (à chargements statiques) et à l'appareil triaxial (à chargements répétés) les comportements réversibles. Ce travail nous permettra finalement d'injecter la succion dans les modèles classiques d'élasticité non linéaire pour le comportement mécanique des sols granulaires saturés