CN108222937A - Secondary top board structure quantitative analysis and its evaluation method based on inclined seam exploitation - Google Patents

Abstract

The invention discloses a secondary roof structure quantitative analysis and evaluation method based on inclined coal seam mining, and belongs to the technical field of coal seam mining in coal mines. It solves many production problems faced in the mining process of the inclined coal seam in the prior art. The evaluation method of the present invention includes: first determining the degree of damage to the bottom plate caused by mining in the upper coal seam, and then determining the degree of damage to the roof caused by mining in the lower part, that is, the damage height of the roof caused by mining in the lower part, and performing analysis by using the loose circle theory of surrounding rock in the roadway, etc. ; Then carry out actual measurement and analysis on the loosening circle of surrounding rock in the roadway, and further divide the surrounding rock into small loosening circle, medium loosening circle and large loosening circle according to the size of the loosening circle; finally, through the influence of double mining, the dynamic pressure secondary Roof structure classification index. The invention has extremely important guiding significance for controlling the surrounding rock of the roadway in the near-distance inclined coal seam under the goaf and ensuring the safe and efficient production of the mine.

Description

Quantitative analysis and evaluation method of secondary roof structure based on inclined coal seam mining

technical field

The invention relates to the technical field of coal seam mining in coal mines, in particular to a quantitative analysis and evaluation method of a secondary roof structure based on inclined coal seam mining.

Background technique

During the downward mining of short-distance coal seams, under the severe influence of repeated mining, the roadway in the lower coal seam forms a dynamic pressure roadway and a secondary roof structure formed by gangue in the goaf after mining in the upper coal seam. Compared with single coal seam mining, the roof overlying rock structure and mechanical mechanism have changed significantly. The specific manifestations are: the approaching amount of the surrounding rock of the roadway increases, the deformation is obvious, the cracks in the surrounding rock develop, the plastic zone expands, the mine pressure appears intense, and even induces the overall instability and collapse of the roadway or rock burst. There are bound to be more production difficulties faced in the mining process of inclined coal seams.

Contents of the invention

Based on the many difficulties faced in the mining process of inclined coal seams in the above-mentioned prior art, the present invention proposes a quantitative analysis and evaluation method for secondary roof structure based on inclined coal seam mining. It is of great guiding significance to control the surrounding rock of coal seam roadway and ensure the safe and efficient production of mines.

The technical problems that need to be solved for realizing above-mentioned purpose are:

Aiming at many technical problems brought about by the downward mining of close-distance inclined coal seams, a "secondary roof structure" formed by the lower coal seam roadway under the influence of repeated mining was proposed for the first time. How to focus on the analysis of the internal relationship between the roadway roof and the goaf above the floor space and the dynamic pressure secondary roof structure; how to realize the quantitative prediction and judgment of the secondary roof structure, and how to determine the judgment standard of the dynamic pressure secondary roof structure .

In order to solve the problems of the technologies described above, the present invention proposes the following technical solutions:

The quantitative analysis and evaluation method of the secondary roof structure based on inclined coal seam mining is characterized in that it includes the following steps in sequence:

a. Determine the degree of damage to the floor caused by the mining of the upper coal seam, that is, the maximum plastic failure depth h _max of the floor rock layer in the goaf, and calculate it by formula (1):

In formula (1), M—mining height, m; k—stress concentration factor, k=2; γ—average bulk density of overlying strata, kN/m ³ ; H—coal seam burial depth, m; c—cohesive force of coal seam, MPa ; φ—coal seam internal friction angle, °; f—coal seam and roof-floor friction coefficient, ξ—triaxial stress coefficient, φ _f — internal friction angle of floor rock formation, °;

b. Determine the degree of damage to the roof caused by mining below, that is, the damage height of the roof caused by mining below, which is determined by using the loose circle theory of surrounding rock in the roadway, which is divided into the following sub-steps:

b1. The surrounding rock after roadway excavation is divided into loose circle, plastic zone and elastic zone;

b2. Carry out theoretical analysis of the loose circle of surrounding rock in the roadway:

After the rock mass is excavated, the stress is redistributed, which makes the secondary stress present the stress distribution characteristics of elastic and plastic coexistence. In order to simplify the theoretical analysis, it is considered that the circular roadway is excavated in a continuous, homogeneous, isotropic, and uniform stress field where the original rock stress is P _0. When the secondary stress of the tunnel wall exceeds the yield stress of the rock mass, the tunnel wall The rock mass produces a plastic zone, which is a limit equilibrium state. According to elastic mechanics, the balance differential equation satisfied by the stress at each point is shown in Equation 2:

In the formula (2), σ _θ — hoop stress, kg/cm ² ; σ _r — radial stress, kg/cm ² ; r — radius of any point examined in the plastic zone, cm;

b3. In the plastic zone, since the rock mass has just begun to fail and is in a state of limit equilibrium, the stress at each point should satisfy the Mohr-Coulomb strength criterion, as shown in formula (3):

In the formula: c—the cohesive force of the surrounding rock, kg/cm ² ; —internal friction angle, °; σ _θ —circumferential stress, kg/cm ² ; σ _r —radial stress, kg/cm ² ; r—radius of any point examined in the plastic zone, cm.

Substituting formula (3) into formula (2), the general solution of the equation is shown in formula (4):

c ₁ is an undetermined constant, determined according to the boundary conditions: on the periphery of the roadway, r = r ₀ , σ _r =P _i ; P _i is the reaction force of the support to the surrounding rock;

Substitute r ₀ and P _i into formula (4) to get formula (5):

b4. Substituting formula (5) into formulas (3) and (4), the stress in the plastic zone is shown in formula (6):

b5. Substituting formula (6) into the condition that the elastic stress satisfies σ _r +σ _θ = 2P ₀ to get formula (7):

c. Carry out actual measurement and analysis on the loosening circle of surrounding rock in the roadway, and further divide the surrounding rock into small loosening circle, medium loosening circle and large loosening circle according to the size L of the loosening circle;

The small loose circle refers to 0≤L≤40cm; the medium loose circle refers to 40<L≤150cm; the large loose circle refers to L>150cm.

d. Based on the degree of damage to the floor caused by mining in the upper coal seam described in step a and the degree of damage to the roof caused by mining in the lower part described in step b, that is, to comprehensively determine the classification index H of the dynamic pressure secondary roof structure through the influence of double mining _G , which is calculated according to formula (8):

H _G = h _max + L (8);

That is to say, when the coal seam spacing h _c ≤ H _G , the roadway roof is regarded as the secondary roof mechanical structure; when h _c > H _G , it is regarded as the primary roof mechanical structure.

As a preferred solution of the present invention, the damage to the floor caused by the mining of the upper coal seam is formed by the movement of the passive stress zone to the goaf under the action of the active stress zone and the transition stress zone.

Preferably, when the surrounding rock is a small loose circle, use shotcrete support or bare; when the surrounding rock is a medium loose circle, use sprayed layer local support or sprayed metal mesh partial support; when the surrounding rock is large loose When the circle is closed, the sprayed metal mesh is used for partial support or comprehensive treatment.

Beneficial technical effects brought by the present invention:

(1) Aiming at the technical difficulties brought about by the down mining of close-distance inclined coal seams, a quantitative analysis and evaluation method for the secondary roof overlying rock structure used in multi-coal seam mining is proposed for the first time;

(2) Based on the loose circle theory of roadway surrounding rock, detect the plastic zone range of dynamic pressure roadway surrounding rock and classify it, objectively evaluate the overall stability of the roadway, and provide theoretical guidance and scientific basis for surrounding rock control and support optimization .

Description of drawings

The present invention will be further described below in conjunction with accompanying drawing:

Fig. 1 is the mechanical structure diagram of secondary roof of dynamic pressure roadway of the present invention;

Fig. 2 is the mechanical morphology diagram of mining damage to the coal seam floor;

Fig. 3 is a schematic diagram of the loose circle of the roadway surrounding rock;

Figure 4 is a diagram of stress distribution around the roadway;

Figure 5 is a mechanical model diagram.

In the figure, 1. roof of upper coal seam, 2. goaf of upper coal seam, 3. isolated rock formation, 4. lower coal seam, 5. loose circle, 6. plastic zone, 7. elastic zone.

In Fig. 1, l, the distance between the dynamic pressure roadway and the secondary roof; α, the inclination angle of the two coal layers; h _c , the distance between the coal seams.

Detailed ways

The present invention proposes a quantitative analysis and evaluation method of secondary roof structure based on inclined coal seam mining. In order to make the advantages and technical solutions of the present invention clearer and clearer, the present invention will be described in detail below in conjunction with specific examples.

Aiming at many technical problems brought about by the downward mining of close-distance inclined coal seams, a "secondary roof structure" formed by the lower coal seam roadway under the influence of repeated mining was proposed for the first time. Based on this, the adverse effect of the secondary roof of the dynamic pressure roadway on the stability of the surrounding rock is analyzed, and then sufficient technical guidance and theoretical basis are provided for the scientific selection of a reasonable support scheme.

1. As shown in Figure 1, the inclination angle of the two coal seams is α, and the distance is h _c . The secondary roof structure refers to the upper coal seam roof 1 formed by rock beam fracture after the mining of the upper coal seam at a certain distance l above the dynamic pressure roadway. The remaining upper coal seam goaf 2, the existence of the upper coal seam goaf 2 has a great impact on the mine pressure of the lower roadway. Its main feature is that the distance between the upper coal seam goaf 2 and the lower coal seam 4 roadway roof is small, and the isolation rock layer 3 between them is affected by double mining, the joints and fissures are relatively developed, and the isolation rock layer 3 is basically in a plastic state; The existence of the goaf has severely damaged the integrity of the rock formation, the potential for horizontal force transmission, and the natural balance structure. The influence of gangue, some basic roof beams and coal pillars in the upper goaf on the mine pressure of the lower roadway is compared. severe, the stability of the roadway is poor, and generally there is obvious deformation, and it is very easy to cause instability and damage.

2. Determination of the judgment standard " _HG " of the dynamic pressure secondary roof structure

The roof structure classification index " _HG " is based on the damage to the floor by upper mining and the damage to the roof by lower mining (entrying), that is, it is determined comprehensively through the impact of double mining.

(1) The damage to the bottom plate caused by the mining of the upper coal seam

According to the existing research: during the mining process, under the action of supporting pressure, the floor rock mass forms three zones according to its location, force characteristics and moving path, Ⅰ is the active stress zone, Ⅱ is the transition stress zone, and Ⅲ It is the passive stress zone, as shown in Figure 2. The passive stress zone moves to the goaf under the action of the active stress zone and the transitional stress zone, resulting in floor rock formation failure.

The maximum plastic failure depth h _max of the floor rock layer in the goaf is:

In the formula: M—mining height, m; k—stress concentration factor, k=2; γ—average bulk density of overlying strata, kN/m ³ ; H—coal seam burial depth, m; c—cohesive force of coal seam, MPa; φ— Coal seam internal friction angle, °; f—coal seam and roof and floor friction coefficient, ξ—triaxial stress coefficient, φ _f — internal friction angle of floor rock formation, °.

(2) Damage to the roof caused by mining (digging) below

The roof strata of roadways with near-distance inclined coal seams are greatly affected by double mining, the strength of the strata is reduced, and the joints and fissures are seriously developed, so it can be approximated as a loose strata. The damage height of the roof caused by mining (entrying) below can be determined by using the loose circle theory of the surrounding rock of the roadway.

The loose circle of surrounding rock is an important basis for evaluating the stability of roadway surrounding rock and making reasonable support design. The loose ring support theory of roadway surrounding rock was proposed by Professor Dong Fangting of China University of Mining and Technology after in-depth research on the state of roadway surrounding rock. The study found that the existence of the surrounding rock loose circle is an inherent characteristic of the roadway, and its range (thickness value L) can currently be measured by drilling imagers (surrounding rock fissure detectors), acoustic wave meters or multi-point displacement meters. The main object of the roadway support is the crushing and deformation force generated during the generation and development of the surrounding rock loose circle. The source of the tensile force of the bolt is the occurrence and development of the loose circle; It is divided into three categories: small, medium and large, and then the corresponding support mechanism and optimization scheme are obtained.

① Stress state after roadway excavation

Before the excavation of the roadway, the rock mass was in the original stress state. After the excavation of the roadway, the internal stress of the rock mass was redistributed, and the stress on the surrounding rock changed from three directions to two directions and one direction, resulting in different rock strengths. The degree of reduction destroys the original state of stress balance and produces stress concentration. If the concentrated stress value of the surrounding rock is less than the characteristic strength of the rock after falling, the surrounding rock is in an elastic-plastic state, and the surrounding rock is stable; otherwise, the surrounding rock will be damaged, and this damage will gradually expand to the deep, until it reaches a new three-dimensional stress balance state So far, a relaxation and fracture zone has appeared in the surrounding rock at this time, and this relaxation and fracture zone due to stress is called the surrounding rock loose zone, and its mechanical characteristics are characterized by stress reduction. The surrounding rock after roadway excavation can be divided into loose and cracked zone (loose circle 5), plastic zone 6 and elastic zone 7, as shown in Figure 3.

The larger the loose circle, the more serious the surrounding rock damage and the more difficult the support. The main factors affecting the size of the roadway surrounding rock loose circle are: surrounding rock stress, rock strength, geological structure, roadway span, roadway shape, excavation time, groundwater effect, Construction methods, support methods and support strength, etc., the loose circle is a comprehensive index under the joint action of the above-mentioned various influencing factors. Reasonable determination of the size of the surrounding rock loose circle of the roadway has important guiding significance for evaluating the stability of the surrounding rock of the roadway and the support effect, optimizing the roadway support method and support parameters, and ensuring the safety of the roadway support.

②Theoretical analysis of the loose circle of surrounding rock in the roadway

After the rock mass is excavated, the stress is redistributed, which makes the secondary stress present the stress distribution characteristics of elastic and plastic coexistence. In order to simplify the theoretical analysis, it is approximately considered that the excavation of the circular roadway is continuous, homogeneous, isotropic, and in a uniform stress field where the original rock stress is P ₀ , and its mechanical model is shown in Figure 4 and Figure 5.

When the secondary stress of the cave wall exceeds the yield stress of the rock mass, the cave wall rock mass produces a plastic zone, which is a limit equilibrium state. According to elastic mechanics, the equilibrium differential equation satisfied by the stress at each point is:

In the plastic zone, since the rock mass has just begun to fail and is in a state of limit equilibrium, the stress at each point should satisfy the Mohr-Coulomb strength criterion:

In the formula: c—the cohesive force of the surrounding rock, kg/cm ² ; —internal friction angle, °; σ _θ —circumferential stress, kg/cm ² ; σ _r —radial stress, kg/cm ² ; r—radius of any point under consideration in the plastic zone, cm.

Substituting formula (2) into formula (1), the general solution of this equation is:

c ₁ is an undetermined constant, which can be determined according to the boundary conditions: on the periphery of the roadway, r = r ₀ , σ _r =P _i (P _i is the reaction force of the support to the surrounding rock). Substitute r ₀ and _Pi into formula (3) to get:

Substituting formula (4) into formula (2) and formula (3), the stress in the plastic zone is:

At the junction of the elastic zone and the plastic zone (r=R ₀ ), the stress is continuous, so σ _r and σ _θ should not only satisfy the stress distribution law of the elastic zone, but also satisfy the stress distribution law of the plastic zone. Usually, this The distance from the elastic boundary point to the axis of the hole is called the radius r of the plastic ring. Substituting equation (5) into the condition σ _r +σ _θ = 2P ₀ satisfied by the elastic stress, we get:

From formula (6), it can be concluded that the greater the stress of the original rock where the roadway is located, the larger the inelastic deformation zone; the greater the reaction force of the support to the surrounding rock, the smaller the radius of the inelastic deformation zone. , the radius of the inelastic deformation zone is the maximum value; the smaller the rock strength, the larger the inelastic deformation zone; reflected in the area of the roadway, the larger the radius of the roadway, the larger the inelastic deformation zone. It shows that the surrounding rock loose circle of the roadway is closely related to the strength of the surrounding rock, the magnitude of the force and the support condition.

③ Actual measurement and analysis of the loose circle of surrounding rock in the roadway

In this test, the CXK6 mining intrinsically safe borehole imager (using existing technology) is used to record the surrounding rock rupture image in real time to determine the size of the surrounding rock loosening circle in the dynamic pressure roadway. CXK6 Mine Intrinsically Safe Borehole Imager is suitable for all kinds of borehole detection under harsh conditions in coal mines, such as observing bolt and cable holes, gas discharge holes, advanced detection holes, coal seam exploration holes, geological exploration holes and The loose circle of surrounding rock can accurately reveal the state of surrounding rock in underground engineering. Combined with relevant analysis software, it can more accurately predict the occurrence of rock formations, the development and expansion of cracks, and the scope of loosening and damage. It can also be used for parameter design of underground engineering bolt support, surrounding rock grouting reinforcement, roadway repair, etc., and evaluation of underground engineering construction quality and support schemes Provide reliable, true and effective measured data and analysis for coal mines and geotechnical engineering.

④ Classification of loose circles of surrounding rock in roadways

Table 1 Classification table of roadway surrounding rock loose circle

The purpose of surrounding rock classification is to correctly evaluate the support difficulty and identify the main support objects, so as to reasonably determine the support parameters and construction technology, and provide a basis for the design of similar roadway support. The size of the surrounding rock loose circle not only includes many factors that affect the stability of the surrounding rock, but also reflects the interaction results of many factors. It is a comprehensive classification index. Classifying surrounding rocks with surrounding rock loose circles has the following outstanding advantages:

a. Bypassing the difficult problems such as original rock stress, surrounding rock strength, structural surface properties measurement, etc., but focusing on their influence results, that is, the loose circle is a comprehensive index including the influence of multiple factors;

b. The loose ring is measured on site, without any assumptions in important aspects;

c. After the size of the loose circle is obtained through actual measurement, it is intuitive and simple to determine the support parameters, and it is convenient for on-site application;

d. Use a single comprehensive index to classify, and the repeatability and reliability of the classification do not vary with people's experience;

e. The theoretical and practical basis of the loose circle classification method is relatively solid, and it forms a technical series with the design of bolt support.

Combined with the support mechanism of bolting and spraying, according to the support theory of the roadway surrounding rock loose circle, according to the size L of the loose circle, the surrounding rock is divided into a small loose circle (0-40cm, including the endpoint value), a medium loose circle (40-150cm, no Including 40, including 150) and large loose circle (greater than 150cm) three categories and six subcategories, see Table 1.

Based on the above analysis, it can be seen that the prediction standard for the overlying rock structure of the secondary roof is H _G =h _max +L. That is to say, when the coal seam spacing h _c ≤ H _G , the roadway roof is regarded as the secondary roof mechanical structure; when h _c > H _G , it is regarded as the primary roof mechanical structure.

It should be noted that any simple replacement of these terms by those skilled in the art under the inspiration of the present invention shall fall within the protection scope of the present invention.

Claims (3)

1. a kind of secondary top board structure quantitative analysis and its evaluation method based on inclined seam exploitation, which is characterized in that successively Include the following steps：

A, determine that superjacent adopts the extent of the destruction to bottom plate, i.e. gob floor rock stratum maximum plastic failure depth h_max, lead to Cross formula (1) calculating：

In formula (1), M-mining height, m；K-the factor of stress concentration, k=2；γ-overlying rock volume-weighted average, kN/m³；H-coal seam Buried depth, m；C-coal seam cohesive force, MPa；φ-coal seam internal friction angle, °；F-coal seam and roof and floor friction coefficient, ξ-triaxial stress coefficient,φ_f- floor strata internal friction angle, °；

B, determine that the extent of the destruction to top plate is adopted in lower section, i.e. the destruction height to top plate is adopted in lower section, utilizes roadway surrounding rock pine Moving-coil theory determines, is divided into following sub-step：

B1, the country rock after roadway excavation is divided into relaxation zone, plastic zone and elastic region；

B2, the theory analysis for carrying out Exploring Loose Rock Country in Tunnels：

Rock mass is after excavating stress redistribution so that the stress distribution feature that secondary stress elastoplasticity occurs and deposits, for letter Change theory analysis, it is believed that circular tunnel digging is P in continuous, homogeneous, isotropism, in the stress of primary rock₀Uniform stress field In, when the secondary stress of hole wall exceeds the yield stress of rock mass, then wall rock mass in hole generates plastic zone, and plastic zone is that a kind of limit is put down Weighing apparatus state, according to Elasticity, shown in the balance differential equation such as formula (2) that each point stress is met：

In formula (2), σ_θ- circumference stress, kg/cm²；σ_r- radial stress, kg/cm²；Any point investigated in r-plastic zone Radius, cm；

B3, in plastic zone, since rock mass just starts to destroy, in state of limit equilibrium, each point stress should all meet More- Coulomb strength criterion, as shown in formula (3)：

In formula：The cohesive force of c-country rock, kg/cm²；- internal friction angle, °；σ_θ- circumference stress, kg/cm²；σ_r- radially should Power, kg/cm²；The radius at any point investigated in r-plastic zone, cm；

Formula (3) is substituted into formula (2) to obtain shown in equation general solution such as formula (4)：

c₁For undetermined constant, determined according to boundary condition：On tunnel-surrounding, r=r₀, σ_r=P_i；P_iIt is stent to country rock Counter-force；

By r₀And P_iSubstitution formula (4) obtains formula (5)：

B4, formula (5) substitution formula (3), (4) are obtained shown in plastic zone stress such as formula (6)：

B5, formula (6) is substituted into the condition σ that elastic stress meets_r+σ_θ=2P₀Obtain formula (7)：

C, Exploring Loose Rock Country in Tunnels surveyed, analyzed, country rock is divided into small loosening according further to the size L of relaxation zone Circle, middle relaxation zone and big relaxation zone；

The small relaxation zone refers to 0≤L≤40cm；The middle relaxation zone refers to 40 ＜ L≤150cm, the big loosening Circle refers to L ＞ 150cm；

D, it is adopted based on the superjacent described in step a and the lower section described in the extent of the destruction of bottom plate and step b is adopted to top plate Extent of the destruction, that is, pass through the dual influence synthesis adopted and determine dynamic pressure secondary top board structure classification indicators H_G, according to formula (8) It calculates and obtains：

H_G=h_max+L (8)；

I.e. as coal seam spacing h_c≤H_GWhen, back is considered as secondary top plate mechanical structure；Work as h_c＞ H_GWhen, then it is considered as primary top plate Mechanical structure.

2. secondary top board structure quantitative analysis and its evaluation method according to claim 1 based on inclined seam exploitation, It is characterized in that：Superjacent adopts the destruction to bottom plate, is in active stressed zone and unfair stress area by passive stressed zone Move what is formed to goaf under effect.

3. secondary top board structure quantitative analysis and its evaluation method according to claim 1 based on inclined seam exploitation, It is characterized in that：When country rock is small relaxation zone, using shotcrete support or nude；When country rock is middle relaxation zone, use The supporting of spray-up part or the part supporting of spray-up metal mesh；When country rock is big relaxation zone, using the part supporting of spray-up metal mesh or Comprehensive treatment.