Browsing by Author "Dakoulas, Panos C."
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Item Aspects of constitutive modeling of soils(1992) Amer, Muhammad; Dakoulas, Panos C.Constitutive models in soil mechanics are used to determine theoretical solutions to a host of geotechnical engineering problems such as pile-soil interaction, pile installation, short and long term stability, consolidation and foundation problems, etc. A fundamental component of the numerical formulation of the constitutive equations is the development of the elastoplastic tangent stiffness matrix. The numerical formulation of the elastoplastic matrix for a work hardening/softening material, as applicable to a general purpose finite element computer code, has been attempted by the author. The derived tangent stiffness matrix is symmetric for the associated flow rule (g = f) but it is non-symmetric for the non-associated flow rule (g $\not=$ f). All the components of the elastoplastic matrix for the Lade's model$\sp{\lbrack 11\rbrack}$ are also given. A parametric study was carried out to determine the validity of the associated and the non-associated flow rules for various stress paths in the deviatoric plane. The results demonstrate the inability of the associated flow rule to capture the soil behavior, except at stress levels very close to the hydrostatic axis.Item Dynamic response of concrete-faced rockfill dams in rectangular canyons(1997) Guo, Yi; Dakoulas, Panos C.An analytical closed-form solution has been developed for steady-state lateral response of concrete-face rockfill dams built in rectangular canyons. The canyon is assumed to be rigid, while the dam is idealized as a three-dimensional linearly-hysteretic elastic body deforming in shear and bending. Both free and base induced oscillations are studied for various canyon geometries. A parametric study of the effects of the canyon narrowness and the dam slope on the response is undertaken. Finally, a more rigorous numerical formulation is used for verification of the closed-form solution.Item Experimental investigation and constitutive modeling of marine clay(1993) Yu, Shinyuan; Dakoulas, Panos C.A new cross-anisotropic elasto-plastic constitutive model is developed based on experimental observations of marine clay from the Gulf of Mexico. Standard triaxial and torsional simple shear tests performed on the aforementioned clay demonstrate that the soil exhibits a cross-anisotropic behavior with lower compressibility in the direction of the deposition than perpendicular to the direction of deposition. A new general cross-anisotropic model for the stress-dependent elastic moduli is presented. In agreement with experimental evidence, the model considers that the Young's modulus and the shear modulus of soil depend on the state of stress, while the three Poisson's ratios are practically constant. The expressions for the stress dependence of the moduli are derived by considering the conservation of energy. Numerical simulations of the undrained elastic reloading demonstrate that the development of pore water pressure depends significantly on the exact representation of the cross-anisotropic elastic parameters. The plastic model is developed based on the assumptions that the material behavior is time independent and that the interaction between mechanical and thermal processes is negligible. The model, having twelve parameters, consists of a failure function, plastic potential function (non-associated flow rule), yield function and hardening law. To describe the degree of cross-anisotropy at failure, a new four-parameter failure criterion is developed. The plastic potential function, having two parameters, determines the directions of the plastic strain increments which are assumed independent of the stress path leading to the current state of stress. The potential surface expands along its center line and may translate in the stress space. The yield criteria are associated with and derived from surfaces of constant plastic work. The yield-plastic work relation requires six parameters in which four parameters define the yield function and two parameters define the plastic work equation. For the special case of isotropic soil, all of the functions may be reduced to those of Lade's isotropic elasto-plastic model. Most of the elastic and plastic parameters can be easily determined by experimental results from standard triaxial tests, but a few parameters need advanced tests such as the torsional simple shear test on hollow cylinder specimens and the cubic triaxial test. Comparisons between results from computer simulation of tests and actual experimental data showed that the model is satisfactory in predicting the behavior of the tested clay under torsional simple shear and conventional triaxial compression and extension tests.Item Experimental investigation for constitutive modeling of fine sand under cyclic loading using hollow cylinder specimens(1991) Sun, Yuanhui; Dakoulas, Panos C.An experimental program for the understanding of the behavior of fine cohesionless soil under monotonic and cyclic loading and for the development and refinement of constitutive models has been undertaken. An experimental program for both loose and dense saturated fine Ottawa Silica sands under monotonic loading using both solid and hollow cylinder specimens has been conducted. The failure surfaces have been established from twenty drained monotonic load tests. Results were found in good agreement with the failure surfaces incorporated in Lade's constitutive model. The soil behavior and deformation characteristics under undrained conditions have been investigated under a cyclic experimental program for loose sand. Results indicated that the circular rotation of principal stress axes with a constant amplitude deviator stress, as well as the stress direction reversals have significant effects on the rate of pore water pressure buildup, the triggering of a liquefaction flow failure in contractive sand and the rate of accumulation of deformation. The rate of excess pore water pressure buildup is faster during a cyclic test with circular rotation of principal stress axes than during a cyclic triaxial shear test or a cyclic torsional shear test having same amplitude of shear stress. The rate is faster during a cyclic triaxial extension test than in a cyclic triaxial compression test or a cyclic torsional simple shear test with the same amplitude of shear stress. The deformation and significant pore water pressures are developed during the first cycle and more remarkably during the last cycle. The pore water pressures (and mean effective stress reduction) and developed fast when the stress path reaches the failure surface which was established in monotonic load tests. The pore water pressure and deformation increase fast in the case of shear stress reversal. The pore water pressure buildup and mean effective stress reduction are more pronounced during extension loading than in compression loading. Moreover, the amplitude of shear stress has significant effects for the pore water pressure development and deformation. Significant pore water pressure and deformation can occur, during the rotation of the principal stress axes, even when the deviator stress is maintained unchanged. (Abstract shortened with permission of author.)Item Hybrid FE-BE formulation for coupled dynamic poroelastoplastic analysis of soil-structure systems(1999) Mohammed-Eltaher, Ayman Hamdy; Dakoulas, Panos C.A hybrid finite element-boundary element formulation is developed for the nonlinear seismic analysis of 2D soil-structure systems that contain fully or partially saturated soils. The proposed formulation aims at combining the sophistication, versatility and nonlinear capabilities of the FEM with the ability of the BEM to model rigorously energy radiation. The FE part of the formulation is used to model the effective stress problem in the near field, incorporating all material and geometric nonlinearities. The BE part is used to model the infinite domain as an elastic medium, by considering strain-dependent equivalent linear properties. The new method is used in a series of analyses and reproductions of published results to verify its validity, demonstrate its versatility and attain more insight on select problems. The new formulation is applied on two different types of structures, with main objective to provide a better understanding of their seismic behavior. The first study is a reanalysis of the Lower San Fernando dam under the 1971 earthquake. The study reproduces remarkably many of the known behavior characteristics and the mode of failure of the dam. In a subsequent analysis aimed at demonstrating the effects of foundation flexibility, the original dam response is compared to the response assuming a hypothetical flexible foundation. The results demonstrate a substantial reduction in the response of the flexible foundation case. The second study examines the dynamic response of waterfront retaining walls. Emphasis is placed on the effects of relative density and stiffness of the backfill and foundation materials. As expected, the study shows that a wall with a dense backfill sand endures the dynamic loading with minimum permanent deformation, whereas a wall with loose backfill sand may experience excessive deformations and liquefaction. Significant deformation is also observed for the model with flexible foundation, despite the reduction of wave energy through radiation damping. The developed FE-BE formulation allows a deformation-based design of soil-structure systems by accounting rigorously for complex wave propagation phenomena and material nonlinearities.Item Nonlinear seismic behavior of retaining wall-soil systems(2000) Inada, Noritake; Dakoulas, Panos C.The prediction of the seismic behavior of waterfront structures has been considered as a challenging problem and attracted significant research interest, especially after the severe damage of such structures in Niigata, Japan, during the 1964 Niigata Earthquake. The objective of the present study is to improve our understanding of the effects of the backfill material and the wall-soil interface on the seismic behavior and safety performance of retaining walls. The nonlinear analyses are conducted by using an explicit finite-difference formulation for large-deformation analysis of soil-structure systems subjected to seismic excitation. The effects of separation at the wall-soil interface are investigated assuming a fix-based rigid wall and a linearly elastic backfill material. These effects are found to be significant, resulting to wall forces and moments that may be, respectively, 25 and 40% larger than those based on the assumption of no separation. The effects of nonlinearity of a typical saturated backfill soil in a waterfront structure are investigated by considering three different materials, namely, loose sand, medium sand and dense sand. The study examines the effects of relative density, intensity of base excitation, frequency of base excitation and number of cycles of loading on the wall pressures, forces, moments, displacements and rotations. The results show the dramatic effect of the excess pore-water pressure buildup that may lead to liquefaction or cyclic mobility.Item Nonlinear seismic response of dams using a coupled boundary element - finite element formulation(1996) Abou-Seeda, Hassan Mohamed; Dakoulas, Panos C.A study of the effects of dam-foundation interaction on the response of earth, rockfill and concrete-faced rockfill dams to obliquely incident P, SV and Rayleigh waves is presented. Emphasis is placed on the effects of the foundation flexibility, the spatial variability of the ground motion and the material nonlinearity. The study is based on a rigorous hybrid numerical formulation that combines the efficiency and versatility of the Finite Element Method (FEM) and the ability of Boundary Element Method (BEM) to account for the radiation conditions at the far field. The developed hybrid method is very powerful, and can be used efficiently to obtain accurate solutions of problems of complex geometry, material heterogeneity and, for time-domain analysis, material nonlinearity. The 2-D frequency-domain formulation is used at first to investigate the linear response of earth and rockfill dams to incident P, SV and Rayleigh waves and the response of concrete-faced rockfill dams to incident Rayleigh waves. Furthermore, the nonlinear time-domain coupled BE-FE formulation is used to investigate the response of earth and rockfill dams to vertically incident SV waves. By accounting rigorously for the energy radiated back into the halfspace, the study demonstrates the dramatic effect of the flexibility of the foundation rock in reducing the overall response of the dam. The effects of the spatial variability of the ground motion across the width of the dam are also shown to be important, but less dramatic than those of the foundation flexibility. Finally, the results from the nonlinear analysis of two different dams, each experiencing various degrees of nonlinearity, have demonstrated the great importance of the material nonlinear behavior on the response of dams subjected to strong ground motion.Item Response of earth dams in canyons subjected to asynchronous excitation (Dams)(1989) Hashmi, Humayun; Dakoulas, Panos C.A mathematical closed-form solution is presented for steady-state lateral response of earth and rockfill dams in canyons subjected to asynchronous excitation consisting of SH waves incident at an arbitrary angle. The dam is idealized with a 2-dimensional shear beam model and the canyon is considered rectangular in shape and consisting of elastic rock. An extensive series of parametric studies is undertaken to investigate the influence of the main parameters on the steady-state response by considering the effects of the dam-canyon interaction. In particular the study focuses on the effects of: (a) the angle of incidence, (b) the impedance ratio, and (c) the canyon narrowness. The solution is extended for a transient arbitrary excitation consisting of inclined SH waves for use in equivalent linear seismic analysis. (Abstract shortened with permission of author.)Item Response of earth dams in semi-elliptical canyons to oblique SH waves(1993) Hsu, Ching-Heng; Dakoulas, Panos C.Two analytical closed-form solutions are developed for steady-state lateral response of earth and rockfill dams built in semi-elliptical canyons. In the first model, the canyon is assumed to be rigid, while the dam is idealized as a two-dimensional linearly-hysteretic elastic body deforming only in shear (shear beam). Both free and base-induced oscillations are studied for various canyon geometries. In the second model, the canyon is assumed to consist of flexible elastic rock, subjected to asynchronous excitation consisting of obliquely incident harmonic SH waves. The solution accounts in a rigorous way for the complex wave reflection, transmission and diffraction phenomena associated with the dam-filled canyon. The study focuses on the effects of: (a) the angle of incidence, (b) the impedance ratio and (c) the canyon narrowness. It is shown that the effects of radiation damping and ground motion spatial variability are very important.Item Seismic effective-stress deformation analysis of waterfront retaining structures(2000) Kastranta, Georgia; Dakoulas, Panos C.A deformation-based method that can be used for the analysis of the seismic response and for the design of waterfront retaining structures is developed. The study is focused on the behavior of gravity caisson-type quay walls. The key element for the success of the method lies on the selection of an appropriate constitutive model that can describe adequately the monotonic and cyclic undrained behavior of soil subjected to arbitrary stress paths. To this end, an existing generalized plasticity model, developed by Pastor et al. (1990), was examined and modified in order to improve its performance. Comparisons with a centrifuge model study and a case history from the recent 1995 Kobe earthquake on gravity quay walls are used for the evaluation of the new method of analysis, based on the modified generalized plasticity model. Particular emphasis is given to the undrained behavior of the foundation and backfill sand, and its effect on the deformational mechanism and damage of the quay walls.