Browsing by Author "Koseki, J"

Now showing 1 - 10 of 10

item: Article-Abstract
A density- and stress-dependent elasto-plastic model for sands subjected to monotonic undrained torsional shear loading
(2015-08-20) Chiaro, G; Koseki, J; De Silva, LIN
A density- and stress-dependent elasto-plastic model for saturated sands undergoing monotonic undrained torsional shear loading is presented in this paper. The model is developed under an extended general hyperbolic equation (GHE) approach, in which the void ratio and stress level dependence upon stress-strain response of sand is incorporated. Most importantly, a state-dependent stress-dilatancy relationship is introduced to account for the effect of density on the stress ratio. Such a stress-dilatancy relation is used for modeling the excess pore water pressure generation in undrained shear conditions as the mirror effect of volumetric change in drained shear conditions. In this paper, details of the model formulation and soil parameters calibration are described. By using the proposed model, numerical simulation of monotonic undrained torsional shear tests have been carried out on Toyoura sand. The model predictions show that undrained shear behavior, described in terms of stress-strain relationship and effective stress path for both loose and dense sands can be modeled satisfactorily by using a single set of soil parameters.
item: Conference-Abstract
An elasto-plastic model to describe the undrained cyclic behavior of saturated sand with initial static shear
Chiaro, G; De Silva, LIN; Kiyota, T; Koseki, J
With the aim of simulating the behavior of saturated sand with initial static shear (i.e., sloped ground) undergoing undrained cyclic loading, which leads to liquefaction and large cyclic shear strain development, an elastoplastic constitutive model which can describe both monotonic and cyclic torsional shear behaviors of saturated sand under drained and/or undrained conditions is presented. It can simulate qualitatively the stress-strain relationship and the effective stress path, even after the specimen enters fully liquefied state. To verify its effectiveness, the proposed model is employed to simulate the results of a series of hollow-cylindrical torsional shear tests on loose Toyoura sand specimens with initial static shear stress under stress-reversal and non-reversal loading conditions.
item: Conference-Abstract
Extremely large post-liquefaction deformations of saturated sand under cyclic torsional shear loading
Chiaro, G; Kiyota, T; De Silva, LIN; Sato, T; Koseki, J
The effect of static shear stress on the undrained cyclic behavior of saturated Toyoura sand was studied by conducting a series of torsional shear tests up to double amplitude shear strain of about 100%. After being isotropically consolidated, the specimens were subjected to drained monotonic torsional shear loading, and then, cyclic torsional shear stress was applied under undrained condition. The amplitude of combined static and cyclic shear stress was kept constant by correcting the measured value for the effect of membrane force. Based on these test results, it was found that the effective stress path and the stress-strain curve were affected by the initial static shear stress. Accumulation of shear strain was clearly noticed in the same direction where the static shear stress was applied. Localization of specimen deformation, which increases with the shear strain level, was observed.
item: Article-Abstract
Modeling the cyclic undrained behaviour of sand with initial static shear stress
Chiaro, G; Koseki, J; De Silva, LIN
This paper proposes a cyclic elasto-plastic model for simulating the undrained behavior of saturated sand with initial static shear (i.e., sloped ground) subjected to cyclic loading (i.e., earthquake). The model can describe both monotonic and cyclic torsional shear behaviors of saturated sand under drained and/or undrained conditions; moreover, it is capable of simulating qualitatively the stress-strain relationship and the effective stress path, even after the specimen achieves fully liquefied state. Its effectiveness is verified by simulating the results of undrained cyclic torsional shear tests on loose Toyoura sand specimens with initial static shear stress under stress-reversal and non-reversal loading conditions.
item: Conference-Abstract
Modelling of drained and undrained cyclic Shear behavior of sand
De Silva, LIN; Koseki, J
An elasto-plastic constitutive model capable of simulating both monotonic and cyclic drained and undrained torsional shear behavior of sand is presented. The generalized hyperbolic equation (GHE) has been employed to model the drained monotonic behavior of sand. Then the cyclic shear behavior is modelled by combining the GHE with extended Masing"s rules considering the hardening behavior of sand during drained cyclic loadings and the damage to soil skeleton at large stress levels. A modified stress-dilatancy relationship that relates the ratio of plastic strain increments (-dEPvoldyPzs)to stress ratio ("'zs/p') is proposed to simulate the volumetric behavior of sand. To model the undrained cyclic shear behavior, it is assumed that the total volumetric strain increment is equal to zero. The proposed model is calibrated by means of the results of a series of drained and undrained cyclic torsional shear tests on hollow cylindrical Toyoura sand specimens. Comparison with experiment data suggests that the propose model can reasonably simulate the stress-strain relationship and volumetric behavior of sand subjected to drained cyclic loadings and the generation of excess pore water pressure and stress-strain relationship during undrained cyclic loading.
item: Conference-Abstract
Stress-dilatancy behavior of loose sand during drained cyclic torsional shear loading
Wahyudi, S; Chiaro, G; De Silva, LIN; Koseki, J
In nature, soil is frequently subjected to cyclic loading under drained or undrained conditions such as traffic loads and earthquakes. In order to investigate the behavior of soil during drained cyclic loading, an accurate volumetric change measurement is one of the key issues to obtain properly the stress-dilatancy relationship. Several researchers such as Pradhan et al. (1989), Shahnazari and Towhata (2002), Nishimura (2002), De Silva (2008) among others, have conducted experimental studies to investigate the stress-dilatancy relationship of sands. However, the effect of over-consolidation on dilatancy properties of loose sand is not well understood; therefore, in this study, investigations on the stress-dilatancy relationship were conducted by performing drained cyclic torsional shear tests.
item: Article-Full-text
Stress-dilatancy relationships of sand in the simulation of volumetric behavior during cyclic torsional shear loadings
(Elsevier, 2014) DeSilva, LIN; Koseki, J; Wahyudi, S; Sato, T
In order to describe the volumetric behavior of soil subjected to shearing, a relationship that deals with the ratio of plastic strain increments to stress ratio (i.e. a stress–dilatancy relationship) is required in addition to the stress–shear strain relationship. In view of the above, stress–dilatancy relationships during cyclic torsional shear loadings were experimentally investigated in the current study. Based on the experimental results, a bilinear non-unique stress–dilatancy model was proposed for stress controlled drained cyclic torsional shear loading. The stress–dilatancy relationships during virgin loading and subsequent cyclic loading were modeled separately by considering the effects of stress history (overconsolidation or normal consolidation). Then the volume change of Toyoura sand specimens subjected to cyclic torsional shear loading was simulated by combining the simulation of stress–shear strain relationship with the proposed stress–dilatancy relationships. It was observed from the comparison of the experiment results with the simulation of volumetric strain that, after combining with accurate modeling of stress–shear strain relationship, the proposed stress–dilatancy relationship can reasonably simulate the volumetric behavior of sand subjected to various drained cyclic torsional shear loadings
item: Article-Full-text
A stress-strain description of saturated sand under undrained cyclic torsional shear loading
(Elsevier, 2015) De Silva, LIN; Koseki, J; Chiaro, G; Sato, T
A constitutive model to describe the cyclic undrained behavior of saturated sand is presented. The increments in volumetric strain during undrained loading, which are equal to zero, are assumed to consist of increments due to dilatancy and increments due to consolidation/swelling. This assumption enables the proposed model to evaluate increments in volumetric strain due to dilatancy as mirror images of increments in volumetric strain due to consolidation/swelling, thus simulating the generation of excess pore water pressure (i.e., reduction in mean effective principal stress) during undrained cyclic shear loading. Based on the results of drained tests, the increments in volumetric strain due to consolidation/swelling are evaluated by assuming that the quasi-elastic bulk modulus can be expressed as a unique function of the mean effective principal stress. On the other hand, in evaluating the increments in volumetric strain due to dilatancy, a normalized stress–plastic shear strain relationship is employed in combination with a novel empirical stress–dilatancy relationship derived for torsional shear loading. The proposed stress–dilatancy relationship accounts for the effects of over-consolidation during cyclic loading. Numerical simulations show that the proposed model can satisfactorily simulate the generation of excess pore water pressure and the stress–strain relationship of saturated Toyoura sand specimens subjected to undrained cyclic torsional shear loading. It is found that the liquefaction resistance of loose Toyoura sand specimens can be accurately predicted by the model, while the liquefaction resistance of dense Toyoura sand specimens may be slightly underestimated. (i.e., the liquefaction potential is higher). Yet, the model predictions are conservative.
item: Conference-Abstract
Three-dimensional modeling of stress-strain relationship of sand subject to large cyclic loading
Namikawa, T; Koseki, J; De Silva
In order to predict ground behavior subjected to cyclic loading such as that induced by earthquake, it is necessary to accurately evaluate the deformation characteristics of soils subjected to cyclic loading and develop a model which expresses these deformation characteristics appropriately. This paper provides a simple three-dimensional elasto-plastic model for sand subjected to large cyclic loading. It is an extension of the work on one-dimensional modeling of the cyclic loading stress-strain relationships of sand (De Silva 2008), in which a series of large amplitude cyclic torsional shear tests were implemented on specimens of sand. The concept of the model with infinite number of nesting surfaces (INS model) (Mrozet al. 1978) was employed to introduce the Masing's rule, which has been often used in the onedimensional modeling of the cyclic stress-strain relationships, into the three-dimensional modeling. In thezconcept of the INS model, the active loading and stress reversal surfaces are defined to express the cyclic deformation behavior. In the proposed model, the effects of cyclic hardening behavior which were observed in the torsional shear test results were taken into account by using the relationship between the accumulated deviator plastic strain. Moreover, the stress-dilatation relationship was defined by the relationship between the ratio of the plastic volumetric strain to the plastic deviator strain and the size of the active loading surface. The proposed model was applied to simulate stress-strain relationships of dense sand subjected to drained large cyclic loading.
item: Article-Abstract
Undrained cyclic torsional shear behavior of saturated sand with initial static shear
Chiaro, G; Kiyota, T; De Silva, LIN; Sato, T; Koseki, J
This paper summarizes the experimental results of a study conducted in order to investigate the effect of initial static shear on the undrained cyclic behavior of saturated sand under cyclic torsional shear loading up to single amplitude shear strain of about 50 (~o. Several hollow cylinder specimens at relative density of about 45 % were performed with varying the initial static shear and the subsequent cyclic hear stress levels. The observed failure behavior of specimens could be distinguished into liquefaction and residual deformation failures depending on the magnitude of' combined static and cyclic shear stress. In addition. it was found that the presence of static shear does not always lead to an increase in the resistances to liquefaction and strain accumulation. They could either increase or decrease with increasing the static shear depending 011 the magnitude of combined shear stress, the type of loading and the failure behavior.