Solution for the preliminary weakening of hard roof for progressive caving in Panel M6 CD-2 level -190/-130 at Mong Duong coal company

Abstract

Panel M6 CD-2 level -190/-130 at Mong Duong coal company is one of the panels where caving sections did not occur as designed, leaving large mined-out voids. The mine pressure generated by hanging roof greatly concentrates on the face area, causing local face spall and roof fall and hindering support movement. To minimise these negative impacts on the panel, the Company has designed a solution to drilling holes of less than 2.5 m length behind the support tail to conduct forced roof caving by means of blasting. The solution helps to reduce the stress concentration on the face and to decrease the risk of unsafety. However, despite the large drilling work volume, roof caving efficiencywas not as high as expected. The large periodic caving span of 14.57 m creates a large void that poses a potential risk of sudden roof collapse or eventually “windblast” which is extremely serious. This paper is based on the roof collapse law to calculate the roof rock's natural caving span. The load from the hanging roof is proposed to transfer to lower strata by a new method of cutting roof along the designed planes. This leads to the redistribution of mine pressure and the release of elastic energy stored in roof strata. The solution using pressurized water is implemented to weaken the rock, change the mechanical properties of rock, and relocate the forced caving points. The paper’s result has established water-pressurized drill holes at intersections between the rib and roof of transportation and ventilation roadways with an interval of 12÷15 m. An associated water pressure of 931.09 kG/cm2is found to induce a regular caving span of 1.6 m as designed