Modeling granular soils liquefaction using coupled lattice Boltzmann method and discrete element method

In this paper, a novel coupled pore-scale model of pore-fluid interacting with discrete particles is presented for modeling liquefaction of saturated granular soil. A microscale idealization of the solid phase is achieved using the discrete element method (DEM) while the fluid phase is modeled at a pore-scale using the lattice Boltzmann method (LBM). The fluid forces applied on the particles are calculated based on the momentum exchange between the fluid and particles. The presented model is based on a first principles formulation in which pore-pressure develops due to actual changes in pore space as particles׳ rearrangement occurs during shaking. The proposed approach is used to model the response of a saturated soil deposit subjected to low and large amplitude seismic excitations. Results of conducted simulations show that at low amplitude shaking, the input motion propagates following the theory of wave propagation in elastic solids. The deposit response to the strong input motion indicates that liquefaction took place and it was due to reduction in void space during shaking that led to buildup in pore-fluid pressure. Soil liquefaction was associated with soil stiffness degradation and significant loss of interparticle contacts. Simulation results also indicate that the level of shaking-induced shear strains and associated volumetric strains play a major role in the onset of liquefaction and the rate of pore-pressure buildup.     

Influence of cyclic loading history on small strain shear modulus of saturated clays

Small strain shear modulus G_max is an essential parameter in soil dynamics, and it is usually estimated based on the Hardin and Richart equation. However, many previous researches on sands have indicated that the Hardin and Richart equation does not consider the influences of cyclic loading history on G_max. In this paper, effects of cyclic loading history on G_max of saturated clays under undrained conditions are studied using a combination device of piezoelectric-ceramic bender element system and cyclic triaxial apparatus. The dynamic pre-loading includes both relatively high amplitudes of cyclic stresses and cyclic strains. G_max without cyclic loading history is also investigated for the comparison purpose. Test results show that, at the same effective stress, both cyclic strain history and cyclic stress history will induce reduction of G_max compared to the corresponding G_max values with non-cyclic loading effects. In strain-controlled tests, the reduction of G_max is slight and relatively stable; while in stress-controlled tests, the reduction of G_max increases suddenly and remarkably when the effective stresses degrade to a certain degree. The comparison between double amplitude axial strain and residual excess pore water pressure behaviors show that the remarkable reduction of G_max can demonstrate the cyclic failure of saturated clays.     

Wave-induced dynamic response of saturated multi-layer porous media: Analytical solutions and validity regions of various formulations in non-dimensional parametric space

In this paper, dynamic response of saturated-layered porous media under harmonic waves is evaluated through a semi-analytical solution. The coupled differential equations governing the dynamics of saturated or nearly saturated porous media such as soils containing all the inertial terms of solid and fluid phases are presented for a multi-layer system. Possible simplifications of the equations which are called formulations are introduced based upon the presence of inertial terms associated with the phases. The semi-analytical solutions to the response of multiple layers for all the formulations are presented in terms of pore water pressure and stress variations considering a set of non-dimensional parameters and their respective ratios. Validity of the formulations is presented in a non-dimensional parametric space. The maximum discrepancies in the pore pressure response of the formulations leading to validity regions are illustrated for typical dynamic problems. Subsequently, the effects of layering and drainage conditions on these regions are also presented. The proposed semi-analytical solution may be served as a benchmark one for validating the coupled numerical solutions, which can be used to deal with real scientific and geo-engineering problems in the emerging field of computational geomechanics.     

A viscous-spring transmitting boundary for cylindrical wave propagation in saturated poroelastic media

Based on the u–U formulation of Biot equation and the assumption of zero permeability coefficient, a viscous-spring transmitting boundary which is frequency independent is derived to simulate the cylindrical elastic wave propagation in unbounded saturated porous media. By this viscous-spring boundary the effective stress and pore fluid pressure on the truncated boundary of the numerical model are replaced by a set of spring, dashpot and mass elements, and its simplified form is also given. A u–U formulation FEA program is compiled and the proposed transmitting boundaries are incorporated therein. Numerical examples show that the proposed viscous-spring boundary and its simplified form can provide accurate results for cylindrical elastic wave propagation problems with low or intermediate values of permeability or frequency content. For general two dimensional wave propagation problems, spuriously reflected waves can be greatly suppressed and acceptable accuracy can still be achieved by placing the simplified boundary at relatively large distance from the wave source.     

饱和砂岩滞弹性弛豫衰减机理的探索

本文采用从法国引进的Metravib热机械分析仪用正弦波加载方式,首次对四种不同孔隙度的饱和砂岩的衰减进行了实验研究,在5~400 Hz的频率,-50 ℃~100 ℃的温度范围获得衰减的热弛豫规律,由此求得它们的激活能和原子振动频率,其激活能和弛豫时间是处在原子和电子的激活能和弛豫时间之间.可见,在饱和岩石的晶粒间界缺陷处参与扩散的是原子、电子.并得出随孔隙度增大,衰减强度和激活能增大,原子的振动速率加快,弛豫时间缩短.在交变应力作用下,由多种矿物晶体胶结而成的饱和砂岩是一种多晶、多相的固体,由于内部结构复杂,损伤、缺陷广布,弛豫衰减是普遍存在的.饱和砂岩中存在的晶界、相界等许多缺陷,以及缺陷间的相互作用,比如饱和岩石中的饱和液体与岩石骨架之间的作用等等都可以产生弛豫衰减峰,弛豫过程还受晶界上原子扩散所控制.由于饱和岩石中的种种缺陷、相界等等导致上述矿物颗粒或晶界之间的多重弛豫,才使弛豫衰减峰变宽,分布宽度增大.用饱和砂岩中特有的饱和液体及砂岩内部结构的复杂性解释了饱和砂岩的衰减机理,很自然地将其宏观衰减特征与微细观结构紧紧联在一起.衰减及其机理的研究既具有科学意义,对地球物理勘探又具有实用价值.     

Dynamic response of a partially debonded pipeline embedded in a saturated poroelastic medium to harmonic plane waves

A practical model of partially debonded pipeline embedded in a saturated poroelastic medium is proposed, and the dynamic response of this model to harmonic plane waves is theoretical investigated. Biot׳s poroelastic theory is introduced to describe the dynamic equations of the saturated poroelastic medium, and the potentials obtained from Helmholtz decomposition theorem are expressed by wave function expansion method. The debonding areas around the pipeline are assumed to be filled with water. The disturbed solutions of basic field equations in these areas are expressed in terms of a scalar velocity potential. Different boundary conditions of bonded and debonded areas are adopted, and the expanded coefficients are obtained. An example of one partially debonded area is presented and analyzed. It is found that the stresses in the perfectly bonded and debonded areas show great difference, and the jump of dynamic stress at the connection points between these two areas is great in the case of low frequency. The effect of debonded areas on the dynamic stress under different thicknesses of lining is also examined.     

A new simple approach to evaluate pedogenic clay transformation in a Vertic Calcisol

The aim of this study is to characterize the pedogenic clay minerals by using simple approach: mixing mineralogical and geochemical findings.The fine clay fractions (< 0.1 μm) of a Vertic Cambisol profile were studied by means of X-ray diffraction (XRD), infrared spectroscopy (FTIR) and cation exchange capacity (CEC).Qualitative and quantitative mineralogical compositions of the clay mixture were determined.Moreover, chemical equilibria and thermodynamic stabilities of minerals (calcite, gypsum, kaolinite, smectites and illites) were studied using results of ionic activities obtained from total concentration of various aqueous species in water extracts from soil-saturated pastes.XRD analysis shows a good homogeneity in the mineralogical composition of the soil material, with depth of soil profiles. The identified clay minerals are mainly illite–smectite mixed layers (I/S) and kaolinite. The chemical analysis of saturated paste extracts with clay minerals shows a slight undersaturation of the illitic phase while smectites and also calcite and gypsum reach the thermodynamic equilibrium along the soil profile.     

冰相雨胚转化水汽密度差的实验研究

我国的降水云系多属混合云。成雨机制中固相雨胚的形成对自然降水和人工增雨都具有十分重要的意义。该文通过飞机外场催化试验指出固相雨胚的形成过程不仅转化过冷云水，还可转化冰水面饱和水汽密度差Δρ（g/m3）。为在低过冷云水含量（SLWC）（g/m3）的云中实施人工增雨提供了重要的实验依据。并将使降水形成理论日臻完善。     

饱和多孔介质近场波动分析的一种黏弹性人工边界

采用有限模型数值方法求解能量开放系统中的波动问题时,虚拟人工边界的处理方式对计算结果的准确性和精度具有重要的影响.本文针对无限域饱和多孔介质中波传播问题的人工边界处理方式进行了研究,提出了饱和多孔介质近场波动分析的一种黏弹性人工边界处理方法.在考虑多孔介质中固相和液相的相互作用的情况下,通过在人工边界处分别施加反映固相和液相介质波传播效应的弹簧及阻尼来模拟饱和多孔介质中波的能量辐射效应影响.算例表明,本文建议的黏弹性人工边界具有较好的模拟效果.     

饱和粗砂、粉土内甲烷水合物形成与分解过程中的水分迁移规律

用可探测土壤水分变化的pF-meter探头组装了新型水合物研究设备,并对两种饱和介质内甲烷水合物形成与分解过程中的水分迁移规律进行研究,以期望能为进一步探索多孔介质内甲烷水合物形成与分解机制提供参考.通过实验发现相同条件下,不同饱和介质内生成的甲烷水合物形态存在较大差异,且形成过程中的水分迁移规律也不同;水合物分解过程...