WO2020252988A1 - Strength test method for rotary jet grouting-reinforced coal body - Google Patents

Abstract

Disclosed is a strength test method for a rotary jet grouting-reinforced coal body. The main steps comprise: manufacturing a coal substrate (1) with a through hole (2) in the middle; filling the through hole (2) with a coal and grout mixed liquid (3), and forming a sample (7) after pressure is maintained for a period of time; then, vertically placing the sample (7) in two direct shear boxes (9, 10) arranged one above the other, and applying an indenter to a surface of the sample (7); adjusting a vertical stress to a set value; using displacement control to slowly move the lower shear box (10), and collecting a shear force in real time; stopping the test after the sample (7) is completely sheared off, and recording parameters, such as displacement, a peak shear force, and a residual shear force; and finally, calculating, by means of data fusion, the peak value of a coal body, residual cohesion, and a residual internal friction angle. By means of providing a control group, a change in the strength of a rotary jet grouting-reinforced coal body is measured. In the method, by taking a real coal body as a substrate, the strength between structural planes of the coal body after grouting can be subjected to quantitative calculation, and a test result is closer to a site situation.

Description

Method for testing strength of rotary jet grouting to strengthen coal body Technical field

The invention relates to a method for testing the strength of a reinforced coal body, in particular to a method for testing the strength of a rotary jet grouting reinforced coal body, and belongs to the field of mining engineering.

Background technique

Due to the influence of geological structure, deep coal seams are often broken, loose and difficult to form. When tunneling in this type of stratum, it is easy to cause roof collapse, large deformation of the two sides and other phenomena that are not conducive to production. At present, in order to prevent the deformation of the coal seam roadway, most mines adopt the combined support method of grouting after trussing and walling, combined support with bolts, meshes and cables, or bolt and cable grouting. However, practice shows that these methods are often difficult to control loose and broken fences. Rock deforms and accidents happen frequently.

In recent years, the proposed method of using rotary grouting technology to strengthen loose and broken surrounding rock is gradually gaining acceptance. The method uses pre-rotation grouting in the soft coal seam in advance to form interlocking coal slurry blasting columns, forming an integral rotary blasting column arched bearing structure around the front of the coal wall, plus U-shaped shed to strengthen the support. A good control effect of surrounding rock is achieved. The on-site effect is shown in Figure 1. During rotary jetting, high pressure cuts and disperses the coal particles around the borehole. With the withdrawal of the nozzle, a uniform coal slurry mixture is formed inside the borehole. After solidification, it becomes the coal slurry jetting column. However, this method is still in the qualitative application of engineering and lacks quantitative evaluation. At present, there is a lack of quantitative method research on the strength of coal after jet grouting reinforcement, the specific increase, and the mechanism of jet grouting reinforcement.

Taking into account that the traditional combined control method is difficult to control the loose coal roadway, the use of jet grouting advanced pre-grouting can form the bearing structure well and achieve good results, but at this stage, there is no quantitative evaluation of the coal after jet grouting reinforcement. Strength characteristics. Therefore, it is very necessary to propose a concrete and operable quantitative evaluation method for the strength of coal after jet grouting.

Summary of the invention

The purpose of the present invention is to provide a method for testing the strength of the rotary jet grouting reinforced coal body, which can quantify the strength of the coal body after the jet grouting reinforcement, and the test result is more instructive.

In order to achieve the above-mentioned objectives, the present invention provides a method for testing the strength of a rotary jet grouting reinforcement coal body, which includes the following steps:

(1) Making the coal matrix: Take coal samples on site, and press the coal into a coal matrix according to the occurrence state of the coal on the site. At the same time, a circular through hole is formed in the middle of the coal matrix. The coal matrix is the size required for the experiment ；

(2) Wrap the sides and bottom of the coal base with heat shrinkable tubes to prevent the slurry from leaking from the bottom surface or immersing into the side walls when the coal slurry mixture is injected under pressure;

(3) Preparation of coal slurry mixture: prepare cement slurry with a water-cement ratio of 1:1; measure the porosity of the loose coal body and add the configured cement slurry to the loose coal according to the maximum filling rate of the cement slurry in the loose coal , Mixing and mixing to achieve uniform mixing of coal and cement slurry;

(4) The grouting chamber is connected to the nitrogen tank through a gas pipe, the gas pipe is equipped with a valve that controls the conduction or closing of the gas pipe, and the gas pipe is equipped with a pressure gauge and a pressure control device;

Place the coated coal substrate in the grouting chamber, inject the mixed coal slurry mixture into the through hole in the middle of the coal substrate, seal the grouting chamber, open the valve, and inject nitrogen under pressure from the nitrogen tank. In the slurry chamber, keep the pressure constant for a certain period of time so that the coal slurry is in full contact with the hole wall and is dense;

(5) Take out the coal slurry reinforced sample from the grouting chamber and place it in a natural state to completely consolidate the coal slurry mixture to form strength for use;

(6) Comprehensive strength test of coal matrix and coal slurry: the sample to be tested formed by the coal matrix reinforced by the coal slurry mixture is placed vertically in the upper and lower direct shear boxes arranged up and down, the upper direct shear box There is a through hole in the middle of the lower direct shear box to contain the sample to limit the position of the sample in the horizontal direction; the top surface of the sample is evenly sprinkled with a certain thickness of fine sand to ensure uniform force on the top surface; the direct shear is activated The instrument, fix the upper shear box, and apply the indenter to the top surface of the sample, adjust the vertical stress to the set value, and adopt servo control, the vertical pressure remains unchanged during the shearing process, and the displacement control is used to slowly move the lower shear Cut the box, collect the shear force in real time, stop the test after the sample is completely cut, record the displacement, peak shear force, residual shear force and other parameters;

(7) Set the next sample reinforced with coal slurry in step (5), reset the new vertical stress, repeat step (6), measure the new peak shear force and residual shear force; determine at least four Set the peak shear force and residual peak force of coal matrix reinforced with coal slurry mixture under different vertical stresses;

(8) Data fitting, calculate the peak and residual cohesion and internal friction angle of the reinforced coal; place the obtained peak and residual test data respectively on the vertical axis as the shear force and the abscissa as the two-dimensional coordinate of the vertical stress Under the system, the straight line fitting method is adopted, the inclination of the straight line obtained is the angle of internal friction, and the intercept between the straight line and the ordinate is the cohesion;

The peak test data uses the formula:

Fitting, where τ is the total peak shear stress of the sample after coal slurry reinforcement, c is the peak cohesive force of the sample after coal slurry reinforcement, and σ is the applied vertical stress,

It is the peak internal friction angle of the sample after coal slurry reinforcement. The residual test data uses the formula:

Fitting, where τ ₁ is the residual shear stress of the sample after coal slurry reinforcement, c ₁ is the residual cohesive force of the sample after coal slurry reinforcement, and σ is the applied vertical stress,

Is the residual internal friction angle of the specimen after coal slurry reinforcement;

(9) Set up a control group, and use the coal body without spray reinforcement as the comparison group, that is, the coal matrix in step (1) is not provided with through holes, and steps (2), (3), (4), ( 5) Repeat steps (6), (7) and (8) to calculate the peak value and residual cohesion and internal friction angle of the coal in its natural state.

Further, the pressing step of the coal matrix in the step (1) is:

A. On-site sampling: take a coal sample on site and record the buried depth H of the coal sample at the same time;

B, coal consistent coefficient f ₁ assay: Step A coal samples taken according to the national standard "coal consistent coefficient measuring method (GB / T23561.12-2010)" measured consistent coefficient f _1;

C. Briquette production: ① Use the forming pressure formula to calculate the forming pressure of the corresponding standard cylindrical size. The standard cylindrical sample size is D×h=50mm×100mm, and the forming pressure formula is: P=πr ² ·γH, where D Is the diameter of the standard cylinder, h is the height of the standard cylinder, r is the radius of the standard cylinder, γ is the bulk density of the overlying coal seam, and H is the buried depth of the coal sample; ②, the forming pressure P calculated by the above steps, The crushed coal sample obtained in step A is pressed into a briquette with a standard cylindrical specimen size, the pressure used is P, and the pressure holding time is t; ③, the briquette is dried;

D. Determination of briquette firmness coefficient f ₂ : Test the uniaxial compressive strength P ₀ of the briquette specimen made in step C, and according to the relationship between uniaxial compressive strength P ₀ and firmness coefficient f ₂ , That is, f ₂ =P ₀ /10, calculate the firmness coefficient f ₂ of the briquette;

E. Compare the coal sample solidity coefficient f ₁ and the briquette solidity coefficient f ₂ to determine the pressure holding time t ₀ :

If f ₁ =f ₂ , it proves that the pressure holding time t of the coal briquette in the above step C is appropriate and the parameters are accurate. The solidity of the coal briquette can reflect the solidity of the coal body. At this time, the pressure holding time t in the step C is t ₀ ；

If f ₁ ＞f ₂ , you need to repeat step C and increase the pressure holding time t, and then repeat step D to step E until the measurement result f ₁ =f ₂ , the solidity of the briquette specimen reflects the solidity of the coal body, at this time The adjusted dwell time t in step C is t ₀ ;

If f ₁ <f ₂ , it is necessary to repeat step C and reduce the holding time t, and then repeat step D to step E until the measurement result f ₁ = f ₂ , the solidity of the briquette specimen reflects the solidity of the coal body, at this time The adjusted dwell time t in step C is t ₀ ;

F. Suppressed coal matrix: the coal sample taken on site is compressed into the shape and size required by the experiment, where the compression time is t ₀ , the forming pressure P ₀ =s×γ×H, s is the upper surface area of the coal matrix, and the compression is completed After drying at low temperature.

Preferably, the step of forming the through hole in step (1) is: when the coal matrix is pressed, a rigid cylinder is vertically placed in the center of the coal matrix in advance, and the rigid cylinder is the same as the diameter of the drilled hole in the field. The height of the coal matrix is equal, and the outer surface is threaded. After the coal matrix is successfully pressed, the rigid cylinder is rotated and taken out.

Preferably, in the step (4), the pressure is kept constant at 2 MPa and maintained for 30 minutes.

Preferably, in the step (5), a sample reinforced by coal slurry is taken from the grouting chamber and placed in a natural state for 28 days.

According to the present invention provides a method for testing the strength of a coal body reinforced by jet grouting. The method quantifies the strength indicators of the coal body after jet grouting reinforcement, such as cohesion and internal friction angle. The coal strength comparison is more conducive to the design of jet grouting parameters and guides on-site engineering practice. The coal samples are collected from the site, and the pressure and time of pressing the coal matrix are calculated and tested to avoid the situation that soft coal is difficult to be processed into samples. At the same time, the compressed coal matrix conforms to the actual situation of coal occurrence on site; When the coal matrix is through holes, a threaded cylinder is placed in advance to make the inner wall of the through hole uneven, which conforms to the situation of cutting the inner wall of the coal with the rotary jet slurry on site.

Description of the drawings

Figure 1 is a schematic diagram of the on-site application of rotary jet grouting to strengthen coal;

Figure 2 is a front view of the coal matrix in step (1) of the present invention;

Figure 3 is a top view of the coal matrix in step (1) of the present invention;

Figure 4 is a schematic view of injecting a coal slurry mixture into the coal in step (4) of the present invention;

Figure 5 is a schematic diagram of the strength test of the test piece in step (6) of the present invention;

Figure 6 is a graph showing the relationship between peak strength and normal stress of a jet-grouting reinforced coal body and a natural coal body obtained according to an embodiment of the present invention;

Fig. 7 is a graph showing the relationship between residual strength and normal stress of a rotary jet-grouting reinforced coal body and a natural coal body obtained according to an embodiment of the present invention;

In the figure, 1. Coal matrix, 2. Through hole, 3. Coal slurry mixture, 4. Grouting chamber, 5. Nitrogen bottle, 6. Valve, 7. Sample, 8. Fine sand, 9. Upper shear Cut box, 10. Lower cut box.

Detailed ways

The present invention will be further described in detail below in conjunction with the drawings.

A method for testing the strength of rotary jet grouting to strengthen coal, including the following steps:

(1) Making coal matrix 1: Take coal samples at the site, and press the coal into coal matrix 1 according to the on-site coal body occurrence state, and at the same time form a circular through hole 2 in the middle of the coal matrix 1, as shown in Figures 2 and As shown in 3, the coal matrix 1 is the size required for the experiment;

Due to the poor strength and looseness of coal, especially soft coal, it is difficult to process into complete and regular-shaped specimens, so it is processed by pressing. Under different buried depth conditions, the physical properties of coal are quite different, so When compressing the coal matrix 1, factors such as the depth of burial must be considered; for the convenience of compression and subsequent testing, the coal matrix 1 can be a cylinder; the diameter of the through hole 2 is consistent with the diameter of the rotary jet, and the outer diameter of the coal matrix 1 is based on actual measurements The reinforcement range of coal slurry is generally 3 times the diameter of the through hole 2.

(2) Wrap the side and bottom of the coal base 1 with heat shrinkable tubes to prevent the slurry from leaking from the bottom surface or immersing into the side walls when the coal slurry mixture 3 is injected under pressure;

(3) Make a coal slurry mixture to simulate the coal slurry formed after the rotary grouting reinforcement of the coal body: prepare cement slurry with a water-cement ratio of 1:1; the measured porosity of the loose coal body is based on the maximum value of the cement slurry in the loose coal The filling rate is mixed with the prepared cement slurry added to the loose coal to achieve uniform mixing of coal and cement slurry;

According to the on-site rotary jet grouting reinforcement process technology, the formation of the sample after coal slurry reinforcement is the coal slurry mixture formed by filling the cement slurry into the pores of the loose coal. Therefore, the laboratory first prepares the required cement slurry to ensure the fluidity and strength of the slurry. The water-cement ratio is usually 1:1, ready for use after preparation. According to the measured porosity of the loose coal, the maximum filling rate of cement slurry, that is, the cement slurry fills all the pores of the loose coal completely, to configure the coal slurry mixture; therefore, the amount of cement slurry added to the loose coal in the laboratory is based on The pore volume ratio is added, that is, coal slurry mixture 3 is configured according to the maximum filling rate of cement slurry, and mixed and stirred for 5 minutes to achieve uniform mixing of coal and cement slurry;

(4) The grouting chamber 4 is connected to the nitrogen tank through a gas pipe, the gas pipe is equipped with a valve that controls the conduction or closing of the gas pipe, and the gas pipe is equipped with a pressure gauge and a pressure control device;

Place the coated coal substrate in the grouting chamber 4, inject the mixed coal slurry mixture 3 into the through hole 2 in the middle of the coal substrate 1, seal the grouting chamber 4, open the valve, and pressurized nitrogen from Inject the nitrogen tank into the grouting chamber 4, keep the pressure constant at 2 MPa, and maintain it for 30 minutes, so that the coal slurry mixture 3 is in full contact with the hole wall of the through hole 2, as shown in Figure 4;

(5) Take out the coal slurry reinforced sample from the grouting chamber 4, and place it in a natural state for 28 days to make the coal slurry mixture completely consolidated and form strength for use;

(6) Comprehensive strength test of coal matrix and coal slurry: As shown in Figure 5, the sample to be tested formed by the coal matrix 1 reinforced by the coal slurry mixture 3 is placed vertically in the upper direct shear box 9 and the lower direct shear. Inside the box 10, the upper direct shear box 9 and the lower direct shear box 10 have through holes for accommodating the sample to limit the position of the sample in the horizontal direction; evenly sprinkle a certain thickness of fine sand on the top surface of the sample to Ensure that the force on the top surface is uniform; start the direct shear instrument, fix the shear box 9, and apply the indenter to the top surface of the sample, adjust the vertical stress to the set value, and adopt servo control to maintain the vertical pressure during the cutting process No change, use displacement control to slowly move the lower shear box 10 to collect the shear force in real time. After the sample is completely sheared, stop the test and record the displacement, peak shear force, residual shear force and other parameters;

(7) Set the next sample reinforced with coal slurry in step (5), reset the new vertical stress, repeat step (6), measure the new peak shear force and residual shear force; determine at least four Set the peak shear force and residual peak force of coal matrix 1 reinforced with coal slurry mixture 3 under different vertical stresses;

(8) Data fitting, calculate the peak and residual cohesion and internal friction angle of the reinforced coal; place the obtained peak and residual test data respectively on the vertical axis as the shear force and the abscissa as the two-dimensional coordinate of the vertical stress Under the system, the straight line fitting method is adopted, the inclination of the straight line obtained is the angle of internal friction, and the intercept between the straight line and the ordinate is the cohesion;

The peak test data uses the formula:

Fitting, where τ is the total peak shear stress of the sample after coal slurry reinforcement, c is the peak cohesive force of the sample after coal slurry reinforcement, and σ is the applied vertical stress,

It is the peak internal friction angle of the sample after coal slurry reinforcement. The residual test data uses the formula:

Fitting, where τ ₁ is the residual shear stress of the sample after coal slurry reinforcement, c ₁ is the residual cohesive force of the sample after coal slurry reinforcement, and σ is the applied vertical stress,

Is the residual internal friction angle of the specimen after coal slurry reinforcement.

(9) Set up a control group, and use the coal body without spray reinforcement as the comparison group, that is, the coal matrix in step (1) is not provided with through holes, and steps (2), (3), (4), ( 5) Repeat steps (6), (7) and (8) to calculate the peak value and residual cohesion and internal friction angle of the coal in its natural state.

In the above experimental steps, the focus is on testing the comprehensive strength of the coal matrix 1 reinforced by the coal slurry mixture 3, and quantitatively calculate the cohesive force, internal friction angle, residual cohesion and internal friction angle of the coal after the rotary grouting reinforcement, and compare without the rotary grouting reinforcement The raw coal body provides a reference basis for the design and optimization of on-site parameter rotary injection parameters.

In order to make the coal matrix 1 more in line with the actual situation on site, further, the pressing steps of the coal matrix 1 in step (1) are:

A. On-site sampling: take a coal sample on site and record the buried depth H of the coal sample at the same time;

B, coal consistent coefficient f ₁ assay: Step A coal samples taken according to the national standard "coal consistent coefficient measuring method (GB / T23561.12-2010)" measured consistent coefficient f _1;

C. Briquette production: ① Use the forming pressure formula to calculate the forming pressure of the corresponding standard cylindrical size. The standard cylindrical sample size is D×h=50mm×100mm, and the forming pressure formula is: P=πr ² ·γH, where D Is the diameter of the standard cylinder, h is the height of the standard cylinder, r is the radius of the standard cylinder, γ is the bulk density of the overlying coal seam, and H is the buried depth of the coal sample; ②, the forming pressure P calculated by the above steps, The crushed coal sample obtained in step A is pressed into a briquette with a standard cylindrical specimen size, the pressure used is P, and the pressure holding time is t; ③, the briquette is dried;

D. Determination of briquette firmness coefficient f ₂ : Test the uniaxial compressive strength P ₀ of the briquette specimen made in step C, and according to the relationship between uniaxial compressive strength P ₀ and firmness coefficient f ₂ , That is, f ₂ =P ₀ /10, calculate the firmness coefficient f ₂ of the briquette;

E. Compare the coal sample solidity coefficient f ₁ and the briquette solidity coefficient f ₂ to determine the pressure holding time t ₀ :

If f ₁ =f ₂ , it proves that the pressure holding time t of the coal briquette in the above step C is appropriate and the parameters are accurate. The solidity of the coal briquette can reflect the solidity of the coal body. At this time, the pressure holding time t in the step C is t ₀ ；

If f ₁ ＞f ₂ , you need to repeat step C and increase the pressure holding time t, and then repeat step D to step E until the measurement result f ₁ =f ₂ , the solidity of the briquette specimen reflects the solidity of the coal body, at this time The adjusted dwell time t in step C is t ₀ ;

If f ₁ <f ₂ , it is necessary to repeat step C and reduce the holding time t, and then repeat step D to step E until the measurement result f ₁ = f ₂ , the solidity of the briquette specimen reflects the solidity of the coal body, at this time The adjusted dwell time t in step C is t ₀ ;

F. Suppressed coal matrix: the coal sample taken on site is compressed into the shape and size required by the experiment, where the compression time is t ₀ , the forming pressure P ₀ =s×γ×H, s is the upper surface area of the coal matrix, and the compression is completed After drying at low temperature.

According to the above-mentioned compression steps of coal matrix 1, through continuous experiments and correction of compression time parameters, the structure of the compressed coal matrix 1 is similar, and the mechanical parameters are consistent with those in the unexcavated environment. Therefore, the aforementioned compressed coal matrix is adopted. 1 method, the test result is also more accurate.

Because the coal body is weak, loose and broken, when the coal body is drilled and the soft coal is cut with high-pressure rotary jetting slurry, the cinder on the inner wall of the borehole is easy to fall, resulting in uneven inner wall. When the coal slurry mixture is injected 3 At this time, the interface friction between the slurry and the coal body 1 is relatively large. In order to make the test method of the present invention closer to the real situation, preferably, the step of forming the circular through hole 2 is: when the coal matrix 1 is pressed, the The center of the coal base 1 is vertically placed into a rigid cylinder, the rigid cylinder is the same diameter as the borehole, the height of the coal base 1 is the same, and the outer surface is threaded. After the coal base 1 is successfully pressed, the rigid cylinder is rotated and taken out. In this way, the inner wall of the formed through hole 2 has an uneven structure, which is closer to the real situation.

According to the present invention, a method for testing the strength of a coal body reinforced by jet grouting is quantified. The method quantifies the strength indicators of the coal body after jet grouting, such as cohesion and internal friction angle. The coal strength comparison is more conducive to the design of jet grouting parameters and guides on-site engineering practice. The coal samples are collected from the site, and the pressure and time of pressing the coal matrix are calculated and tested to avoid the situation that soft coal is difficult to be processed into samples. At the same time, the compressed coal matrix conforms to the actual situation of coal occurrence on site; When the coal matrix is through holes, a threaded cylinder is placed in advance to make the inner wall of the through hole uneven, which conforms to the situation of cutting the inner wall of the coal with the rotary jet slurry on site.

The following is an example based on the above specific implementation:

(1) Soft coal is taken from 600m underground, and its forming stress is calculated to be 15MPa based on the buried depth. At the same time, the pressure holding time during pressing is 20min measured according to the coal matrix production steps disclosed in the above specific implementation method; For soft coal, a cylindrical mold with an inner diameter of 90mm and a height of 100mm is used, and a threaded steel column with a diameter of 30mm is placed vertically in the middle. The forming stress during pressing is 15MPa and the pressure holding time is 20min. The compressed coal matrix is dried at low temperature, the diameter of the coal matrix is 90mm, the height is 100mm, and the diameter of the through hole is 30mm. Set up the comparison group, no through holes are left when the coal is pressed, and the forming stress and the pressure holding time are the same.

(2) Wrap the side and bottom of the formed coal body with heat-shrinkable tubes to prevent the slurry from leaking from the bottom surface or immersing into the side wall during coal slurry injection under pressure.

(3) Making a coal slurry mixture. The cement slurry is made according to the water-cement ratio of 1:1. According to the porosity of the coal measured on site, a suitable amount of coal particles are added to the cement slurry of equal pore volume and mixed and stirred for 5 minutes to achieve uniform mixing of the coal and cement slurry.

(4) Put the coated coal matrix in the grouting chamber, inject the coal slurry mixed in step 3, lock the chamber bolts, open the valve, and pressurized nitrogen is injected into the grouting chamber from the gas storage tank In the middle, keep the pressure constant at 2 MPa and maintain it for 30 minutes to allow the coal slurry to fully contact the pore wall and make it dense.

(5) Take out the coal slurry reinforced sample from the chamber and place it in a natural state for 28 days to fully consolidate the coal slurry to form strength for use.

(6) Comprehensive strength test of coal slurry reinforced coal matrix. The sample to be tested formed by the coal matrix anchored by the coal slurry is placed in the upper and lower direct shear boxes, and 2mm thick fine sand is evenly sprinkled on the top surface of the sample to ensure uniform stress on the top surface. Start the direct shear instrument, fix the shear box, and apply the indenter to the top surface of the sample, adjust the vertical stress to 1MPa, and adopt servo control, the vertical pressure remains unchanged during the shearing process, and the displacement control is 0.02mm/min Move the lower shear box slowly to collect the shear force in real time. After the sample is completely sheared, stop the test and record the displacement, peak shear force, residual shear force and other parameters.

(7) Take the next step 5 to make the coal slurry reinforced sample, reset the new vertical stress to 1.5 MPa, repeat step 6 to determine the new peak shear force and residual shear force. Measure the peak shear force and residual peak force of coal slurry strengthened with coal under at least four groups of different vertical stresses (1, 1.5, 2, 2.5 MPa).

(8) Put the obtained peak value and residual test data respectively in a two-dimensional coordinate system with the ordinate being the shear stress and the abscissa being the vertical stress, using a straight line fitting method, the obtained straight line inclination is the internal friction angle, The ordinate intercept is the cohesion. The peak test data uses the formula:

The result of fitting is shown in Fig. 6. The peak cohesion of coal slurry and solid is 0.69 MPa, and the peak internal friction angle of coal slurry and solid is 37.4°. The residual test data uses the formula:

The result of fitting is shown in Figure 7. The residual cohesive force of adding solid is 0.31MPa, and the residual internal friction angle of coal slurry and solid is 33°.

(9) The coal without jet grouting reinforcement is used as the comparison group. According to the method in step 8, the results are shown in Figures 6 and 6. The peak and residual cohesive forces of the coal in the natural state are calculated to be 0.41MPa and 0.11MPa, and the peak and residual internal friction angles are 26.4° and 23.7°. The comparison shows that the peak and residual cohesion and internal friction angle of the coal body reinforced by the rotary jet coal slurry have been greatly improved, and the effect of the rotary jet grouting coal body has been quantitatively evaluated.

Claims (5)

1. A method for testing the strength of rotary jet grouting to strengthen coal, which is characterized in that it comprises the following steps:

   (1) Making the coal matrix: Take coal samples on site, and press the coal into a coal matrix according to the occurrence state of the coal on the site. At the same time, a circular through hole is formed in the middle of the coal matrix. The coal matrix is the size required for the experiment ；

   (2) Wrap the sides and bottom of the coal base with heat shrinkable tubes to prevent the slurry from leaking from the bottom surface or immersing into the side walls when the coal slurry mixture is injected under pressure;

   (3) Preparation of coal slurry mixture: prepare cement slurry with a water-cement ratio of 1:1; measure the porosity of the loose coal body and add the configured cement slurry to the loose coal according to the maximum filling rate of the cement slurry in the loose coal , Mixing and mixing to achieve uniform mixing of coal and cement slurry;

   (4) The grouting chamber is connected to the nitrogen tank through a gas pipe, the gas pipe is equipped with a valve that controls the conduction or closing of the gas pipe, and the gas pipe is equipped with a pressure gauge and a pressure control device;

   Place the coated coal substrate in the grouting chamber, inject the mixed coal slurry mixture into the through hole in the middle of the coal substrate, seal the grouting chamber, open the valve, and inject nitrogen under pressure from the nitrogen tank. In the slurry chamber, keep the pressure constant for a certain period of time so that the coal slurry is in full contact with the hole wall and is dense;

   (5) Take out the coal slurry reinforced sample from the grouting chamber and place it in a natural state to completely consolidate the coal slurry mixture to form strength for use;

   (6) Comprehensive strength test of coal matrix and coal slurry: the sample to be tested formed by the coal matrix reinforced by the coal slurry mixture is placed vertically in the upper and lower direct shear boxes arranged up and down, the upper direct shear box There is a through hole in the middle of the lower direct shear box to contain the sample to limit the position of the sample in the horizontal direction; the top surface of the sample is evenly sprinkled with a certain thickness of fine sand to ensure uniform force on the top surface; the direct shear is activated The instrument, fix the upper shear box, and apply the indenter to the top surface of the sample, adjust the vertical stress to the set value, and adopt servo control, the vertical pressure remains unchanged during the shearing process, and the displacement control is used to slowly move the lower shear Cut the box, collect the shear force in real time, stop the test after the sample is completely cut, record the displacement, peak shear force, residual shear force and other parameters;

   (7) Set the next sample reinforced with coal slurry in step (5), reset the new vertical stress, repeat step (6), measure the new peak shear force and residual shear force; determine at least four Set the peak shear force and residual peak force of coal matrix reinforced with coal slurry mixture under different vertical stresses;

   (8) Data fitting, calculate the peak and residual cohesion and internal friction angle of the reinforced coal; place the obtained peak and residual test data respectively on the vertical axis as the shear force and the abscissa as the two-dimensional coordinate of the vertical stress Under the system, the straight line fitting method is adopted, the inclination of the straight line obtained is the angle of internal friction, and the intercept between the straight line and the ordinate is the cohesion;

   The peak test data uses the formula:

   

   Fitting, where τ is the total peak shear stress of the sample after coal slurry reinforcement, c is the peak cohesive force of the sample after coal slurry reinforcement, and σ is the applied vertical stress,

   

   It is the peak internal friction angle of the sample after coal slurry reinforcement. The residual test data uses the formula:

   

   Fitting, where τ ₁ is the residual shear stress of the sample after coal slurry reinforcement, c ₁ is the residual cohesive force of the sample after coal slurry reinforcement, and σ is the applied vertical stress,

   

   Is the residual internal friction angle of the specimen after coal slurry reinforcement;

   (9) Set up a control group, and use the coal body without spray reinforcement as the comparison group, that is, the coal matrix in step (1) is not provided with through holes, and steps (2), (3), (4), ( 5) Repeat steps (6), (7) and (8) to calculate the peak value and residual cohesion and internal friction angle of the coal in its natural state.
2. The method for testing the strength of rotary jet grouting-reinforced coal body according to claim 1, wherein the step of pressing the coal matrix in the step (1) is:

   A. On-site sampling: take a coal sample on site and record the buried depth H of the coal sample at the same time;

   B, coal consistent coefficient f ₁ assay: Step A coal samples taken according to the national standard "coal consistent coefficient measuring method (GB / T23561.12-2010)" measured consistent coefficient f _1;

   C. Briquette production: ① Use the forming pressure formula to calculate the forming pressure of the corresponding standard cylindrical size. The standard cylindrical sample size is D×h=50mm×100mm, and the forming pressure formula is: P=πr ² ·γH, where D Is the diameter of the standard cylinder, h is the height of the standard cylinder, r is the radius of the standard cylinder, γ is the bulk density of the overlying coal seam, and H is the buried depth of the coal sample; ②, the forming pressure P calculated by the above steps, The crushed coal sample obtained in step A is pressed into a briquette with a standard cylindrical specimen size, the pressure used is P, and the pressure holding time is t; ③, the briquette is dried;

   D. Determination of briquette firmness coefficient f ₂ : Test the uniaxial compressive strength P ₀ of the briquette specimen made in step C, and according to the relationship between uniaxial compressive strength P ₀ and firmness coefficient f ₂ , That is, f ₂ =P ₀ /10, calculate the firmness coefficient f ₂ of the briquette;

   E. Compare the coal sample solidity coefficient f ₁ and the briquette solidity coefficient f ₂ to determine the pressure holding time t ₀ :

   If f ₁ =f ₂ , it proves that the pressure holding time t of the coal briquette in the above step C is appropriate and the parameters are accurate. The solidity of the coal briquette can reflect the solidity of the coal body. At this time, the pressure holding time t in the step C is t ₀ ；

   If f ₁ ＞f ₂ , you need to repeat step C and increase the pressure holding time t, and then repeat step D to step E until the measurement result f ₁ =f ₂ , the solidity of the briquette specimen reflects the solidity of the coal body, at this time The adjusted dwell time t in step C is t ₀ ;

   If f ₁ <f ₂ , it is necessary to repeat step C and reduce the holding time t, and then repeat step D to step E until the measurement result f ₁ = f ₂ , the solidity of the briquette specimen reflects the solidity of the coal body, at this time The adjusted dwell time t in step C is t ₀ ;

   F. Suppressed coal matrix: the coal sample taken on site is compressed into the shape and size required by the experiment, where the compression time is t ₀ , the forming pressure P ₀ =s×γ×H, s is the upper surface area of the coal matrix, and the compression is completed After drying at low temperature.
3. The method for testing the strength of the rotary jet grouting to strengthen the coal body according to claim 2, wherein the step of forming the through hole in step (1) is: when the coal matrix is pressed, the center of the coal matrix is vertically erected in advance. Put straight into a rigid cylinder, the rigid cylinder is the same as the diameter of the drilled hole in the site, the height of the coal matrix is the same, and the outer surface is threaded. After the coal matrix is successfully pressed, the rigid cylinder is rotated and taken out.
4. The method for testing the strength of rotary jet grouting-reinforced coal body according to any one of claims 1 to 3, characterized in that, in the step (4), the pressure is kept constant at 2 MPa and maintained for 30 minutes.
5. A method for testing the strength of a rotary jet grouting reinforced coal body according to any one of claims 1 to 3, in the step (5), a sample of the coal slurry reinforced by the coal slurry is taken out of the grouting chamber, and placed In the natural state for 28 days.

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