松散土体中细颗粒运移的微观过程研究
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摘要
自然界的松散土是泥石流、滑坡等灾害发生的重要物质之一,其结构的松散性和长期降雨渗流作用为细颗粒发生运移、形成内部侵蚀提供了空间和动力条件,颗粒大量流失和孔隙通道堵塞造成土体结构变化和稳定性下降,从而演变为滑坡或坍塌。渗透、水槽和人工降雨等试验方法对认识土体内部细颗粒运移的宏观特征具有重要作用,但无法直接分析孔隙通道内细颗粒分布、位移等随时间的变化特征。作者结合上海同步辐射光源3维CT技术,为获得拟静态下1维柱体渗流过程中细颗粒侵蚀形态特征,以不同粗细颗粒粒径之比为变量设计微观渗流试验,通过耦合离散元与Darcy流体方程计算分析整体和局部区域内细颗粒数量和平均动能的变化特征。结果表明:离散元与Darcy流耦合是计算土体内部细颗粒运移的有效手段。CT扫描和数值计算结果均表明土样入流口和出流口存在优先侵蚀现象。计算至2.5 s时,已分别有37.05%和31.95%细颗粒被侵蚀,其他位置侵蚀程度相对较低。在渗流方向上土体内部细颗粒存在流失补给平衡和逐渐侵蚀的现象,细颗粒的平均动能沿渗流方向总体呈逐渐增高的趋势。长期性堵塞形成过程中,细颗粒的平均动能呈现随时间逐渐降低的趋势;临时性堵塞区域内细颗粒数量的增加相对于此区域内细颗粒平均动能的增高存在滞后效应。微观尺度上土体内部细颗粒运移特征主要受流体状态和孔隙特征影响,其研究对于理解松散土坡体破坏机制具有重要价值。
Field observations show that loose soils are common deposit in gullies after an earthquake which provides ideal sources of material for debris flows and landslides. During rainfall infiltration process, pore structure provided by coarse soil skeleton provids a natural flow channel inside soil for fine particles to migrate, thus degrading the stability of soil structure and causing further slope failure. Pervious study of fine particle migration in soil mainly focused on seepage experiments, mid-scale flume test with rainfall as a boundary condition, and other macro-scale methods. However, these methods could not directly obtain the parameters such as pore structure, velocity of fine particles, and pore pressure inside soil sample to quantify the internal erosion process. In the current study, a series of one-dimensional cylinder microscopic seepage tests were designed, the coarse to fine particle size ratio was chosen as a controlling parameter, the three-dimensional computed tomography technology in Shanghai Synchrotron Radiation Facility(SSRF) was then used to quantify the characteristics of fine particle migration during seepage process.The numerical simulation using discrete element method(DEM) coupled with Darcy's flow was then used to back analysis the physical test. The results of CT and numerical calculations revealed that there existed preferential erosion in the inflow and outflow region of the soil sample. When the numerical simulation is calculated to 2.5 s, 37.05% and 31.95% of the fine particles had been eroded in above two regions, respectively, while the erosion at other locations is relatively low. The form of fine particle erosion inside the soil in the direction of seepage includs the loss balance of loss and supply and gradual erosion. The average kinetic energy of the fine particles tends to increase along the seepage path. On the one hand,the forming of long-term clogging to flow channels is reflected by the decreasing of average kinetic energy and quantity of fine particles with time; On the other hand, the temporary clogging to flow channels is reflected by the rate of increase of the number of fine particles in the measurement region larger than the rate of increase of average kinetic energy. The fine particle migration characteristics in the soil on the microscopic scale are mainly affected by the fluid state and pore characteristics, and its research is of great value for understanding the mechanism of loose soil slope failure.
Field observations show that loose soils are common deposit in gullies after an earthquake which provides ideal sources of material for debris flows and landslides. During rainfall infiltration process, pore structure provided by coarse soil skeleton provids a natural flow channel inside soil for fine particles to migrate, thus degrading the stability of soil structure and causing further slope failure. Pervious study of fine particle migration in soil mainly focused on seepage experiments, mid-scale flume test with rainfall as a boundary condition, and other macro-scale methods. However, these methods could not directly obtain the parameters such as pore structure, velocity of fine particles, and pore pressure inside soil sample to quantify the internal erosion process. In the current study, a series of one-dimensional cylinder microscopic seepage tests were designed, the coarse to fine particle size ratio was chosen as a controlling parameter, the three-dimensional computed tomography technology in Shanghai Synchrotron Radiation Facility(SSRF) was then used to quantify the characteristics of fine particle migration during seepage process.The numerical simulation using discrete element method(DEM) coupled with Darcy's flow was then used to back analysis the physical test. The results of CT and numerical calculations revealed that there existed preferential erosion in the inflow and outflow region of the soil sample. When the numerical simulation is calculated to 2.5 s, 37.05% and 31.95% of the fine particles had been eroded in above two regions, respectively, while the erosion at other locations is relatively low. The form of fine particle erosion inside the soil in the direction of seepage includs the loss balance of loss and supply and gradual erosion. The average kinetic energy of the fine particles tends to increase along the seepage path. On the one hand,the forming of long-term clogging to flow channels is reflected by the decreasing of average kinetic energy and quantity of fine particles with time; On the other hand, the temporary clogging to flow channels is reflected by the rate of increase of the number of fine particles in the measurement region larger than the rate of increase of average kinetic energy. The fine particle migration characteristics in the soil on the microscopic scale are mainly affected by the fluid state and pore characteristics, and its research is of great value for understanding the mechanism of loose soil slope failure.
引文
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[14]Zhang Kai.Research on the fluid flow and sample mixing in the microfluidic devices[D].Hangzhou:Zhejiang University,2007.[张凯.微器件中流体的流动与混合研究[D].浙江:浙江大学,2007.]
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[19]Wang Zhichao.Research on droplet impact to discrete particle based on a SPH-DEM coupling method[D].Tianjin:Tianjin University,2015.[王志超.基于SPH-DEM耦合方法的液滴冲击散粒体运动机理研究[D].天津:天津大学,2015.]
[20]Tang Y,Chan D H,Zhu D Z.A coupled discrete element model for the simulation of soil and water flow through an orifice[J].International Journal for Numerical and Analytical Methods in Geomechanics,2017,41(14):1477-1493.
[21]Cui Y F,Nouri A,Chan D,et al.A new approach to DEMsimulation of sand production[J].Journal of Petroleum Science and Engineering,2016,147:56-67.
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[25]Maeda K,Wood D M,Kondo A.Micro and macro modeling of internal erosion and scouring with fine particle dynamics[C]//Proceedings of 6th International Conference on Scour and Erosion.Paris:ISSMGE,2012:321-328.
[2]Huang Runqiu.After effect of geohazards induced by the Wenchuan earthquake[J].Journal of Engineering Geology,2011,19(2):145-151.[黄润秋.汶川地震地质灾害后效应分析[J].工程地质学报,2011,19(2):145-151.]
[3]Liu Chuanzheng.Anaysis on genetic model of Wenjiagou debris flows in Wenchuan earthquake area,Sichuan[J].Geological Review,2012,58(4):709-716.[刘传正.汶川地震区文家沟泥石流成因模式分析[J].地质论评,2012,58(4):709-716.]
[4]Wang G,Sassa K.Pore-pressure generation and movement of rainfall-induced landslides:Effects of grain size and fineparticle content[J].Engineering Geology,2003,69(1/2):109-125.
[5]Cui Peng,Guo Chaoxu,Zhou Jiawen,et al.The mechanisms behind shallow failures in slopes comprised of landslide deposits[J].Engineering Geology,2014,180:34-44.
[6]Chen Xiaoqing,Cui Peng,Feng Zili,et al.Artificial rainfall experimental study on landslide translation to debris flow[J].Chinese Journal of Rock Mechanics and Engineering,2006,25(1):106-116.[陈晓清,崔鹏,冯自立,等.滑坡转化泥石流起动的人工降雨试验研究[J].岩石力学与工程学报,2006,25(1):106-116.]
[7]Jiao Bintian,Lu Xiaobing,Wang Shuyun,et al.The movement of fine grains and its effects on the landslide and debris flow caused by raining[J].Chinese Journal of Underground Space and Engineering,2005,1(S1):36-38.[矫滨田,鲁晓兵,王淑云,等.土体降雨滑坡中细颗粒运移及效应[J].地下空间与工程学报,2005,1(S1):36-38.]
[8]Wang Zhibing,Li Kai,Wang Ren,et al.Impact of fine particle content on mode and scale of slope instability of debris flow[J].Advances in Science and Technology of Water Resources,2016(2):35-41.[王志兵,李凯,汪稔,等.细粒含量对泥石流斜坡失稳模式与规模的影响[J].水利水电科技进展,2016(2):35-41.]
[9]Cui Yifei,Zhou Xiaojun,Guo Chaoxu.Experimental studyon the moving characteristics of fine grains in wide grading unconsolidated soil under heavy rainfall[J].Journal of Mountain Science,2017,14(3):417-431.
[10]Valdes J R,Santamarina J C.Clogging:Bridge formation and vibration-based destabilization[J].Canadian Geotechnical Journal,2008,45(2):177-184.
[11]Zhou Xiaojun,Cui Peng,Jia Shitao,et al.Flume test study on the movement of fine grains based on orthogonal design[J].Journal of Sichuan University(Engineering Science edition),2012,44(Supp 1):83-88[周小军,崔鹏,贾世涛,等.基于正交设计的土体细颗粒迁移积聚水槽实验研究[J].四川大学学报(工程科学版),2012,44(增刊1):83-88.]
[12]Zhou Hu,Li Wenzhao,Zhang Zhongbin,et al.Characterization of multi-scale soil structure with X-ray computed tomography[J].Acta Pedologica Sinica,2013,50(6):1226-1230.[周虎,李文昭,张中彬,等.利用X射线CT研究多尺度土壤结构[J].土壤学报,2013,50(6):1226-1230.]
[13]Yin Zhenyu.Micromechanics-based analytical model for soils:review and development[J].Chinese Journal of Geotechnical Engineering,2013,35(6):993-1009.[尹振宇.土体微观力学解析模型:进展及发展[J].岩土工程学报,2013,35(6):993-1009.]
[14]Zhang Kai.Research on the fluid flow and sample mixing in the microfluidic devices[D].Hangzhou:Zhejiang University,2007.[张凯.微器件中流体的流动与混合研究[D].浙江:浙江大学,2007.]
[15]Cundall P A.A computer model for simulating progressive,large-scale movements in block rock systems[C]//Proceedings of International Symposium Fracture.Nancy,1971.
[16]Potyondy D O,Cundall P A.A bonded-particle model for rock[J].International Journal of Rock Mechanics and Min-ing Sciences,2004,41(8):1329-1364.
[17]Zou Yuhua,Chen Qun,Chen Xiaoqing,et al.Discrete numerical modeling of particle transport in granular filters[J].Computers and Geotechnics,2013,47:48-56.
[18]Galindo-Torres S A,Scheuermann A,Mühlhaus H B,et al.A micro-mechanical approach for the study of contact erosion[J].Acta Geotechnica,2013,10(3):357-368.
[19]Wang Zhichao.Research on droplet impact to discrete particle based on a SPH-DEM coupling method[D].Tianjin:Tianjin University,2015.[王志超.基于SPH-DEM耦合方法的液滴冲击散粒体运动机理研究[D].天津:天津大学,2015.]
[20]Tang Y,Chan D H,Zhu D Z.A coupled discrete element model for the simulation of soil and water flow through an orifice[J].International Journal for Numerical and Analytical Methods in Geomechanics,2017,41(14):1477-1493.
[21]Cui Y F,Nouri A,Chan D,et al.A new approach to DEMsimulation of sand production[J].Journal of Petroleum Science and Engineering,2016,147:56-67.
[22]Tsuji Y,Kawaguchi T,Tanaka T.Discrete particle simulation of two-dimensional fluidized bed[J].Powder Technology,1993,77(1):79-87.
[23]Wen C Y,Yu Y H.Mechanics of fluidization[J].Chemical Engineering Progress Symposium Series,1966,62:100-111.
[24]Ergun S.Fluid flow through packed columns[J].Chemical Engineering Progress,1952,48(2):6.
[25]Maeda K,Wood D M,Kondo A.Micro and macro modeling of internal erosion and scouring with fine particle dynamics[C]//Proceedings of 6th International Conference on Scour and Erosion.Paris:ISSMGE,2012:321-328.
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