Prediction method for dynamic exposure time of mine roof considering rock mass structural creep

Abstract

The present invention discloses a method for predicting the dynamic exposure time of a mine roof considering the structural creep of rock mass, belonging to the technical field of mine mining safety. The specific steps are as follows: Step S1: Measure the rock mass structure of the roof rock mass to obtain the rock mass integrity coefficient of the roof rock mass; Step S2: Drill test rock samples of roof rock masses with different buried depths on site, conduct indoor creep-fatigue interaction mechanical tests on the test rock samples, apply stress disturbances during the indoor creep mechanical test, and determine the corresponding times at the creep stable stage and the creep unstable stage of the test rock samples, and initially obtain the dynamic exposure time interval of the mine roof; Step S3: Revise the initial dynamic exposure time interval of the mine roof according to the rock mass integrity coefficient to obtain the predicted dynamic exposure time of the mine roof. The prediction accuracy is high, which is shortened from "month level" to "day level", facilitating the guidance of the mine roof management work.

Description

Technical Field

[0001] The present invention relates to the technical field of mine safety in mining, and in particular to a method for predicting the dynamic exposure time of a mine roof considering the structural creep of rock mass. Background Art

[0002] In underground metal mine mining, the physical and mechanical properties of the stope roof rock mass are closely related to the differences in the rock's own composition and the rock mass structure. To a large extent, the rock mass structure plays a crucial role in the stability of the stope roof. There are many weak surfaces such as faults, joints, and fissures of different sizes and directions in the rock formation. The existence of these weak surfaces causes extreme unevenness in the quality of the rock mass, resulting in different mechanical parameters for each part of the rock mass. In addition, in a complex rock mechanics environment, continuous stope excavation will continuously change the stress conditions and states of the stope roof and surrounding rocks. The originally distributed balanced stress field will change again, generating a new secondary stress field. Among them, the tensile stress is the largest at the center position of the roof, and its magnitude exceeds the failure strength of the rock mass, resulting in tensile failure; at the corner position of the roof and the side wall, the shear stress on the rock is the largest, and when its magnitude exceeds the shear strength, the rock will undergo shear failure. Due to the influence of factors such as the gravity of the overlying rock strata and the creep characteristics of the rock on the stope roof, if the exposed area of the stope roof is too large or the exposure time is too long, the roof support strength is insufficient or the support is not timely enough, and the design of the stope structure parameters is not reasonable enough, roof fall accidents are likely to occur. According to relevant data statistics, roof accidents in metal mines account for 46.5% of various underground accidents, ranking first among fatal accidents. Mine safety should always be given top priority. The current stope roof stability evaluation methods and monitoring and early warning means have conducted a relatively systematic study on the blasted area of the stope. If a long exposure time is given during the roof management process, and if the roof cannot be scientifically managed during this period, the roof risk will increase, resulting in mine safety accidents. Therefore, the accurate prediction of the roof exposure time in metal mines is of great significance for guiding safe mining such as mine blasting ore drawing, stoping ore drawing, and filling connection.

[0003] At present, in the research on the stope exposure time, numerical simulation, instrument monitoring and empirical formula methods are mostly used. The roof positions studied by such methods are usually fixed. Through numerical simulation analysis, the deformation laws of roof stress and displacement over time are obtained. Monitoring instruments are installed on the roadway roof, with stress or deformation as the warning value, and the time from stability to failure is recorded. The exposure times obtained by these two methods correspond to the limit time when the roof fails, which belong to post-event data. It is very difficult to predict the exposure time and it cannot guide the stope roof management. Empirical formulas mostly use stability images, introduce time factors, and give a relatively large time span for roof stability, and the accuracy needs to be improved. The above research methods are for the case where the stope roof position remains unchanged. However, when the stope roof position changes with the mining height, such as the upward horizontal slicing and filling mining method, it is relatively difficult to obtain the roof exposure time. Therefore, there is an urgent need to find a method to predict the roof exposure time in advance.

[0004] At present, there are many studies on the occurrence mechanism of roof rock fall in metal mine stopes. Although it explains the roof fall phenomenon in the stope to a certain extent and reflects that the roof fall is related to the stress distribution and rock mass structure characteristics, the rock mass structure elements are crucial and cannot be ignored in the study of roof stability. In terms of the stope roof stability evaluation, it mainly includes on-site monitoring methods, theoretical calculation and numerical simulation methods, and multivariate statistical methods. When using many methods for stability evaluation, the exposed area and limit span affecting the stope stability are calculated. However, the influence of time factors on roof stability, that is, the roof exposure time, is not considered. Foreign scholars analyzed the roof exposure time through statistical probability methods, corrected the stability number in the MATHEWS stability diagram by introducing time factors, and gave the recommended values for roof exposure. The exposure time span is in months, divided into four types: more than 12 months, 5 to 12 months, 3 to 5 months, and less than 3 months. This approach artificially magnifies the roof stability state to a certain extent and cannot reflect the true stress state of the roof, with great potential safety hazards. Moreover, the MATHEWS stability diagram of the stability isoprobability line does not consider the influence of rock mass structural creep on the roof stability state. Due to the influence of factors such as the gravity of overlying strata, rock creep, and mining disturbance characteristics on the stope roof, it is urgent to carry out roof time-dependent stability evaluation and exposure time prediction. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for predicting the dynamic exposure time of a mine roof considering rock mass structural creep to solve the above technical problems.

[0006] To achieve the above purpose, the present invention provides a method for predicting the dynamic exposure time of a mine roof considering rock mass structural creep, and the specific steps are as follows:

[0007] Step S1: Measure the rock mass structure of the roof rock mass to obtain the rock mass integrity coefficient of the roof rock mass;

[0008] Step S2: Drill test rock samples of the roof rock mass at different burial depths on site, conduct indoor creep-fatigue interaction mechanical tests on the test rock samples, apply stress disturbances during the indoor creep mechanical test, and determine the corresponding times at the stable creep stage and the unstable creep stage of the test rock samples to obtain the preliminary dynamic exposure time interval of the mine roof;

[0009] Step S3: Revise the preliminary dynamic exposure time interval of the mine roof according to the rock mass integrity coefficient to obtain the predicted dynamic exposure time of the mine roof.

[0010] Preferably, in Step S1, the calculation formula for the rock mass integrity coefficient is as follows:

[0011] ;

[0012] where is the wave velocity coefficient, is the joint coefficient, is the rock mass quality index of the current roof rock mass, is the maximum value of the rock mass quality index of the roof rock mass in the whole mining area;

[0013] ;

[0014] where is the wave velocity of the roof rock mass, is the wave velocity of the rock block;

[0015] ;

[0016] where is the number of regional joints obtained by non-contact measurement and digital panoramic borehole camera of the roof rock mass; is the rock mass volume of the measurement area.

[0017] Preferably, Step S2 is specifically as follows:

[0018] Step S21: Obtain test rock samples at different burial depths by drilling;

[0019] Step S22: Install a disturbance stress monitoring system at the mining site, extract the disturbance stress amplitude and disturbance frequency according to the real-time collected disturbance stress wave data; convert the disturbance stress wave into a sine wave by the rain flow counting method or the equivalent amplitude method to obtain the spectral characteristic parameters of the disturbance stress wave;

[0020] Step S23: Conduct indoor staged creep loading on test rock samples with different burial depths. Considering the differences between the sample size and in-situ rock mass, convert the in-situ measured data into laboratory data for staged creep loading. The time for each creep stage in the mechanical test is 48 hours. Apply disturbances during the staged creep loading according to the spectral characteristic parameters of the disturbance stress wave. The time calculation formula is as follows:

[0021] ;

[0022] where, is the disturbance loading time, is the disturbance frequency, is the number of disturbance loading cycles;

[0023] The loading parameters from the in-situ disturbance stress to the indoor disturbance stress are transformed by introducing the stress similarity ratio. The stress similarity ratio kc is defined as:

[0024] kc = C L ×C ρ

[0025] C L is the geometric similarity ratio, C ρ is the density similarity ratio;

[0026] The calculation formula for the loading amplitude in the indoor mechanical test is as follows:

[0027] ;

[0028] where, is the loading amplitude, is the stress similarity ratio, is the recorded stress wave amplitude;

[0029] After the test, a strain-time curve is obtained;

[0030] Step S24: Construct a creep model;

[0031] Step S25: Use the damping least squares method to fit the creep model parameters according to the strain-time curve, and obtain the time intervals [T1, T2] of the creep stable stage and the creep unstable stage; T1 is the creep time corresponding to the loading stage when the deformation rate is 0, and T2 is the creep time corresponding to the (n - x)th loading stage; n is the number of stages when the test rock sample fails, and x is the safety factor;

[0032] Step S26: Determine that the preliminary dynamic exposure time interval of the mine roof is .

[0033] Preferably, the creep model adopts the Burgers model. When calculating the creep time, it is carried out when the stress is not greater than the yield stress of the rock. The constitutive equation, creep equation, unloading equation and relaxation equation of the Burgers model are as follows:

[0034] ;

[0035] ;

[0036] ;

[0037] ;

[0038] Wherein, , and are the stress, the first-order derivative of the stress with respect to time, and the second-order derivative of the stress with respect to time, respectively; , and are the strain, the first-order derivative of the strain with respect to time, and the second-order derivative of the strain with respect to time, respectively; and are the elastic modulus of the first spring and the elastic modulus of the second spring, respectively; and are the viscosity coefficient of the first viscous pot element and the viscosity coefficient of the second viscous pot element, respectively; is the unloading time, and are the first integral constant and the second integral constant related to the initial conditions, respectively; and are the first relaxation parameter and the second relaxation parameter related to the change characteristics of the material stress-strain, respectively; is the time.

[0039] Preferably, the stress function after loading perturbation is as follows:

[0040] ;

[0041] Wherein, is the stress function after loading perturbation, is the angular frequency of the perturbation wave.

[0042] Preferably, in step S3, the revised formula for the preliminary dynamic exposure time interval of the mine roof is as follows:

[0043] ;

[0044] Wherein, is the predicted dynamic exposure time of the mine roof.

[0045] Therefore, the present invention adopts the above-mentioned method for predicting the dynamic exposure time of the mine roof considering the structural creep of rock mass, and the beneficial effects are as follows:

[0046] (1) Revise the preliminary dynamic exposure time interval of the mine roof by combining the rock mass integrity coefficient characterizing the structural characteristics of the roof rock mass. The introduced rock mass integrity coefficient is determined by the volumetric joint density and the rock mass wave velocity, which improves the prediction accuracy.

[0047] (2) Apply loading disturbances during the indoor creep mechanics test, considering the mining blasting disturbance. The disturbance frequency and amplitude are equivalently reduced according to the stress wave characteristics recorded by the mine blasting vibration. And determine the time difference corresponding to the creep stable stage and the creep unstable stage of the test rock sample through the Burgers model to obtain the preliminary dynamic exposure time interval of the mine roof, revealing the whole process evolution characteristics of the fault zone activation and roadway instability.

[0048] Next, through the drawings and embodiments, the technical solution of the present invention will be further described in detail. Description of the Drawings

[0049] Figure 1 It is the roof position map of the west mining area of the 4059 section of the present invention;

[0050] Figure 2 It is the roof position map of the east mining area of the 4059 section of the present invention;

[0051] Figure 3 It is the flow chart of the method for predicting the dynamic exposure time of the mine roof considering the structural creep of rock mass of the present invention. Detailed Embodiments

[0052] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "upper", "lower", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the present invention is usually placed during use. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention. In the description of the present invention, it should also be noted that unless otherwise clearly specified and defined, the terms "set", "installed", "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0053] Next, in conjunction with the drawings, the embodiments of the present invention will be described in detail.

[0054] Taking the east and west mining areas of the 4059m section of Hongniu Copper Mine as the research object, the section height from 4059m to 4097m is 38 meters. As Figure 1 and Figure 2 shown, 12 meters have been mined in the west mining area. The stope has a large span and a long empty roof time, posing serious safety risks. It is necessary to predict the exposure time of the roof at different mining depths for upward mining. The mining stratified thickness of the ore body is 6 meters.

[0055] As Figure 3 shown, a method for predicting the dynamic exposure time of mine roof considering rock mass structural creep is as follows:

[0056] Step S1: Conduct rock mass structure measurement on the roof rock mass, and comprehensively use the method of combining structural plane photography imaging, borehole television and ultrasonic testing to obtain rock mass structure data, and obtain the rock mass integrity coefficient of the roof rock mass.

[0057] The calculation formula of the rock mass integrity coefficient is as follows:

[0058] ;

[0059] Among them, is the wave velocity coefficient, is the joint coefficient, is the rock mass quality index of the current roof rock mass, is the maximum value of the rock mass quality index of the roof rock mass in the whole mining area;

[0060] ;

[0061] Among them, is the wave velocity of the roof rock mass, is the wave velocity of the rock block;

[0062] ;

[0063] Among them, is the number of regional joints obtained by non-contact measurement and digital panoramic borehole imaging of the roof rock mass; is the rock mass volume of the measurement area.

[0064] Step S2: Drill test rock samples of roof rock masses with different buried depths on site, conduct indoor creep-fatigue interaction mechanical tests on the test rock samples, and apply stress disturbances during the indoor creep mechanical test, and determine the corresponding time at the stable creep stage and the unstable creep stage of the test rock samples to obtain the preliminary dynamic exposure time interval of the mine roof.

[0065] Step S2 is specifically as follows:

[0066] Step S21: Drilling is used to obtain test rock samples at different roof burial depths. In this embodiment, the west mining area and the east mining area obtain test rock samples at burial depths of 0 meters, 6 meters, 12 meters, 18 meters and 24 meters. At least 5 test rock samples are drilled at each burial depth to eliminate the discreteness of the rock test results at the same layer height as much as possible.

[0067] Step S22: Install a disturbance stress monitoring system at the mining site, extract the disturbance stress amplitude and disturbance frequency based on the disturbance stress wave data collected in real time; convert the disturbance stress wave into a sine wave using a rain flow counting method or an equivalent amplitude method to obtain the frequency spectrum characteristic parameters of the disturbance stress wave.

[0068] Step S23: Perform indoor graded creep loading on the test rock samples with different burial depths. Considering the difference between the sample size and the on-site rock mass, the field measured data is converted into laboratory data for graded creep loading. The time of each creep stage of the mechanical test is 48 hours. The disturbance is loaded during the graded creep loading according to the frequency spectrum characteristic parameters of the disturbance stress wave. The time calculation formula is as follows:

[0069] ;

[0070] in, is the disturbance loading time, is the disturbance frequency, The number of cycles for the disturbance loading.

[0071] The loading parameters of field disturbance stress to indoor disturbance stress are transformed by introducing stress similarity ratio. The stress similarity ratio kc is defined as:

[0072] kc = CL × Cρ;

[0073] CL is the geometric similarity ratio, and Cρ is the density similarity ratio.

[0074] The calculation formula for the loading amplitude of indoor mechanical tests is as follows:

[0075] ;

[0076] in, is the loading amplitude, is the stress similarity ratio, is the recorded stress wave amplitude.

[0077] After the test, the strain-time curve was obtained.

[0078] Step S24: construct a creep model; the creep model adopts the Burgers model, and the constitutive equation, creep equation, unloading equation and relaxation equation of the Burgers model are as follows:

[0079] ;

[0080] ;

[0081] ;

[0082] ;

[0083] wherein, 、 and are stress, the first-order derivative of stress with respect to time, and the second-order derivative of stress with respect to time, respectively; 、 and are strain, the first-order derivative of strain with respect to time, and the second-order derivative of strain with respect to time, respectively; and are the first spring elastic modulus and the second spring elastic modulus, respectively; and are the viscosity coefficient of the first dashpot element and the viscosity coefficient of the second dashpot element, respectively; is the unloading time, and are the first integral constant and the second integral constant related to the initial conditions, respectively; and are the first relaxation parameter and the second relaxation parameter related to the variation characteristics of the material stress-strain, respectively; is time.

[0084] When t = 0, only the spring works, , when t → ∞, → ∞, the strain change rate tends to be constant, belonging to the metastable creep type. Similarly, the unloading equation of the Burgers body can be obtained by superimposing the unloading equations of the Maxwell body and the Kelvin body. When t → ∞, , there is still a part of the permanent deformation remaining after unloading. When t → ∞, σ → 0, the stress can be relaxed to 0, and the relaxation curve is a decreasing exponential curve. Through the step loading creep test, it is found that the creep curves of rock specimens show two forms: stable creep and unstable creep at different stress levels. For the prediction of the roof exposure time, for the sake of conservatism, the Burgers model does not calculate the time corresponding to the unstable creep, but calculates the creep time corresponding to the creep deformation rate of zero and the xth creep loading stage from the bottom, which is the initially determined roof exposure time interval. x can be set according to actual needs, and the larger x is, the higher the safety factor is.

[0085] The stress function after loading perturbation is as follows:

[0086] ;

[0087] wherein, Load the stress function after perturbation, is the angular frequency of the perturbation wave.

[0088] Step S25: Use the damped least squares method to perform creep curve fitting on the creep model parameters according to the strain-time curve, and obtain the time interval [T1, T2] of the creep stable stage and the creep unstable stage; T1 is the creep time corresponding to the loading stage when the deformation rate is 0, and T2 is the creep time obtained corresponding to the (n - x)-th loading stage; n is the number of stages when the test rock sample fails, and x is the safety factor.

[0089] Step S26: Determine that the preliminary dynamic exposure time interval of the mine roof is .

[0090] Step S3: Revise the preliminary dynamic exposure time interval of the mine roof according to the rock mass integrity coefficient to obtain the predicted dynamic exposure time of the mine roof.

[0091] The revision formula for the preliminary dynamic exposure time interval of the mine roof is as follows:

[0092] ;

[0093] Among them, is the predicted dynamic exposure time of the mine roof.

[0094] To verify the effectiveness of the technical solution of this embodiment, it is verified with the existing prediction scheme. The existing prediction scheme uses the MATHEWS stability diagram with the addition of the stability equiprobability line.

[0095] Table 1 Calculated values of roof creep parameters and exposure time at different burial depths in the west mining area

[0096] ;

[0097] Table 2 Calculated values of roof creep parameters and exposure time at different burial depths in the east mining area

[0098] ;

[0099] Table 3 Roof exposure time at different burial depths in the west mining area based on the stability equiprobability line

[0100] ;

[0101] Table 4 Roof exposure time at different burial depths in the east mining area based on the stability equiprobability line

[0102] ;

[0103] In Tables 3 and 4, 0 and f are respectively represented as the corrected stability number and the predicted probability value.

[0104] Comparing the above results, it can be seen that the predicted dynamic exposure time of the mine roof all falls within the range of the existing technical solutions, which demonstrates the effectiveness of the prediction results of this application. Moreover, the roof exposure time interval predicted by using structural creep is short, and the roof exposure time prediction is shortened from the "month level" to the "day level", with a smaller error and more accurate results, which can well guide the management of the mine roof.

[0105] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for predicting the dynamic exposure time of a mine roof considering rock mass structural creep, characterized in that: The specific steps are as follows: Step S1: measuring the rock mass structure of the roof rock mass to obtain the rock mass integrity coefficient of the roof rock mass; Step S2: Drill test rock samples of roof rock masses with different buried depths on site, conduct indoor creep-fatigue interactive mechanical tests on the test rock samples, apply stress disturbances during the indoor creep mechanical tests, and determine the time corresponding to the stable creep stage and the unstable creep stage of the test rock samples, so as to obtain a preliminary dynamic exposure time interval of the mine roof; Step S3: Revise the preliminary mine top dynamic exposure time interval according to the rock mass integrity coefficient to obtain the predicted mine top dynamic exposure time.

2. A method for predicting the dynamic exposure time of a mine top considering rock mass structural creep according to claim 1, characterized in that: In step S1, the rock mass integrity coefficient is calculated as follows: ； in, is the wave velocity coefficient, is the joint coefficient, is the rock quality index of the current roof rock mass, It is the maximum value of rock quality index of roof rock mass in the whole mining area; ； in, is the wave velocity of the roof rock mass, is the rock wave velocity; ； in, The number of regional joints obtained by non-contact measurement of the roof rock mass and digital panoramic borehole photography; is the rock volume in the measurement area.

3. A method for predicting the dynamic exposure time of a mine roof considering rock mass structural creep according to claim 2, characterized in that: Step S2 is specifically as follows: Step S21: obtaining test rock samples at different burial depths by drilling; Step S22: installing a disturbance stress monitoring system at the mining site, extracting the disturbance stress amplitude and disturbance frequency according to the disturbance stress wave data collected in real time; converting the disturbance stress wave into a sine wave by using a rain flow counting method or an equivalent amplitude method, and obtaining the frequency spectrum characteristic parameters of the disturbance stress wave; Step S23: Perform indoor graded creep loading on the test rock samples with different burial depths. Considering the difference between the sample size and the on-site rock mass, the field measured data is converted into laboratory data for graded creep loading. The time of each creep stage of the mechanical test is 48 hours. The disturbance is loaded during the graded creep loading according to the frequency spectrum characteristic parameters of the disturbance stress wave. The time calculation formula is as follows: ； in, is the disturbance loading time, is the disturbance frequency, The number of cycles for the disturbance loading; The loading parameters of field disturbance stress to indoor disturbance stress are transformed by introducing stress similarity ratio. The stress similarity ratio kc is defined as: kc= C L ×C ρ； C L is the geometric similarity ratio, C ρ is the density similarity ratio; The calculation formula for the loading amplitude of indoor mechanical tests is as follows: ； in, is the loading amplitude, is the stress similarity ratio, is the recorded stress wave amplitude; After the test, the strain-time curve is obtained; Step S24: constructing a creep model; Step S25: Use the damped least square method to fit the creep curve of the creep model parameters according to the strain-time curve to obtain the time interval [T1, T2] of the creep stable stage and the creep unstable stage; T1 is the creep time corresponding to the loading stage corresponding to the deformation rate of 0, and T2 is the creep time corresponding to the nxth loading stage; n is the stage number when the test rock sample is destroyed, and x is the safety factor; Step S26: Determine the initial mine top dynamic exposure time interval .

4. A method for predicting the dynamic exposure time of a mine roof considering rock mass structural creep according to claim 3, characterized in that: The creep model adopts the Burgers model. The creep time is calculated when it is not greater than the rock yield stress. The constitutive equation, creep equation, unloading equation and relaxation equation of the Burgers model are as follows: ； ； ； ； in, , as well as are stress, first-order derivative of stress with respect to time, and second-order derivative of stress with respect to time, respectively; , as well as are strain, first-order derivative of strain with respect to time, and second-order derivative of strain with respect to time, respectively; and are the first spring elastic modulus and the second spring elastic modulus respectively; and are the viscosity coefficient of the first viscosity pot element and the viscosity coefficient of the second viscosity pot element respectively; For uninstallation time, and are the first and second integration constants related to the initial conditions, respectively; and are the first relaxation parameter and the second relaxation parameter respectively related to the variation characteristics of material stress-strain; For time.

5. The method for predicting the dynamic exposure time of a mine roof considering rock mass structural creep according to claim 4, characterized in that: The stress function after loading disturbance is as follows: ； in, The stress function after loading disturbance, is the angular frequency of the disturbance wave.

6. A method for predicting the dynamic exposure time of a mine roof considering rock mass structural creep according to claim 4, characterized in that: In step S3, the revised formula for the dynamic exposure time interval of the preliminary mine top is as follows: ； in, To predict the dynamic exposure time of mine roof.

Images