Volcanic particle aggregation in explosive eruption columns. Part II: Numerical experiments

The goal of this paper is to determine the parameters that control the aggregation efficiency and the growth rate of volcanic particles within the eruption column. Numerical experiments are performed with the plume model ATHAM (Active Tracer High resolution Atmospheric Model). In this study we employ the parameterizations described in a companion paper (this issue). The presence of hydrometeors promotes the aggregation of ash particles, which strongly increases their fall velocities and thus their environmental impact. The tephra mass is about two orders of magnitude greater than that of hydrometeors during typical Plinian eruptions without interaction of external water. Ice is highly dominant in comparison to liquid water (> 99% by mass). This is caused by the fast column rise (> 100 m s^− 1 on average) to very cold altitudes. Most particles occur in the form of frozen aggregates with low ice content.The collection efficiency is governed by the availability of hydrometeors acting as adhesives at the particles’ surface in our study, and wet ash particles have a higher sticking capacity than icy ones. Therefore, aggregation is fastest during the eruption within the column when limited regions of liquid water exist and when particle concentrations are very high (of the order of 10⁵ cm^− 3). Increased humidity in the background atmosphere generally leads to enhanced ice formation, but shows only a weak influence on the aggregation process. First sensitivity studies showed, however, a significant increase of the liquid water fraction when considering salinity effects. The availability of water or ice at the particles' surfaces is also governed by the surface properties, the porosity and permeability of ash, which are not well established to date. Particle growth is significantly enhanced for greater differences in the sizes and fall velocities among particles, as gravitational capture becomes more efficient. Our experiments indicate a major influence of the erupted particle size distribution. First sensitivity studies show that electrostatic forces result in a significant enhancement of aggregated particles.The present exploratory study provides new insights into the sensitivity of the ash aggregation process to a number of key parameters. Our results indicate the need of further constraining particle composition, size, porosity, permeability, and surface properties at low temperatures by in situ observations in the laboratory and in the field. In addition further research on electrostatic aggregation would be desirable.     

抗拔钢筋对提高混凝土承台-钢柱脚锚固承载力的研究

在试验研究基础上,采用ANSYS有限元分析程序,对不同抗拔钢筋配筋率的混凝土承台-钢柱脚在单向加载下的性能进行了弹塑性计算分析。结果表明,随着抗拔钢筋配筋率提高,其抗拔承载力、后期刚度、耗能能力均比未配抗拔钢筋的混凝土承台-钢柱脚逐步提高,但其提高的比例逐渐趋慢。用承载力桁架模型对各混凝土承台-钢柱脚承载力进行了计算,结果与有限元计算结果符合较好。     

钢框架结构的震害及其原因

在美国的北岭地震和日本的阪神地震中,钢框架结构发生的震害引起了各国学者注意。震害主要发生在梁柱焊接节点处。在阪神地震中还发生了钢柱的脆断和柱脚的破坏。结合各国学者的研究,对北岭和阪神地震震害进行比较,分析震害发生的原因。     

顶空型与顶实型陷落柱的成因与导水性能的差异

本文运用弹性理论分析了顶空型与顶实型陷落柱压实应力的差异。利用Griffith理论分析了陷落柱活化的判据，认为顶空型陷落柱比顶实陷落柱更易于活化导水。     

煤田陷落柱的超声模拟分析

通过超声波模拟试验, 研究了煤层陷落柱的反射波组和绕射波组特征以及绕射波组对反射波组特征的影响。结果表明, 煤田高分辨率地震勘探资料在目的层埋深200～500 m时, 经过三维偏移后, 可以将菲涅耳带半径收缩到1/5～1/3, 能够准确地解释直径大于30 m的陷落柱;对直径为15～30 m的陷落柱解释结果也较为可靠, 有效地提高了勘探精度。     

综合物探技术在探测突水隐伏陷落柱中的应用

以太行山东麓东庞矿突水陷落柱探测应用为例，介绍了三维地震和可控源音频大地电磁法工作原理、施工方法及效果。理论分析与实践结果表明，在解决煤矿采区构造和水文地质问题方面，综合物探技术以其成本低、见效快、精度高等优势得到了广泛应用，取得了良好的地质效果。     

Dynamic performance of cable-stayed bridge tower with multi-stage pendulum mass damper under wind excitations—Ⅱ: Experiment

The possibility of using a multi-stage pendulum mass damper (MSPMD) to control wind-induced vibration of a single column tower of a cable-stayed bridge during construction was studied theoretically in part I of this work. In this paper, the performance of the MSPMD for reducing bridge tower vibration is studied experimentally. A MSPMD model and a tower model of the bridge with geometry scaling of 1:100 were designed and manufactured. Calibration of the MSPMD model with different wire lengths is conducted to verify the analytical model of the damper. A series of tests for the uncontrolled freestanding tower, tower with cables, and tower with MSPMD model are then performed under harmonic and white noise excitations. The experimental results show that the responses of the tower model signif icantly decrease with the installation of the MSPMD model, which demonstrates the effectiveness of the MSPMD to mitigate the vibration of the bridge tower.     

强柱系数基于结构易损性的抗震性能评估

增大柱端抗弯承载力是抗震"能力设计"措施中引导钢筋混凝土框架结构形成梁铰型有利耗能机构的关键措施。本文以6层确定性钢筋混凝土框架结构为分析对象,通过结构易损性分析评估了不同强柱系数取值对钢筋混凝土框架结构抗震性能的影响。结构易损性分析表明增大柱端抗弯承载力是改善结构抗震性能的有效措施,增大强柱系数提高了结构的变形能力,使不同破坏极限状态之间形成较大的"梯度",对防止强烈地震作用下结构的突然倒塌提供了预示。结构易损性曲线对评估结构抗震性能、选用合适的目标强柱系数提供了量化标准。     

寻找隐伏导水陷落柱的有效方法——综合探测逐渐逼近

隐伏导水陷落柱的存在是矿井安全生产的重大隐患，是矿井防治水的重点、难点。目前单一探测手段难以准确探测隐伏导水陷落柱。根据地质、水文条件在宏观上圈出隐伏导水陷落柱范围，然后利用三维地震、电磁法、化探、放水试验、水质分析、钻探等综合勘探手段，逐步缩小异常区范围。实践证明，这种从宏观到微观，从整体到局部，有效利用各种勘探手段，逐步逼近，能够准确定位隐伏导水陷落柱。     

基岩风化带的工程地质特性与缩小防护煤柱机理研究

以大量实验数据和现场测试资料为依据，研究了隐伏煤田基岩风化带岩体的分布特点及工程地质特性，重点论述了邻近风化带开采的覆岩破坏和矿山压力显现规律以及缩小防护煤柱的机理，为类似矿井的资源回收与安全开采提供了依据。