Research On Key Technologies And Application Of Distributed Fiber Optic Sensing For Overlying Rock Deformation During Coal Mining

During the course of deep coal mining,the overlying rock mass is easy to deform and collapse due to the change of its initial stress.On the one hand,it is a threat to the safety of mining roadway;on the other hand,it may cause land subsidence,which affects the safety of the ground buildings and the geological environment.Therefore,it is of great significance to carry out real-time monitoring of the deformation of overlying strata during coal mining.In this paper,the distributed optical fiber sensing technique is used to study the deformation and failure mechanism of overlying strata in coal seam based on field and laboratory tests.First,a special sensing cable that is suitable for monitoring the deformation of overlying strata is developed.Then a distributed optical fiber monitoring system for overlying rock deformation detection is established.On this basis,a physical model test is carried out to study the deformation of the rock mass,and the feasibility and accuracy of the proposed system are verified using numerical simulation and close range photography methods.The results show that the deformation and failure mechanism of the overlying strata can be determined using the proposed method.Finally,a distributed optical fiber monitoring system is designed and installed in a 600-m borehole in Yangliu mine,Huaibei,China.The evolution process of the separation in deep rock mass is acquired,which provides the basis for the safety assessment of mining roadway.The main work of the thesis can be summarized as follows:(1)In this paper,the deformation mechanism and evolution process of the overlying rock mass during coal mining are studied through an in-depth literature review.On this basis,the requirement of deformation monitoring of coal seam is summarized.(2)The classifications of the distributed optical fiber sensing technology are introduced,and the characteristics and applicability of different techniques,such as BOTDR,BOTDA,BOFDA and so on,are compared.The comparison results show that the BOTDR technique has the advantage of single-ended measurement,which is very suitable for field monitoring of the deformation of overlying rock mass during mining.(3)A special sensing cable that is suitable for monitoring the deformation of overlying strata is developed,and its performance and properties are tested and calibrated.The package and protection methods of the sensing cable are introduced in detail.The strain transfer efficiency of the sensing cable is also analyzed.(4)Two problems are solved:the coupling effect between the sensing optical cable and the rock or soil mass;the application of distributed optical fiber sensing system for large deformation monitoring.Through a series of laboratory tests,the coupling capability between sensing optical cable and rock or soil mass is analyzed,and a force transfer model is proposed for the cable-sand interface.The use of different fixed-point cables solves the problem of large deformation measurement using the current method.Finally,the properties of the refill material and the installation process of the cable are presented in detail.(5)A framework for overlying rock deformation monitoring is established based on the DFOS technology.The design principle of the monitoring system,and the acquisition,transmission and processing of the data are introduced.The wavelet analysis and abnormal data recognition methods are also presented.(6)A laboratory model test is carried out to study the deformation of overlying rock.The optical cables are vertical and horizontal buried into the model,which forms an optical fiber sensor-based monitoring system.Using the proposed method,the distributed deformation information is acquired,which reveals the deformation mechanism and evolution process of the overlying rock mass during coal mining.In addition,the feasibility and accuracy of the proposed system are verified using numerical simulation and close range photography methods.The numerical simulation results show that the maximum displacement of coal seam roof is about 4cm,it is highly consistent with the test results of the optical cable.We can obtain the height of water flowing fractured zone is 30.63cm by using empirical formula,it is consistent with the resule of optical fiber monitoring data,which is about 30cm.(7)A field test is carried out in the Yangliu mine,Huaibei,China.A distributed optical fiber monitoring system is designed and installed in a 600-m borehole,and the evolution process of the separation in deep rock mass is acquired.The height of caving zone and fracture zone were 12.7m and 56.7m,respectively by analyze optical fiber monitoring data,and we calculated that the height of the caving zone was 13.4+2.5m,and the height of the fracture zone was 53.6+5.6m due to empirical formula,the two results are basically consistent.Based on two years monitoring data,the overburden rock deformation and separation mechanism of Yangliu mine can be analyzed by using the key strata theory,finally,it is concluded that the two layer of hard igneous rock is the key strata,which controling the deformation of the overlying rock.A distributed monitoring-based model is proposed to study the deformation of overlying rock mass.The proposed model is applied to investigate the stability of overlying rock in Yangliu mine during mining,and some suggestions are given about early warning and treatment of the deformation of the overlying rock.