Generation of a realistic 3D sand assembly using X‐ray micro‐computed tomography and spherical harmonic‐based principal component analysis

Three‐dimensional particle morphology is a significant problem in the discrete element modeling of granular sand. The major technical challenge is generating a realistic 3D sand assembly that is composed of a large number of random‐shaped particles containing essential morphological features of natural sands. Based on X‐ray micro‐computed tomography data collected from a series of image processing techniques, we used the spherical harmonics (SH) analysis to represent and reconstruct the multi‐scale features of real 3D particle morphologies. The SH analysis was extended to some highly complex particles with sharp corners and surface cavities. We then proposed a statistical approach for the generation of realistic particle assembly of a given type of sand based on the principle component analysis (PCA). The PCA aims to identify the major pattern of the coefficient matrix, which is made up of the SH coefficients of all the particles involved in the analysis. This approach takes into account the particle size effect on the variation of particle morphology, which is observed from the available results of micro‐computed tomography and QICPIC analyses of sand particle morphology. Using the aforementioned approach, two virtual sand samples were generated, whose statistics of morphological parameters were compared with those measured from real sand particles. The comparison shows that the proposed approach is capable of generating a realistic sand assembly that retains the major morphological features of the mother sand. Copyright © 2016 John Wiley & Sons, Ltd.     

A three‐phase thermo‐hydro‐mechanical finite element model for freezing soils

Artificial ground freezing (AGF) is a commonly used technique in geotechnical engineering for ground improvement such as ground water control and temporary excavation support during tunnel construction in soft soils. The main potential problem connected with this technique is that it may produce heave and settlement at the ground surface, which may cause damage to the surface infrastructure. Additionally, the freezing process and the energy needed to obtain a stable frozen ground may be significantly influenced by seepage flow. Evidently, safe design and execution of AGF require a reliable prediction of the coupled thermo‐hydro‐mechanical behavior of freezing soils. With the theory of poromechanics, a three‐phase finite element soil model is proposed, considering solid particles, liquid water, and crystal ice as separate phases and mixture temperature, liquid pressure, and solid displacement as the primary field variables. In addition to the volume expansion of water transforming into ice, the contribution of the micro‐cryo‐suction mechanism to the frost heave phenomenon is described in the model using the theory of premelting dynamics. Through fundamental physical laws and corresponding state relations, the model captures various couplings among the phase transition, the liquid transport within the pore space, and the accompanying mechanical deformation. The verification and validation of the model are accomplished by means of selected analyses. An application example is related to AGF during tunnel excavation, investigating the influence of seepage flow on the freezing process and the time required to establish a closed supporting frozen arch. Copyright © 2013 John Wiley & Sons, Ltd.     

Continuous versus event‐based sampling: how many samples are required for deriving general hydrological understanding on Ecuador's páramo region?

As a consequence of the remote location of the Andean páramo, knowledge on their hydrologic functioning is limited; notwithstanding, these alpine tundra ecosystems act as water towers for a large fraction of the society. Given the harsh environmental conditions in this region, year‐round monitoring is cumbersome, and it would be beneficial if the monitoring needed for the understanding of the rainfall–runoff response could be limited in time. To identify the hydrological response and the effect of temporal monitoring, a nested (n = 7) hydrological monitoring network was set up in the Zhurucay catchment (7.53 km²), south Ecuador. The research questions were as follows: (1) Can event sampling provide similar information in comparison with continuous monitoring, and (2) if so, how many events are needed to achieve a similar degree of information? A subset of 34 rainfall–runoff events was compared with monthly values derived from a continuous monitoring scheme from December 2010 to November 2013. Land cover and physiographic characteristics were correlated with 11 hydrological indices. Results show that despite some distinct differences between event and continuous sampling, both data sets reveal similar information; more in particular, the monitoring of a single event in the rainy season provides the same information as continuous monitoring, while during the dry season, ten events ought to be monitored. Copyright © 2016 John Wiley & Sons, Ltd.     

方解石811N:一种可能的微区碳氧锶同位素分析标准

同位素标准物质是同位素分析的基准物质和数据比较的重要依据。方解石811作为碳酸盐碳氧同位素的实验室标准物质,具有良好的适用性,但由于制备时间久远,存量日益减少,且性状信息缺乏,影响了该标准在微区微量分析中的运用。本次研究选用未制备的811方解石(811N)作为研究对象,结合多种分析技术从不同尺度对其矿物组成、元素含量分布、碳氧同位素组成进行分析,揭示其相关信息及规律,并探讨了其Sr同位素组成,从而为下一步811N的制备和使用提供参考信息,有助于该样在不同碳酸盐分析和研究,特别是微区微量元素和同位素中的运用。实验结果表明,方解石811N较为纯净,主要为方解石,只含有微量的白云石、针铁矿、黏土矿物;除主要化学成分(CaCO₃)外,含有微量的Mg、Mn、Sr、Si、REE、U、Th等元素,Mg、Mn、Sr含量在整体分布上存在一定的变化,但在局部相对均匀。碳氧同位素组成整体差异程度相对较小,且其差异程度和元素均匀性呈对应关系,主要区域碳氧同位素组成均匀。Sr同位素组成在不同区域中无明显差异。本次研究认为811N是合适的碳酸盐碳氧同位素分析标准物质,该样制备后能够满足碳氧同位素分析和研究的需要,同时该样具备适用于微区微量C、O、Sr同位素分析和研究的潜力。     

干湿-冻融循环条件下膨胀土的压缩及渗透特性变化规律

北疆供水一期工程穿越膨胀土区域,历经多次干湿-冻融循环后力学特性衰减严重,易产生渠坡局部浅层滑坡和冻胀破坏等现象。为深入研究其劣化机制,通过干湿-冻融循环条件下的压缩试验、渗透试验和SEM微观扫描试验,从宏-细-微观多角度分析其压缩和渗透指标的变化规律。研究结果表明：随干湿-冻融循环次数的增加,膨胀土整体压缩性增大,其压缩曲线可分为拟弹性段与拟塑性段;随循环次数的增加,回弹指数呈波动趋势,压缩指数与循环次数呈指数正相关,与细-微观裂隙呈线性正相关。黏土颗粒在循环作用下组成“团聚体-孔隙-填充颗粒”形式的较松散的临时结构,絮凝结构增加,各向异性减少;土样承受竖向压力时,膨胀土孔隙间距减少,压缩性较大;压力超过固结屈服应力时,团聚体颗粒扁角化、极角频率增加、孔隙压密,压缩性逐渐稳定。渗透系数在循环过程中变化分为缓慢、迅速、稳定3个阶段;渗透系数在第5次循环变化较大,第7次循环后逐渐稳定,与循环次数及表面裂隙率呈正相关趋势。渗透系数与各项微观参数的灰色关联度均大于0.65,微观孔隙率是最主要的影响因素;循环作用下微观孔隙发育明显,形成新渗流通道,渗透系数与微观孔隙率呈线性正相关。     

基于微雨雷达的六盘山区地形云降水宏微观特征观测分析

利用2017年6—11月宁夏六盘山区收集的微雨雷达和微波辐射计等探测资料,对比分析了六盘山区山脊和山谷对流云降水、层状云和浅积云降水过程中的云微物理特征及亮带,针对典型层状云降水事件山脊和山谷站的亮带及以下各层的雨滴谱分布特征,探索了亮带以上几层水凝物的分类。结果发现:六盘山区三类降水云山脊的反射率及反射率衰减程度均高于山谷,表明地形强迫使得山脊降水云的物理和动力过程较山谷更剧烈,层状云降水过程中山脊0℃等温线以上的反射率明显高于山谷,表明山脊在0℃等温线以上有更多水凝物。分析一次典型层状云降水过程发现:六盘山区降水液滴自亮带下落的过程中,碰并过程占主导;亮带以下各层DSD符合Gamma分布,山脊站Gamma分布的相关性比山谷站强且拟合优度更优;降水开始的前3 min,推测-4～0℃等温线之间的水凝物主要是霰,此后水凝物主要是雪颗粒和霰,而山谷在降水开始的前5 min,-4～0℃等温线层之间的水凝物主要是雪颗粒和较大的霰,此后水凝物主要是霰。     

Tephrochronological dating of varved interglacial lake deposits from Piànico‐Sèllere (Southern Alps,Italy) to around 400 ka

The sediment record from the Piànico palaeolake in the southern Alps is continuously varved, spans more than 15 500 years, and represents a key archive for interglacial climate variability at seasonal resolution. The stratigraphic position of the Piànico Interglacial has been controversial in the past. The identification of two volcanic ash layers and their microscopic analysis provides distinct marker layers for tephrochronological dating of these interglacial deposits. In addition to micro‐facies analyses reconstructing depositional processes of both tephra layers within the lake environment, their mineralogical and geochemical composition has been determined through major‐element electron probe micro‐analysis on glass shards. Comparison with published tephra data traced the volcanic source regions of the Piànico tephras to the Campanian volcanic complex of Roccamonfina (Italy) and probably the Puy de Sancy volcano in the French Massif Central. Available dating of near‐vent deposits from the Roccamonfina volcano provides a robust tephrochronological anchor point at around 400 ka for the Piànico Interglacial. These deposits correlate with marine oxygen isotope stage (MIS) 11 and thus are younger than Early to Middle Pleistocene previously suggested by K/Ar dating and older than the last interglacial as inferred from macrofloral remains and the geological setting. Copyright © 2006 John Wiley & Sons, Ltd.     

建立隧道掘进机产业促进我国地下空间开发

论述了国际地下工程建设动态，指出在地下工程建设中，优先采用全断面隧道掘进机（TBM）进行快速开挖已成料总的发展趋势；对我国开发隧道掘进机的历史和现状进行了综述，提出了存在的问题有利条件，对在我国建立隧道掘进机产业提出了意见和建议。     

边界层辐合线发展成飑线的数值试验

利用高分辨率η坐标模式，以一个实际飑线个例的常规资料为初值，通过嵌入一定强度的边界中层尺度辐合线，进行了边界层辐合线型飑线激发的数值试验，结果表明，飑线实例的大尺度环境场提供了累积的有效浮力能，小尺度积云对流则通过对流翻腾提供高层水汽凝结潜热加热和低层降温冷却，而中尺度幅合线通过提供带状辐合上升运动，起着胚胎和组织积云对流的作用，这样大、中、小三种尺度相互作用导致了边界层中尺度辐合线发展成为飑线。