非线性大气化学动力学方程组数值解法的比较

大气化学模式中描述化学反应动力学的是一组高度耦合、刚性、非线性的常微分方程组,高效精确的计算方法在空气质量模拟预测、气候化学相互作用研究中尤为重要。本文介绍了求解此类非线性常微分方程组(ODEs)的七种常见的数值计算方法,包括QSSA、MQSSA、HYBRID、MHYBRID、LSODE、GongCho以及TWOSTEP方法,并基于同一反应机理进行24 h的数值模拟,通过对模拟结果的讨论分析,比较了各种方法的优劣。研究结果表明,就计算效率而言,QSSA方法最为省时。就计算精度而言,LSODE方法最优。若综合考虑计算精度和效率,HYBRID、MHY-BRID是比较适中的数值解法。     

Dynamics of cracking and swelling clay soils: Displacement of skeletal grains,optimum depth of slickensides,and rate of intra-pedonic turbation

Grumusols (Vertisols), though having a uniformly high clay content with depth, commonly exhibit a coarsening upward in the sand fraction, either of skeletal grains or of nodules. The non-uniform way in which the wetting front advances in swelling clay soils, with preferred advance and swelling around the coarse grains, produces upward directional forces and uplifting of the grains, thus producing the observed textural differentiation in the coarse fraction. From observations of slickenside distribution in the profile, and using published data on the ratio of lateral to vertical stresses and swelling pressure distribution with depth, a concept of optimum depth for slickensides is developed. Generally at 150–200cm, below the depth of cracking, this depth depends in part on climatic conditions. Below the optimal depth increased overburden pressure and smaller moisture variations restrain the extent of swelling and deformation, above it drying produces cracks and obliterates large slickensides, though a strong bimasepic microfabric remains. The extent of intrapedonic turbation was estimated from measurements of the volume of cracks and of surface material falling into the cracks. It indicates a turnover time of several hundred to a few thousand of years. This agrees with the known increase of radiocarbon ages of organic matter (MRT) with depth which in Vertisols (Grumusols) is only slightly slower than in non-turbating Udolls (Chernozems) and Udalfs (Lessivés). The moderate rate of homogenization due to intrapedonic turbation does not prevent the development of a normal organic carbon profile and it is slower than the upswelling of coarse grains by upward directional forces. The development of shear planes and slickenside structures is a rapid process and not dependent on turbation.     

Can the diffuse double layer theory describe changes in hydraulic conductivity of compacted clays?

Observations related to modifications of the hydraulic conductivity of compacted soils as a consequence of intentional compositional variation of the percolating fluid are often attributed vaguely to calculated changes in double layer thickness. The same calculated decrease or increase in double layer thickness is interpreted differently, depending on which (unconsciously held) boundary conditions authors take into account. The interpretations of different authors seem to be paradoxical only if the boundary conditions have not been mentioned. On the basis of a literature survey and own previously published test results the different boundary conditions have been grouped and divided into classes. With this division the interpretation of changes in double layer thickness will lead to unique interpretations and predictions of changes in hydraulic conductivity.     

Evaluation and Prediction of the Swelling Characteristics of Nigerian Black Clays

The swelling characteristics of black clays from two major areas of occurrence in Borno State, Nigeria, were investigated in the laboratory. Clay samples derived from lagoonal clay deposits showed high swelling tendencies while those samples derived from Olivine basalts showed medium to high swelling tendencies. Although interparticle swelling was the dominant swelling mechanism, soil samples with higher clay contents exhibited higher swelling tendencies. The developed predictive models show that the free swell percentage as well as the swelling pressure can be predicted from measured values of electrical conductivity, specific gravity, clay content and plasticity index. On the other hand, swell percent can be predicted from measured values of electrical conductivity and specific gravity only.     

柱撑粘土的晶体结构模型

根据X射线衍射、电子探针等实验数据综合分析和理论计算，讨论了柱撑基团[Al13O4(OH)24(OH2)12]^7 和[Ti20O32(OH)12(OH2)18]^4 在粘土矿物结构单元层层间的分布，铝基和钛基的笼状离子作用范围分别相当于粘土矿物的10个和6个晶胞。羧基铝柱撑蒙脱石层间孔道高度为0.9-1.2nm，直径为1.5nm；理论孔道率为84％，有效孔道率为56％。钛基柱撑蒙脱石的层间孔道高度为1.34-1.60nm，直径为1.0nm，理论也道率为75％，有效孔道率为50％。根据柱撑离子基团的两种类型，可把柱撑粘土分为Keggin型柱撑粘土和Dawson型柱撑粘土。     

Argiles et zéolites dans l'altération d'un volcan bouclier en milieu tropical (Le Piton des Neiges,La Réunion)

The drilling of a gallery through the Roche-Écrite massif (Reunion Island) allowed to sample the old formations of the ‘Piton des Neiges’ volcano. The mineralogical study of the alteration products of the formations showed significant variations of the zeolites/clay minerals ratio depending on the location of the rocks in the volcano. The abundance of zeolites in the formation near the surface hydrographic network or the underground water drainage system seems to be due to the physicochemical processes favouring zeolites in more open systems in their competition with secondary clays. To cite this article: L. Bret et al., C. R. Geoscience 335 (2003).     

A THERMO-VISCOPLASTIC CONSTITUTIVE MODEL FOR CLAYS

The effect of heat on clay behaviour is characterized by non-linearity and irreversibility. Due to the complex influence of temperature, thermomechanical factors have to be taken into account for the numerical simulation of the behaviour of such materials. A cyclic thermo-viscoplastic model is developed for this purpose. It includes thermal hardening and the evolution of yield surfaces with temperature. From the physical point of view, it is built on the basis of available experimental results for a temperature range in which no phase change occurs. Conceptually, it is the generalization of an isothermal multimechanism cyclic model. A thermoplastic formulation of the model is also derived. The results obtained from numerical simulations compare well with experiments. © 1997 by John Wiley & Sons, Ltd.     

Modeling of embankment beneath marine deposited soft sensitive clays considering straightforward creep degradation

Abstract

One method straightforwardly describing the creep degradation behavior of soft marine clay is proposed and applied to the embankment modeling. Based on the experimental phenomena, the evolution of creep coefficient of soft structured clay is identified comparing with reconstituted clay, and formulated using the creep coefficient of reconstituted clay and a creep-based structure parameter relating to the inter-particle bonding. The contributions of inter-particle bonding and debonding to creep coefficient are thus considered and the creep degradation behavior is then captured straightforwardly. The creep coefficient is extended to 3D and incorporated into a newly developed elasto-viscoplastic model to describe the creep degradation in a direct way. Based on the correlations, the liquid limit is adopted as the viscosity related input parameter. The model is derived using Newton–Raphson algorithm and implemented into a Finite Element code for coupled consolidation analysis. The general applicability on creep degradation of the model is validated by simulating 1D creep, 1D CRS (constant strain rate) and 3D undrained creep tests. Finally, the enhanced model considering creep degradation is applied and validated by simulating one test embankment and one test fill on marine deposited soft sensitive clays.     

Non‐uniqueness of critical state line in compression and extension conditions

Experimental evidence has indicated that the critical state line determined from undrained compression tests is not identical to that determined from undrained extension tests. The purpose of this paper is to investigate a modelling method that accounts for the non‐uniqueness of critical state lines in the compression and the extension testing conditions. Conventional elastic–plastic cap models can predict only a unique critical state line for the compression and the extension tests. A new micromechanical stress–strain model is developed considering explicitly the location of critical state line. The model is then used to simulate undrained triaxial compression and extension tests performed on isotropically consolidated samples with different over‐consolidated ratios. The predictions are compared with experimental results as well as that predicted by models with kinematic hardening of yield surface. All simulations demonstrate that the proposed micromechanical approach is capable of modelling the undrained compression and the undrained extension tests. Copyright © 2009 John Wiley & Sons, Ltd.     

Compressibility characteristics of soils

Compressibility characteristics of soils forms one of the important soil parameters required in design considerations. Compression index, C_c, which is the slope of the linear portion of void ratio, e vs. logaritham of effective pressure p(log p) relationship, is extensively used for settlement determination. The e–log p is most often assumed to be linear at higher pressure range and hence C_c is taken as a constant. Both published experimental results as well as results obtained in this investigation reveal that the e vs. log p could be curved, concave upwards or concave downwards depending upon the soil plasticity characteristics and initial water content. Thus, assuming C_c to be constant may not be valid for many cases. In this paper, an alternate procedure is given to characterise the compressibility of a soil. Experimental results show that the deformation expressed as a percent of thickness of the soil, (strain) vs. effective pressure could be treated as a rectangular hyperbola and the behaviour could be characterised by two parameters, a and b. Characterising the compressibility behaviour with effective pressure by two parameters is better than a single parameter, C_c.