An Experimental Investigation On Water And Sand Inrush Due To Coal Mining Under Unconsolidated Layers

This paper presents an experimental investigation on movement mechanism of quicksand from thick sand layers due to coal mining, based on theory of granular flow and water and sand movement. The research background is based on the engineering geological condition and hydrogeological condition of the Xiaojihan Coalmine, Northern Shanxi with coal mining in a large-scale and high-intensity. The basic characteristics of quicksand due to mining based on the quicksand cases before in Northern Shanxi have been summarized for the research. The indoor model tests have been conducted to study the factors that influence quicksand and mechanisms. The main research achievements including:(1)The basic features, including large volume of water, high sand ratio, spreading to the surface are summarized based on the quicksand instances. The engineering geological conditions and hydrogeological conditions of the Xiaojihan coalmine have been analyzed. The factors which influence quicksand of the coal mining are analyzed, including water and sand source, pathway and hydraulic condition. The results show that sand and water is from the Salawusu Formation. The pathways of quicksand mainly include the natural pathway in the area of thick sand, thin soil and thin bedrock, as well as fractured zone due to coal mining.(2)The process of quicksand shows that dry sand has a good fluidity, water-saturated sand also has a good liquidity, but the unsaturated sand is stable because of the pseudo-cohesion. There is pseudo-cohesion of unsaturated soil when sand containing certain water due to capillary, and therefore the sand presents stable. When water content increases from 0 to saturated, there is a point at which pseudo-cohesion becomes 0 and mixed sand and water will flow. The turning point where pseudo-cohesion is 0 can be gotten by direct shear tests. The starting moisture that pseudo-cohesion is 0 is defined in this paper. The starting moisture is defined as the water content of unsaturated sand at which the mixed sand will transfer its state from static to flowing state in self gravity. The concept of starting moisture for quicksand has been verified qualitatively by model tests.(3)The direct shear tests show that the angle of internal friction is increases first and then decreases with increasing moisture. It reaches the minimum when the water content is 10%. The pseudo-cohesion increases first and then decreases with increasing water content in a way of cubic curve. The water content decreases exponentially with the particle size when the pseudo-cohesion is the maximum. The pseudo-cohesion decreases when decreasing particle size at the same water content.(4)The characteristics of the mass flow of quicksand has investigated by 9 tests. The mass flow increases at the beginning and then decreases cyclically. The ratio between sand thickness and water height is smaller; the funnel opening is bigger and more similar to the cone. Under water table, the sand particles flow on the funnel slope with the gravity and flowing water. The pore water pressure around the fissures at the bottom of sand layer will change immediately when the fissure opens, which decreases fluctuates during the quicksand. The force pressure increases about 15%-20%. The soil arching effect is obvious on the interface of sand layer and bedrock.(5)The conceptual quicksand model has been proposed combining the characteristics of 2D profile of quicksand. A formula for calculating the pore water pressure in the interface of sand layer and bed rock during quicksand was introduced. Referring the idea of radius of influence for well flow in groundwater dynamics, the radius of influence of instantaneous pore water pressure has been proposed and the semi-empirical formula combined the hydrogeology dynamics has been fitted. When the pathway of quicksand is turning on the aquifers, the pore pressure in the sand on the bedrock will response immediately. A pore pressure funnel in space will form at the same tame. The time of hydraulic gradient induced by variation of pore water pressure is earlier than that in induced by water table changes. This variation of gradient encourages the formation of quicksand.