饱和悬浮塑料砂流动特性的试验研究

Laboratory tests on flow characteristics of saturated suspended plastic sand

饱和砂土在液化状态下呈现出非牛顿流体的特性,而常规动力试验要获得稳定的液化状态比较困难。采用塑料砂和相等密度的氯化钠溶液配制成了有效应力接近于0的饱和悬浮塑料砂试样,基于流体力学低雷诺数绕球流动理论并结合粒子成像测速(PIV)技术,开发了饱和悬浮塑料砂流动特性的试验装置。利用紧贴透明模型箱内壁的半球在饱和悬浮塑料砂中运动时塑料砂颗粒的运动来分析其流动特性,并通过拉力与速度的关系计算饱和塑料悬浮砂的表观黏度。试验结果表明,饱和塑料悬浮砂试样是剪切稀化非牛顿流体,可作为液化砂土的相似材料用于液化问题的研究。基于试验中的细观规律,提出了零速度线、剪切区、剪切角、剪切区高度等实用概念。研究表明,拖球速度对剪切角有重要影响,速度越大,剪切角也越大,相应剪切区也越大。剪切区高度反映了拖球运动方向的影响范围,提出拖球试验中应保持球体形心与边界之间的距离要大于6倍球体直径。分析了拖球试验中的孔洞产生机制,提出在拖球和拖管试验中,应降低拖球的运动速度,以符合低雷诺数绕球(管)流动的基本理论。

Saturated sand in liquefied state exhibits the characteristics of a non-Newtonian fluid, while it is difficult to obtain a stable liquefied state using a conventional dynamic test. A sample mixing plastic sand and the sodium chloride solution of equal density is prepared to obtain a stable state of almost zero effective stress. According to the theory of low Reynolds number flows around a sphere, the test device for flow characteristics of saturated suspended plastic sand is developed based on the particle image velocimetry （PIV） technology. A hatf sphere on the inner wall of the transparent model box is embedded in the saturated suspended plastic sand, and it can move in a vertical direction. The movement of the particles around the half sphere which recorded by a PIV device indicates the flow characteristics of the plastic sand in a almost zero effective stress state. The apparent viscosity of the plastic sand is obtained by measuring the movement velocity and force applied on the half sphere. The test results show that the saturated suspended plastic sand is a non-Newtonian fluid, and it can be used as an equivalent material of liquefied sand. Same important concepts, including the zero-velocity bounding line, shearing zone, shearing angle and height of the shearing zone, are proposed based on the micro investigation on the pulling ball tests. The shearing angle of the shearing zone increase with the ball＇s velocity increasing. The influencing zone in the direction of dragging ball can be described in the height of the shearing zone. The minimum distance between the ball center with the bounding wall of the model box should be larger than six times of the ball diameter. The hole below the moving ball is due to the high apparent viscosity of the saturated suspended plastic sand. The moving velocity of the ball or pipe in the dragging tests should be as low as that satisfying the theory of the low Reynolds number flows.