The origin of accretionary lapilli   总被引：1，自引：0，他引：1  

J. S. Gilbert  S. J. Lane 《Bulletin of Volcanology》1994,56(5):398-411

Experimental investigations in a recirculating wind tunnel of the mechanisms of formation of accretionary lapilli have demonstrated that growth is controlled by collision of liquid-coated particles, due to differences in fall velocities, and binding as a result of surface tension forces and secondary mineral growth. The liquids present on particle surfaces in eruption plumes are acid solutions stable at 100% relative humidity, from which secondary minerals, e.g. calcium sulphate and sodium chloride, precipitate prior to impact of accretionary lapilli with the ground. Concentric grain-size zones within accretionary lapilli build up due to differences in the supply of particular particle sizes during aggregate growth. Accretionary lapilli do not evolve by scavenging of particles by liquid drops followed by evaporation — a process which, in wind tunnel experiments, generates horizontally layered hemispherical aggregates. Size analysis of particles in the wind tunnel air stream and particles adhering to growing aggregates demonstrate that the aggregation coefficient is highly grain-size dependent. Theoretical simulation of accretionary lapilli growth in eruption plumes predicts maximum sizes in the range 0.7–20 mm for ash cloud thicknesses of 0.5–10 km respectively.  相似文献   

钻孔灌注桩施工对邻近运营隧道变形影响研究     

周志良  仇欢  黄晓东  杜平  杨平  吕伟华 《岩土工程技术》2021,(1)

桩基施工将引起周边地层位移和邻近隧道结构变形,以南京龙津桥改建项目桩基工程为背景,通过现场实测,分析研究钻孔灌注桩施工全过程各工况条件对地层位移场和邻近既有隧道结构变形的影响规律。结果表明:由于钢套管的护壁作用,钻孔灌注桩施工过程中产生的最大地层位移和隧道结构变形较小,说明采用“钢套管边旋压边取土”、“群桩间跳施工”等工艺,对周围土体扰动影响程度较小,建议近隧桩基工程采用全套管灌注桩的施工方法,监测分析结果可为类似工程提供技术参考。  相似文献   

密集建筑群中的深基坑施工技术     

张维正 《探矿工程》2006,33(6):1-3,7

以沈阳市某深基坑支护及降水工程在密集建筑群中施工为例，介绍了该支护工程的设计与施工，阐述了该工程变形监测方法并对其结果进行了分析，总结了在沈阳地区密集建筑群中进行基坑开挖的设计与施工经验。  相似文献   

大变形有限元法分析边坡稳定性     

杨涛  周德培  苏金蓉 《山地学报》2006,24(1):101-104

边坡稳定性分析的重要任务是确定边坡潜在滑面的位置及相应的安全系数,鉴于坡体位移可在现场监测中直接得到,提出边坡潜在滑面确定的位移判据,采用大变形有限元首先计算出边坡的位移场,然后在开挖面的不同高度作塑性区内的水平位移参考线,依次连接参考线上的位移突变点即得到边坡失稳的潜在滑面位置,分析滑面上的应力以及岩体抗剪强度即得到边坡的整体安全系数。  相似文献   

风沙起电的风洞实验研究   总被引：3，自引：0，他引：3  

张鸿发  屈建军  言穆弘 《高原气象》2002,21(4):402-407

利用大型风沙物理风洞实验装置 ,模拟铺沙地面在不同风速条件下对不同粒径的沙样和混合沙的起电电场、电位和荷质比作实验测量。结果表明 :存在风沙起电效应。风洞内电场多为负极性 ,最大电场达到 - 2 9kV·m- 1 ,导线电位可达 12 0mV。沙粒越细 ,起电越强 ,且随风速增加而增大  相似文献   

Near Wall Flow over Urban-like Roughness   总被引：3，自引：35，他引：3  

Hong Cheng  Ian P. Castro 《Boundary-Layer Meteorology》2002,104(2):229-259

In this study, comprehensive measurements over a number of urban-type surfaces with the same area density of 25% have been performed in a wind tunnel. The experiments were conducted at a free stream velocity of 10 m s^-1 and the main instrumentation was 120 ° x-wire anemometry, but measurement accuracy was checked using laser Doppler anemometry.The results haveconfirmed the strong three-dimensionalityof the turbulent flow inthe roughness sublayer and the depths of the inertial sublayer (log-law region) and roughness sublayer for each surface have been determined. Spatial averaging has been used to remove the variability of the flow in the roughness sublayer due to individual obstacles and it is shown that the spatially averaged mean velocity in the inertial sublayer and roughness sublayer can,together, be described by a single log-law with a mean zero-plane displacement and roughness length for the surface, provided that the proper surface stress is known. The spatially averaged shear stresses in the inertial sublayer and roughness sublayer are compared with the surface stress deduced from form drag measurements on the roughness elements themselves.The dispersive stress arising from the spatial inhomogeneity in the mean flow profiles was deduced from the data and is shown to be negligible compared with the usual Reynolds stresses in the roughness sublayer. Comparisons have been made between a homogeneous (regular element array) surface and one consisting of random height elements of the same total volume. Although the upper limits of the inertial sublayer for both surfaces were almost identical at equivalent fetch, the roughness sublayer was much thicker for the random surface than for the uniform surface, the friction velocity and the roughness length were significantly larger and the `roughness efficiency' was greater. It is argued that the inertial sublayer may not exist at all in some of the more extreme rough urban areas. These results will provide fundamental information for modelling urban air quality and forecasting urban wind climates.  相似文献   

Near-Wall Flow Development After A Step Change In Surface Roughness     

H. Cheng  Ian P. Castro 《Boundary-Layer Meteorology》2002,105(3):411-432

An experimental study of the initial flow field downstream of a step change in surface roughness is presented. The roughness length of the downstream surface was approximately tenfold that of the upstream roughness and, unlike all previous studies, attention was concentrated on the roughness sublayer region beneath the inertial (log-law) region. The experiments were conducted at a boundary layer Reynolds number of about 6 × 10⁴ (based on layer thickness andfree-stream velocity) and around a longitudinal location where the (downstream) roughness length, z_o2, was about 1% of the boundary-layer thickness atthe roughness change point.The thickness of the roughness sublayer was found for the two roughness. It was observed that the vertical profiles of mean velocity and turbulence characteristics started to show similarity after about 160z₀₂ downstream of the roughness change. The presence of a shear stress overshoot is shown to depend strongly on the precise location (with respect to the roughness elements) at which the measurements are made and the thickness of the equilibrium layer is shown to be very sensitive to the way it is defined. It is demonstrated that the growing equilibrium layer has first to encompass the roughness sublayer before any thickness of inertial sublayer can be developed. It follows that, in somepractical cases, like flows across some urban environments, the latter(log-law) region may never exist at all.  相似文献   

土钉墙技术在长春市北方大厦基坑工程中的应用实践   总被引：2，自引：1，他引：2  

代国忠  杨国春 《地质与勘探》2002,38(2):88-90

结俣长春市北方大厦基坑支护工程实际，着重介绍了土钉墙支护方案的设计，施工及应用情况，为土钉墙支护技术在长春市深基开挖工程中的推广应用积累了经验。  相似文献   

隧道塌方的尖点灾变模型及应用   总被引：8，自引：0，他引：8  

刘贵应  刘新喜  魏新颜 《地质灾害与环境保护》2002,13(2):59-62

针对隧道塌方失稳问题，运用灾变理论建立隧道塌方失稳的尖点灾变模型。基于平衡曲面方程，可求得隧道围岩体内承载区介质刚度与对应的松驰区弱化介质本构曲线拐点处理刚度之比值K，并给出了塌方与否的判据，当K≤1时，隧道将发生塌方失稳；当K＞1时，隧道将不会发生塌方失稳。根据此模型，笔者解释了隧道中几种常见的灾变破坏机理，并提出了相应的治理方案。  相似文献   

深埋长隧道有害气体发生的地质条件初探   总被引：1，自引：0，他引：1  

任光明  赵志祥  聂德新  符文熹 《山地学报》2002,20(1):122-125

有害气体的深埋长隧道的主要地质灾害现象之一。本文在现场调查及查阅国内外文献资料基础上，探讨了有害气体发生的地质条件，为有害气体地质灾害的预测提供了依据。  相似文献