An Experimental Investigation On The Flow Pattern Of Quicksand Due To Mining By Using Transparent Soils

Quicksand threatens the safety of underground engineering construction and production strictly, such as mining, tunneling. A transparent soil composed of fused quartz and mineral oils which has similar properties with natural sands is adopted to investigate the patterns of sand flow during quicksand. According to the geological characteristics of quicksand due to mining, the geological conditions are integrated and simplified. An experimental set up is designed and made to obtain the speckle images inside quicksand under different conditions. Then, a digital image correlation method which is programmed by the writer is used to realize the movement inside quicksand. The process, stages and flow patterns of quicksand are analyzed according to the experimental results. The influence of particle size, fracture width and water head on the quicksand is also analyzed. Meanwhile, the total volume of quicksand is calculated under geological conditions. The main conclusions are as follows.(1)The process of quicksand is investigated by the experiment. At initial time of quicksand, only the sand near the orifice starts to flow, while the majority of sand in other region keeps static. However, quicksand develops very fast, and a columnar flow region, which is named for its similarity to a column, forms fast and a depression cone at the top of columnar pathway appears. Then, a funnel region forms. The funnel region is the movement region affected by funnel. With the development of quicksand, the height of columnar flow region reduces continuously while the funnel region expands. After columnar flow region disappears, the funnel region will still propagate for a period until it reaches its limit. Then, the sand keeps static and quicksand finishes. According to the variation of flow velocity and flow rate with time, the process of quicksand is divided into four stages; initiating stage, stable stage, unstable stage and end stage.(2)The results show that there are two kinds of flow patterns of quicksand; the columnar type flow and the funnel type flow. The movement of quicksand in a columnar flow region is vertical; the velocity field distributes similar as a series of ellipses which is tangential at the same point and enlarge continually. The value of velocity is larger along with decreasing distance to the orifice. The velocity field in the funnel type flow region shows a stratified distribution. The sand in the surface moves fast and slows down inside the and until it stops. The velocity direction is downwards the slope surface. Moreover, compared with columnar flow region, the value of the velocity in a funnel region is quite smaller. Then, a mathematical formula for describing the velocity in the columnar flow region is obtained by analysis and fitting. Meanwhile, the experiment identified two kinds of flow patterns in the surface movement during quicksand. They are called the uniformly slope declining type and the non-uniformly slope declining type, respectively.(3)The results also show that quicksand can be influenced by particle size, fracture widths and water head. With the increasing particle size, fracture width and water head, the flow velocity and flow rate increase and starting speed is accelerated. Moreover, the increasing particle size and fracture width can enlarge the width of the columnar flow region. All of the three factors have an influence on the flow pattern. Commonly the non-uniformly slope declining type happens more easily with small particle size, wide fracture and high water head.(4)The factors which affect the total volume of quicksand are analyzed and the methods of calculating total volume of quicksand in different conditions are studied. The total volume of quicksand has a relationship with the condition whether there is overburden above the sand layer. It can be found that the total volume of quicksand in the condition without overburden is larger than the volume with overburden and in the non-uniformly slope declining type. The volume of quicksand in the condition with overburden and in the uniformly slope declining type is the smallest among these three kinds conditions.