CN113790961A - Physical simulation test-based method for determining stability of end slope mining supporting coal pillar - Google Patents
Physical simulation test-based method for determining stability of end slope mining supporting coal pillar Download PDFInfo
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Abstract
The invention provides a physical simulation test-based method for determining stability of an end slope mining supporting coal pillar, which comprises the steps of firstly establishing a physical simulation test model of engineering geology to be monitored, simulating the process of coal pillar instability of the supporting coal pillar along with load increase, collecting vertical stress of each measuring point to draw a stress curve, judging whether the coal pillar is unstable or not according to the shape of the stress curve, wherein the vertical stress of the coal pillar is distributed in a saddle shape and is in a stable state in the mining process; in the grading loading process, the width of the coal pillar is sequentially sheared and damaged from small to large, the sudden increase of the vertical stress in the middle of the coal pillar is a precursor of instability, and the sudden change point of the vertical stress is used as an early warning criterion of the instability of the coal pillar; the invention can accurately simulate the actual situation of the engineering field and provide reasonable design basis for the relevant parameters of the supporting coal pillar.
Description
Technical Field
The invention relates to the technical field of stability of end slope mining supporting coal pillars of open pit coal mines, in particular to a method for determining stability of end slope mining supporting coal pillars based on a physical simulation test.
Background
In China, coal production bases such as Xinjiang, inner Mongolia, Shanxi, Qinghai and Tibet contain a large amount of shallow coal beds and are suitable for open-pit mining, so that a large amount of end slope pressed coal (retained coal) is generated. A large amount of high-quality coal still exists in the retained coal, and huge resource waste is caused by abandonment and mining. The end mining process is successfully applied in a large range abroad, and in recent years, with the continuous development of the end mining process in China, a new method is provided for the recovery of the retained coal. When the end slope is mined, a coal mining machine is used for carrying out tunneling mining on the exposed coal seam to form a plurality of independent mining tunnels, and the mining tunnels support the overlying strata through reserved coal pillars. If the support coal pillar is unstably damaged, equipment burying accidents can be caused, a series of engineering disasters such as strip mine side slope landslide and the like can be caused, normal mining work of a mining area is seriously influenced, and casualties and economic losses are further caused.
When the end slope is mined, the end slope coal mining machine is remotely controlled to carry out tunneling mining on the exposed coal seam to form a plurality of independent mining caves, and the overlying strata are supported by reserved coal pillars among the mining caves. If the support coal pillar is unstably damaged, not only equipment burying accidents can be caused, but also a series of engineering disasters such as strip mine side slope landslide and the like can be caused, the safe and normal mining work of a mining area is seriously influenced, and inestimable casualties and economic losses are caused.
The key point for solving the problem is to research a destabilization mechanism for supporting the coal pillars under the end slope mining condition, further obtain a criterion for generating the destabilization, help technicians to avoid a destabilization damage period, avoid a series of engineering disasters such as equipment burying accidents and slope landslide of strip mine slopes, and guarantee the safe production of the coal mine.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a physical simulation test-based method for determining the stability of an end slope mining supporting coal pillar, which comprises the following steps:
step 1: constructing a physical simulation test model of the engineering geology to be monitored;
step 2: simulating the coal pillar instability supporting process with the load increasing by using a physical simulation test model, and collecting the vertical stress of each measuring point;
and step 3: fitting the vertical stress of the corresponding measuring point of each coal pillar to obtain a stress curve of the corresponding coal pillar;
and 4, step 4: judging whether the coal pillar is unstable or not according to the shape of the stress curve;
and 5: and in the process of the coal pillar evolving from the critical state to the destabilization state, judging the moment of the destabilization of the coal pillar according to the change of the slope of the stress curve, and taking the moment of the sudden increase of the slope corresponding to the stress curve as the starting moment of the destabilization of the coal pillar.
The step1 comprises the following steps:
step 1.1: determining the geometric similarity ratio of a physical simulation test model according to the geometric dimension of the comprehensive simulation test platform and the geological distribution condition of the project to be monitored;
step 1.2: determining the geometric dimension and the stress boundary of the model according to the geometric similarity ratio;
step 1.3: and determining the mix proportion of each simulated rock stratum by comparing the cementing materials with the design strength through an orthogonal test, and carrying out layered cast-in-place simulation on the coal seam distribution condition on a comprehensive simulation test platform according to the mix proportion.
The step2 comprises the following steps:
step 2.1: loading the model according to the design requirement to enable the model to reach a converted original rock stress state;
step 2.2: excavating the underground mining after the stress is applied stably;
step 2.3: after the excavation is finished, keeping for a certain time, then applying overload pressure in a grading manner, and stopping loading when through cracks appear in the coal pillar or the stress drops below the original rock stress in the loading process;
step 2.4: and arranging pressure sensing elements at 4 equal dividing points of the width of the coal pillar, and collecting the vertical stress of each measuring point of the coal pillar with different widths.
The step4 is specifically expressed as follows: if the shape of the curve is saddle-shaped, the applied load does not exceed the ultimate strength of the coal pillar, and the coal pillar is in a stable state; if the shape of the curve is a platform shape, the applied load is equal to the ultimate strength of the coal pillar, and the coal pillar is in a critical state; if the shape of the curve is arched, the applied load is larger than the ultimate strength of the coal pillar, and the coal pillar is in a destabilizing state.
The invention has the beneficial effects that:
the invention provides a physical simulation test-based method for determining stability of an end slope mining supporting coal pillar, which simulates the process of coal pillar instability of the supporting coal pillar along with increase of load by constructing a physical simulation test model, judges whether the coal pillar is unstable or not according to the shape of a vertical stress curve, and takes a vertical stress sudden change point as an early warning criterion of the coal pillar instability; the method has reliable results, provides reasonable basis for parameter design of the support coal pillar, and timely sends out early warning before the coal pillar is unstable.
Drawings
FIG. 1 is a flow chart of a method for determining stability of an end slope mining supporting coal pillar based on a physical simulation test according to the present invention;
FIG. 2 is a schematic diagram of the design of the test process of the present invention;
FIG. 3 is a schematic view of the load on the support pillars of the present invention;
FIG. 4 is a distribution diagram of lithology of each stratum of the side slope in the invention;
FIG. 5 is a graph of the geometry of the model of the present invention;
FIG. 6 is a schematic diagram of a simulation test system according to the present invention;
FIG. 7 is a schematic view of a coal pillar stress monitoring scheme according to the present invention;
FIG. 8 is an evolution mechanism curve of vertical stress at each measuring point of a coal pillar with different widths, wherein (a) shows the evolution mechanism curve when the length of a supporting coal pillar is 4.5m, (b) shows the evolution mechanism curve when the length of the supporting coal pillar is 5.0m, (c) shows the evolution mechanism curve when the length of the supporting coal pillar is 5.5m, and (d) shows the evolution mechanism curve when the length of the supporting coal pillar is 6.0 m;
FIG. 9 is a graph showing the vertical stress distribution relationship of coal pillars of different widths after completion of excavation by the underground mining system;
FIG. 10 is a vertical stress variation mechanism curve of coal pillars of different widths at the same position measuring point in the present invention;
FIG. 11 is a graph of the evolution mechanism of vertical stress at different width coal pillar measuring points during the staged loading process in the present invention;
FIG. 12 is a plot of the plastic zone distribution for a 6.0m wide coal column at different loading levels according to the present invention; wherein (a) is the plastic zone distribution diagram after the 6 th level loading; (b) a plastic zone distribution diagram after the 7 th level loading is finished; (c) the plastic zone profile for the loaded 8 th stage.
Detailed Description
The invention is further described with reference to the following figures and specific examples.
As shown in fig. 1, a method for determining stability of an end slope mining supporting coal pillar based on a physical simulation test includes:
step 1: constructing a physical simulation test model of the engineering geology to be monitored; the method comprises the following steps:
step 1.1: determining the geometric similarity ratio of a physical simulation test model according to the geometric dimension of the comprehensive simulation test platform and the geological distribution condition of the project to be monitored;
step 1.2: determining the geometric dimension and the stress boundary of the model according to the geometric similarity ratio;
step 1.3: determining the mix proportion of each simulated rock stratum by comparing the cementing materials with the design strength through an orthogonal test, and carrying out layered cast-in-place simulation on the coal seam distribution condition on a comprehensive simulation test platform according to the mix proportion;
step 2: simulating the coal pillar instability supporting process with the load increasing by using a physical simulation test model, and collecting the vertical stress of each measuring point; the method comprises the following steps:
step 2.1: loading the model according to the design requirement to enable the model to reach a converted original rock stress state;
step 2.2: excavating the underground mining after the stress is applied stably;
step 2.3: after the excavation is finished, keeping for a certain time, then applying overload pressure in a grading manner, and stopping loading when through cracks appear in the coal pillar or the stress drops below the original rock stress in the loading process;
step 2.4: arranging pressure sensing elements at 4 equal dividing points of the width of the coal pillar, and collecting vertical stress of each measuring point of the coal pillar with different widths;
and step 3: fitting the vertical stress of the corresponding measuring point of each coal pillar to obtain a stress curve of the corresponding coal pillar;
and 4, step 4: judging whether the coal pillar is unstable or not according to the shape of the stress curve; if the shape of the curve is saddle-shaped, the applied load does not exceed the ultimate strength of the coal pillar, and the coal pillar is in a stable state; if the shape of the curve is a platform shape, the applied load is equal to the ultimate strength of the coal pillar, and the coal pillar is in a critical state; if the shape of the curve is an arch, the applied load is larger than the ultimate strength of the coal pillar, and the coal pillar is in a destabilization state;
and 5: and in the process of the coal pillar evolving from the critical state to the destabilization state, judging the moment of the destabilization of the coal pillar according to the change of the slope of the stress curve, and taking the moment of the sudden increase of the slope corresponding to the stress curve as the starting moment of the destabilization of the coal pillar.
To verify the effectiveness of the method of the present invention, the verification process is shown in fig. 2, and the mechanical load model is shown in fig. 3, and is designed as follows:
in order to research a destabilization mechanism of a support coal pillar under an end slope mining condition, a certain lignite open-pit mine is taken as a research background, No. 21 coal seam outcrop coal is dug and stoped for recovering end slope retained coal, the cross section of a stoping chamber is rectangular, the width is 2m, the height is 2.5m, the mining depth is 100m, and 1 stoping is completed in 3 days. The height of the west-side slope is about 100m, the strata are layered and distributed nearly horizontally, the lithology of the strata does not change obviously in the trend direction, and the lithology of the strata of the top-down slope is surface soil, coarse sandstone, siltstone, No. 21 coal and basal sandstone respectively, as shown in figure 4. The top and bottom plates of the coal bed are mainly made of sandstone, and the physical and mechanical parameters of each rock stratum are shown in table 1.
TABLE 1 coal-rock physical-mechanical parameter table
According to the geometric dimension of the comprehensive simulation test platform and the geological distribution condition of the west end slope field engineering of a certain lignite open pit, determining the geometric similarity ratio of the model as Cl1:50, volume-weight similarity ratio CγWhen 1:1.6, the stress similarity ratio can be calculated as Cσ＝Cγ×Cl＝1:80。
The physical simulation test model has the overall dimensions of 300cm wide and 180cm high, wherein the coal seam burial depth is 100cm, and the simulated actual equivalent burial depth is 50 m. The width of the coal mining cave is 4cm, the height is 5cm, the reserved coal pillars are designed to be 4 widths of 9 cm, 10 cm, 11 cm and 12cm respectively, and the corresponding actual supporting coal pillar widths are 4.5m, 5m, 5.5m and 6 m. In order to reduce the test times, different coal pillars are arranged in the same model, the distance between the coal pillars with different widths is 40cm, and the tunnel width is 10 times, so that the boundary effect is eliminated. In addition, 8 levels of overload pressurization, such as 200, 300, 200, 400, 200, 300, and 200kPa, were applied to the model to simulate the coal pillar destabilization evolution mechanism as the load increases for supporting the coal pillar. The concrete sizes of the models, the formation distribution and the drift numbers are shown in figure 5.
In order to obtain the material which accords with the mechanical properties of similar models, river sand is used as fine aggregate, a mixture of cement and gypsum is used as a cementing material, the mixture ratio of each simulated rock stratum is finally determined by comparing the cementing material with the design strength through an orthogonal test, and the physical and mechanical parameters are shown in table 2.
TABLE 2 model test material parameter Table
When vertical stress is collected, a simulation test system is adopted to carry out a simulation loading process of graded loading on a designed physical simulation test model, as shown in fig. 6, the simulation test system mainly comprises an evenly distributed pressure loading system and a measurement counterforce device, and provides a displacement boundary and a stress boundary for the model; the stress monitoring system comprises a pressure box and a stress monitoring system, and monitors the vertical stress of the coal pillar. The model is manufactured on a simulation test system in a layered cast-in-place mode according to the design mixing proportion, and after the last layer is poured and solidified, demolding and maintaining are carried out for 28 days, so that the coal bed can be better identified, and the model is painted black by using paint. The test steps of simulating the coal pillar instability supporting process along with the increase of the load based on the physical simulation test model are as follows:
step 1: and according to the design requirement, loading the model to reach the converted original rock stress state.
step 2: and after the stress is applied stably, excavating the drift, and excavating No. 1 drift, No. 2 drift and No. 3 drift in sequence, wherein the excavation time of each drift is about 70min, and the load is kept for about 30min after the excavation is finished, and the test section 1 is formed.
step 3: after the excavation of the excavation section 1 is finished, according to the step2, excavation test sections 2 (No. 4, No. 5 and No. 6 mining tunnels), 3 test sections (No. 7, No. 8 and No. 9 mining tunnels) and 4 test sections (No. 10, No. 11 and No. 12 mining tunnels) are sequentially performed.
step 4: and (3) after the excavation is finished, keeping for 30min, then applying overload pressure in a grading manner, and stopping loading if through cracks appear in the coal pillar or the stress is obviously reduced in the loading process.
Further, in order to monitor the pillar change, a pressure sensing element (pressure cell) is previously disposed at a designated monitoring position. Taking the test section 1 as an example, as shown in fig. 7, the coal pillar monitoring points are equally distributed along the horizontal direction of the central point of the No. 2 mining height according to the width of the coal pillar, the distance is about 2.25cm, and the distance is about 1.25cm from the bottom plate. And the other 3 test sections are based on the underground mining of No. 5, No. 8 and No. 11, and monitoring points are arranged according to the mode of figure 6. There are 24 stations in total, numbered sequentially from left to right.
The change mechanism of the vertical stress of each measuring point of the coal pillar with different widths in the process of excavation of the underground mining is shown in figure 8. As can be seen from fig. 8: the vertical stress of the test section where each measuring point is located before excavation is 0.018MPa, and the fact that different test sections are excavated step by step sequentially shows that the excavation of the previous test section does not influence the vertical stress distribution of the coal pillar in the next test section, and therefore the coal pillar interval of the test design can eliminate the boundary effect generated by the formation of the previous coal pillar with different widths. The stress of the measuring point of each test section is linearly increased or kept at a fixed value along with the excavation of the underground mining, and the stress does not have the phenomenon of sudden increase and sudden decrease, which indicates that concentrated stress is formed in the coal pillar in the excavation process, but the supporting stress of the coal pillar is not redistributed, no obvious crack is generated in the macro of the coal pillar, and the coal pillar is still in a stable state. Taking the measuring point 1 as an example, after the excavation of the No. 1, No. 2 and No. 3 chambers is completed, the vertical stress of the chambers is respectively increased by 0.014, 0.001 and 0MPa, which shows that the stress distribution of the coal pillar is not affected by the next chamber excavation after the coal pillar where the measuring point 1 is located is formed, so that the stability of the whole coal pillar can be predicted only by considering the stability of the coal pillar in a specific chamber excavation range in the actual engineering.
The vertical stress distribution relationship of the coal pillars with different widths after the excavation of each underground is shown in fig. 9, and because the relative positions (4 equal points) of each 6 measuring points in the coal pillars with the respective widths are the same, all the measuring points are drawn on the same abscissa and are numbered as measuring points in the coal pillars with the widths of 4.5m, 5.0m, 5.5m and 6.0m from top to bottom respectively. As can be seen from fig. 9: the vertical stress in each coal pillar is symmetrically distributed by the central shaft of the coal pillar, the vertical stress of the coal pillar in each test stage is symmetrically distributed by the central shaft of the middle mining cave, the stress reaches the peak value at the measuring point close to the free surface, and when the stress exceeds the critical stress, the stress of the coal pillar suddenly drops and releases near the free surface to be damaged. The vertical stress of the measuring points (such as the measuring points 1, 7, 13 and 19) at the same positions is linearly reduced along with the increase of the width of the coal pillar, and the evolution relation is shown in figure 9, so that the concentrated stress generated by the extraction can be reduced by reasonably increasing the width of the coal pillar, and the stability of the coal pillar is improved.
And after all the excavation is finished, the model is loaded to all the coal pillars for instability in a grading manner, and the instability evolution mechanism of the coal pillars is observed. The mechanism of vertical stress change of the coal pillar measuring points with different widths in the graded loading process is shown in fig. 10. Because the vertical stresses in the coal pillars are symmetrically distributed, only the characteristic measuring points of the coal pillars with different widths shown in fig. 10 are selected in fig. 11 for explanation, and the evolution mechanism of the vertical stresses of the coal pillar measuring points with different widths in the graded loading process is shown in fig. 11. As can be seen from fig. 11: the vertical stress in the coal pillar is increased along with the increase of the grading load, when the ultimate strength of the coal pillar with different widths is exceeded, the stress is sharply reduced, through shear failure cracks appear on the surface of the coal pillar, and the instability failure sequentially occurs on the width of the coal pillar from large to small. After the coal pillar is damaged, the vertical stress is transferred to the solid coal seams at two sides, the internal vertical stress is not suddenly reduced to 0, and the upper load of the coal pillar is supported by the residual strength. Taking the measuring points 19 and 20 in the 6.0m long coal pillar as an example, when the load is increased to the 7 th level, the stress growth rate of the middle part (measuring point 20) of the coal pillar is increased from 23.9Pa/s to 67.1 Pa/s, the stress growth rate exceeds the vertical stress in the coal pillar near the adjacent empty surface (measuring point 19) before instability damage, the vertical stress distribution form is changed from saddle shape to arch shape, the plastic zone of the coal pillar near the adjacent empty surface is fully developed due to the increase of the load, the stress is redistributed, the stress migrates to the middle part of the coal pillar with the incompletely developed plastic zone, the middle part stress is rapidly increased, and finally instability damage occurs. In actual engineering, the stress sudden-change inflection point of the coal bed in the middle of the support coal pillar can be used as an early warning sign of coal pillar instability, and the early warning sign is used as a judgment basis for early warning of instability disasters. The corresponding peak stress is increased when the coal pillar is unstably damaged along with the increase of the width of the coal pillar, and the contact area between the coal pillar and the top bottom plate is increased due to the increase of the width of the coal pillar, so that the restraint effect of the top bottom plate on the generation of the lateral deformation of the top bottom plate is increased, the development of microcracks in the top bottom plate is limited, the ultimate strength of the top bottom plate is improved macroscopically, and the unstability damage is difficult.
And further constructing a numerical simulation model to verify the effectiveness of the method, wherein the numerical simulation adopts FLAC3D finite difference software, the numerical model adopts a geometric dimension and a simulated material test prototype which are the same as those in the figure 5, physical and mechanical parameters are the same, the constitutive model adopts a Mohr-Coulomb model, the periphery of the model adopts normal displacement constraint, the bottom surface adopts three-way displacement constraint, and the top surface is a stress boundary and applies the same stress in the model test. In the simulation, the yield width of the coal pillar is used as a destabilization judgment basis, and when the ratio of the yield area width of the coal pillar to the width of the coal pillar exceeds 88%, the destabilization of the coal pillar is judged.
The criterion delta of coal pillar instability is as follows:
in the formula, Y is the single-side yield width of the support coal pillar, m; the width of a yield region in the coal pillar is 2Y; wpIs the width of the coal pillar, m; width of elastic nucleus region Wp–2Y；HpIs the thickness of the coal bed; psIs the yield zone load; peIs the elastic nuclear region load; wmIs the width of the pit, m; e is the initial elastic modulus of the coal pillar, GPa; u. of0The compression amount of the coal pillar under a certain load is m;
when delta is 0, the supporting coal pillar is in a critical state of system balance, and when delta is 0<The system can jump across the bifurcation set at 0, namely the coal pillar is destabilized and destroyed, and the inequality delta is solved<0 to 2Y>0.88WpNamely, when the ratio of the width of the plastic zone of the coal pillar to the width of the coal pillar exceeds 88%, the coal pillar is destabilized and damaged.
Taking the numerical simulation result of a 6.0m wide coal pillar as an example, the distribution range of the plastic zone near the final 3 stages of the load of the destruction is shown in FIG. 12. When the 6 th stage load is applied, the ratio of the yield zone width of the coal pillar to the coal pillar width is 80%, and the instability damage does not occur, but the ratio of the yield zone width to the coal pillar width is only 8% of the damage critical value. When the 7 th stage load was completed, the yield zone width did not increase in fig. 12(b), but the yield mass increased by 6%, indicating that: this level of loading, although increasing the yield zone of the pillar, still did not result in its destabilization. After the 8 th-level load is loaded, the yield region in fig. 12(c) is suddenly changed compared with the first 2-level load, the ratio of the width of the yield region to the width of the coal pillar is suddenly increased from 80% to 100%, which is greater than the critical value 88%, and the whole coal pillar is subjected to overall yield. Shows that: when the bearing stress of the coal pillar exceeds the limit strength, the coal pillar is subjected to shearing instability damage, the development mechanism of the plastic zone in the instability process of the numerical simulation support coal pillar is basically consistent with the vertical stress evolution mechanism corresponding to a similar material simulation test, and the instability damage simulation results of the coal pillars with other widths are consistent with the test result, so that the test result has high reliability.
Claims (4)
1. A method for determining stability of an end slope mining supporting coal pillar based on a physical simulation test is characterized by comprising the following steps:
step 1: constructing a physical simulation test model of the engineering geology to be monitored;
step 2: simulating the coal pillar instability supporting process with the load increasing by using a physical simulation test model, and collecting the vertical stress of each measuring point;
and step 3: fitting the vertical stress of the corresponding measuring point of each coal pillar to obtain a stress curve of the corresponding coal pillar;
and 4, step 4: judging whether the coal pillar is unstable or not according to the shape of the stress curve;
and 5: and in the process of the coal pillar evolving from the critical state to the destabilization state, judging the moment of the destabilization of the coal pillar according to the change of the slope of the stress curve, and taking the moment of the sudden increase of the slope corresponding to the stress curve as the starting moment of the destabilization of the coal pillar.
2. The physical simulation test-based method for determining the stability of the end slope mining supporting coal pillar according to the claim 1, wherein the step1 comprises the following steps:
step 1.1: determining the geometric similarity ratio of a physical simulation test model according to the geometric dimension of the comprehensive simulation test platform and the geological distribution condition of the project to be monitored;
step 1.2: determining the geometric dimension and the stress boundary of the model according to the geometric similarity ratio;
step 1.3: and determining the mix proportion of each simulated rock stratum by comparing the cementing materials with the design strength through an orthogonal test, and carrying out layered cast-in-place simulation on the coal seam distribution condition on a comprehensive simulation test platform according to the mix proportion.
3. The physical simulation test-based method for determining the stability of the end slope mining supporting coal pillar, according to claim 1, is characterized in that the step2 comprises the following steps:
step 2.1: loading the model according to the design requirement to enable the model to reach a converted original rock stress state;
step 2.2: excavating the underground mining after the stress is applied stably;
step 2.3: after the excavation is finished, keeping for a certain time, then applying overload pressure in a grading manner, and stopping loading when through cracks appear in the coal pillar or the stress drops below the original rock stress in the loading process;
step 2.4: and arranging pressure sensing elements at 4 equal dividing points of the width of the coal pillar, and collecting the vertical stress of each measuring point of the coal pillar with different widths.
4. The method for determining the stability of the end slope mining supporting coal pillar based on the physical simulation test as claimed in claim 1, wherein the step4 is specifically expressed as: if the shape of the curve is saddle-shaped, the applied load does not exceed the ultimate strength of the coal pillar, and the coal pillar is in a stable state; if the shape of the curve is a platform shape, the applied load is equal to the ultimate strength of the coal pillar, and the coal pillar is in a critical state; if the shape of the curve is arched, the applied load is larger than the ultimate strength of the coal pillar, and the coal pillar is in a destabilizing state.
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