Study On Transmissivity And Safety Assessment Method Of Sealing Boreholes

As a potential water conducting channel, poor sealed boreholes(PSB) are Common hidden dangers in underground engineering. Hydrogeological Conditions and engineering geological conditions of strata, which PSB drilling through, determine the Transmissivity of PSB and the safety of underground engineering.In this paper, based on actual hydrogeological conditions and engineering geological conditions of Shandong Beizao coal mine, theoretical researchs and numerical simulations are carried out in three aspects: Influence of swelling weak rock Hygroscopic deformation on borehole structure, Influence of Mining activities on borehole structure, Discharge(drawdown) hydrograph of aquifer–vertical well system, influence of borehole structure on Discharge(drawdown) hydrograph. The main content and conclusions are as follows:(a). According to the uniaxial compressive strength of rock and expansion test results, respectively based on geological weak rock classification standard and expansive weak rock classification standard, lithologies of roof strata are classified. Considering grouting sealing quality of PSB, a conceptual borehole structure, which is a component of water channel, could be divided into five sections: diameter shrink section caused by hygroscopic deformation, overburden failure section caused by borehole structure, aperture diameter section, grouting sealing section and original structure section.(b). Considering swelling weak rock hygroscopic deformation as a weak coupling process, respectively based on the Richards equation and the humidity stress field theory, the fundamental Equations of Moisture diffusion and elastic deformation was established in cylindrical coordinates. Based on the similarity of heat conduction problems and moisture diffusion problems, Hygroscopic elastic deformation coupled with moisture diffusion in swelling weak rock was numerically simulated by using ANSYS. And the simulation results show that: with the increase of moisture content, swelling weak rock has been softening, and the humidity stress would have a peak, which would migrate to the deep surrounding rock with moisture diffusion, and the final steady shrinkage of borehole wall was 3cm.(c).The height of fractured zone without borehole structure was evaluated by empirical formulas and numerical simulations. And displacement process over time at the Interface between fractured zone and bending zone was extracted, which was used as one of the initial conditions to assessment the Influence of Mining activities on borehole structure by simulation.and the simulation results show that: fractured zone could continue to develop along borehole, which would not more than 3.42 m in length and 1.1m in radius, at the case of commonly used apertures and engineering geological conditions of study area.(d). Fundamental equations of ?aquifer & vertical well‘ system were established and a calculation program was developed based on the conceptual borehole structure. The Discharge(drawdown) hydrograph of aquifer–vertical well system was iterative calculated by this calculation program.The calculation results show that: water head at the top of borehole could quickly reduced to confined aquifer roof and maximum discharge would not more than 5.02 L/s at the case of commonly used apertures and hydrogeological conditions of study area.And the maximum discharge could used as water conducting capacity of PSB to evaluate the safety of PSB.