Computer modelling of hydrogen defects in the clinopyroxenes diopside and jadeite

Atomistic computer simulation techniques have been employed to model mechanisms of hydrogen incorporation in the clinopyroxenes diopside and jadeite. Calculation of solution reaction energies for the pure phases indicates that hydrogen is most easily incorporated via the formation of [V_Si(OH)₄] ^x hydrogarnet type defects. When components of the two phases are mixed, then solution energies can become exothermic. The substitution of Al for Si in diopside and of Mg or Ca for Al in jadeite, provides favourable routes for hydrogen incorporation, with exothermic values of solution energy. Thus the amount of water present in these minerals in the Earth’s upper mantle will vary with composition. Simulation of IR frequencies associated with O–H stretching at specific defect clusters has also been carried out. An analysis of hydrogen–oxygen bond lengths gives good agreement, although comparison of experimental and calculated IR frequencies are problematic. This is partly due to the complexity of experimental spectra, but may also be due in part to deficiencies in the ability of the model to accurately describe the O–H stretching frequency.     

Mechanisms of OH defect incorporation in naturally occurring,hydrothermally formed diopside and jadeite

The IR spectrum of an alpine, hydrothermally formed diopside containing 17 wt ppm H₂O consists of three main OH absorption bands centred at 3647, 3464 and 3359 cm⁻¹. Jadeite from a Californian vein occurrence is characterised by bands at 3616 and 3557 cm⁻¹ and contains about 197 wt ppm H₂O. Based on the pleochroic scheme of the OH absorption bands in diopside, OH defect incorporation models are derived on the basis of fully occupied cation sites and under the assumption of M1 and M2 site vacancies; OH defects replacing O2 oxygen atoms are most common. The less pronounced OH pleochroism and the broad band absorption pattern of jadeite indicate a high degree of OH defect disordering. The pleochroic scheme of the main absorption bands at 3616 and 3557 cm⁻¹ implies partial replacement of O2 oxygen atoms by OH dipoles pointing to vacant Si sites. Under the assumption of M1 and M2 site vacancies, O1–H and O2–H defects are also derivable. OH incorporation modes assuming Si-vacancies should be considered for jadeite-rich clinopyroxenes formed in deep crust and upper mantle regions.     

Determination of unconfined compressive strength and Young's modulus of porous materials by indentation tests

The formation of a compacted zone under the indenter seems to be the major factor controlling the indentation process in porous rocks. In the case of very porous materials, where the pore structure fails and deformation (by structural collapse) proceeds with almost no increase in the applied load and with very limited damage to the surrounding material, no chipping is observed. The extent of the compacted zone is controlled by the porosity of the material and by the strength of its porous structure. This paper presents an interpretation model developed by the authors to obtain the uniaxial compressive strength of porous materials from the results of indentation tests. It is based on the model proposed by Wilson et al. (Int. J. Mech. Sci., 17, 1975, 457) for the interpretation of indentation tests on compressible foams and on an estimation by the authors of the extent of the compacted zone under the indenter. The results of indentation tests can also be used to obtain the Young's modulus of the material with a model proposed by Gill et al. (Proceedings of the 13th Canadian Symposium on Rock Mechanics, 1980, 1103). Uniaxial compression and indentation tests have been performed on artificial porous materials showing porosities varying between 44 and 68%. The uniaxial compressive strength values obtained from both types of test show a very good agreement. For the Young's modulus, the values obtained from the two types of test are different but the variation of the moduli with porosity is the same. Finally, a parameter called permanent penetration modulus is proposed as a means of characterizing the uniaxial compressive strength of porous materials.     

扩底桩的上拔试验及其颗粒流数值模拟

通过原型试验和应用颗粒流理论及其PFC2D程序数值模拟,研究了黄土中扩底桩承受上拔荷载的宏观力学性能及其细观结构,以及扩底桩在上拔荷载下的位移、极限上拔承载力和破坏机理.结果表明增加扩底桩扩大端的高度对提高承载力是有效的;破坏机理为土的减压软化和损伤软化的渐进性破坏.并提出了扩底桩极限上拔承载力计算式,计算值与实测值相吻合.应用颗粒流理论分析了颗粒细观结构、受影响区域、扩底桩的上拔位移和土中滑裂面.颗粒流模拟的土中滑裂面、荷载与位移关系和宏观试验实测结果相一致.     

Flow laws of multiphase materials and rocks from end-member flow laws

Rheological properties of polyphase rocks play an important role in the dynamics of the lithosphere and asthenosphere. However, flow laws for large portions of the polyphase rocks in the Earth's crust and mantle have not been well determined. An analysis based on the theory of mixtures has been made to calculate the general flow laws of coarse (≥5 μm), nearly equant-grained (aspect-ratio ≤3), and massive polyphase rocks and materials for which only the flow laws and volume fractions of the constituents are taken into consideration in the modeling of the bulk rheology and effects of microstructure could be ignored. The theoretical analysis is based on three assumptions: (1) the polyphase composite and the monophase aggregates of its constituents obey the same kind of flow laws (linear, power or exponential), (2) there is no change in the operative deformation mechanism of each phase when it is in the composite as compared to when it is in a monophase aggregate, and (3) neither chemical (metamorphic) reactions take place among the constituent phases nor eutectic melting occurs due to the phase mixing. The proposed iterative process allows to predict, to the first approximation, the flow laws for a large number of polyphase rocks in terms of the experimentally determined flow laws of a relatively small number of monomineralic aggregates. Applications of this approach to typical polyphase rocks such as granite, diorite, diabase, aplite and websterite as well as to synthetic two-phase materials such as forsterite–enstatite mixtures and water ice–ammonia dehydrate aggregates yield quite accurate approximations to the experimental values.     

Updating reliability of single piles and pile groups by load tests

Pile load tests are used to refine designs and for quality assurance. They can also be used to verify the reliability of piles and pile groups. Stochastic methods have previously been developed to verify the reliability of single piles. A general stochastic method to verify the reliability of pile groups is developed in this paper. The method can be used to assess the reliability of groups where pile tests have been conducted to the ultimate capacity, to below the ultimate capacity but exceeding specified capacity, and where pile tests fail to achieve the specified capacity. In the latter case, the method allows decisions to be made as to whether the reliability of the entire pile group is satisfactory or whether additional piles need to be installed.     

颗粒流强度折减法和重力增加法的边坡安全系数研究

将强度折减法和重力增加法的思路引入离散单元法,对土坡安全系数的评价作尝试性研究。运用颗粒流软件,采用强度折减法和重力增加法对边坡进行稳定分析。结果表明颗粒流计算得到的结果与有限元和条分法计算结果比较接近,为计算边坡稳定开辟了一条新的途径。采用颗粒流求解边坡的安全系数不需要条分,也即不需要对条间力做假定,同时不需要假定滑移面的位置和形状,颗粒根据所受到的接触力调整其位置,最终从抗剪强度最弱面发生剪切破坏。最后对强度折减法的不等比例折减提出了不同观点,为今后的研究提供思路。     

Slope stability analysis by means of finite element limit analysis and finite element strength reduction techniques. Part II: Back analyses of a case history

This paper deals with a back analysis of a slope failure. The case history investigated is located in an alpine environment in central Europe and is characterized by a very steep original terrain, indicating in situ soil with high strength. To study the factor of safety, two different approaches applying the so-called φ′/c′ reduction are used, namely finite element limit analysis and strength reduction finite element analysis. Comparison of a strength reduction technique with rigorous finite element limit analysis confirms that the factors of safety (FoS) obtained are very similar for associated plasticity, an intrinsic assumption of limit analysis. For non-associated plasticity, a modified version of the so-called Davis approach has been applied because it has been shown that the original formulation proposed by Davis works well when the FoS is defined in terms of loads but is not appropriate when the FoS is defined in terms of soil strength. The results show that, with the modified Davis parameter, both strength reduction finite element analyses and finite element limit analyses provide very similar factors of safety. The key advantage of limit analysis, however, is that the value of the FoS can be bracketed from above and below with upper and lower bound calculations.     

Characterisation of deformation and flow mechanics around porphyroclasts in a calcite marble ultramylonite by means of EBSD analysis

This paper studies the flow heterogeneity around porphyroclasts associated with greenschist facies deformation of a calcite marble shear zone. Microstructural data from electron backscatter diffraction analyses (EBSD) are used to constrain the flow mechanics of this dominantly non-coaxial type of deformation. The microstructure of the undisturbed ultramylonite (grain-size range 5–100 μm, mean 40 μm) is interpreted to represent steady-state (time-independent) flow conditions with flow planes parallel to the shear zone boundary. Single calcite porphyroclasts (grain-size 1–3 mm) caused flow perturbation in the fine-grained marble ultramylonite. It is the shape, in particular, of these rigid porphyroclasts that controls their rotational behaviour during deformation and, therefore, the development of specific flow fabrics. The flow planes around elongated-rhomboidal, stable porphyroclasts change the orientation to become roughly parallel to the porphyroclast margin, whereas the geometry of flow planes around nearly equant, rotating porphyroclasts describes a δ-type flow pattern. We infer that to some extent decoupling at the clast–matrix interface has occurred to guarantee a stable orientation of elongated porphyroclasts, but was not sufficient to reduce the rotation rate of equant clasts to zero. According to the flow deflection, the general crystallographic preferred orientation (CPO) with its single c-axis maximum perpendicular to the flow plane is rotated about an axis which is (sub)parallel to the kinematic rotation axis of the shear zone. Ultramylonite microstructures, CPOs and misorientation data are best explained by the dual operation of grain-size-insensitive (dislocation creep with recovery and recrystallization) and grain-size-sensitive (diffusion creep) mechanisms. The limited grain-size reduction around porphyroclasts suggests that the grain-size-insensitive mechanisms controlled rheology.