Mechanism And Failure Criterion Of Seepage In Surrounding Rocks Of Water Retention Bulkheads In Coal Mines

Design and construction of water retention bulkheads are the common measures to prevent and control water inrush in coalmines with complicated hydrogeological conditions and frequent mine water disasters in our country.However,water leakage,even water inrush still occurs,because most bulkheads are designed only from the aspect of structural stability and mechanical strength according to regulations,without consideration of the permeability of surrounding rock.This threats the mine production safety.When the water pressure is high or bulkhead runs for a long time,potential fractures ws in surrounding rock will become water-conducting pathway,leading water to flow around bulkhead,causing leakage at last.The seepage in bulkhead and its surrounding rock were investigated systematically by means of theoretical analysis and calculation,scale model test and case studies.The main conclusions can be drawn as follows:⑴ On the basis of the analysis of seepage pathways in surrounding rocks,a theoretical criterion of hydraulic gradient was proposed,and the factor of safety of preventing leakage by bulkhead and its surrounding rock was determined.Through the analysis of ground water,bulkhead and its surrounding rock,three seepage pathways were identified,including the interface of the bulkhead and surrounding rock,water-conducting fault and fratures in surrounding rock.Several indicators,representing conditions of engineering geology and hydrological geology,and construction qualities,were selected to quantify and evaluate the forming possibilities of those three water-conducting pathways,and then the connectivity coefficient of waterconducting pathway was defined,which reflecting whether the water-conducting pathway could go through the surrounding rock and reach the tunnel of the reverse side of bulkhead.A formula,regarded as the theoretical criteria for estimating the critical hydraulic gradient when connective seepage occurred,was deduced on the basis of calculation of the steady hydraulic gradient,considering the connectivity coefficient of water-conducting pathway,permeability and rock-mass integrity index of surrounding rock.Feasibilities to obtain the calculating parameters were explained at the same time.According to statistical results of existing bulkheads,the relationship between the ratio,that of critical hydraulic gradient and steady hydraulic gradient,and weather the seepage would occur or not,were analyzed.Factor of safety of preventing leakage by bulkhead and its surrounding rock was determined as 0.6.If the ratio exceeds the factor of safety of preventing leakage,seepage from the bulkhead and its surrounding rock will occur.⑵ An experimental setup for seepage simulation of bulkhead and its surrounding rock was designed and developed,and similar materials for fluid-solid coupling were compounded in accordance with proper mechanical and hydrological properties.Based on similarity theory and fluid-solid coupling theory,an experimental system for seepage simulation of bulkhead and surrounding rock was designed and developed.The setup was a horizontal steel cylinder,having the same axial direction with that of the tunnel.During the laying of similar materials for surrounding rock and pre-made tabular bulkhead,the tunnel space was reserved and sensors were embedded.During test,pressurized water was supplied to simulate the blocked water on the closed side.On the other open side,seepage from bulkhead and surrounding rock was observed,leakage water was gathered and weighed.Pore water pressures were obtained by signal monitoring and data acquisition system.⑶ Seepage from different structure surrounding rock were simulated by the physical model tests,and change rules of pore water pressures,hydraulic gradient and seepage discharge were summarized.Seepage from surrounding rocks with different structures and different water pressures were simulated when a tabular bulkheas was used to resist the water pressure.By using signal monitoring and data acquisition system,the pore water pressures in surrounding rock and seepage discharges were measured,the process of seepage formation was revealed,and a large number of basic data were obtained.Test results indicate that pore water pressure in homogeneous surrounding rock changed constantly,generally with five stages,including initial value maintaining stage,rising stage,peak value lasting stage,declining stage and stable stage.In the test,heterogeneous surrounding rocks were composed by loose circle and outside surrounding rocks.The change process of the pore water pressure of in outside surrounding rocks,were similar to that of heterogeneous surrounding rocks,but the change process of loose circle was complex.Hydraulic gradients were calculated by pore water pressures.And according to the change process,five types of hydraulic gradient and their relationships and differences among them were further defined.The five types of hydraulic gradient incude initial hydraulic gradient,instantaneous hydraulic gradient,peak hydraulic gradient,critical hydraulic gradient and steady hydraulic gradient.The relationship between those hydraulic gradients and seepage in surrounding rocks was put forward clearly,which was that if the instantaneous hydraulic gradient reached the critical value,that meant the seepage went on further.If the instantaneous hydraulic gradient reached peak value,that meant the seepage had flowed though and out of the surrounding rock.Based on seepage observations in tests,the changes of seepage flow were analyzed,hydraulic conductivities of surrounding rock were also back deduced by test results,and then used to be compared with hydraulic conductivity measured in permeability test,aiming to gain the gaps and reasons.⑷ The reliability of the theoretical failure criterion of bulkheads was verified by case studies,and the factor of safety of preventing leakage by bulkhead and its surrounding rock was verified as well.On the basis of selection of the Sanhejian Coalmine,the Hanqiao Coalmine and the Hongyang Coalmine,the three typical bulkheads,and combination with underground engineering and geological data,the connectivity coefficients of water-conducting pathway were determined.The ratios of critical hydraulic gradient and steady hydraulic gradient of the three coalmines were calculated by the theoretical criterion formula,and they all exceeded the factor of safety 0.6.The more the ratios were beyond the factor of safety,the leakage were more serious,which were consistent with the engineering practices and proved the reliability of the theoretical criterion formula.In addition,the influence of water pressure on seepage flow was revealed,by analyzing and comparing variation tendencies of seepage flow in engineering and model tests.The reliability of the model test results were also verified by above results.The results of theoretical analysis and calculation,scale model test and engineering examples of water retention bulkheads indecate that the seepage of surrounding rock was an important incentive of water retention failure of bulkheads.The research achievements of seepage from water retention bulkhead and its surrounding rock in this paper are significant to optimize design and improve water retention performance of bulkhead.