An efficient finite–discrete element method for quasi‐static nonlinear soil–structure interaction problems

An efficient finite–discrete element method applicable for the analysis of quasi‐static nonlinear soil–structure interaction problems involving large deformations in three‐dimensional space was presented in this paper. The present method differs from previous approaches in that the use of very fine mesh and small time steps was not needed to stabilize the calculation. The domain involving the large displacement was modeled using discrete elements, whereas the rest of the domain was modeled using finite elements. Forces acting on the discrete and finite elements were related by introducing interface elements at the boundary of the two domains. To improve the stability of the developed method, we used explicit time integration with different damping schemes applied to each domain to relax the system and to reach stability condition. With appropriate damping schemes, a relatively coarse finite element mesh can be used, resulting in significant savings in the computation time. The proposed algorithm was validated using three different benchmark problems, and the numerical results were compared with existing analytical and numerical solutions. The algorithm performance in solving practical soil–structure interaction problems was also investigated by simulating a large‐scale soft ground tunneling problem involving soil loss near an existing lining. Copyright © 2011 John Wiley & Sons, Ltd.     

A discrete element approach to elastic properties of layered geomaterials

Discrete element methods (DEMs) are used for layered geomaterials to investigate the dependency of traditional engineering constants on material properties and loading conditions. Shear deformations and compression tests parallel and perpendicular to layering are conducted on samples of varying kerogen volume fractions, confining pressures, porosities, and layer geometries. The goal of this article is to develop a method to better characterize oil shale (a transversely isotropic layered geomaterial) while eliminating high experimental costs. The DEM simulations conducted in this study demonstrate strong dependencies of Young's modulus, Poisson's ratio, and shear modulus on kerogen volume fraction and porosity. Furthermore, a rule of thumb for layer thickness and particle resolution is proposed for simulation design. Results agree well with robust effective medium theories, solidify the ability of DEM to model the mechanical properties of layered heterogenous materials, and encourage the use of DEM to study more complicated layered media and material failure. Copyright © 2011 John Wiley & Sons, Ltd.     

Solution of quasi‐static large‐strain problems by the material point method

A procedure for solving quasi‐static large‐strain problems by the material point method is presented. Owing to the Lagrangian–Eulerian features of the method, problems associated with excessive mesh distortions that develop in the Lagrangian formulations of the finite element method are avoided. Three‐dimensional problems are solved utilizing 15‐noded prismatic and 10‐noded tetrahedral elements with quadratic interpolation functions as well as an implicit integration scheme. An algorithm for exploiting the numerical integration procedure on the computational mesh is proposed. Several numerical examples are shown. Copyright © 2010 John Wiley & Sons, Ltd.     

Using estimated thermodynamic properties to model accessory phases: the case of tourmaline

Accessory phases and minor components in minerals are commonly ignored in thermodynamic modelling. Such an approach seems unwarranted, as accessory phases can represent a significant element reservoir and minor components can substantially change their host mineral's stability field. However, a lack of thermodynamic data prohibits assessment of these effects. In this contribution, the polyhedron method is used to estimate the thermodynamic properties of tourmaline, a common and widespread accessory phase, stable over a range of P–T–X conditions. The polyhedron method allows Δ H , S , V , C _P and V _m ( T , P ) properties to be estimated from a linear stoichiometric summation over the fractional properties of its polyhedron constituents. To allow for estimates of tourmaline, fractional thermodynamic properties for B^III and B^IV polyhedra were derived. Mixing contributions to molar volume were evaluated and symmetrical mixing parameters derived for Al-Mg, Al-Fe and Al-Li interaction on tourmaline's Y-site and T-site Al-Si interaction. Evaluation of the estimated properties using experimental and natural equilibria between tourmaline and melts, minerals and hydrothermal fluids, shows that reliable semi-quantitative results are obtained. The boron contents in fluids coexisting with tourmaline are calculated to within an order of magnitude of measured content, and where anchor-points are available, agreement improves to within a factor of 2. Including tourmaline in petrogenetic modelling of metamorphic rocks indicates that its presence leads to disappearance of staurolite and garnet, among others, and modifies the X _Mg of coexisting phases, in line with observations on natural rocks.     

Considerations of the discontinuous deformation analysis on wave propagation problems

In rock engineering, the damage criteria of the rock mass under dynamic loads are generally governed by the threshold values of wave amplitudes, such as the peak particle velocity and the peak particle acceleration. Therefore, the prediction of wave attenuation across fractured rock mass is important on assessing the stability and damage of rock mass under dynamic loads. This paper aims to investigate the applications of the discontinuous deformation analysis (DDA) for modeling wave propagation problems in rock mass. Parametric studies are carried out to obtain an insight into the influencing factors on the accuracy of wave propagations, in terms of the block size, the boundary condition and the incident wave frequency. The reflected and transmitted waves from the interface between two materials are also numerically simulated. To study the tensile failure induced by the reflected wave, the spalling phenomena are modeled under various loading frequencies. The numerical results show that the DDA is capable of modeling the wave propagation in jointed rock mass with a good accuracy. Copyright © 2009 John Wiley & Sons, Ltd.     

Implicit integration of a mixed isotropic–kinematic hardening plasticity model for structured clays

In recent years, a number of constitutive models have been proposed to describe mathematically the mechanical response of natural clays. Some of these models are characterized by complex formulations, often leading to non‐trivial problems in their numerical integration in finite elements codes. The paper describes a fully implicit stress‐point algorithm for the numerical integration of a single‐surface mixed isotropic–kinematic hardening plasticity model for structured clays. The formulation of the model stems from a compromise between its capability of reproducing the larger number of features characterizing the behaviour of structured clays and the possibility of developing a robust integration algorithm for its implementation in a finite elements code. The model is characterized by an ellipsoid‐shaped yield function, inside which a stress‐dependent reversible stiffness is accounted for by a non‐linear hyperelastic formulation. The isotropic part of the hardening law extends the standard Cam‐Clay one to include plastic strain‐driven softening due to bond degradation, while the kinematic hardening part controls the evolution of the position of the yield surface in the stress space. The proposed algorithm allows the consistent linearization of the constitutive equations guaranteeing the quadratic rate of asymptotic convergence in the global‐level Newton–Raphson iterative procedure. The accuracy and the convergence properties of the proposed algorithm are evaluated with reference to the numerical simulations of single element tests and the analysis of a typical geotechnical boundary value problem. Copyright © 2007 John Wiley & Sons, Ltd.     

Is being in work good for wellbeing? Work Integration Social Enterprises in regional Australia

ABSTRACT

Work Integration Social Enterprises (WISE) offer supported work environments for people experiencing disadvantage, including people with disability. This paper reflects on a research project that is mapping the ways in which social enterprises in regional Australian cities produce wellbeing for their employees. Through supported employment programs, these organisations are transforming individual lives, helping to build a sense of self-worth and purpose, and increase individuals’ social skills and capacities. Many of the perceived benefits are associated with a belief in the innate good of paid work. Based upon interviews with employees in the social enterprises, it is apparent that the hopes attached to providing ‘meaningful work’ are considerable. At the same time social enterprises are taking on social responsibilities that were once provided by the state, while also selling their services as employers of disabled and disadvantaged members of society and maintaining commercial viability of the enterprise. Using feminist political economy alongside discussion of the distributive economy, this paper explores how contemporary policy conditions, productivist biases and normative discourses about the value of wage work inflect the forms of wellbeing that can be experienced by WISE workers.     

An alternative interpretation for the formation of doubly plunging folds in sandstone terrains

Folds are marvellous features of mountain terrains, but despite extensive research, many fundamental problems have still not been solved. In terrains of sandstone, fold hinges are rarely straight lines but curved, forming a pattern characterized by doubly plunging, elliptical dome‐and‐basin structures. Such structures are an obvious manifestation of coeval or successive shortening deformation in two orthogonal principal directions in the horizontal plane. Based on an anatomic investigation of the fold pattern of sandstone beds at the rocky beaches of Saint‐Jean‐Port‐Joli (Quebec, Canada), we propose that the doubly plunging folds may result from a transition from a plane deformation to a constrictional deformation due to auxetic effects of quartz‐rich rocks. The sandstone beds possessed potentially such negative values of Poisson's ratio that, when placed under compression in one direction, they become contracted in the transverse direction, producing a series of doubly plunging folds. Further work is needed to approve or disapprove the interpretation.     

Controls on evapotranspiration from jack pine forests in the Boreal Plains Ecozone

The exchanges of water, energy and carbon between the land surface and the atmosphere are tightly coupled, so that errors in simulating evapotranspiration lead to errors in simulating both the water and carbon balances. Areas with seasonally frozen soils present a particular challenge due to the snowmelt-dominated hydrology and the impact of soil freezing on the soil hydraulic properties and plant root water uptake. Land surface schemes that have been applied in high latitudes often have reported problems with simulating the snowpack and runoff. Models applied at the Boreal Ecosystem Research and Monitoring Sites in central Saskatchewan have consistently over-predicted evapotranspiration as compared with flux tower estimates. We assessed the performance of two Canadian land surface schemes (CLASS and CLASS-CTEM) for simulating point-scale evapotranspiration at an instrumented jack pine sandy upland site in the southern edge of the boreal forest in Saskatchewan, Canada. Consistent with past reported results, these models over-predicted evapotranspiration, as compared with flux tower observations, but only in the spring period. Looking systematically at soil properties and vegetation characteristics, we found that the dominant control on evapotranspiration within these models was the canopy conductance. However, the problem of excessive spring ET could not be solved satisfactorily by changing the soil or vegetation parameters. The model overestimation of spring ET coincided with the overestimation of spring soil liquid water content. Improved algorithms for the infiltration of snowmelt into frozen soils and plant-water uptake during the snowmelt and soil thaw periods may be key to addressing the biases in spring ET.     

The impact of flow discharge on the hydraulic characteristics of headcut erosion processes in the gully region of the Loess Plateau

Headcut erosion is associated with major hydraulic changes induced by the gully head of concentrated flow. However, the variation in the hydraulic characteristics of the headcut erosion process is still not clear in the gully region of the Loess Plateau. A series of rainfall combined scouring experiments (flow discharges ranging from 3.6 to 7.2 m³ hr⁻¹, with 0.8 mm min⁻¹ rainfall intensity) were conducted on experimental plots to clarify the variation in the hydraulic parameters induced by gully head and erosion processes under different flow discharges. The results showed that concentrated flows in the catchment area and gully bed were turbulent (Reynolds number ranging from 1,876 to 6,693) and transformed between supercritical and subcritical (Froude number ranging from 0.96 to 3.73). The hydraulic parameters, such as the flow velocity, Reynolds number, shear stress, stream power, Darcy–Weisbach friction factor, and unit stream power in the catchment area were 0.45–0.59 m s⁻¹, 2086–6693, 1.96–5.33 Pa, 0.89–2.86 W m⁻², 0.08–0.16, and 0.023–0.031 m s⁻¹, respectively. When the concentrated flows dropped from the gully head, the hydraulic parameters in the gully bed decreased by 3.39–26.07%, 1.49–29.99%, 65.19–67.14%, 67.25–74.96%, 28.53–61.31%, and 67.82–77.14%, respectively, which contributed to the flow energy consumption at the gully head. As flow discharge increased, Reynolds number, shear stress, and stream power increased, while flow velocity, Froude number, unit stream power, and Darcy–Weisbach friction factor did not. The flow energy consumption at the gully head was 9.66–10.13, 13.25–13.74, 15.68–16.41, and 19.28–20.25 J s⁻¹, respectively, under different flow discharges and accounted for 60.58–68.50% of the flow energy consumption of the experimental plots. Generally, the sediment discharges increased rapidly at the initial stage, then increased slowly, and finally reached a steady state condition, which showed a significant declining logarithmic trend with experimental duration (P<.01) and increased with increasing flow discharge. Accordingly, the flow energy consumption was significantly correlated with the sediment yield. These findings could improve our understanding of the hydraulic properties and flow energy characteristics of headcut erosion.