Jinhyun Choo

Hydromechanical Modeling of Unsaturated Flow in Double Porosity Media

AbstractGeomaterials with aggregated structure or containing fissures often exhibit a bimodal pore size distribution that can be viewed as two coexisting pore regions of different scales. The double-porosity concept enables continuum modeling of such materials by considering two interacting pore scales satisfying relevant conservation laws. This paper develops a thermodynamically consistent framework for hydromechanical modeling of unsaturated flow in double-porosity media. With an explicit treatment of the two pore scales, conservation laws are formulated incorporating an effective stress tensor that is energy-conjugate to the rate of deformation tensor of the solid matrix. A constitutive framework is developed on the basis of energy-conjugate pairs identified in the first law of thermodynamics, which is then incorporated into a three-field mixed finite-element formulation for double-porosity media. Numerical simulations of laboratory- and field-scale problems are presented to demonstrate the impact of d...

Patterns of Nonlinear Shear Stiffness Degradation of Reconstituted Clay with Different Stress Histories

This paper describes patterns of nonlinear shear stiffness degradation with respect to the stress history of clay. An experimental study using undrained triaxial compression tests was conducted on specimens cut from reconstituted clay samples of kaolinite. The nonlinear pattern of stiffness degradation was analyzed within the frameworks of both the conventional overconsolidation ratio (OCR) and the stress path rotation angle. The experimental data were subsequently interpreted based on the concept of the kinematic sub-yield surface. The pattern of stiffness degradation is more relevant to the rotation angle of the current stress path than the OCR value. The sizes of the sub-yield surfaces are variable. Results show that the kinematic movements of sub-yield surfaces within the overall bounding surface may provide an insufficient tool to fully describe the pattern of stiffness degradation.

Effect of Pre-Shear Stress Path on Nonlinear Shear Stiffness Degradation of Cohesive Soils

The nonlinear degradation of soil stiffness from very small to small strain is a key consideration for reliable prediction of ground behavior and its interactions with structures under dynamic excitation and working load conditions. Despite high sensitiveness of stiffness measurement to testing conditions, the effect of the pre-shear stress path on the stiffness degradation has not been properly discussed. Here the authors investigate the effect of pre-shear stress path on nonlinear shear stiffness degradation of cohesive soils. Reconstituted kaolinite specimens were consolidated to be the overconsolidation ratio (OCR) = 1, 2, and 4 along Ko and isotropic stress paths. The shear stiffness degradations of the specimens during undrained shear were measured using on-specimen linear variable differential transformers. Experimental results show that the pre-stress stress path has a strong influence on the degree of shear stiffness degradation at different OCRs. This influence is interpreted within the context of the rotation angle of shear stress path, which provides a good qualitative explanation of the inconsistent observations in the literature.

Pore directivity of soils subjected to shearing: Numerical simulation and image processing

ABSTRACT: Soils subjected to shearing experience dilation or contraction depending on their initial porosity, and the relative displacement of individual particles determines a soil’s unique particle-pore microstructure during volume change. It has been suggested that soil microstructure tends to be stabilized as pores are aligned parallel to the loading direction as particles are mobilized. We explore the evolution of internal pore fabric and directivity during direct shear conditions in which a constrained boundary hampers the full mobilization of particles. Two representative volumetric responses for dense and loose granular soils during direct shear are simulated via the discrete element method. The arbitrarily shaped pore structure in 3D space is quantified using best-fitting ellipsoids to evaluate pore characteristics. Changes in pore fabric are analyzed based on local porosity, pore size distribution, and geometrical configurat ion of fitted ellipsoids. Results show that initial porosity determines the characteristic pore evolution during shearing. Numerical results also demonstrate that a pore elongation oriented in the direction of the shear manifests under dense packing, while randomly distributed pore directivity is observed under loose packing.

Applicability Study on Deep Mixing for Urban Construction

Abstract The deep mixing method, which is generally considered as a method for improving soft ground, is assessed in terms of its applicability for urban construction. Using small equipment tailored to perform deep mixing in congested urban areas, deep mixing was performed to reinforce the foundation ground of a retaining wall in a redevelopment site in Seoul. Strengths characteristics, construction vibrations and displacements induced to an adjacent old masonry wall were evaluated by laboratory tests and field monitoring. The results indicate that the strength of ground was improved appropriately whilst the vibrations and displacements induced by deep mixing were slight enough to satisfy the general requirements for construction works in urban environments. Therefore, it is concluded that deep mixing method can be a practical option for foundation methods in urban construction works where minimizing noise and vibrations is an important concern.Key Words : Deep mixing method, Soil mixing, Soil-cement, Ground improvement, Urban construction

Liquid CO2 Fracturing: Effect of Fluid Permeation on the Breakdown Pressure and Cracking Behavior

Liquid CO2 fracturing is a promising alternative to hydraulic fracturing since it can circumvent problems stemming from the use of water. One of the most significant differences between liquid CO2 and hydraulic fracturing processes is that liquid CO2 permeates into matrix pores very rapidly due to its low viscosity. Here we study how this rapid permeation of liquid CO2 impacts a range of features during the course of the fracturing process, with a focus on the breakdown pressure and cracking behavior. We first conduct a series of laboratory fracturing experiments that inject liquid CO2, water, and oil into nominally identical mortar specimens with various pressurization rates. We quantitatively measure the volumes of fluids permeated into the specimens and investigate how these permeated volumes are related to breakdown and fracture initiation pressures and pressurization efficiency. The morphology of the fractures generated by different types of fluids is also examined using 3D X-ray computed tomographic imaging. Subsequently, the cracking processes due to injection of liquid CO2 and water are further investigated by numerical simulations employing a phase-field approach to fracture in porous media. Simulation results show that rapid permeation of liquid CO2 gives rise to a substantial pore pressure buildup and distributed microcracks prior to the major fracture propagation stage. The experimental and numerical results commonly indicate that significant fluid permeation during liquid CO2 fracturing is a primary reason for its lower breakdown pressure and more distributed fractures compared with hydraulic fracturing.

Enriched Galerkin Finite Element Method for Locally Mass Conservative Simulation of Coupled Hydromechanical Problems

Numerical simulation of coupled hydromechanical processes is crucial to address many problems in geotechnical engineering. Conventionally, the seepage flow equation in a hydromechanical model is solved by the continuous Galerkin (CG) finite element method. However, the CG method does not ensure mass conservation in a local (element-wise) manner, which may be a critical drawback in a number of situations. Here we introduce an enriched Galerkin (EG) finite element method that allows locally mass conservative simulation of coupled hydromechanical problems. The major advantage of the EG method is that its computational cost is appreciably less than that of the discontinuous Galerkin (DG) method. We describe the proposed method’s key idea and demonstrate its performance and significance through an example simulation of wetting collapse of a heterogeneous unsaturated soil.

Multiscale Poromechanics: Fluid Flow, Solid Deformation, and Anisotropic Plasticity

Natural geomaterials such as fissured rocks and aggregated soils often exhibit pore size distributions with two dominant porosity scales. In fractured rocks the dominant porosities are those of the fractures and rock matrix, whereas in aggregated soils the micropores and macropores comprise the two relevant porosity scales. When infiltrated with fluids this type of materials may also exhibit two permeability scales. In this paper we present a framework for so-called ‘dual porosity-dual permeability’ materials that covers both steady-state and transient fluid flow responses. The formulation revolves around a thermodynamically consistent effective stress previously developed for porous media exhibiting two porosity scales. Apart from the aspect of multiscale poromechanics, some geomaterials such as shale also exhibit pronounced anisotropy in their mechanical behavior due to the presence of distinct bedding planes. A transversely isotropic constitutive model is appropriate for this type of material behavior. Anisotropic plasticity models can easily be integrated into the aforementioned dual porosity-dual permeability framework.

Deep Mixing Improvement of Soft Ground Adjacent to a Historic Masonry Wall: Performance and Impacts on Surroundings

This paper presents a case study of deep mixing performed to improve soft ground adjacent to a historic masonry wall in an urban redevelopment site. A challenge of the site was to perform the redevelopment without impairing the masonry wall of historic importance. For the foundation of a retaining wall adjacent to the masonry wall, deep mixing was applied and its impacts on the masonry wall were monitored during the construction. Using small construction equipment customized to cramped conditions, 193 soil-cement columns were installed by deep mixing technique. Strength characteristics of the improved ground, vibration velocities with respect to the distance from the equipment, and displacements of the masonry wall throughout the construction were evaluated by laboratory tests and field measurements. The results indicate that not only the strength of ground was improved to meet the design strength but also the vibration levels and displacements induced by deep mixing were less than regulations. INTRODUCTION Construction works are often required to minimize disturbances on surroundings (e.g. vibration) as well as to satisfy design criteria. In congested urban areas, for example, driven pile is rarely used due to its harsh sound and vibration, and other pile types whose installations scarcely perturb adjacent environments are used instead. In this regard, currently various methods are evaluated in terms of their ‘construction impacts’, such as vibration generated and displacement induced to adjacent structures. One of the merits of deep mixing is suggested as its relatively vibration-free construction (Bruce and Bruce, 2003). However, there are few (if any) quantitative supports for this 46 3A _5 08 35 _A S C E _V ol _0 1_ T xt _R es iz e_ A A .jo b_ P ro ce ss C ya n_ 08 /0 1/ 20 12 _1 0: 49 :4 0 46 3A _5 08 35 _A S C E _V ol _0 1_ T xt _R es iz e_ A A .jo b_ P ro ce ss M ag en ta _0 8/ 01 /2 01 2_ 10 :4 9: 40 46 3A _5 08 35 _A S C E _V ol _0 1_ T xt _R es iz e_ A A .jo b_ P ro ce ss Y el lo w _0 8/ 01 /2 01 2_ 10 :4 9: 40 46 3A _5 08 35 _A S C E _V ol _0 1_ T xt _R es iz e_ A A .jo b_ P ro ce ss B la ck _0 8/ 01 /2 01 2_ 10 :4 9: 40