Mehter M. Allam

Effect of clay mineralogy on coefficient of consolidation

The coefficient of consolidation (Cv) signifies the rate at which a saturated clay undergoes 1-dimensional consolidation when subjected to an increase in pres- sure. Measured in cm2/s, C~ is a function of hydraulic conductivity, k (crn/s), coefficient of volume change, mv (cm2/kg), and unit weight of pore fluid, ~w (kg/cm3), and is expressed as:

Novel procedure for the estimation of swelling pressures of compacted bentonites based on diffuse double layer theory

Bentonite clays are proven to be attractive as buffer and backfill material in high-level nuclear waste repositories around the world. A quick estimation of swelling pressures of the compacted bentonites for different clay–water–electrolyte interactions is essential in the design of buffer and backfill materials. The theoretical studies on the swelling behavior of bentonites are based on diffuse double layer (DDL) theory. To establish theoretical relationship between void ratio and swelling pressure (e versus P), evaluation of elliptic integral and inverse analysis are unavoidable. In this paper, a novel procedure is presented to establish theoretical relationship of e versus P based on the Gouy–Chapman method. The proposed procedure establishes a unique relationship between electric potentials of interacting and non-interacting diffuse clay–water–electrolyte systems. A procedure is, thus, proposed to deduce the relation between swelling pressures and void ratio from the established relation between electric potentials. This approach is simple and alleviates the need for elliptic integral evaluation and also the inverse analysis. Further, application of the proposed approach to estimate swelling pressures of four compacted bentonites, for example, MX 80, Febex, Montigel and Kunigel V1, at different dry densities, shows that the method is very simple and predicts solutions with very good accuracy. Moreover, the proposed procedure provides continuous distributions of e versus P and thus it is computationally efficient when compared with the existing techniques.

Methylene Blue Surface Area Method to Correlate with Specific Soil Properties

Specific surface area (SSA) of soils is calculated by determining the amount of adsorbed substance such as methylene blue (MB), ethylene glycol monoehtyl ether (EGME), or water required to form a monolayer. Free swell index and liquid limit of soils, which are well correlated with each other, and with engineering properties of soils, can be used to check the relative advantage of determination of a specific area by different methods in geotechnical engineering. The water adsorption method yields different values depending on the relative humidity (RH) at which the soils are equilibrated with water and gives significantly high adsorption values at 100 % RH. The water adsorption method, comparatively simpler, gives more accurate values for kaolinitc soils but generally has lower values for soils with montmorillonitic mineral owing to insufficient saturation of exchangeable ions. The SSA values obtained by EMGE correlates reasonably well with free swell index. Improved correlation is observed in specific surface area values obtained by MB methods. The SSA values obtained by methylene blue titration (MBT) or as methylene blue spot method (MBS) are comparable.

Robust solver based on modified particle swarm optimization for improved solution of diffusion transport through containment facilities

Accurate estimation of mass transport parameters is necessary for overall design and evaluation processes of the waste disposal facilities. The mass transport parameters, such as effective diffusion coefficient, retardation factor and diffusion accessible porosity, are estimated from observed diffusion data by inverse analysis. Recently, particle swarm optimization (PSO) algorithm has been used to develop inverse model for estimating these parameters that alleviated existing limitations in the inverse analysis. However, PSO solver yields different solutions in successive runs because of the stochastic nature of the algorithm and also because of the presence of multiple optimum solutions. Thus the estimated mean solution from independent runs is significantly different from the best solution. In this paper, two variants of the PSO algorithms are proposed to improve the performance of the inverse analysis. The proposed algorithms use perturbation equation for the gbest particle to gain information around gbest region on the search space and catfish particles in alternative iterations to improve exploration capabilities. Performance comparison of developed solvers on synthetic test data for two different diffusion problems reveals that one of the proposed solvers, CPPSO, significantly improves overall performance with improved best, worst and mean fitness values. The developed solver is further used to estimate transport parameters from 12 sets of experimentally observed diffusion data obtained from three diffusion problems and compared with published values from the literature. The proposed solver is quick, simple and robust on different diffusion problems.

Accurate parameter estimation of contaminant transport inverse problem using particle swarm optimization

Swarm Intelligence techniques such as particle swarm optimization (PSO) are shown to be incompetent for an accurate estimation of global solutions in several engineering applications. This problem is more severe in case of inverse optimization problems where fitness calculations are computationally expensive. In this work, a novel strategy is introduced to alleviate this problem. The proposed inverse model based on modified particle swarm optimization algorithm is applied for a contaminant transport inverse model. The inverse models based on standard-PSO and proposed-PSO are validated to estimate the accuracy of the models. The proposed model is shown to be out performing the standard one in terms of accuracy in parameter estimation. The preliminary results obtained using the proposed model is presented in this work.

Discussion on "Direct Shear Interface Test for Shaft Capacity of Piles in Sand" by E. Saibaba Reddy, D. N. Chapman, and V. V. R. N. Sastry

The authors are to be congratulated for presenting some interesting results on the interfacial friction angle between sands and solid surfaces. The authors results are consistent with some of the findings of the writers, which they are presenting in this discussion.

Numerical modeling of the contaminant Transport through stratified soil

The transport processes of the dissolved chemicals in stratified or layered soils have been studied for several decades. In case of the solute transport through stratified layers, interface condition plays an important role in determining appropriate transport parameters. First‐ type and third‐ type interface conditions are generally used in the literature. A first‐type interface condition will result in a continuous concentration profile across the interface at the expense of solute mass balance. On the other hand, a discontinuity in concentration develops when a third‐ type interface condition is used. To overcome this problem, a combined first‐ and third‐ type condition at the interface has been widely employed which yields second‐ type condition. This results in a similar break‐through curve irrespective of the layering order, which is non‐physical. In this work, an interface condition is proposed which satisfies the mass balance implicitly and brings the distinction between the breakthrough curves for different layering sequence corroborating with the experimental observations. This is in disagreement with the earlier work by H. M. Selim and co‐workers but, well agreement with the hypothetical result by Bosma and van der Zee; and Van der Zee.

Novel implicit automated time stepping algorithm for contaminant transport through soil.

Accurate estimation of mass transport parameters of chemical contaminants through a soil system is a prerequisite for realistic inverse modeling. In this situation, a mathematical model of the transport process is solved numerically to optimize the design parameters. As the stability and convergence requirements of the numerical procedures limit the usage of large numerical time steps, these time- integral numerical methods either become computationally expensive or yield inaccurate solutions. A detailed convergence and error analyses of different numerical methods is performed in this paper. Further, an automated time stepping implicit algorithm is proposed by smartly adjusting the numerical time step for implicit time-integral numerical procedure using the numerical time step of the explicit schemes obtained by stability and convergence criteria. Thus, the procedure to estimate the numerical solution is rapid due to the implicit nature of the solution yet accurate as it satisfies stability and convergence requirements.

Preconsolidation pressure of desiccated soils

Abstract During desiccation soils are subjected to very high shrinkage stresses, often of the order of 1000 kPa. However, studies on re-saturated desiccated soils do not always show evidence of these soils being subjected to very high stresses. This paper address this important issue and the reason can be attributed to the type of saturation, either by raising the relative humidity or by inundation, which has been found to influence the volume change behavior of these soils. Data shows that resaturation of the dried soil by prolonged exposure to an atmosphere of high relative humidity causes negligible changes in void ratio with very high apparent preconsolidation pressure. Inundation in water, which results in considerable swelling, is found to greatly diminish the effects of the desiccation stresses so that the soils exhibit low apparent preconsolidation pressures. Permeability measurements and Scanning Electron Microscopy indicates that the method of saturation adopted can have significant influence on soil fabric, which in turn affect the compressibility characteristics of soils.