变化水环境中散体沙休止角的初步研究

发布时间：2018-07-08 14:46

本文选题：散体沙 + 休止角　；参考：《武汉大学》2014年博士论文

【摘要】：休止角是反映散体内在力学关系的宏观参数,可作为评价散体流动性好坏和边坡稳定性的量化指标。在变化的水环境中,干湿交替变化和水动力的作用将对散体沙的力学性质和边坡稳定性产生影响,这些影响可以通过休止角的宏观变化展现出来。变化水环境中散体沙休止角的研究对于散体介质力学、泥沙运动力学以及河道中散体沙边(岸)坡、堤坝、洲体失稳等实际问题的研究具有重要的科学理论意义和工程实践价值。本文采用理论分析和静、动态试验结合的方法,考虑干湿变化和侧向水流作用两种变化水环境,建立了散体沙堆积体边坡稳定的力学模式,研究了散体沙的颗粒特性、含水状态和水动力条件对休止角的影响规律与机理。并根据实测数据,分析了三峡蓄水后坝下游不饱和水流冲刷作用下散体沙洲滩休止角、坡角的变化及其对边坡稳定性的影响。主要结论和认识如下： (1)在干燥状态下,散体沙的内摩擦力仅包含咬合力和分子吸附作用力,复合粗糙度较大的散体沙的内摩擦力主要受咬合力影响,复合粗糙度较小的散体沙的内摩擦力主要受分子吸附作用力影响。当散体沙不饱和含水时,颗粒接触区的液桥形成弯月面,产生液桥力,引起阻碍颗粒相对运动的液桥吸附作用力。此时,内摩擦力中增加了液桥吸附作用力,其大小和对内摩擦力的影响程度随水膜厚度的增大而增大,随散体沙的复合粗糙度增大而减小。当散体沙饱和含水时,液桥吸附作用力不存在,咬合力和分子吸附作用力因为水膜的润滑作用减到最小。散体沙的内摩擦系数是水膜厚度和散体沙复合粗糙度的函数。 (2)当散体沙边坡不受外力作用时,休止角接近但不等于散体沙的内摩擦角。在干燥状态下,散体沙的休止角主要受其颗粒特性影响,与复合粗糙度有关。当散体沙含水率很小时,休止角随含水率增加缓慢,当含水率超过一定值后,休止角随含水率迅速增大至最大值并保持基本稳定不变。休止角随含水率变化的过程受颗粒特性影响,休止角增速变化的转折点含水率的大小与散体沙的复合粗糙度成正比。散体沙休止角的最大值远大于其干燥状态下的休止角,呈现类似粘性土的性质。当散体沙处于水下淹没状态时,休止角减至极小值,休止角随颗粒特性的变化规律与干燥状态下一致。本文构造了休止角的函数表达式,并结合试验实测数据进行了初步验证,计算值与试验实测值接近,能够表达休止角随含水率变化的规律。 (3)基于考虑侧向水流作用的散体沙边坡力学平衡模式,提出相应动水休止角的基本表达式和边坡泥沙起动流速公式。在侧向水流作用下,当流速小于散体沙边坡上泥沙的起动流速,边坡是稳定的,动水休止角等于静水休止角；当流速超过坡面泥沙起动流速又不达到扬动流速时,边坡经过坡角调整可以达到“静”／“动”平衡,此时动水休止角随流速增大而减小,随散体沙粒径的增大而增大；当流速超过扬动流速时,边坡发生冲刷失稳。 (4)当水流饱和时,动水休止角仅与坡面泥沙组成和流速、水深等水流条件有关；当水流过饱和或不饱和时,坡体将发生冲淤变化,直接改变坡角大小和边坡形态,引起局部水流条件的变化,从而影响休止角的大小。三峡蓄水后坝下游不饱和水流的沿程冲刷,使得河床组成粗化,存在部分二元结构岸滩散体沙砂层出露,岸滩抗冲性变差,休止角减小；枯水流量增大使得最小流速增大,散体沙休止角减小；同流量下枯水位降低和水流动力轴线摆动影响岸滩附近流速和水深的大小及横向分布,进而对散体沙休止角产生复杂影响；江心洲(滩)、边滩的冲刷使得洲缘、岸坡变陡,坡角与休止角的差距缩小,易发生滑塌。
[Abstract]:The repose angle is a macroscopic parameter that reflects the mechanical relations in the bulk , and can be used as a quantitative index to evaluate the dispersion fluidity and the stability of the slope . In the changing water environment , the alternation of dry and wet alternation and the action of hydrodynamic force will have an influence on the mechanical properties and the stability of the slope .

Based on the measured data , the effects of particle characteristics , water - bearing state and hydrodynamic conditions on the angle of repose are studied .

( 1 ) In the dry state , the internal friction force of the bulk sand only contains the biting force and the molecular adsorption force , and the internal friction force of the bulk sand with larger composite roughness is mainly influenced by the molecular adsorption force . When the bulk sand is saturated with water , the influence degree of the particle contact area is increased with the increase of the thickness of the water film , and the influence degree of the size and the internal friction force of the liquid bridge is increased with the increase of the thickness of the water film .

( 2 ) The repose angle is close to but not equal to the internal friction angle of the bulk sand when the bulk sand slope is not acted by the external force . In the dry state , the repose angle of the bulk sand is mainly influenced by the particle characteristics . When the water content of the bulk sand is more than a certain value , the repose angle decreases with the water content rapidly to the maximum value and keeps the basic stable .

( 3 ) Based on the mechanical balance model of bulk sand slope considering lateral water flow , the basic expression of corresponding moving water stop angle and the formula of slope sediment start flow velocity are proposed . Under the action of lateral water flow , when the flow velocity is smaller than the starting flow rate of sediment at the bulk sand slope , the slope is stable , the moving water repose angle is equal to the rest angle of the static water ;
When the flow velocity exceeds the start - up flow velocity of the slope surface and does not reach the raising flow velocity , the slope angle adjustment can reach the " static " / " dynamic " balance , and the angle of repose of the moving water decreases with the increase of the flow rate , and increases with the increase of the particle diameter of the bulk sand ;
When the flow velocity exceeds the flow velocity , the slope is eroded and unstable .

( 4 ) when the water flow is saturated , the angle of repose of the moving water is only related to the sediment composition and the flow rate of the slope surface , the water depth and the like ;
When the water flow is too saturated or unsaturated , the slope body will change , directly change the slope angle and the slope form , cause local water flow conditions to change , thereby affecting the size of repose angle .
the flow rate of the low water flow is increased so that the minimum flow velocity is increased , and the repose angle of the bulk sand is reduced ;
The flow velocity and the water depth in the vicinity of the beach are affected by the decrease of the low water level and the fluctuation of the water flow power axis at the same flow rate .
The erosion of the beach and the beach makes the continental margin , the bank slope becomes steep , the gap between the slope angle and the repose angle is reduced , and the collapse is easy to occur .
【学位授予单位】：武汉大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：O347.7;TV223

【参考文献】