An evaluation on the degradation evolutions in three constitutive models for bonded geomaterials by DEM analyses

This paper presents a numerical evaluation on the degradation evolutions in three well-known constitutive models for bonded geomaterials using the Distinct Element Method (DEM). A series of isotropic, constant stress ratio and biaxial compression tests on the bonded geomaterials were carried out by a two-dimensional (2D) DEM code, NS2D, for this purpose. The constitutive models examined are the disturbed state constitutive model (DSC model), the superloading yield surface model and Nova’s model. First, a microscopic interpretation of the degradation variable, which is used to describe the degradation evolution in the models, was presented based on the micromechanics theory to compare the degradation evolutions used by the models with the DEM results. Then, the macromechanical responses of the numerical specimens in the DEM tests were examined in comparison with the experimental data on the artificially bonded sands. Finally, the degradation evolutions observed in the DEM tests were compared with the results predicted by the constitutive models. The study shows that the DEM results are able to capture the main features of the artificially bonded sands. The degradation evolutions used in the three constitutive models are all overally in agreement with the DEM results. In addition, two simple evolution equations for the degradation variable, which can well describe the degradation evolution observed in the DEM tests, were presented and discussed.     

On the failure of a discrete axial chain using a continualized nonlocal Continuum Damage Mechanics approach

The failure of a discrete elastic‐damage axial system is investigated using both a discrete and an equivalent continuum approach. The Discrete Damage Mechanics approach is based on a microstructured model composed of a series of periodic elastic‐damage springs (axial Discrete Damage Mechanics lattice system). Such a discrete damage system can be associated with the finite difference formulation of a Continuum Damage Mechanics evolution problem. Several analytical and numerical results are presented for the tensile failure of this axial damage chain under its own weight. The nonlocal Continuum Damage Mechanics models examined in this paper are mainly built from a continualization procedure applied to centered or uncentered finite difference schemes. The asymptotic expansion of the first‐order upward difference equations leads to a first‐order nonlocal model, whereas the asymptotic expansion of the centered finite difference equations leads to a second‐order nonlocal Eringen's approach. To complete this study, a phenomenological nonlocal gradient approach is also examined and compared with the first continualization methods. A comparison of the discrete and the continuous problems for the chains shows the effectiveness of the new micromechanics‐based nonlocal Continuum Damage modeling, especially for capturing scale effects. For both continualized approaches, the length scale of the nonlocal models depends only on the cell size, while for the so‐called phenomenological approach, the length scale may depend on the loading parameter. This apparent load‐dependent length scale, already discussed in the literature with numerical arguments, is found to be sensitive to the postulated structure of the nonlocal model calibrated according to a lattice approach. Copyright © 2015 John Wiley & Sons, Ltd.     

A one-dimensional viscoelastic and viscoplastic constitutive approach to modeling the delayed behavior of clay and organic soils

Accurate modeling of the time-dependent behavior of geomaterials is of great importance in a number of engineering structures interacting with soft, highly compressible clay layers or with organic clays and peats. In this work, a uniaxial constitutive model, based on Perzyna’s overstress theory and directly extendible to multiaxial stress conditions, is formulated and validated. The proposed constitutive approach essentially has three innovative aspects. The first concerns the implementation of two viscoplastic mechanisms within Perzyna’s theory in order to distinguish between short-term (quasi-instantaneous) and long-term plastic responses. Similarly, elastic response is simulated by combining an instantaneous and a long-term viscous deformation mechanism. The second innovative aspect concerns the use of a bespoke logarithmic law for viscous effects, which has never been used before to simulate delayed soil behavior (as far as the authors are aware). The third concerns the model’s extensive validation by simulating a number of different laboratory test results, including conventional and unconventional oedometer tests with small and large load increments/decrements and wide and narrow loading/unloading cycles, constant rates of stress and strain tests, and oedometer tests performed in a Rowe consolidation cell with measurement of pore pressure dissipation.     

预应力锚索框架地梁的互作用变形耦合分析

由于压缩变形的传播效应,预应力锚索框架地梁与边坡各接触点之间存在互作用变形的耦合效应,而现有预应力锚索框架地梁分析方法大多忽略了地梁基底接触点之间的相互影响。从可考虑变形传播效应的弹性半空间地基理论出发,建立预应力锚索框架地梁与边坡岩土体不同接触点之间的互作用变形耦合分析模型,并通过实例对比分析表明,考虑互作用变形耦合效应的分析结果是合理的。     

Discrete element modelling of creep of asphalt mixtures

Creep tests on asphalt mixtures have been undertaken under four stress levels in the laboratory while the discrete element model (DEM) has been used to simulate the laboratory tests. A modified Burger’s model has been used to represent the time-dependent behaviour of an asphalt mixture by adding time-dependent moment and torsional resistance at contacts. Parameters were chosen to give the correct stress-strain response for constant strain rate tests in Cai et al. (2013). The stress-strain response for the laboratory creep tests and the simulations were recorded. The DEM results show reasonable agreement with the experiments. The creep simulation results proved to be dependent on both bond strength variability and positions of the particles. Bond breakage was recorded during the simulations and used to investigate the micro-mechanical deformation behaviour of the asphalt mixtures. An approach based on dimensional analysis is also presented in this paper to reduce the computational time during the creep simulation, and this analysis is also a new contribution.     

Are real-world shallow landslides reproducible by physically-based models? Four test cases in the Laternser valley,Vorarlberg (Austria)

In contrast to the complex nature of slope failures, physically-based slope stability models rely on simplified representations of landslide geometry. Depending on the modelling approach, landslide geometry is reduced to a slope-parallel layer of infinite length and width (e.g., the infinite slope stability model), a concatenation of rigid bodies (e.g., Janbu’s model), or a 3D representation of the slope failure (e.g., Hovland’s model). In this paper, the applicability of four slope stability models is tested at four shallow landslide sites where information on soil material and landslide geometry is available. Soil samples were collected in the field for conducting respective laboratory tests. Landslide geometry was extracted from pre- and post-event digital terrain models derived from airborne laser scanning. Results for fully saturated conditions suggest that a more complex representation of landslide geometry leads to increasingly stable conditions as predicted by the respective models. Using the maximum landslide depth and the median slope angle of the sliding surfaces, the infinite slope stability model correctly predicts slope failures for all test sites. Applying a 2D model for the slope failures, only two test sites are predicted to fail while the two other remain stable. Based on 3D models, none of the slope failures are predicted correctly. The differing results may be explained by the stabilizing effects of cohesion in shallower parts of the landslides. These parts are better represented in models which include a more detailed landslide geometry. Hence, comparing the results of the applied models, the infinite slope stability model generally yields a lower factor of safety due to the overestimation of landslide depth and volume. This simple approach is considered feasible for computing a regional overview of slope stability. For the local scale, more detailed studies including comprehensive material sampling and testing as well as regolith depth measurements are necessary.     

八字岭分岔式隧道的三维地质力学模型试验研究

用地质力学模型试验成功地模拟一段分岔式隧道是一项系统工程,它包括确定模型试验模拟的范围和比尺,正确地选择模型试验的相似材料,设计并加工模型试验的台架和模型的制作,开挖与测试。对模型试验这一系统工程做了较为详细的介绍。模型试验结果表明,按照设定的试验工序,试验的新材料适合各类模型试验,试验台架满足试验所需要的刚度,模型易于制作和开挖、测试效果良好。     

格子Boltzmann方法地震波场模拟

格子Ｂｏｌｔｚｍａｎｎ方法是细胞自动机在某些学科中的具体化和应用。它根据微观运动过程的某些基本特征建立简化的、时间和空间完全离散的动力学模型，这种模型的平行行为符合宏观的微分方程。     

Evaluation of deformation parameter for deep excavation in sand through case histories

This study back analyzed deformation parameters of in situ sand through two excavation case histories in Kaohsiung, Taiwan. Two main features are highlighted; deformation prediction based on monitoring data at the first excavation stage and in situ Young’s modulus evaluation for sand considering monitoring data at the overall excavation stages. The former tends to establish a reliable method to predict the wall deflection at the critical stage based on the data at the first stage and the latter to enrich the limited database of Young’s modulus correlation for sand, specifically applicable for deep excavations analysis. The two constitutive models, linear elastic perfectly plastic and non-linear stress–strain constitutive models, were selected. The stiffness parameters of the models were discretely distributed along the subdivided soil body mesh to reflect the effect of overburden pressure on the in situ soil. In addition, relationship between Standard Penetration Test value (SPT-N value) and Young’s modulus and relationships for estimating the in situ Young’s modulus of the Kaohsiung sand as a function of depth were evaluated. The results greatly enhanced a framework for estimating the in situ Young’s modulus of sand.     

Improved discrete element modeling for proppant embedment into rock surfaces

Brinell indentation tests were performed on Montney siltstone, and the results were compared with discrete element indentation simulations that use the micro-parameters calibrated using compression test data from the same siltstone samples. The simulated proppant indentation into the rock surface can be 15% less than the laboratory measurements. A lower effective particle–particle modulus and thus a lower Young’s modulus are needed in discrete element models for proper simulation of indentation. An equation to find the appropriate value of Young’s modulus for indentation simulation is proposed using Brinell indentation tests including 198 laboratory tests and 32 discrete element simulations. This equation can improve the prediction of Young’s modulus and thus the particle–particle effective modulus for indentation simulations to match the measured force–indentation depth curve in the laboratory. Using the improved micro-parameters, a parametric analysis of the influence of rock Young’s modulus and proppant particle size on proppant embedment was performed. An equation to estimate Brinell hardness as a function of Young’s modulus and closure stress was derived. A practical procedure was developed to predict proppant embedment from the estimated hardness. The predictions agree with the laboratory measurements in a case study on the Montney Formation.

     

Micromechanics of undrained response of dilative granular media using a coupled DEM-LBM model: A case of biaxial test

In this paper the Discrete Element Method (DEM) is coupled with the Lattice-Boltzmann Method (LBM) to model the undrained condition of dense granular media that display significant dilation under highly confined loading. DEM-only models are commonly used to simulate the micromechanics of an undrained specimen by applying displacements at the domain boundaries so that the specimen volume remains constant. While this approach works well for uniform strain conditions found in laboratory tests, it doesn’t realistically represent non-uniform strain conditions that exist in the majority of real geotechnical problems. The LBM offers a more realistic approach to simulate the undrained condition since the fluid can locally conserve the system volume. To investigate the ability of the DEM-LBM model to effectively represent the undrained constraint while conserving volume and accurately calculating the stress path of the system, a two dimensional biaxial test is simulated using the coupled DEM-LBM model, and the results are compared with those attained from a DEM-only constant volume simulation. The compressibility of the LBM fluid was found to play an important role in the model response. The compressibility of the fluid is expressed as an apparent Skempton’s pore pressure parameter B. The biaxial test, both with and without fluid, demonstrated particle-scale instabilities associated with shear band development. The results show that the DEM-LBM model offers a promising technique for a variety of geomechanical problems that involve particle-fluid mixtures undergoing large deformation under shear loading.     

Thermodynamic properties and equation of state of fcc aluminum and bcc iron, derived from a lattice vibrational method

We use a lattice vibrational technique to derive thermophysical and thermochemical properties of the pure elements aluminum and iron in pressure–temperature space. This semi-empirical technique is based on either the Mie–Grüneisen–Debye (MGD) approach or an extension of Kieffer’s model to incorporate details of the phonon spectrum. It includes treatment of intrinsic anharmonicity, electronic effects based on the free electron gas model, and magnetic effects based on the Calphad approach. We show that Keane’s equation of state for the static lattice is better suitable to represent thermodynamic data for aluminum from 1 bar to pressures in the multi-megabar region relative to Vinet’s universal and the Birch–Murnaghan equation of state. It appears that the MGD and Mie–Grüneisen–Kieffer approach produce similar results, but that the last one better represents heat capacity below room temperature. For iron we show that the high temperature behavior of thermal expansivity can be explained within the Calphad approach by a pressure-dependent Curie temperature with a slope between –1 and 0 K/GPa.     

The elastic solid solution model for minerals at high pressures and temperatures

Non-ideality in mineral solid solutions affects their elastic and thermodynamic properties, their thermobaric stability, and the equilibrium phase relations in multiphase assemblages. At a given composition and state of order, non-ideality in minerals is typically modelled via excesses in Gibbs free energy which are either constant or linear with respect to pressure and temperature. This approach has been extremely successful when modelling near-ideal solutions. However, when the lattice parameters of the solution endmembers differ significantly, extrapolations of thermodynamic properties to high pressures using these models may result in significant errors. In this paper, I investigate the effect of parameterising solution models in terms of the Helmholtz free energy, treating volume (or lattice parameters) rather than pressure as an independent variable. This approach has been previously applied to models of order–disorder, but the implications for the thermodynamics and elasticity of solid solutions have not been fully explored. Solid solution models based on the Helmholtz free energy are intuitive at a microscopic level, as they automatically include the energetic contribution from elastic deformation of the endmember lattices. A chemical contribution must also be included in such models, which arises from atomic exchange within the solution. Derivations are provided for the thermodynamic properties of n-endmember solutions. Examples of the use of the elastic model are presented for the alkali halides, pyroxene, garnet, and bridgmanite solid solutions. Elastic theory provides insights into the microscopic origins of non-ideality in a range of solutions, and can make accurate predictions of excess enthalpies, entropies, and volumes as a function of volume and temperature. In solutions where experimental data are sparse or contradictory, the Helmholtz free energy approach can be used to assess the magnitude of excess properties and their variation as a function of pressure and temperature. The formulation is expected to be useful for geochemical and geophysical studies of the Earth and other planetary bodies.     

The Mechanical. Behaviors and Analog Analytical Method of Joints and Weak Intercalations in Rock Mass

In this paper, the mechanical behaviors of rock joints and internationals etc. are described. According to the filed results of the shear tests, several basic types of shear displacement are discussed and classified。 based on the nonlinear properties of these weak plants ,the analytical models and methods suggested in this paper, the influence of normal stress on the shear deformation may be considered .It also can be applied to the complicated conditions of loading-unloading-reloadings. The non-linear and the problems of the weak plane’s reclosing after opening can be handled simultaneously. The same analytical method is also acceptable to the shear deformation properties of various types only by choosing different parameters. Wish this analytical method, analogic calculations for the in-situ weak-plane shear tests are made and results are well checked by curves .This method has been used in some practical engineering problems .     

The Mechanical. Behaviors and Analog Analytical Method of Joints and Weak Intercalations in Rock Mass (Continued)

In this paper, the mechanical behaviors of rock joints and internationals etc. are described. According to the filed results of the shear tests, several basic types of shear displacement are discussed and classified。 based on the nonlinear properties of these weak plants ,the analytical models and methods suggested in this paper, the influence of normal stress on the shear deformation may be considered .It also can be applied to the complicated conditions of loading-unloading-reloadings. The non-linear and the problems of the weak plane’s reclosing after opening can be handled simultaneously. The same analytical method is also acceptable to the shear deformation properties of various types only by choosing different parameters. Wish this analytical method, analogic calculations for the in-situ weak-plane shear tests are made and results are well checked by curves .This method has been used in some practical engineering problems .     

岩体中节理面、软弱夹层等的力学性质和模拟分析方法(一)

岩体中的软弱结构面、特别是软弱夹层和软弱的节理面以及基岩和建筑物的结合面或称界面(以下对这些面简称为弱面)对于水工建筑物的抗滑稳定,天然或人工岩石边坡和地下洞库的稳定性常常起控制作用.     

场地类别分类方案研究

基于性能的抗震设计,要求工程师设计出具有预期抗震性能的结构,一个关键因素是地震作用的确定,这在很大程度上取决于局部场地条件。通过收集和分析北京、苏州和唐山城区956个钻孔资料,建立地表20 m和30 m深土层走时平均剪切波速VS20和VS30的关系式;现场钻探获取北京城区深105 m的典型钻孔原状土样,试验给出各类土体动剪切模量和阻尼比曲线;建立北京城区170个钻孔的场地反应计算模型,采用Nakamura提出的HVSR法和陈国兴等提出的弱震法估算场地基本周期TS值,结合国内外现行抗震规范的场地分类及一些学者对场地分类的研究成果,提出两种新的场地分类建议方案：基于等效剪切波速VSE和覆盖土层厚度H（地表至剪切波速VS ≥ 500 m/s的基岩深度）的双指标场地分类方案及基于VSE、H和TS的三指标场地分类方案。提出的场地分类方案对我国现行抗震规范场地分类方法的改进有参考价值。     

岩体中节理面、软弱夹层等的力学性质和模拟分析方法(续)

六、滑移条件的实验依据和节理单元的塑性变形问题采用关联流动理论来推导节理单元的弹塑性物性矩阵可以明显看出它将使处于继续加载状态的单元的新应力点将落在残余强度线上,也就是说是满足“滑移条件”的,但这与实验结果是否接近呢? 现在我们列举一九七○年在某矿区所作的野外剪切“单点法”试验结果以资说明.     

A Critical Comparison of Three Methods for Time-Lapse Time-Shift Calculation

Time-shift, one of the most popular time-lapse seismic attributes, has been widely used in dynamic reservoir characterization by linking it with pressure and geomechanical changes. Therefore, it is important to select appropriate calculation methods according to different time-lapse seismic data quality and time-shift magnitude. To date, there have been various published works comparing different time-shift calculation methods and discussing their advantages and disadvantages. However, most of these comparisons are based only on synthetic tests or single field applications. As the quality of time-lapse seismic data and time-shift magnitude can vary in different fields, one method may not work consistently well for each case. In this paper, a critical comparison of three different time-shift calculation techniques (Hale’s fast cross-correlation, Rickett’s non-linear inversion, and Whitcombe’s correlated leakage method) is provided. The three methods are applied to a set of synthetic data sets that are designed to account for various seismic noise and time-shift magnitudes. They are also applied to four real time-lapse seismic data sets from three North Sea fields. The calculated time-shift results are compared with the input (in synthetic tests) or the real observations from information such as seabed subsidence and compaction (in field applications). Both qualitative and quantitative comparisons are performed. At the end, each of the time-shift methods is evaluated based on different aspects, and the most appropriate method is suggested for each data scenario. All three time-shift methods are found to successfully measure time-shifts. However, Rickett’s non-linear inversion is the most outstanding method, as it gives smooth time-shifts with relatively good accuracy, and the derived time strains are more stable and interpretable.