Application of borehole protection technique and equipment for screen pipe borehole in soft and weak seam

According to a shallow completed depth of the gas drainage borehole and poor borehole protection effect problems in Sijjazhuang Mine, based on the pri nciples of the air auger drilling technique and the screen pipe borehole protection technique, the equipment selection suitable to the strata, drilling tools structure and th e construction technique optimization were completed. A site test was conducted and test results showed that with the application of the set equipment to the constructi on, the average depth of the borehole was 145 m, the borehole completed depth was improved by 30% in comparison with the previous depth and the max depth of the borehole was 180 m. The screen pipe placing rate of the borehole protection trial was 83.7%, axial dragging distance of the fixed suspended device at the bottom of the borehole was 2 ~ 3 m, the effective borehole protection rate of the screen pipe was 89% after 135 days and the screen pipe borehole protection technique could protect the screen pipe placing depth and the borehole protection effect. The average rock penetration rate of the enhanced model three wings open and close type drilling bit was 51 m and the drilling bit structure was stable and could effectively improve the rock penetration depth of the borehole. In a same borehole deep drilling, the effectiv e seam borehole section (coal and rock ratio) and the gas drainage effect would have no correlation and the borehole depth and the screen pipe effective borehole prot ection rate would be the main factors affected to the gas drainage.