Yunming Yang

Application of a non-coaxial soil model in shallow foundations

The influence of a non-coaxial model for granular soils on shallow foundation analyses is investigated. The non-coaxial plasticity theory proposed by Rudnicki and Rice (J. Mech. Phys. Solids 1975, 23, 371–394) is integrated into a Drucker–Prager model with both perfect plasticity and strain hardening. This non-coaxial model is numerically implemented into the finite-element program ABAQUS using a substepping scheme with automatic error control. The influence of the non-coaxial model on footing settlement and bearing capacity is investigated under various loading and boundary conditions. Compared with the predictions using conventional coaxial models, the non-coaxial prediction results indicate that the settlement of a footing increases significantly when the non-coaxial component of plastic strain rate is taken into consideration, although ultimate footing bearing capacities are not affected significantly. The non-coaxial model has a different effect on footing settlements under different loading and boundary conditions. In general, the discrepancies between coaxial and non-coaxial predictions increase with increasing rotation of principal stresses of the soil mass beneath a footing. It can be concluded that if the non-coaxial component of plastic strain rate is neglected in shallow foundation problems using the finite-element method, the results tend to be non-conservative when designs are dominated by settlement of footings.

Monotonic Direct Simple Shear Tests on Sand under Multidirectional Loading

AbstractStress–strain responses of Leighton Buzzard sand are investigated under bidirectional shear. The tests are conducted by using the variable direction dynamic cyclic simple shear (VDDCSS). Soil samples are anisotropically consolidated under a vertical normal stress and horizontal shear stress and then sheared in undrained conditions by applying a horizontal shear stress acting along a different direction from the consolidation shear stress. The influence of the orientation and magnitude of the consolidation shear stress is investigated in this study. There have been only a few previous studies on soil responses under bidirectional shear, of which most studies do not consider the impact of the magnitude of the consolidation shear stress. They are compared with current studies, indicating both similarities and differences. Generally, all test results indicate that a smaller angle between the first and second horizontal shear stress leads to more brittle responses with higher peak strengths, and a larg...

A Soil Model Considering Principal Stress Rotations

This paper presents an elastoplastic soil model considering the principal stress rotation (PSR). The model is developed on the basis of a well-established kinematic hardening soil model using the bounding surface concept.The impact of the stress rate generating the PSR is treated independently, and is added to the base kinematic hardening model. The significance of independent treatment of the PSR stress rate in the soil model is demonstrated through comparing the simulations of soil stress-strain responses by using the base soil model and the modified model in the paper. Various test results in different sands under both drained and undrained conditions are simulated. The paper also discusses the simulations of sand responses under multiple PSRs.

Finite element analysis of anchor plates using non-coaxial models

Abstract The non-coaxial model simulating the non-coincidence between the principal stresses and the principal plastic strain rates is employed within the framework of finite element method (FEM) to predict the behaviors of anchors embedded in granular material. The non-coaxial model is developed based on the non-coaxial yield vertex theory, and the elastic and conventional coaxial plastic deformations are simulated by using elasto-perfectly plastic Drucker-Prager yield function according to the original yield vertex theory. Both the horizontal and vertical anchors with various embedment depths are considered. Different anchor shapes and soil friction and dilation angles are also taken into account. The predictions indicate that the use of non-coaxial models leads to softer responses, compared with those using conventional coaxial models. Besides, the predicted ultimate pulling capacities are the same for both coaxial and non-coaxial models. The non-coaxial influences increase with the increasing embedment depths, and circular anchors lead to larger non-coaxial influences than strip anchors. In view of the fact that the design of anchors is mainly determined by their displacements, ignoring the non-coaxiality in finite element numerical analysis can lead to unsafe results.

Undrained Soil Behavior under Bidirectional Shear

In practice, the soil is subject to more than one shear stress in many cases. For instance, the soil in an embankment is subject to a static shear stress along sloping direction, and the sloping direction is usually different from the direction of an earthquake loading. Therefore, there is a necessity to investigate undrained soil behaviors under shear stresses along multiple directions. This paper employs the first commercially available bidirectional direct simple shear apparatus (VDDCSS) to investigate the soil responses of Leighton Buzzard sand under two directional shear stresses. Sand samples are first subject to a static shear stress under drained conditions along different directions from 0° to 180°, followed by a monotonic or cyclic shear stress along 0° until failure occurs. In static tests, the soil strength is the lowest when the angle between these two shear stresses is near 90°, and the strength is the highest at 0°. In addition, a smaller angle leads to a more brittle response, and a greater angle leads to a more ductile response. In dynamic tests, liquefaction resistance is decreased from the angle of 0° to 90°, and then increased from 90° to 180°.

Simulation and experimental study of soil behaviors under principal stress rotations

This paper presents an elastoplastic soil model considering the Principal Stress Rotation (PSR) and an experimental facility associated with the PSR. The model is developed on the basis of a well-established kinematic hardening soil model using the bounding surface concept. The impact of the stress rate generating the PSR is treated independently, with its own hardening and flow rules. The new PSR model gives better simulations of soil behaviors under loading paths involving the PSR than the base model without special consideration of the PSR. A new experimental facility, Variable Direction Dynamic Cyclic Simple Shear (VDDCSS), manufactured by GDS is also introduced. It can independently exert two shear stresses to a soil specimen along two orthogonal directions. Test results indicate that directions of shear consolidation have a great impact on following undrained soil behaviors under both monotonic and cyclic shearing.

Modeling soil behaviors under principal stress rotations

This paper presents an elastoplastic soil model considering the Principal Stress Rotation (PSR). The model is developed on the basis of a well-established kinematic hardening soil model using the bounding surface concept. The impact of the stress rate generating the PSR is treated independently, and is added to the base kinematic hardening model. The significance of independent treatment of the PSR stress rate in the soil model is demonstrated through comparing the simulations of soil stress-strain responses by using the base soil model and the modified model in the paper. Various test results in different sands under both drained and undrained conditions are simulated, and the new model gives better simulations involving the PSR induced volumetric strain and liquefaction. The paper also discusses the simulations and experiments of sand responses under multiple PSRs.

Finite Element Computations of Yield Vertex Non-Coaxial Models

This paper concerns the issues on finite element numerical implementations of yield vertex non-coaxial models, and approaches to mitigate the numerical difficulties. According to the yield vertex non-coaxial theory, in addition to the plastic strain rate normal to a yield surface, the plastic strain rate tangential to a yield surface is generated by principal stress rotations. This tangential plastic strain rate can easily direct inside a yield surface, which becomes an elastic strain rate. This alternate occurrence of plastic and elastic strain rates makes numerical iterations difficult to converge in the presence of large principal stress rotations. As a result, the numerical applications of yield vertex models can be regarded as moderate discontinuous problems, similar to the use of contact elements with alternate closing and opening. Two approaches are presented in the paper to mitigate the non-convergence problem. The approach in the implicit finite element procedure is to choose appropriate model parameters to limit the amount of tangential plastic strain rate compared to the normal one. The other is to use the explicit finite element procedure, characterized with a large number of computational steps but without numerical iterations. The computation of load-settlement responses for a shallow foundation is used as an example to show the numerical difficulty of yield vertex models, and how the two approaches mitigate the difficulties.