Experimental Research On Overburden Deformation Characterized By Fiber Frequency Shift Variation Degree

The deformation and damage of overburden strata caused by mining activities is one of the basic reasons of mine disasters.Mining strata movement and formation rock structure is a multi-scale deformation which has large range of motion and other significant characteristics of rock mass mechanics.It is an obvious structural and characteristics of ‘black box' phenomenon which often cannot be directly observed.Distributed optical fiber sensing technology can detect the deformation of overlying strata with high precision,wide range and internal measurement.However,the optical fiber monitoring study on the macroscopic deformation of sedimentary rocks still has the problems such as the structure and system of the fiber optic sensor,the co-deformation of the fiber and the rock mass,and the identification of the deformation characteristics of the rock strata.The current researches on monitoring microscopical deformations of sedimentary rocks focuses on representations of optical fiber test parameters for deformation laws and evolution characteristics of mining overburden rocks.It is of great significance to set up the system of representations of optical fiber test parameters for deformation laws of mining overburden rocks,which can effectively reveal the microscopical deformation rule of strata movements and promote the development of innovation technologies in the field of ground pressure and strata control.The thesis focuses on the representations of the optical frequency shifts caused by deformation of rock strata through the methods of theoretical research,similar material model test and numerical simulation.(a)The concept of the optical fiber frequency shift variation degrees we proposed represent deformations of overburden strata in different optical fiber sensing sections.In the thesis,the mathematical expression of the fiber frequency shift variation degree is given.The influencing factors and applicable scopes of variation degree are analyzed.The more the number of sampling points,the smaller the sampling interval and the smaller the distances between optical fiber sensors,the more accuracy of the representation for the deformation of overburden strata.The corresponding relation between the optical fiber frequency shift and the evolution characteristics of overlying strata is derived.The mechanics model of the optical fiber representations for broken overburden structure breaking is established.The optical fiber representations for different forms of key stratum(voussoiring beam or clamped-clamped beam)is proposed.When the key stratum is in the clamped-clamped beam state,the distribution curves of frequency shift is a single-peak form.When the key layer is in the voussoiring beam state,the distribution curves is a double-peak form.When the key stratum is broken resulting in large deformation,the distribution curves is a saddle form.(b)The experimental study on similarity material model of the variation degree the mining overburden evolution structure was carried out.The experimental results show that when the peak of distribution curve appears with advancing working face,the variation degree is greater than 20 MHz.In this time,roof pressurization is also happened.By comparing the measured results of the abutment pressures of the working face with variation degrees,it verifies the correctness of the optical fiber representations for roof pressurizations.To study the law of roof pressurizations in the similarity material model,we propose the concept of spatial reliability of roof pressurizations and evaluate the reliabilities of representations for roof pressurizations in each stage of rock deformation.The discriminant index of the roof pressurization we proposed is 2mm that accords with the result of variation degree.(c)We establish a representational relationship between variation degrees and evolution characteristics of mining overburden rocks to reveal the dynamic response relationship between mining overburden structure and optical fiber location.Meanwhile,variation degrees also can represent the three zones of overburden strata in the vertical direction.With advancing working face,distribution curves exhibit ‘Step' characteristics.When the working face passes through and away from the vertical optical fiber,curves appear a one-step form in the lower part of the overburden strata.As the working face continues to move forward,curves appear a two-step form.It shows that the more obvious the steps are,the movement of the rock layer is more intense.The representation model of optical fiber frequency variation degree for key stratum breaking in the mining condition was established to represent the first and periodic breaking of key stratum.The correctness of the relationship is verified by comparing the mechanical representations of the overlying rock structures.(d)Through the optical fiber monitoring physical model test and numerical simulation test on the structure and evolution law of the large thick breccia with deep ultra-thick conglomerate,the relation between the variation degree of the frequency shift of the pressure optical fiber in the deep mega breccia working face is established.The initial pressure step distance of the deep ultra-thick conglomerate working face is 640 mm,and the average pressure step distance is 132 mm.The average characteristic distance of the frequency shift of the working face pressure fiber frequency shift is 860 mm,and the average pressure times are 7.5 times.The optical fiber characterization model of the caving angle is established.With the excavation of the working face,the left side collapse angle of the overlying strata is 52-53 degrees,and the right-side caving angle is 53-61 degrees.The compression space reliability of overlying strata in the lower conglomerate,ultra-thick conglomerate above is 78.6%,92.9% and 57.1% respectively,which can be used to characterize the working surface pressure by the frequency shift variation of overlying breccia or below the conglomerate.The initial break distance of the key layer is 1160 mm and the average break distance is 240 mm.By comparing with the key layer breaking level fiber characterization,the correctness of the broken characterization result of the key layer of the frequency shift degree of the fiber frequency shift is verified.Through numerical simulation,the evolution of the three zone of overlying rock and the fracture characteristics of the key layer are compared and analyzed.The results are consistent with the three-band range of the overburden and the fracture structure of the key layer of the physical similar material model test,and further verify the characteristic model of the frequency shift of the overburden surface pressure and the fracture characteristic of the overburden structure.