Preliminary Research On Repose Angle Of Granular Sand In Changing Water Environment

Repose angle is a macroscopical parameter that reflects the internal mechanics relationship in granular accumulation, and it may serve as an evaluation index of fluidity and slope stability. In changing water environment, the mechanical property of granular sand and the slope stability may be influenced by dry-wet cycle and hydrodynamic force, and showed by the change of repose angle. For medium mechanics, mechanics of sediment transportation, and the practical problems such as buckling failure of bank slope, dyke dam or sandbar, the research on repose angle of granular sand in changing water environment not only has scientifically theoretical significance, but also has engineering practice values.Through theoretical analysis, indoor static and dynamic experiment methods, the mechanical model of the granular sand slope stability under the influence of dry-wet change and longitudinal flow was established, the repose angle’s changing rules and mechanical mechanism of granular sand of different particle characteristics, in different moisture condition, or be affected by different hydrodynamic force were studied. Based on the field data, the change of repose angle, slope angle and slope stability of the granular sandbar and bank in the downstream of the dam under the effects of scouring after the construction of The Three Gorges Project were analyzed. The specific contents and main results are as follows:When the granular accumulation is dry, the internal friction force is composed of occlusal force and molecular adsorption force, the key factor of granular sand with bigger composite roughness is occlusal force, while for the smaller ones, the key factor is molecular adsorption force. When the granular accumulation is unsaturated, concave meniscus appears at the granules contact zone, caused cohesive force between granules and viscous resistance which will hinder the relative motion between the granules. Since adding viscous resistance, the coefficient of internal friction grows, the viscous resistance and its effect grows with water film thickness and decreases with composite roughness. When the granular accumulation is under water, the viscous resistance does not exist, the occlusal force and molecular adsorption force minimize because of fluid lubrication. The internal friction coefficient is a function that depends on the water film thickness and the composite roughness of granular sand.When the granular accumulation is free of outside interference, the repose angle is close but not equal to the internal friction angle. In the case of dry sand, and the composite roughness is relatively big, the repose angle grows with the composite roughness. For the granular sand with relatively small composite roughness, the repose angle grows with molecular adsorption. When the moisture content of granular accumulation is low, with the increasing of moisture content, the repose angle increases at first, reaches the maximum and then keeps basically stable. In the growing process, there are two turning points, the corresponding moisture content grows with the composite roughness of granular sand. The maximum of repose angle is considerably larger than that in dry condition, seems like cohesive sand. When the granular accumulation is under water, the repose angle minimizes, its changing rules with particle characteristics are the same as in dry condition. A formula which contains the effect of moisture content on repose angle is preliminary established and tested combining the experimental data. The rule that repose angle’s changing with moisture content can be expressed by the formula.Base on the mechanical equilibrium of granular sand slope affected by longitudinal flow, the expression of the corresponding repose angle and the incipient velocity formula of granular sand on slope were established. Affected by longitudinal flow, when the flow velocity is smaller than the incipient velocity, the slope is stable, the corresponding repose angle is equal to that in still water. When the flow velocity is bigger than the incipient velocity, but smaller than the stirring-up velocity, the slope will change the angle to reach a static or dynamic balance. In this case, the repose angle decreases with flow velocity, increases with grain size. When the flow velocity is bigger than the stirring-up velocity, the slope will be scoured and hard to keep stable.When the flow is saturated, the repose angle is only related to the sand composition and the flow condition such as flow velocity and depth of water. When the flow is supersaturated or unsaturated, scouring-silting change will take place, the slope form and angle will change, lead to the change of flow condition, and influence the repose angle. After the construction of The Three Gorges Project, the flow in the downstream of the dam is unsaturated, leads to erosion development in long-distance and long periods. Because of these, the bed material’s grain size increase, on some bank slope or sandbar, the emergence of the granular sand bed will lead to the decrease of anti-scourability and repose angle. The minimum flow increases, so the minimum velocity increases, the repose angle reduces. The water level at the same quantity of flow lowering and the position of power axis of flow changing will influence the value and transverse distribution of flow velocity and water depth above the slope, caused complex influence on repose angle. The edge of sandbar is being scoured, the slope becoming steeper, as the gap between the slope angle and repose angle minishing, the slope stability reducing, bank collapse will happen.