Blasting technology of gateway retaining along goaf pressure release by roof cutting in hard and weak complex roof

In order to have a good pre-fracturing effect of the bilateral accumulative tensile blasting in the hard and weak complex roof obtained in the gateway reta ined along the goaf with the roof cutting and pressure released, with the site test and analysis, the rational bilateral accumulative tensile blasting technology was applied to the effective pre-fracturing blasting under the conditions of the complex roof with the low top coal strength above the gateway, soft and weak immediate roof and hard basic roof. The results showed that in the sot and weak rock stratum, the blasting energy would cause a wedging role along the in-situ cracks of the borehole wall and the energy accumulation effect was poor. In the blasting borehole with a depth of 12 m, the cracking length was 1.2 m only,a blasting funnel would be easily occurred att he orifice of the borehole and would cause a failure to the previous support. As for the hard rock, with the charge line density increased, the cracking rate along the accu mulative direction would be steadily increased. When an explosive charge of a single borehole was even cartridges, the ratio of the cartridge length and the air columnl ength was 3.348. When the filling location was at the hard rock, in medium sandstone section with a Protodyakonov coefficient of 12, all tensile cracking would be occured along the pre-fracturing direction within the borehole, the cracking length would be 5 m and cracking rate would be 100%. Therefore, as the complex roof, the hard roc k would cause effective cracks along the pre-cracking direction and the soft and weak rock would automatically collapse and would have the satisfactory pre-cracking ef fect.