Swelling pressure of a divalent-rich bentonite: Diffuse double-layer theory revisited

Physicochemical forces are responsible for the swelling pressure development in saturated bentonites. In this paper, the swelling pressures of several compacted bentonite specimens for a range of dry density of 1.10–1.73 Mg/m3 were measured experimentally. The clay used was a divalent-rich Ca-Mg-bentonite with 12% exchangeable Na+ ions. The theoretical swelling pressure–dry density relationship for the bentonite was determined from the Gouy-Chapman diffuse double-layer theory. A comparison of experimental and theoretical results showed that the experimental swelling pressures are either smaller or greater than their theoretical counterparts within different dry density ranges. It is shown that for dry density of the clay less than about 1.55 Mg/m3, a possible dissociation of ions from the surface of the clay platelets contributed to the diffuse double-layer repulsion. At higher dry densities, the adsorptive forces due to surface and ion hydration dominated the swelling pressures of the clay. A comparison of the modified diffuse double-layer theory equations proposed in the literature to determine the swelling pressures of compacted bentonites and the experimental results for the clay in this study showed that the agreement between the calculated and experimental swelling pressure results is very good for dry densities less than 1.55 Mg/m3, whereas at higher dry densities the use of the equations was found to be limited

OPTIMAL SENSOR LOCATION FOR PARAMETER IDENTIFICATION IN SOFT CLAY

Performing parameter identification prior to numerical simulation is an essential task in geotechnical engineering. However, it has to be kept in mind that the accuracy of the obtained parameter is closely related to the chosen experimental setup, such as the number of sensors as well as their location. A well considered position of sensors can increase the quality of the measurement and to reduce the number of monitoring points. This Paper illustrates this concept by means of a loading device that is used to identify the stiffness and permeability of soft clays. With an initial setup of the measurement devices the pore water pressure and the vertical displacements are recorded and used to identify the afore mentioned parameters. Starting from these identified parameters, the optimal measurement setup is investigated with a method based on global sensitivity analysis. This method shows an optimal sensor location assuming three sensors for each measured quantity, and the results are discussed

Probabilistic analysis of a gas storage cavity mined in a spatially random rock salt medium

In most engineering problems the material parameters spread over spatial extents but this variability is commonly neglected. Analyses mostly assign the mean value of a variable to the entire medium, while in the case of heterogeneous materials as geomaterials, this may lead to an unreliable design. The existing scatter in such materials can be represented in the design procedure using the random field concept. In this paper, the random field method is used in a probabilistic analysis of a gas storage cavern in rock salt. The rock salt formation, as a porous media with low permeability and particular creep features, has been used for decades as the host rock for the hydrocarbon storage. To achieve a reliable design, a probabilistic model is presented to compute the failure probability of a cavern mined in a spatially varying salt dome. Here, the nodilatant region around the cavity is regarded as the failure criterion. In this regard, a thermo-mechanical model of a natural gas storage in rock salt, employing BGRa creep law, is developed. Afterwards, the most effective input variable on the model response is identified, using global sensitivity analysis. The Karhunen-Loève expansion is introduced to generate random field. In the following, the subset simulation methodology is utilised to facilitate the execution of Monte-Carlo method. The findings of this study emphasize that considering spatial variability in rock properties significantly affects the reliability of a solution-mined cavity

Back analysis of a coupled thermo-hydro-mechanical model based on instrumented constant volume column test

This study contributes to identification of the constitutive model parameters for coupled THM models for unsaturated sand-bentonite mixtures via back analysis approach. The approach strategy consists of: definition of the forward model, sensitivity analysis, selection of optimization algorithm, selection of a set of parameters to be optimized, setup of the parameter’s constraints, and assessing the reliability and accuracy of the identified model and material parameters. For this analysis the iterative direct approach based on numerical solution of the direct problem and minimization of an objective function has been selected. It is given an example of application of the selected inverse analysis procedure to identification of parameters involved in a modified Barcelona Basic Model taking into account of variation of temperature

Benchmarking of Optimization Algorithms

In this paper, we present an empirical approach for objective and quantitative benchmarking of optimization algorithms with respect to characteristics induced by the forward calculation. Due to the professional background of the authors, this benchmarking strategy is illustrated on a selection of search methods in regard to expected characteristics of geotechnical parameter back calculation problems. Starting from brief introduction into the approach employed, a strategy for optimization algorithm benchmarking is introduced. The benchmarking utilizes statistical tests carried out on well-known test functions superposed with perturbations, both chosen to mimic objective function topologies found for geotechnical objective function topologies. Here, the moved axis parallel hyper-ellipsoid test function and the generalized Ackley test function in conjunction with an adjustable quantity of objective function topology roughness and fraction of failing forward calculations is analyzed. In total, results for 5 optimization algorithms are presented, compared and discussed

Coupled thermo-hydro-mechanical modelling of crack development along fossil dinosaur’s footprints in soft cohesive sediments

The fossil footprints have been used to back calculate the properties of the soil in the Age of Dinosaurs. The interpretation of fossil footprints requires the simulation of the processes during as well as after the footprint was generated. Some radial and circumferential cracks were observed occurring on track walls of the footprints. It is supposed that the origin of these cracks can be elucidated by means of footprint’s drying after the footprint is generated. In order to verify this hypothesis and to allow for a precise interpretation of the dinosaur tracks, a series of laboratory and numerical simulation tests was carried out. The tests were designed to mimic the shape evolution of the footprint of the dinosaur during drying. Within the experiments the change of environmental humidity and temperature was monitored and recorded. The laboratory experiment showed that both the radial and circumferential cracks appear during the drying process. The numerical simulation has been performed to better understand and to account for the cracking mechanism in dinosaur’s tracks. In this study the behaviour of a silty soil during drying is numerically simulated by means of a 3D model and performing coupled thermo-hydro-mechanical analysis utilizing the finite element program CODE BRIGHT. Based on the analysis of the tensile stresses along the sample, it was found that the highest tensile stress is on the track wall and it is due to soil shrinkage. It can be concluded that the high tensile stress induced during drying is the most possible reason for cracks to appear in radial and circumferential direction along the foot’s imprint

The undrained shear strength characteristics of silty sand: an experimental study of the effect of fines

This laboratory investigation has been conducted to elucidate how the fines fraction affects the undrained residual shear strength and liquefaction potential of sand-silt mixtures (Algeria). A series of monotonic and cyclic undrained triaxial tests were carried out on undrained, reconstituted, saturated samples of sand with varying fines content ranging from 0 to 50%. These were undertaken in order to evaluate the effect of the fines fraction on the undrained residual shear strength and liquefaction potential of loose, medium dense, and dense sand-silt mixtures (Dr = 17%, 53%, 62% and 91%), under an initial confining pressure of 100 kPa. The results of the monotonic tests indicate that the stress-strain response and shear strength behaviour is controlled by the percentage of the fines fraction and the samples become contractive for the studied relative density (Dr = 17% and 91%). The undrained residual shear strength decreases as the gross void ratio decreases, and the fines content increases up to 30%. Above this level of fines, it decreases with increasing gross void ratio. Moreover, the undrained residual strength decreases linearly as the fines content and the intergranular void ratio increase. Cyclic test results show that for the studied amplitude, the increase in fines content leads to an acceleration of liquefaction. The liquefaction resistance decreases with the increase in gross void ratio and the loading amplitude.</p