Hossein Nowamooz

Effective stress concept for the effect of hydraulic hysteresis on the resilient behaviour of low traffic pavements

This paper presents the influence of the unsaturated state and especially the different hydraulic behaviours during the wetting and drying cycles (hysteresis phenomenon) on the resilient behaviour of unbound granular materials for roads. In this context, the soil water retention curve (SWRC) of a compacted clayey sand was initially obtained for both wetting and drying paths. These experimental results were then complemented by a series of direct shear tests. Finally, repeated load triaxial tests (RLTTs) were also performed at the same water contents on both wetting and drying paths. It can be stated that the samples prepared on the wetting path showed higher resilient volumetric strains compared to those prepared on the drying path, especially at low water content. However, the influence of the hysteresis phenomenon is less evident for the resilient shear strains. Different effective stress concepts based on the SWRC and shear test results were also added to the classical Boyce model generally used to estimate the resilient behaviour based on the total stress concept. The estimated effective stresses of the RLTT give the best correlation coefficient for all water contents of the wetting and drying paths. Finally, finite element calculations, carried out with CAST3M, were performed to compare the deflection of a low traffic pavement at saturated/dry conditions for the subgrade soil. The asphalt concrete layer and the base layer were taken the same for both conditions. The influence of the hysteresis phenomenon on the deflections is more pronounced at the dry condition.

Influence of fine content and water content on the resilient behaviour of a natural compacted sand

Granular materials are often used in pavement structures. The influence of fine content and water content on the mechanical behaviour is very important. In this work, first, we present the resilient behaviour of compacted clayey natural sands with two different fine contents using the repeated load triaxial tests. The samples were compacted at different water contents with the same dry density. It can be stated that an increase in the fine content (from 4% to 7.5%) causes an increase in the volumetric strains and shear strains for all the water contents. The influence of water content can be considered negligible comparing to the influence of fine content. Then, the experimental results were simulated using the nonlinear elastic model (modified Boyce model) generally used for the resilient behaviour of the unsaturated granular soils. The results show that the model can correctly predict the variation of the volumetric strains and the shear strains with the mean stress for all the stress paths. Finally, finite element calculations carried out with CAST3M were performed to compare the deflection of a low traffic pavement with sub-base layers containing two different fine contents. The surface asphalt concrete layer and the base layer were taken the same for both structures. The modelling results show that the sub-base soil with higher fine content produces 37% higher deflection in the whole structure and that the effect of the water content is lower than that of the fine content.

Effect of dry density, soil texture and time-spatial variable water content on the soil thermal conductivity

Study of the heat transfer process in saturated and unsaturated soils requires, basically, a relationship between thermal conductivity and the characteristics of the soil, such as water content, dry density and texture of the soil. This study intends to produce a generic model that can predict soil thermal conductivity with the help of easily measurable parameters. The proposed model is first calibrated using measured thermal conductivities from literature data. In order to validate the proposed model the predicted thermal conductivity of this proposed model as well as existing ones are compared with the measured thermal conductivity in literature for different soils. Validation of the proposed model was also performed on our experimental results obtained for a compacted Misillac sand and in-situ clay loam soils. The results show an average of 15% improvement in prediction accuracy for the proposed model compared to the existing models, considering all soil textures. Moreover, we perform a model to estimate thermal conductivity over time throughout the profile of soil in the context of seasonal variation of temperature. The proposed model shows an important effect of heterogeneity on the thermal conductivity variations of a double layered soil.

Permanent deformation behaviour of a granular material used in low-traffic pavements

Granular materials are usually used in low-traffic pavement structure as base layer or sub-base layer. The influence of fine content on permanent axial deformation behaviour is significant as well as the water content. This study aims to investigate the permanent axial deformation behaviour of the granular material under cyclic loading at various water contents and various fine contents. A triaxial apparatus is used to obtain permanent axial deformation on the samples prepared with the same dry density at different water contents between 7% and 11% and at different fine contents of 4%, 7.5% and 15.3%. The results show the significant influence of water content and fine content on permanent axial deformation behaviour. The permanent axial deformation increases with the increase of water content while the influence of fine content depends on the water sensitivity of fine particles and their initial water contents. The modified empirical–analytical models are proposed for describing the evolution of permanent axial deformation based on the results in the single-stage test and the multi-stage tests. It takes into account the number of cycles, the stress level, the water content and the fine content of the granular material. Two approaches are used: one based on the water contents and fine contents and the other based on suction values. The approach based on suction values needs less number of parameters to describe permanent axial deformation compared with the approach based on the water contents and fine contents while the two approaches present more or less the same accuracy. The simulation results show a very good capacity of the proposed approaches. These findings reduce the number of tests required to predict permanent axial deformation.

Effect of hydraulic hysteresis on low-traffic pavement deflection

In many European countries, low-traffic pavement design is based on linear elastic calculations, and to limit the risk of rutting, a strain criterion, limiting the vertical elastic strain at the top of the soil layer is used. However, the unsaturated state and especially the different hydraulic behaviours during the drying and wetting cycles (hysteresis phenomenon) have been rarely taken into account in modelling of road pavements. The objective of this work is to study the effect of the hydraulic hysteresis of the granular materials on the deflection of pavement. The Soil Water Retention Curves as well as the repeated load triaxial tests provide the necessary experimental results to characterise the hydro-mechanical behaviour of these granular materials. A finite elements modelling of a low-traffic pavement has been carried out with CAST3M at different saturated, intermediate and dry conditions for the granular materials. The influence of the hysteresis phenomenon on the deflections is more pronounced at the dry condition.

Effect of Anisotropy on the Resilient Behaviour of a Granular Material in Low Traffic Pavement

Granular materials are often used in pavement structures. The influence of anisotropy on the mechanical behaviour of granular materials is very important. The coupled effects of water content and fine content usually lead to more complex anisotropic behaviour. With a repeated load triaxial test (RLTT), it is possible to measure the anisotropic deformation behaviour of granular materials. This article initially presents an experimental study of the resilient repeated load response of a compacted clayey natural sand with three fine contents and different water contents. Based on anisotropic behaviour, the non-linear resilient model (Boyce model) is improved by the radial anisotropy coefficient γ3 instead of the axial anisotropy coefficient γ1. The results from both approaches (γ1 and γ3) are compared with the measured volumetric and deviatoric responses. These results confirm the capacity of the improved model to capture the general trend of the experiments. Finally, finite element calculations are performed with CAST3M in order to validate the improvement of the modified Boyce model (from γ1 to γ3). The modelling results indicate that the modified Boyce model with γ3 is more widely available in different water contents and different fine contents for this granular material. Besides, based on the results, the coupled effects of water content and fine content on the deflection of the structures can also be observed.

Influence of Hydraulic Hysteresis on the Resilient Behavior of a Natural Compacted Sand

This paper presents the influence of the unsaturated state (suction) and especially the different hydraulic behaviour during the wetting and drying cycles (hysteresis phenomenon) on the resilient behavior, of unbound granular materials for roads. First, the Soil Water Retention Curve (SWRC) of a compacted clayey sand will be obtained for both wetting and drying paths. These experimental results were then completed by a series of repeated load triaxial tests at a range of water content between 8% and 11% on both wetting and drying paths. Finally, the experimental results were simulated using the modified Boyce model. Generally, it can be stated that the hydraulic hysteresis phenomenon plays an important role on the mechanical modelling of the resilient behaviour.

Seasonal Heat Storage in Unsaturated Soils with Variable Thermal Properties

Hydrothermal properties of soil are prerequisites of the models of heat storage in unsaturated soils. Variation of water content as well as soil characteristics change its thermal properties. Therefore, to give comprehensive effects of hydrothermal properties of soil on the characteristics of heat storage, we need to take into account the dependecy of those thermal properties to the water content and other soil characteristics in the heat transfer models. In this study we aim to model heat distribution within the depth of unsaturated soil while its thermal conductivity varies with time and depth. The proposed model is based on the fundamental solution of the one-dimensional heat equation and the decomposition method. We consider that thermal conductivity varies exponentially with depth and sinusoidally with time. We calibrate our model with the experimental data from different soil textures in the literature. We recently developed a model which can estimate soil thermal conductivity from its characteristics such as dry density, degree of saturation, and the texture of soil. Water content of soil at different depth is back-estimated with the help of our previous model.

Effect of Density in the Water Retention Curve of a Compacted Silt-Bentonite Mixture

In unsaturated swelling-shrinking soils, like clay soils, water content change results in significant volume change and drying is associated with high water content gradients.This work presents initially the soil water retention curve (SWRC) of a bentonite-silt mixture compacted at three different initial dry densities (1.27, 1.35 and 1.55 Mg.m− 3). The SWRCs were fitted by the van Genuchten (1980) and Brooks and Corey (1964) type functions. The estimated parameters were used to model the SWRC of the intermediate samples.

Using a Complementary Evapotranspiration Relationship to Estimate Surface Suction for Soil-Atmosphere Interaction Analysis

The estimation of soil surface condition from climatic parameters is of great importance in soil-atmosphere analysis. This paper deals with the estimation of the surface suction of soils using meteorological data. The rate of evapotranspiration (AE/PE) is related to the soil surface suction by the Wilson equation and it is generally used when the potential evapotranspiration (PE) and the soil surface suction are known while hydrological models allow the calculation of the evapotranspiration rate (AE/PE) by introducing complementary relationships. In this paper, a complementary hydrological model was combined with the Wilson equation in order to estimate the soil surface suction in time. To test the validity of the results, the approach was applied to the Toulouse region in the south of France in 3 different years (1999, 2003, 2004). The 2003 year was distinguished as a high intensity drought period in France that has caused many damages by triggering shrinkage and swelling in clays. By comparing the estimated total surface suction for these three years, the 2003 year showed higher values compared to the two other years as expected.