Effect Of Working Face Advance Speed On Strata Behaviors In High-Intensity Mining

Rocks show different physical and mechanical characteristics under different loading rate, based on it, and using different research methods such as indoor experiments on rock mechanics properties, the thin plate theory, the plane elastic complex variable function method, the energy principle, the elastic dynamics method, ANSYS/ls-dyna numerical simulation and similar material simulation experiment method, this article studies the strata pressure behavior in high intensity mining working face under different advance speed. By using the thin plate theory and plane elastic complex variable function method and so on, the author conclude the relation between the size of working face and internal stress changing, and then get the relation between working face advance speed and loading rate by solving the time derivative and referring to boundary conditions of the force. By laboratory experiments on the physical and mechanical properties of sandstone, the rule of rock’s strength changing under different loading rate are obtained, that is, the faster the loading rate, the greater the strength of the rock. Thus, the strength of the stope roof is always different with different working face advance speed, and the faster the working face advancing, the stronger the roof breakage resistance. Combining the calculation formula of interval of main roof breakage we establish the connection between working face advance speed and main roof breaking interval, then further analyze the working face support working resistance and so on under different roof breaking interval. In addition, the law of working face advance speed influence on roof subsidence and sink velocity are analyzed according to roof deformation rate, and the roof’s dynamic load impact effect in high intensity mining face is also explained based on the energy principle and the stress-strain curve of rock. In the analysis process, we used the principle of work-power in molecular scale and elastic dynamics method to explain the mechanism of loading rate influence on rock strength, and used ANSYS/ls-dyna numerical simulation to validate the law of loading rate’s influence on rock’s tensile strength in Brazilian splitting experiments, and we also simulated the roof’s dynamic load impact effect in high intensity mining face and mine pressure behavior under different working face advance speed by improving the similar material simulation experiment platform. In general, we conclude the main results as follows:(1)With the working face advancing, stope space exposed is bigger and bigger, the upper strata compress main roof gradually. And main roof’s internal force is function of mining height and advance length. By using the thin plate theory and plane elastic complex variable function method, the author establishes relational expression of main roof’s internal force and size. By solving the time derivation, the internal force changes into stress rate, and the advancing length turn to advance speed. So we can get the relationship between stress rate and the working face advance speed. Taking main roof as the research object and referring to boundary conditions of the force, we can establish connection between main roof’s internal force changing rate and the loading rate on main roof from the upper strata, finally get the relation between the loading rate and the working face advance speed. Defining the correlation coefficient between them as “i”, and the expression of “i” can be obtained by using the thin plate theory or plane elastic complex variable function method. Results show that, the faster the working face advancing, the bigger the loading rate on main roof.(2) Rocks show different physical and mechanical characteristics under different loading rate, so we select nearly 150 pieces of sandstone block to carry out uniaxial compression, shear and Brazilian splitting experiments for observing rock’s failure process, strength and the energy change rule under different loading rate. It was found that:(a) The loading rate has a great influence on rock’s failure process, while the rate is bigger, the damage time of specimen is shorter, the destroying sound is bigger, the internal native crack interaction is weaker, and the experimental results are less volatility.(b) The bigger the loading rate, the greater the uniaxial compressive strength and elastic modulus of sandstone.(c) The bigger the loading rate, the greater the cohesion and internal friction angle of sandstone.(d) Within a certain range, the greater the loading rate, the bigger the tensile strength. Under the same displacement loading rate, there will be different external force changing rates, and the greater the changing rate, the bigger the tensile strength. The external force changing rate is always bigger under the bigger displacement loading rate.(e) From the thermal imaging observation in Brazilian splitting test, we found that, the faster the loading rate, the higher the temperature on the specimen surface and the more energy released while breaking. On the contrary, in shear test, the bigger the loading rate, the smaller the temperature.(3) According to the principle of work-power in molecular scale, the mechanism of displacement loading rate influence on rock strength was explained. Result shows that, in rocks’ physical and mechanical properties experiment, external force not only need to overcome molecular force to work, but also transforms some energy into kinetic energy of molecules. And the value of the work is only determined by composition of objects and the final state while destroying, and has nothing to do with the loading rate. But the bigger the displacement loading rate, the greater the total kinetic energy of rock and the more external force work needed. So there will need bigger external force for the rock breaking under the condition of the same ultimate strain. That is to say, the bigger the loading rate, the stronger the strength shows.According to the elastic dynamics method can also explain the mechanism of external force loading rate influence on rock strength. While considering the influence of loading rate, the equilibrium equation in the elastic statics method must be replaced by the equations of motion. Because we have to take the effect of inertial force into account, which will result in different outcomes from using the elastic statics method, confirming the conclusion that the loading rate affect the rock’s strength.In addition, the law of the loading rate influence on tensile strength is verified by using the explicit dynamic finite element analysis program ANSYS/ls-dyna. In transient analysis or the static analysis on rate(such as creep or viscoelasticity) by using ANSYS/ls-dyna, time represents the actual, expressed in seconds, minutes or hours, and has a natural advantage on analyzing the mechanism of the loading rate. The simulation calculation results further confirm that the bigger the displacement or external force loading rate, the greater the rock’s tensile strength.(4)Combined with roof deformation rate, the law of working face advance speed influencing on roof subsidence and sinking velocity is analyzed. Combined with the law of working face advance speed influencing on roof tensile strength and the solving formula of the main roof’s breaking interval, the regulation of working face advance speed influencing on main roof’s breaking interval and support capacity is analyzed. Which points out that the faster the advance speed, the larger the main roof’s breaking interval and the greater the support working resistance while weighting at the same condition.A last, according to the stress-strain curve of rock, the roof’s dynamic load impact effect in high intensity mining face is analyzed by using energy principle. Concretely, the actuating range of dynamic load impact effect is obtained by building the beam structure model on elastic foundation. Combining with the stress-strain curve of rock, the expression of maximum impact load under different conditions is obtained by establishing the dynamic model of roof movement. The results show that, the enough initial supporting resistance can avoid the occurrence of dynamic load impact. When the supporting force is insufficient, the maximum impact load equals to twice of the roof weight minus the initial supporting resistance if the compression process of immediate roof is completely in conformity with Hooke’s law. Because the stress-strain relationship is not all linear elastic, the dynamic load coefficient fluctuates around "2" varying with initial supporting resistance, breaking roof weight and the properties of immediate roof. In the end, the correctness of theoretical analysis is validated by similar simulation experiment and numerical simulation of high intensity mining working face. And the author also put forward the measures to control dynamic load impact disasters by filling the mined-out area, selecting reasonable support and increasing the initial supporting resistance.(5)Using the similar simulation experiment platform improved in simulation material, loading system and observation methods and so on to simulate the law of roof subsidence, the sinking velocity, the weighting interval and support working resistance at different working face advance speeds, which further verifies the law of the working face advance speed influencing on mining pressure of stope.