Investigation on Effect of Cyclic Confining Pressure on Excess Pore Water Pressure Variation inside Clay-Aggregate Mixtures

This paper describes findings of a series of numerical studies to investigate the effect of Variable Confining Pressure (VCP) on Excess Pore Water Pressure (EPWP) distribution in a mixture of gravelly inclusions floating in matrix of clay, called “mixed clay” material. Mixed clay material has frequently been used as core of embankment dams in seismically active areas of Iran. The mixture naturally exists in rainfall active areas and also is made manually. The matrix provides sealing while the inclusions increase strength and deformation modulus. Previous studies reveal noticeable increase of EPWP among inclusions under cyclic or dynamic loadings, keeping the initial confining pressure constant. The confining pressure is usually held constant based on the triaxial testing routine. However, recent studies have discovered evident variation of confining pressure in different locations of the shaking embankment-core during an earthquake. After reviewing the literature of analytical and experimental studies regarding the magnitudes of VCP caused by different input motions on an embankment, and also those regarding mixture behavior especially in terms of EPWP variation caused by cyclic loads, results of the conducted semi-coupled dynamic analyses on models of triaxial specimens having limited inclusions inside are presented herein. Making use of PLAXIS 2D 2016, specimens with different inclusion sizes and inclusion distances, experiencing VCP are analyzed. The geometry of a triaxial specimen is modelled in PLAXIS environment in an axisymmetric mode. The inclusions are modeled by half-circle geometry at the center of the specimen. The material behavior is undrained, capable of liquefaction resembling, which means that the degradation of the soil is also modeled. Calibration of the model is based on experiments of previous studies on similar materials, taking into account slight modifications to postpone liquefaction generation. This would clarify EPWP generation caused by VCP variation in pre-liquefaction stage; the UBC model is not precise enough in post-liquefaction stage. The bulk modulus of the soil and water are combined and the consequent slight volumetric strain multiplied by the huge bulk modulus of the water provides the excess pore water pressure, while the relatively slight soil skeleton modulus (compared with that of water) leads to nearly zero effective stress. To assure providing of enough precision, two types of calculations were compared: dynamic semi-coupled and static fully-coupled analyses were conducted and results were compared. It was evident that giving permission to EPWP to redistribute in a coupled analysis was not conservative. The parameter that defines VCP effect is the ratio of the confining pressure cycle amplitude to that of deviatoric stress, called “h”. The results of this study shows that cyclic variation of confining and deviatoric stress at the same time would cause the increase of EPWP within inclusions of the matrix. By decrease of inclusion distance to diameter, EPWP increase among inclusions is dominant, which weakens as the distance increases. The EPWP increase is maximum at the minimum h (0.3), which represents the constant confining pressure situation. This shows that neglecting the increase of h as the consequence of VCP is conservative regarding the stability of structures made up of such mixtures during strong ground motions. Findings of this study is in agreement with those of previous ones: increase of EPWP among inclusions affected by loading direction relative to inclusions locations and geometry was evident. Further studies are required to reveal the effect of loading frequency on the results.