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.     

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内蒙古苏右旗毕力赫金矿区安山岩LA-ICP-MS锆石 U-Pb年龄、元素地球化学特征及其形成的构造环境

毕力赫大型斑岩型金矿位于华北克拉通北缘早古生代增生造山带。矿区出露一套岩性为安山岩-英安岩-流纹岩的火山岩组合,安山岩属于亚碱性、高钾钙碱性系列,具有较高的MgO含量(2.05%~10.29%), 低的FeO^T/MgO比值和TiO₂含量(0.21%~0.85%);Cr (44×10^-6~473×10^-6)、Ni (19.3×10^-6~249.4×10^-6)含量高,亏损Nb、Ta、Ti、P,无或弱的Eu/Eu^*异常,具有安底斯型活动大陆边缘火山岩特征。LA-ICP-MS锆石U-Pb测年获得安山岩和安山质凝灰岩成岩年龄分别为281±4.3Ma和281±12Ma,为早二叠世,这与毕力赫金矿成矿年代以及与成矿有关的花岗闪长斑岩结晶年龄一致,表明了矿区成岩-成矿作用与晚古生代华北板块与西伯利亚板块之间俯冲-造山作用有关,指示华北克拉通北缘寻找晚古生代浅成低温-斑岩型金(铜)矿具有巨大潜力。火山岩以及与成矿相关侵入岩出现U-Pb年龄为1.8Ga、2.0Ga和2.4Ga的古老继承性锆石,暗示矿区存在华北克拉通基底,古老变质基底岩石可能是制约本区金矿成矿主要因素之一。     

基于空间信息源的多尺度地形图智能化缩编体系研究与实践

各种比例尺地形图担负着不同的社会、经济职能,多尺度地形图生产和建库,是基础地理信息系统建设的重要内容,也是信息库建设的重点和难点。以往获得较小比例尺地形图成果,一是按照尺度划分规则分别测绘,二是采用制图综合软件逐级梯度缩编,无论哪种方法都存在着效率低、时间长、更新困难、质量因人而异的缺陷。研究采用新的技术方法,基于空间信息源的多尺度地形图智能化缩编,借助空间信息数据库的整体性、适应性、拓展性、算法先进性和超强分析能力,运用空间信息拓扑关系,建立基本比例尺图形数据库,在信息数据库中智能化完成梯级比例尺缩编、跨比例尺缩编及任意比例尺缩编,自动建立相应比例尺图形数据库,同时保障多尺度图形数据库及时更新和维护,克服传统方法测制与缩编各级比例尺地形图的弊端,保障系列比例尺地形图与基本比例尺图形数据库的一致性、实时性,满足各层次、各行业用图需求,方便快速建立城市DSM地表模型和空间真三维地理模型。该方法具有成本低、速度快、精度高、通用性强、按需供图的特点和优势。湘潭市数字城市工程实践证明:基于基础信息源数据进行地形图智能化缩编的体系、架构、理论和方法是可行的,且具有广泛的社会推广应用价值。     

Weathering, mineralogical evolution and soil organic matter along a Holocene soil toposequence developed on carbonate-rich materials

A toposequence of Holocene soils located between 1100–2400 m asl in the Italian Alps served as the basis for the following analyses: the weathering of limestone and dolomite, the calculation of mass balances, understanding the formation of pedogenic Fe and Al, the determination of soil mineral and clay mineral reactions and transformation and the measurement of accumulation and stabilisation mechanisms of soil organic matter. Leaching of carbonates is most intense at the lower elevations, although calcite and dolomite have a higher solubility at low temperatures. The pCO₂ in the soil is higher at lower elevations and weathering is driven mainly by carbonic acids. At higher elevations, organic acids appear to determine the mineral transformations and weathering reactions to a greater extent. This suggests that two very different weathering regimes (carbonic and organic acid weathering) exist along the toposequence. The transformation of mica into vermiculite is the main process in both the clay and fine-earth fraction. Weathering of silicate minerals started even before the carbonates had been completely removed from the soils. The transformation mechanisms of silicate minerals in the A and O horizon at higher elevations was at least as intensive as that at the climatically warmer sites. The neoformation of pedogenetic clays at climatically cooler sites was slightly greater than that at the warmer sites. However, the formation rate of secondary Fe and Al phases was more pronounced at lower elevation, which means that this process seemed to be driven dominantly by carbonic acid (weathering of primary minerals). Soil organic matter (SOM) abundance in the mineral soil is nearly 15 kg/m² at all sites and, surprisingly, no climate-driven effect could be detected. In general, the preservation and stabilisation of SOM was due to poorly crystalline Al and Fe phases and vermiculite, regardless of some variations in the composition of the parent material (varying calcite/dolomite ratio).     

岩石试件端面摩擦效应数值模拟研究

试件端面摩擦效应直接影响试件内的塑性等效应变、侧向位移的分布和单元应力应变曲线。本文运用ANSYS中的接触单元模拟了平面应变状态下端面摩擦效应对塑性等效应变、侧向位移和单元应力应变曲线的影响,得到了不同摩擦系数时塑性等效应变及侧向位移的渐进变化形式。当接触面摩擦较小时,塑性等效应变图案为上下两个X形网络,侧向位移上下分布均匀;当接触面摩擦增大时,塑性等效应变网络向中部靠拢并且明显增大,侧向位移上下分布不均匀,中部较上下端面位移大;当试件端面侧向位移被限制,即摩擦力很大时,塑性等效应变网络变为一个X形局部化带,侧向位移分布更加不均匀,中部明显隆起。     

In-plane soil–structure interaction in layered,fluid-saturated,poroelastic half-space I: Structural response

Linear in-plane soil–structure interaction in two dimensions (2D) is studied in fluid-saturated, poroelastic, layered half-space using the Indirect Boundary Element Method (IBEM). The structure is a shear wall supported by a rigid embedded foundation. Exact stiffness matrices for the soil layer and half-space, and Green׳s functions of uniformly distributed loads and pore pressure on an inclined line are derived. Results of the system response in the frequency domain are presented for the special case of single soil layer over bedrock, semi-circular foundation and zero seepage force. The effects of water saturation, soil porosity, depth of soil layer, rigidity contrast between layer and bedrock are investigated in the frequency domain for incident plane P- and SV waves. The results suggest that water saturation may cause increase of the system frequency by more than 10%.     

紫坪铺水库造成孔隙弹性耦合变化及其对2008年汶川地震触发作用

位于川西地区龙门山断裂带附近的紫坪铺水库于2005年9月开始蓄水.约2.7年后,2008年5月12日,M_w7.9级汶川地震在龙门山断裂带上发生,两事件在时间和空间上的接近,揭示其可能相互关联,但前人的诸多研究给出了不同甚至是相反的结果.本研究基于完全耦合孔隙弹性理论,利用二维有限元模型(FEM),模拟水库蓄水造成的区域孔隙压力场和应力场的演化过程,基于库仑应力演化探讨其对龙门山断裂带活动的影响.模拟结果显示紫坪铺水库蓄水打破了原来的区域孔隙压力平衡,形成孔隙压力梯度源,向周围地壳传播;进而造成龙门山断裂带上库仑应力正值范围不断扩大,由浅入深影响到整条断层,尤其对浅层范围的加载作用明显,达上百千帕,为整个断层面的失稳提供了基础.震源区域库仑应力呈持续增长趋势,汶川地震发震前,增长了约数千帕~数十千帕,即使初期库仑应力为负,在未来某时刻库仑应力仍可能由负转正,并不断增大.通过计算汶川地震震源及其附近区域内相对于多种可能断层倾角的库仑应力,发现库仑应力随断层倾角增大而增加.因此整体来看,紫坪铺水库蓄水对龙门山断裂带起加载作用,有可能触发地震.对紫坪铺库区周围的小震分析也显示,蓄水以来小震明显增多、且不同时段小震发生密集区与水库距离逐渐增大,与孔隙压力扩散趋势一致.以上结果表明紫坪铺水库的蓄水增加了汶川地震的危险性.     

Structure,Orientation and Finite Element Analysis of the Tail Club of Mamenchisaurus hochuanensis

Abstract: The structure and orientation of the posterior extremity (tail club) of the caudal vertebrae of Mamenchisaurus hochuanensis Young and Chao, 1972 from the Upper Jurassic Shangshaximiao Formation has been analyzed to determine the tail club function using Finite Element Analysis. Of the four caudal vertebrae composing the tail club, the second largest (C"1") was probably the most proximal, and is fixed with the preceding sequence of the caudal vertebrae, whereas the smallest (C"4") is free and forms the termination of the tail club. Our analysis also suggests that the tail club is more efficient in lateral swinging rather than up-and-down motion, and that the best region for the tail club to impact is at the spine of the largest of the four caudals (C"2"), with a maximum load for impact at about 450 N. The tail club of Mamenchisaurus hochuanensis probably also had limitations as a defense weapon and was more possibly a sensory organ to improve nerve conduction velocity to enhance the capacity for sensory perception of its surroundings.     

Metasomatic alteration of chromite from parts of the late Archaean Sittampundi Layered Magmatic Complex (SLC), Tamil Nadu,India

Chromite deposits associated with layered anorthosite complexes in the Archaean high-grade terranes are rare in the world. The late Archaean Sittampundi Layered Magmatic Complex, Tamil Nadu, India is one of the few such deposits in the world where layers of Fe-Al rich chromites are associated with extremely calcic (An>95) anorthosite. ‘Frozen in’ magmatic mineralogy of the chromitite and the enclosing anorthosite suggest successive crystallization of chromite + clinopyroxene and chromite + clinopyroxene + anorthite from a hydrous Al-rich basaltic melt that was emplaced in a suprasubduction zone setting. Intense deformation and upper amphibolite facies metamorphism at ∼2.45 Ga converted the magmatic assemblages to hitherto unreported hornblende + gedrite + Mg-Al rich spinel ± chlorite bearing assemblages. During metamorphic reconstitution, chromite was pseudomorphically replaced by green spinel in the domains rich in secondary amphiboles. Mass-balance calculation and algebraic analyses of the observed mineralogy suggest that a number of chemical species including chromium became mobile during the formation of spinel pseudomorph in response to infiltration driven metamorphism. Aluminium became mobile in the length scale of chromite grain but remained immobile in the length scale of a thin section.