【作者】 王飞；

【导师】 王常明；

【作者基本信息】 吉林大学 ， 地质灾害防治工程， 2018， 博士

【摘要】 北京市北部和西部山区分布有大量的泥石流沟,受季风气候和山脉影响,在每年的7~8月北京市容易形成特大降雨,这导致北京市历史上山洪与泥石流多发,因此对北京市泥石流灾害的调查研究工作至关重要。受北京市科委课题“北京泥石流灾害预测预警技术研究”资助,本项目针对南窖沟小流域进行详细地质调查,资料收集,物理力学性质试验和泥石流模拟研究,可为北京市泥石流灾害预警研究提供依据。同时,对泥石流启动过程中侵蚀破坏过程的研究和细观机理的揭示,具有较强的学术价值和实用价值。对北京市房山区南窖沟小流域进行野外地质调查,在获得南窖沟小流域的地质地貌概况、降雨情况和灾害历史的基础上,对主支沟内物源堆积状态、岸坡结构特征以及物源堆积总量进行调查分析,并对泥石流活动特征和危险性进行评价。通过配置不同含水率的土样进行大直剪试验,获得南窖沟堆积区土样的抗剪强度指标。借助大比例泥石流模拟试验台,研究了在一次洪水过程作用下泥石流启动过程中的侵蚀-破坏机制。依托小比例模型试验台,通过9组正交试验,对影响泥石流启动的雨强、坡度和含水率进行最优组合,并深入分析泥石流启动机理。通过PFC分别对含水率为1.5%、5%和11%的土样进行应力-应变曲线拟合,以确定最优模拟参数。根据拟合结果获得的土体细观参数进行不同含水率条件下的泥石流启动试验模拟,在泥石流模拟过程中实时监测泥石流的运动速度、物源冲出量、颗粒运动轨迹以及堆积体的孔隙率变化情况,并进行深入分析,揭示泥石流启动的细观机理,并对物源启动的演化机制进行了分析和验证。论文主要得到以下结论:(1)南窖沟属于大型、沟谷型、稀性以及坡面侵蚀型泥石流沟,其致灾能力较强,已经处于衰退期,属于中度危险。沟内有较多早期形成的崩坡积物及松散堆积物,节理裂隙发育较好,为泥石流的发生提供充沛物源,对居民区的威胁较大;粒径范围为60~200mm的颗粒较多,约占总质量的47.15%,有一定的搬运能力。堆积物样本以原生矿物为主,粘土矿物主要为伊利石和高岭石。(2)南窖沟堆积物土样应力-位移曲线为应变软化型,大致可分为三个阶段:弹性变形阶段、塑性变形阶段及破坏阶段。在相同含水率下,法向应力越大,土样的屈服强度越大,抗剪强度越高,软化程度越低。在相同法向应力条件下,随着含水率的增加,土样的屈服强度、抗剪强度及残余强度减小,抗变形能力减弱,软化程度增大。(3)在降雨和一次洪水过程的共同作用下,南窖沟物源体破坏机制为降雨-径流-面蚀-坡脚掏蚀-切沟侵蚀-侵蚀终止,切沟侵蚀发生在洪峰来临之前。三种雨型的坡脚位置均达到饱和且最先发生破坏,饱和含水率在25%~30%;直接暴雨作用下,土体尚未饱和就已发生切沟侵蚀破坏。土体侵蚀破坏是随机的阵发性过程,孔隙水压力曲线也具有阵发性特征。切沟侵蚀破坏时,间歇前期降雨+暴雨、暴雨、小雨转暴雨后方洪水流量分别为1.2 m~3/h、2.2 m~3/h和1.92m~3/h,均小于泥石流的洪峰流量,切沟破坏时间分别为10min、14min和20min。(4)沟槽坡度为5°和8°时,十年雨强作用下,沟口易发生堵塞和淤积现象;百年雨强作用下,沟口不易堵塞,沟口物质易发生推移和溜滑现象。雨强较大时,堆积土体形成以表面冲蚀为主的溜滑性泥石流。表层土体含水率的变化一般表现为四个阶段:快速上升→平缓上升→快速上升→饱和。土体的前缘坡脚位置是水流的汇聚处,孔隙水压力大。坡度是切沟形成时间和物源冲出量的决定性影响因素。雨强是切沟形成时的累计雨量的决定性影响因素。含水率对物源冲出量的影响最大,对切沟形成时间和累计雨量的影响最小。(5)颗粒流程序可以准确模拟大直剪试验。1.5%含水率条件下,土体破坏模式为坡脚侵蚀、坡脚阻塞和溯源侵蚀;5%含水率条件下,土体破坏模式主要为坡脚侵蚀破坏和后缘拉张破坏;11%含水率条件下,土体破坏模式为后缘拉张破坏和土体整体溜滑。物源启动过程中,其速度表现为先增大后减小,并逐步趋向稳定。(6)建立了泥石流启动不同阶段的演化模型,得出了物源颗粒在降雨和径流条件下的受力情况和颗粒启动的临界流速条件。分别对南窖沟内直径为1m和2m的粒径进行了流速计算,得出坡脚侵蚀阶段两种颗粒发生启动的临界速度分别2.2和3.1 m/s。更多还原

【Abstract】 There are a large number of debris flow gullies in the northern and western mountainous areas of Beijing.Due to the the monsoon climate and mountain ranges,Beijing is prone to heavy rainfall in the annual 7~8 month,which leads to the frequent occurrence of mountain torrents and debris flows.Therefore,it is crucial to investigate and study the debris flow disaster in Beijing.Funded by the Beijing Municipal Science and Technology Commission,detailed geological investigation,data collection,test of physical and mechanical properties and simulation of debris flow in Nanjiao gully were carried out,which provided evidence for early warning research of debris flow in Beijing.At the same time,it is of great academic value and practical value to study the erosion process and the mesoscopic mechanism in the start-up procedure of debris flow.Geological survey for Nanjiao gully was carried out in Fangshan District of Beijing.Based on the information of geomorphology,rainfall and disaster history of Nanjiao gully,the accumulation of the source in the main and the branch gully and the structure of the rock mass were investigated and analysed.The characteristics and the risk of debris flow were evaluated.The shear strength parameters of the soil with different water content in Nanjiao gully were obtained by direct shear test.With the aid of a large scale simulation flume,the erosion and destruction mechanism of debris flow start-up procedure in a flood process was studied.Based on the small scale flume,the optimal combination of rainfall intensity,gradient and moisture content was carried out by orthogonal test,and the mechanism of debris flow start-up was analysed.The stress-strain curves of soil with 1.5%,5%and 11%water content were fitted by PFC to determine the optimal simulation parameters.Simulation of debris flow starting tests was carried out with soil micro-parameters under different moisture content according to the fitting results.In the process of simulation,velocity of debris flow,output of material source,motion trajectory of particle and porosity change of congeries were monitored in real time and analyzed in depth to reveal the microscopic mechanism of debris flow initiation.The main conclusions were as follows:(1)Nanjiao gully was the large,diluted and slope erosion debris flow gully,which had strong ability to cause disaster.But it had been in a recession and belonged to moderate risk.There were many early formed slope deposits and loose deposits in the gully and joint was developed,which provided abundant source for the occurrence of debris flow and had great threat to residential areas.The soil particle with size in the range of 60~200mm was more,accounting for about 47.15%of the total mass,which had a certain transporting capacity.The soil was dominated by primary minerals,and clay minerals were mainly illite and kaolinite.(2)The stress-displacement curves of soil in Nanjiao gully were strain softening type,which could be roughly divided into three stages:elastic deformation stage,plastic deformation stage and failure stage.With same water content,the greater the normal stress was,the greater the yield strength was,the higher the shear strength was and the lower the softening degree was.With same normal stress,as water content increasing,the yield strength,shear strength,residual strength and resistance to deformation of soil decreased,and softening degree increased.(3)With joint action of rainfall and a flood process,the failure mechanism of Nanjiao source was rainfall,runoff,surface erosion,erosion at toe of slope,gully erosion and erosion termination.Gully erosion occurred before the arrival of flood peak.The toe of slope reached saturation with water content of 25%~30%and damaged first at 3 kinds of rainfall.Gully erosion damage had occurred when the soil was not saturated with directly heavy rain.The soil erosion damage was a random paroxysmal process,and the pore water pressure curve also had paroxysmal characteristics.When the gully erosion destruction occurred,the flood flow volumes of the intermittent antecedent rainfall&rainstorm,rainstorm and light rain to rainstorm were 1.2m~3/h,2.2m~3/h and 1.92m~3/h,respectively,which were less than the peak discharge of debris flow,and it spent 10min,14min and 20min respectively.(4)When the gradient was 5 degrees and 8 degrees,the outfall was prone to clogging and siltation at ten years’raininess.The outfall was not easy to be blocked,but prone to be passing and slippery at hundred years’raininess.When the raininess was large,debris flow with surface erosion occurred mainly.Water content of surface soil changed at four stages,rapid rise→gentle rise→rapid rise→saturation.The water gathered at the toe of slope,where the pore water pressure was high.The grandient was the decisive factor of the formation time of the trench and the output of the source.The raininess was the decisive factor of the cumulative rainfall during the formation of the trench.Moisture content had the greatest impact on the amount of runoff,and had the least influence on the formation time of gully and cumulative rainfall.(5)The large direct shear test could be simulated accurately by the grain flows program.The main failure modes of soil at 1.5%moisture content were erosion and siltation at toe of slope and retrogressive erosion.The main failure modes at 5%moisture content were erosion at toe of slope and tension destruction at trailing edge.The main failure modes at 11%moisture content were tension destruction at trailing edge and whole slippery.During the starting process of debris flow,the velocity of debris flow first increased,then decreased,and tended to stability.(6)The evolution models of debris flow initiation were established.The stress condition and the critical velocity of particles under rainfall conditions were obtained.The velocity of the particle with diameter of 1m and 2m was calculated and the critical speed of the two kinds of particles with stage of erosion at toe of slope was 2.2m/s and 3.1 m/s,respectively.更多还原