Conventions on thermal expansion modelling of radio telescopes for geodetic and astrometric VLBI

Thermal expansion of radio telescopes has long been recognized as an effect which cannot be neglected in geodetic and astrometric VLBI data analysis if millimeter accuracy is desired. In this article, the author documents the conventions which are being set by the International VLBI Service for Geodesy and Astrometry (IVS) for a consistent modelling of this effect in its routine product generation. For the largest telescopes, the annual cycle of thermal expansion may change the height of the VLBI reference point by as much as 20 mm. However, for telescopes which are used in present-day IVS operations, the variations rather range from 4 to 6 mm.     

基于循环孔洞扩张模型的X型圆管节点超低周疲劳寿命预测

海洋工程钢结构在服役过程中,受风、浪、流或地震等极端循环载荷的影响,易发生超低周疲劳断裂破坏,造成人员伤亡及财产损失,因此超低周疲劳断裂分析及寿命预测对于海工结构安全性评估至关重要。然而,现阶段基于累积损伤理论提出的多种超低周疲劳寿命预测模型无法对多尺度节点实现统一预测,造成了实际工程应用的不便。因此文中基于循环孔洞扩张模型开展X型圆管节点超低周疲劳寿命预测。首先,开发了基于循环孔洞扩张模型的VUSDFLD程序,实现ABAQUS与FORTRAN子程序联合应用,利用有限元分析验证循环孔洞扩张模型在X型圆管节点超低周疲劳断裂分析中的有效性;其次,根据多组X型圆管节点超低周疲劳断裂有限元分析结果,在宏观层面提出了一种基于Manson-Coffin公式的超低周疲劳寿命预测公式;最后,依据Miner理论,将适用于等幅加载的超低周疲劳寿命公式扩展至变幅加载情况,验证了多种节点尺寸下超低周疲劳公式的适用性,为工程应用提供了理论依据。     

Cavity expansion analyses of crushable granular materials with state‐dependent dilatancy

Crushability is one of the important behaviors of granular materials particularly under high stress states, and affects both the deformability and strength of the materials that are in essence associated with state‐dependent dilatancy. In this presentation, first, a new critical state model is proposed to take into account the three different modes of compressive deformation of crushable granular materials, i.e. particle rearrangement, particle crushing and pseudo‐elastic deformation. Second, the governing equations for cavity expansion in crushable granulates are introduced, in which the state‐dependent dilatancy as well as the bounding surface plasticity model are used. Then, the procedure to obtain semi‐analytical solutions to cavity expansion in the material is described in detail, in which a commercial differential equation solver is employed. Finally, cavity expansion analyses are carried out on Toyoura sand, a well‐documented granular material, to demonstrate the effects of crushability and state‐dependent dilatancy. The study shows that particle crushing does occur at both high stress and critical states and affects the stress fields and the deformation behavior of the material surrounding the cavity in association with state‐dependent dilatancy. This leads to conclusion that particle crushing and state‐dependent dilatancy have to be taken into account when cavity expansion theory is used to interpret cone penetration tests and pressuremeter tests. Copyright © 2011 John Wiley & Sons, Ltd.     

Cavity expansion in cross‐anisotropic rock

Schistous rock can be considered—in a first approximation—as cross‐anisotropic linear elastic material. The determination of the corresponding material constants on the basis of the laboratory investigation of rock samples often fails, as the extraction of appropriate cores proves to be unfeasible (the cores disintegrate if the schistosity is pronounced). In this paper a new method is presented to determine the material constants of a linear elastic cross‐anisotropic rock on the basis of cavity expansion field tests, e.g. with a radial jack. To this purpose, an analytic approximation for the deformation of a hydrostatically loaded cylindrical cavity in cross‐anisotropic rock is derived which serves to the inverse analysis of the material parameters. Copyright © 2010 John Wiley & Sons, Ltd.     

Drained cavity expansions in soils of finite radial extent subjected to two boundary conditions

The problem of drained cavity expansion in soils of finite radial extent is investigated. Cylindrical and spherical cavities expanded from zero radius subjected to either constant stress or zero displacement at the finite boundary are considered. The generalised analytical solution procedure presented enables more advanced constitutive models to be implemented than have been possible in previous studies. Results generated for Sydney quartz sand highlight substantial differences between cavity limit pressures for the two boundary conditions and for boundaries of finite and infinite radial extent. This is of significance in accounting for boundary effects when interpreting cone penetration tests conducted in calibration chambers. Copyright © 2011 John Wiley & Sons, Ltd.     

Modelling of groundwater flow in heterogeneous porous media by finite element method

The HySuf‐FEM code (Hydrodynamic of Subsurface Flow by Finite Element Method) is proposed in this article in order to estimate the spatial variability of the transmissivity values of the Berrechid aquifer (Morocco). The calibration of the model is based on the hydraulic head, hydraulic conductivity and recharge. Three numerical tests are used to validate the model and verify its convergence. The first test case consists in using the steady analytical solution of the Poisson equation. In the second, the model has been compared with the hydrogeological system which is characterized by an unconfined monolayer (isotropic layer) and computed by using PMWIN‐MODFLOW software. The third test case is based on the comparison between the results of HySuf‐FEM and the multiple cell balance method in the aquifer system with natural boundaries case. Good agreement between the Hydrodynamic of Subsurface Flow, the numerical tests and the spatial distribution of the thickening of the hydrogeological system is deduced from the analysis and the interpretations of hydrogeological wells. Copyright © 2011 John Wiley & Sons, Ltd.     

岩石圈深层扩张与青藏高原隆升——岩石圈深层扩张模式初论

在综述有关青藏高原隆升的各种观点和模式后,依据青藏高原新生代变形和岩石圈组构特征,提出了高原隆升的新的动力学模式——岩石圈深层扩张模式。其基本观点是强调由于印度板块向欧亚板块的强烈俯冲造成岩石圈组构和变形的不均一性及岩石圈中存在的碰撞→收缩(重力不均衡)→伸展(均衡调整)的正向转变和由伸展→收缩的逆向转变的反复交替。据此将青藏高原的造山带划分为三种成因类型:喜马拉雅型、冈底斯型和昆仑型。     

校正大地电磁测深中静态效应的首枝重合法

着重论述如何用首枝重合法校正静态效应。提出用等权正交多项式拟合视电阻率曲线真实位置；指出了选择参考频率的原则；以及如何自动识别是否有静态效应的影响，产生了何种形式的静位移。分析静校正处理后的资料可知，静态效应影响几乎无所不在，但在盆地沉积中心有可能无静位移情形；静位移的形式是多样的，但主体形式是ρ＿（ＴＥ）曲线向上，ρ＿（ＴＭ）曲线向下位移，且ρ＿（ＴＥ）曲线在ρ＿（ＴＭ）曲线之上；ρ＿（ＴＥ）曲线与ρ＿（ＴＭ）曲线基本上一样容易产生静位移。     

膨胀土判别与分类的人工神经网络方法

提出了运用神经网络理论对膨胀土进行判别与分类的方法。通过对安康膨胀土的实测数据分析，建立了研究膨胀土判别与分类的计算机智能专家系统。结果表明，用人工神经网络方法对膨胀土进行判别与分类的准确率很高，它无需在判别因子与预测目标之间建立基于某种理论的经验统计关系，该方法具有重要的实用价值。     

Gully‐head erosion processes on a semi‐arid valley floor in Kenya: a case study into temporal variation and sediment budgeting

A three year monitoring programme of gully‐head retreat was established to assess the significance of sediment production in a drainage network that expanded rapidly by gully‐head erosion on the low‐angled alluvio‐lacustrine Njemps Flats in semi‐arid Baringo District, Kenya. This paper discusses the factors controlling the large observed spatial and temporal variation in gully‐head retreat rates, ranging from 0 to 15 m a^?1. The selected gullies differed in planform and in runoff‐contributing catchment area but soil material and land use were similar. The data were analysed at event and annual timescales. The results show that at annual timescale rainfall amount appears to be a good indicator of gully‐head retreat, while at storm‐event timescale rainfall distribution has to be taken into account. A model is proposed, including only rainfall (P) and the number of dry days (DD) between storms: which explains 56 per cent of the variation in retreat rate of the single‐headed gully of Lam1. A detailed sediment budget has been established for Lam1 and its runoff‐contributing area (RCA). By measuring sediment input from the RCA, the sediment output by channelized flow and linear retreat of the gully head for nine storms, it can be seen that erosion shifts between different components of the budget depending on the duration of the dry period (DD) between storms. Sediment input from the RCA was usually the largest component for the smaller storms. The erosion of the gully head occurred as a direct effect of runoff falling over the edge (GH_waterfall) and of the indirect destabilization of the adjacent walls by the waterfall erosion and by saturation (GH_mass/storage). The latter component (GH_mass/storage) was usually much larger that the former (GH_waterfall). The sediment output from the gully was strongly related to the runoff volume while the linear retreat, because of its complex behaviour, was not. Overall, the results show that the annual retreat is the optimal timescale to predict retreat patterns. More detailed knowledge about relevant processes and interactions is necessary if gully‐head erosion is to be included in event‐based soil erosion models. Copyright © 2001 John Wiley & Sons, Ltd.