Wenxiong Huang

Polar extension of a hypoplastic model for granular materials with shear localization

A new formulation for polar extension of hypoplastic model is presented for cohesionless granular materials. The formulation is proposed based on the analysis of stationary states, which is a generalization of the concept of critical state in soil mechanics. The model includes stress, couple stress and void ratio as state variables and takes into account the mean grain diameter and inter-granular friction resistance to grain sliding and rotation. A coupled limit condition embedded in the model is derived from the analysis for stationary states and the physical interpretation of frictional parameters are provided. The performance of the model is studied by modeling the plane shearing of an infinite granular layer, which shows the capability of the model in capturing the phenomenon of shear localization with a finite thickness. Numerical results are presented to show the evolution of a localized zone and of the state variables. Parametric studies are performed to investigate the dependence of the thickness of shear bands on various factors. A correlation is proposed to correlate the thickness of shear bands with grain size, frictional parameters and pressure-dependent relative density.

Bifurcation analysis for shear localization in non-polar and micro-polar hypoplastic continua

In this paper, shear localization in granular materials is studied as a bifurcation problem based on a conventional (non-polar) and a micro-polar continuum description. General bifurcation conditions are formulated for a non-polar hypoplastic model and its micro-polar continuum extension. These conditions define stress, couple stress and density states at which weak discontinuity bifurcation may occur. The stress states for bifurcation are then compared with the peak stress states, which define a bounding surface for the accessible stress domain in the principal stress space. The results show that, in a micro-polar continuum description, the constitutive model may no longer be associated with weak discontinuity bifurcation.

Formulation of non-standard dissipative behavior of geomaterials

Abstract.In this paper, fundamental mathematical concepts for modeling the dissipative behavior of geomaterials are recalled. These concepts are illustrated on two basic models and applied to derive a new form of the evolution law of the modified Cam-clay model. The aim is to discuss the mathematical structure of the constitutive relationships and its consequences on the structural level. It is recalled that non-differentiable potentials provide an appropriate means of modeling rate-independent behavior. The Cam-clay model is revisited and a standard version is presented. It is seen that this standard version is non-dissipative, which at the same time explains why a non-standard version is needed. The partial normality is exploited and an implicit variational formulation of the modified Cam-clay model is derived. As a result, the solution of boundary-value problems can be replaced by seeking stationary points of a functional.

Water Content Measurement in Expansive Soils Using the Neutron Probe

Capacitance-type methods for measuring soil water content are known to be unreliable in expansive soils, as cracking disrupts the intimate contact between the soil and the measuring device. The neutron probe, which infers water content from the thermalisation of a cloud of neutrons, is potentially less affected by cracking. The effect of cracking on neutron probe measurements was investigated by a series of numerical simulations using an axisymmetric finite element model based on seven-group neutron-diffusion theory. The simulations employed a consistent soil cracking model based on Maryland clay, in which crack volumes are determined from the changes in void ratio in the shrinking bulk soil. The results show that the presence of cracks in a clay soil affects the inferred water content and that measurements affected by air-filled cracking under-predict not only the water content in the uncracked soil peds but also the average water content in the larger cracked soil mass. The reason for this under-prediction is understood by considering the spatial distribution of the thermalised neutrons in the cracked and uncracked soils. The fast neutrons emitted from the source are seen to diffuse preferentially along air-filled cracks, traveling a large distance from the detector before they become thermalised, thus reducing their likelihood of being back-scattered to the detector where they can be counted. The proximity of the first crack to the probe in the ground also affects the measurement. Water-filled cracks are seen to have the opposite (but lesser) effect to air-filled cracks. A comparison of a simple uniform width crack model to a more realistic model in which crack width varies with changing water content shows that the model is sensitive to crack distribution and that the linear calibration expressions that are typically employed for neutron probes are likely to be unreliable in cracked clay soils.

Finite Element Simulation of Cone Penetration with Finite-Sliding Contact Interface

Rigorous coupled displacement-pore pressure finite element calculations are performed to analyze the Cone Penetration Test (CPT) in sandy soil. The soil is simplified as an elastic-perfect-plastic material. The penetrometer is assumed to be rigid. With the capacity of modeling the large sliding on the penetrometer-soil interface the full penetration process from the surface of the ground to a certain depth is simulated. Parametric study for the influences of various factors to cone resistance is performed. Results are compared with empirical correlations based on the bearing capacity theory and the cavity expansion theory.