Tahar Ayadat

The Influence of Hydraulic Gradient and Rate of Erosion on Hydraulic Conductivity of Sand-Bentonite Mixtures

The use of sand-bentonite mixtures as liner materials for waste disposal is very common. In the laboratory, this study investigated hydraulic conductivities of such mixtures at different hydraulic pressure (hydraulic gradient), dry unit weights, and bentonite contents. The bentonite content and the dry unit weight of the samples were both important factors, significantly affecting the hydraulic conductivity of the liner material. A bentonite content of 5% was found to be sufficient in reaching a hydraulic conductivity under 10−9 m/s, when the liner material was compacted under near optimum moisture content. Nevertheless, hydraulic conductivity was found to increase with hydraulic pressures, especially for the 5% bentonite mixtures subjected to pressure above 40 kPa, suggesting some degree of internal erosion (washing out of particles). Therefore, this paper discuses the influence of internal erosion of the mixtures under a given hydraulic gradient, on the final value of k. The internal erosion of the tested mixtures was found to be influenced mainly by porosity, which can be reduced by properly selecting the sand particle size distribution and the bentonite percentage. Furthermore, this study proposed an empirical expression to predict the risk of internal erosion in the sand-bentonite mixtures, and therefore of k being higher than planned. This expression can be used for designing bentonite content and compaction to achieve very low permeability.

Identification of collapsible soil using the fall cone apparatus

Soils that go through a great loss of volume upon wetting with or without additional loads are identified as collapsible. In recent years, there has been an increasing awareness of this type of soil due to the expansion of urban developments to arid regions. Also man-made earth structures often exhibit collapsing behavior when compacted at water content less than the optimum moisture content. In the literature, methods can be found to predict this behavior based on field and laboratories test results. These methods, however, are time consuming and developed for the type of soils tested. This paper presents the results of an experimental investigation on collapsible soils using the fall cone and the oedometer apparatuses. The fall cone method, originally developed to determine liquid and plastic limits of soils, was adopted in this investigation to identify its collapse potential. A cone penetration limit (Plim) is introduced to identify collapsible soil and a correlation between the collapse potential, CP, and the cone penetration, P, was developed and validated with the present experimental results and those available in the literature. Furthermore, a simple procedure is introduced to determine the optimum Proctor Moisture Content for collapsible soils from the results of the cone test. The proposed procedure is simple and fast to evaluate soil collapsibility by a single reading of the cone penetration.

Prediction of collapse behaviour in soil

ABSTRACT The drastic decrease in volume of a natural, unsaturated soil deposit or compacted fill that occurs upon wetting at virtually constant total stresses is termed “collapse”. Several methods can be found in the literature that predict collapse behavior based on easily obtained soil parameters. Nevertheless, discrepancies were reported among the results of these methods. This paper presents the results of an experimental investigation on carefully conducted laboratory prepared collapsible soils in the oedometer apparatus. These results were used to develop an empirical model for predicting soil collapse in terms of the initial dry unit weight, initial water content and the soil gradation. The values predicted by the proposed model agreed well with the experimental results of the present investigation and those available in the literature.

Effects of hydraulic shear stress and rate of erosion on the magnitude, degree, and rate of collapse

Arid regions worldwide are plagued by collapsible soils. Collapsible soil is characterised by the sudden decrease in volume that occurs when it is subjected to inundation under constant stress. This volume change manifest itself as drastic and unpredicted foundation settlement, which may lead to further catastrophic failure of the supported structures. Collapse settlement is the term applied to the additional settlement of a foundation due to wetting of the underlying soils. The results of an experimental investigation of the effects of the saturation of soil with water, kerosene, and crude oil, and of the effects of the fluid head on the magnitude, degree, and rate of collapse of the underlying soil are presented in this paper. Soil erodibility is presented in terms of the applied hydraulic shear stress and the rate of erosion. The relationship between soil erosion and the magnitude and rate of collapse is examined. Empirical methods for the prediction of the magnitude and rate of collapse of a soil saturated with the test fluids and subjected to a hydraulic constant head are proposed.

Modeling of supercritical fluid extraction of phenanthrene from clayey soil

The supercritical fluid (SFC) extraction efficiency of phenanthrene from clayey soils was modeled. The model accounts for effective diffusion of the phenanthrene in the solid pores, axial dispersion in the fluid phase, and external mass transfer to the fluid phase from the particle surface. This model, involving partial differential equations, was solved using the finite difference. The model showed the relationship between diffusivity, mass transfer coefficient, and properties of porous media (clay texture). The porous media analysis was performed with a microscope and by an image analysis. The proposed model compared well with the experimental data available in the literature.