A method for computing unsupported roof distance in roadway advancement and its in-situ application

A reasonable unsupported roof distance (URD) when advancing underground coal mine roadways can contribute greatly to safe and rapid roadway development. A mechanical model of the roof, using the relationship between the roof stress distribution and URD, obtained by the difference method, and roof stability according to the in-situ roof stress and rock mass strength was developed. We subsequently designed a proper range of URD, developed a testing method of URD with the function of mining protection, evaluated roof stability through analyzing the test data and then determined a reasonable URD. Considering the factors of the geological conditions, the immediate roof stability and the efficiency of the labor arrangement system, the URD of the advancing roadway of 9802 working face in Zhangshuanglou coal mine was determined to be 6 m using the proposed method. The results show that, when a 2 m length of roadway was reinforced by temporary support and high pre-stressed bolt support after the roadway advancement of 6 m per cycle, the speed and the security of the roadway development can be achieved and the advance rate can reach more than 400 m per month.