To study the influence of blasting vibration on the broken rock zone around a seepage roadway and provide guidance for design of the roadway support, the broken rock zones around rock of seepage roadways under production blasting vibration are determined by onsite tests in a mining area, Daye iron mine. During the testing process, it is found that blasting vibration causes internal cracks of surrounding rocks to initiate and develop, the fracture density increases, the acoustic wave velocity of rock mass decreases, and the broken rock zones expand. At the same time, through onsite observation, it is found that blasting vibration results in crack development and formation of a water pathway to lead to surface water into the ground. The mechanical response around rock of the seepage roadway under blasting vibration is simulated by the two-dimensional realistic fracture progress analysis calculation software (RFPA2D). It is found that internal cracks of roadway surrounding rock initiate, propagate, and join up gradually, and the fracture range is expanding under the seepage water pressure, ground stress, and cyclic loads, and the broken rock zones also expand. The results from numerical simulation are consistent with the results of onsite tests. It is also found that the tensile stress appears around some cracks, leading to part of the cracks more likely to generate shear failure under the seepage water pressure during simulation.
During deep mining, the surrounding rock has always high ground stress and high hydraulic pressure [
Many previous researchers have provided qualitative descriptions about the broken zone of the surrounding rock. Jiang et al. proposed the theoretical method to predict the development around the rock plastic zone in soft rock tunnels and discussed the effect of the mechanical properties of soft rock on loosening pressure [
For a proper investigation of the width of broken rock zones in the surrounding rock, in situ tests and computer simulations are needed. The tests of broken rock zones are conducted in the seepage roadway of one mining area in Daye iron mine, after blasting is carried out.
The highest elevation in the testing mining area is +250 m, and the sublevel caving method is used in the mining area. The ore body is the concealed one, and its hanging wall rock is metamorphic diorite. Part of the contacting zone between ore and rock is with frequent geological activity, rock crushing, which belongs to the unstable area type IV and V [
At present, there are many methods of measuring the surrounding rock broken zone. These methods can mainly be divided into two categories: electromagnetic wave detection and acoustic detection methods [
The
One transmitting double receiving refraction path diagram.
As shown in Figure Sent the transceiver test probe to the hole test position with steel pipe and fix it with bracket Inflate the balloon below the hole probe by manual airbag, make it close to the hole wall, tighten the screws, and make the air the airtight gasbag to the bottom hole section form a closed space Pump water into the closed space, until it is full and the probe is completely submerged Keep the probe completely submerged state, and stimulate emission probe to test Select the appropriate stable waveform as a result of the test, record in the disk, and complete a test Loosen the screw, discharged air out of the airtight bag, and retreat the rod back to the next measuring point position according to the test step distance, repeat the above process on to the next test
The diagram for testing the surrounding rock broken zone of an underground roadway.
The measuring points are arranged in the 48th extracting drift at the -50 m level of the mining area, as shown in Figure
Layout of measuring points and holes.
The relationship between the velocity and hole depth is obtained from the data, as shown in Figure
Corresponding curve of wave velocity and hole’s depth of one hole in row 3.
In the blasting vibration caused by rock crack initiation and propagation, the test wave velocity is reduced, and the repeated production blasting vibration makes crack continue to expand, its scope is increased, resulting in surrounding broken rock zones increased.
The upper part of the rock mass is in the fissure zone nearby ground, and the roadway is in a wet state affected by the surface runoff. Due to the effect of blasting excavation, the surrounding rock of seepage roadway has more internal microcracks. The surrounding rock away from the roadway has good integrity, its internal fissure has less fracture compared with the upper zone and the surrounding broken rock zone, and it becomes a less penetrated layer, as shown in Figure
Crack distribution diagram of roadway surrounding rock.
The seepage roadway section is U-shaped; the section is 3.4 m wide and 3 m high. According to the Saint Venant principle, the calculation model is determined as 17 m wide and 15 m high, and the roadway layout is in the middle.
Considering the scene, there is only water dropping from the roadway walls before blasting mining; after a period of production blasting, there are two sites obviously changing with the weather: larger water flow when it rains and less when it is sunny. It is estimated that there is a crack through the earth surface. Using the RFPA2D-Flow system, the calculation model is established [
Calculation mode.
Mechanical parameters of the calculation model for the seepage tunnel.
Homogeneous degree (m) | Average elastic modulus (MPa) | Friction angle (°) | Average compressive strength (MPa) | Poisson ratio |
3 | 90000 | 37 | 200 | 0.25 |
Permeability coefficient (m d-1) | Porosity | Pore pressure coefficient | Injury mutation coefficient | Coupling coefficient |
The upper 1, the lower 0.001 | The upper 0.3, the lower 0.1 | 1 | 5 | 0.1 |
The actual stress condition of the research part of the mining area is as follows: the vertical stress from the above is 7.47 MPa, the horizontal pressure is from 16 MPa to 17.1 MPa from the top down and the periodical falling ore blasting vibration from the upper and horizontal direction. Therefore, the initial condition of the calculation model is set as follows: the vertical stress on the above surface is 7.47 MPa and the horizontal pressure is from 16 MPa to 17.1 MPa compliance with liner change. Upper boundary subjected to blasting vibration is assumed as the loading and unloading process, the loading process from the smallest stress to maximum stress is divided into four step, and the unloading process is divided into four steps too; the minimum is 0 MPa, and the maximum is 20 MPa; the inertia acceleration is 9.8 m/s, direction downward; the under surface constraints by displacement. The penetration pressure from the above is about 3 MPa.
The seepage roadway elevation is -50 m, and the surface vertical lever is +250 m high, so the osmotic pressure head is 300 m. When the production is not on, the water is infiltrated into the rock mass through the main crack and the branch cracks. So, the upper central has a large crack connected to the surface, and water through the crack penetrates into the rock. Because there is 300 m deep from the surface, the penetration pressure is about 3 MPa.
The internal fissure in the surrounding rock nearby the roadway is developed, and rock permeability is good and water in the fissure seeps down quickly into the roadway, so the water pressure in the surrounding rock nearby the roadway is almost zero.
Through simulation, it is clear to see the activation, extension progress that the main cracks in the upper part of the surrounding rock of the seepage roadway are in under the action of seepage pressure, ground stress, and cyclic loading. Cracks of the upper and lower surrounding rocks subjected to cyclic loading gradually expand and run through at a certain moment, as shown in Figure
Distribution of the seepage field in the surrounding rock of the seepage tunnel.
After the roadway excavation, surrounding rock is in equilibrium under the ground stress and seepage pressure, the maximum principal stress has a stress concentration phenomenon, and the minimum principal stress under the influence of seepage water pressure exists with local tension stress in some defects, as shown in Figure
Stress distribution of roadway surrounding rock after excavation.
Maximum principal stress
Minimum principal stress
The cyclic loading is loaded to simulate the vibration load produced by the blasting production. Under cyclic loading, the stress distribution in the first cycle is shown in Figures
Maximum principal stress evolution process in the first cycle.
At the peak time
At the wave valley time
Minimum principal stress evolution process in the first cycle.
At the peak time
At the wave valley time
Shear stress evolution process in the first cycle.
At the start
At the peak time
At the wave valley time
In the end
During the course of the first cycle, the minimum principal stress decreases with cycle load increasing, and the number of the areas in which the tensile stresses exist has a tendency to increase, as shown in Figures
The evolution of shear stress is similar to that of the maximum principal stress, as shown in Figure
The surrounding rock damage occurs under the action of hydraulic pressure, ground stress, and cyclic loadings, and the stress concentration area will transfer to the deep surrounding rock. According to the phenomenon of stress concentration area transferring, the damage state of the surrounding rock can be surveyed and the broken rock zones of surrounding rock can be determined. When the surrounding rock is subjected to the cyclic loading in the ground stress field and the seepage field, the new cracks in the surrounding rock will generate, the local stress will be released, and the concentrated stress area gradually transferred to the deep rock. Therefore, the lower stress area of roadway surrounding rock is the damaged area [
According to the onsite measured hole position, measured holes are arranged on the simulate section, as shown in Figure
Test diagram of the roadway surrounding broken rock zone.
Comparison between the measured and simulated results of broken rock zones in surrounding rock.
Name of holes | First | Second | Third | ||||||
---|---|---|---|---|---|---|---|---|---|
Measure (m) | Simulation (m) | Error (%) | Measure (m) | Simulation (m) | Error (%) | Measure (m) | Simulation (m) | Error (%) | |
1# | 1.28 | 1.14 | 10.9 | 1.42 | 1.26 | 11.3 | 1.50 | 1.27 | 15.3 |
2# | 1.51 | 1.26 | 16.6 | 1.54 | 1.41 | 8.4 | 1.63 | 1.46 | 10.4 |
3# | 1.51 | 1.49 | 1.3 | 1.54 | 1.50 | 2.6 | 1.71 | 1.53 | 10.5 |
4# | 1.48 | 1.25 | 15.5 | 1.56 | 1.34 | 14.1 | 1.61 | 1.35 | 16.1 |
5# | 1.37 | 1.18 | 13.9 | 1.54 | 1.26 | 18.2 | 1.70 | 1.37 | 19.4 |
The simulation value of the broken rock zones of the roadway surrounding rock is smaller than the measured value, the maximum error is 19.4%, and most of the error is within the allowable range. It shows that the results of the numerical simulation are consistent with that of the test.
Through testing the broken rock zone of surrounding rock in the water seepage roadway of Daye iron mine mining area, it is found that blasting load makes the broken zone gradually expand; the test data obtained can provide the basis for roadway support design Through the numerical simulation on surrounding broken rock zones in the seepage roadway, it is found that the cracks in the surrounding rock gradually initiate, expand, and run through under the effect of seepage pressure, ground stress, and cyclic loading, and the broken scope is enlarged. Under the effect of seepage water pressure, tensile stress appears around some cracks, which leads to the partial cracks more likely to generate shear failure. In the numerical simulation, with repeated cyclic load, part of the surrounding rock is damaged, the stress concentration area gradually transferred to the deep rock, and the range of surrounding broken rock zone of the roadway increases
The data in this article is raw data.
The authors declare that they have no conflicts of interest.
The support provided by the National Natural Science Foundation of China (Grant Nos. 51204068, 51404309), Key Laboratory of Anhui Province on Mining Response, Research Foundation of Education Bureau of Hunan Province, China (Grant No. 20A180 ), and Disaster Prevention in Deep Coal Mine (Grant No. KLDCMERDPC15102) is gratefully acknowledged.