Experimental research on instability mechanism of silty mudstone roofs under action of water and its application

Abstract

Bearing capacity of silty mudstone roofs is likely to become weaker after reacting with water during excavation-and-support process, which has resulted in many roof collapse accidents in some mining areas. Based on the practical situation of one coal mine, we carried out experimental research on instability mechanism of silty mudstone roofs under action with water, including: (1) quantitative analysis on clay minerals, consisting of 15% illite, 19% montmorillonite and 12% illite/montmorillonite interstratified materials, who have high water absorption capacity and they are the internal cause of instability of silty mudstone roofs, (2) experiments of disintegration and expansion, which shows the completely disintegration time is 1.5-2h and expansion pressure is 0.87MPa and 3) experiment of complete stress-strain curves under natural condition as well as situation under action of water, which indicates that the peak strength decrease by 21.6% and 17.3% respectively when test block reacts with water in elastic and plastic stage, but residual strength decrease by 4.1%, 33.8%, 9.6% and 55.9% during elastic, plastic, strain-softening and residual stage under action of water. Overall, our study indicates that silty mudstone roofs contain high water absorption capacity minerals, which makes it disintegrate easily and expand strongly, another aspect is that its peak strength and residual strength decrease after reacting with water, which is the instability mechanism of silty mudstone roofs. In addition, we proposed a technology for controlling roofs stability, including water retention and improving support system stiffness. Field test indicates that a combined support system with high support system stiffness bolts can maintain the roof stability in this kind of coal drifts.