Study on the Blasting Effect of Radial Uncoupled Parameters on Limestone

To analyze the influence of radial decoupling parameters on the blasting effect of limestone, finite element analysis was established using ANSYS/ls-dyna to analyze the effects of different radial decoupling coefficients and different media on blasting. Research has found that limestone has more cracks and more pronounced cracking when air is uncoupled. Under the condition of air decoupling, the stress waves in the rock mass are more concentrated and have higher energy, while under the condition of water decoupling, the stress waves in the rock mass diffuse faster, have a larger range of action, and have lower energy. Reasonably increasing the radial decoupling coefficient can effectively reduce damage around the borehole. For limestone formations, it is recommended to use an air decoupling method during construction, with a radial decoupling coefficient kd=2.0 for the borehole.


Introduction
China's infrastructure construction is still in a period of vigorous development, with various types of highways, railways, and municipal transportation infrastructure about to start construction, and market share will still remain high.As of the end of 2020, there were a total of 16798 railway tunnels in operation in China, with a total length of approximately 19630 kilometers [1].As of the end of 2019, the total length of highway tunnels in China reached 18.9666 million meters.Underground projects represented by tunnels usually use blasting construction methods during excavation [2][3][4].In order to fully utilize the performance of explosives and achieve good blasting effects, scientific researchers have conducted research on blasting parameters [5][6][7][8][9].Liu Xiangyu et al. [10] proposed a method for designing blasting parameters based on the complex coupling relationship between multiple blasting parameters in conjunction with the Guanyinqiao Tunnel in Chongqing.Kang Yongquan et al. [11] summarized the basic classification, functional characteristics, and applicability of interval charging and uncoupled charging, and analyzed the commonly used charging structure design parameters for tunnel excavation blasting.Gong Jiu et al. [12] found that compared to air uncoupled charges, water uncoupled charges have a higher utilization rate of explosion energy and are more conducive to improving the blasting effect.Pan Qiang et al. [13] found that the damage factor of uncoupled charge blasting decays in a power function, and the decay rate increases with the increase of uncoupled coefficient when K<K0.Zhang Liwei et al. [14] found that as the spacing between holes increases, the degree of damage between the connecting lines of the blast holes decreases, and the damage area between the connecting lines changes from continuous to discontinuous.Luo Zhihua et al. [15] used numerical simulation to study the effect of radial uncoupled charges on powder ore control.They found that when the diameter of the blast hole is 110 m, the powder ore rate is low when the diameter of the charge is 80 mm, and the bulk rate meets the requirements.
On the basis of existing research, this article further uses ANSYS/LS-DYNA to establish a finite element model, analyzes the impact of uncoupled media, radial uncoupling coefficient, and axial uncoupling coefficient on tunnel construction, and proposes an optimized design scheme.

Rock Fragmentation Mechanism and Influencing Factors
After the explosive is detonated, the surrounding rock will be subjected to a very strong impact effect in a short period of time, and impact stress waves will be generated within the rock mass.Extreme detonation gas pressure can cause the strain of the surrounding rock to exceed the limit strain, leading to failure.During the process of stress wave spreading outward from the borehole, it forces the displacement of surrounding rock particles and generates tensile stress inside the rock mass.When the tensile stress is greater than its ultimate tensile strength, the rock mass will be subjected to tensile failure, generating a series of radial cracks around the blast hole and extending to the area where the tensile stress is less than the ultimate tensile strength of the rock mass.The main failure mode of the rock mass is tensile failure, except for compressive failure around the blast hole.
The charging method where the diameter of the explosive is smaller than the diameter of the blast hole is called uncoupled charging.The uncoupled charging structure is divided into radial uncoupled charging and axial uncoupled charging, with corresponding radial uncoupled charging coefficients and axial uncoupled charging coefficients.The radial decoupling coefficient refers to the ratio of blast hole diameter b . This article only explores radial uncoupled charges.
A reasonable decoupling coefficient must meet two conditions: firstly, it cannot cause damage to the rock on the borehole wall; The second is that the cracks in the connection between the blast holes must be connected.According to engineering experience, the radial uncoupling coefficient is usually taken as values ranging from 1.5 to 3.5.

Simulation Model
The analysis of the impact of uncoupled media on the blasting effect is achieved by changing the parameters of the coupled media in the model.
In the process of analyzing the impact of radial decoupling on the blasting effect, single hole blasting was used for the rock mass, and the linear charge density was set at 0.25kg/m.The diameter of the model is 300mm, the blast hole is located in the center of the model, and the diameter of the cartridge is 32mm.Adjust the uncoupling coefficient by changing the diameter of the blast hole.The decoupling coefficient kd is set to 1.0, 1.5, 2.0, 2.5, and 3.0 in sequence.
To save computational time, the model adopts a "thin sheet" 3D computational model with a thickness of one unit size, and the unit type is 3D SOLID 164.The grid division adopts a mapping division method.X displacement constraints are applied to the left and right boundaries of the model, Y displacement constraints are applied to the upper and lower boundaries, Z displacement constraints are applied to the thickness direction, and non reflective boundary conditions are added around the edges.
The explosives and air are modeled using ALE grids, and the elements are modeled using a multi material algorithm.The rock adopts the HJC damage constitutive model and is modeled using Lagrange grids.The interaction between rocks, explosives, and air is achieved by defining coupling algorithms.

Effective Stress Field
Figure 1 shows the effective stress cloud map of the molten tuff at t=600μs after explosive initiation when the uncoupled media are air and water, and the radial uncoupled parameters are 1.0, 1.5, 2.0, 2.5, and 3.0, respectively.Analysis shows that: 1) When coupled with air, the uncoupling coefficient has a significant impact on the propagation of explosive stress waves and crack propagation.As the uncoupling coefficient increases, the barrier effect of detonation wave propagation to the hole wall is smaller, and the pressure on the hole wall decreases.However, there are more cracks and the cracking is more obvious.
2) Water can accelerate the propagation speed of detonation waves, reduce the attenuation of explosion energy during the propagation process, and the energy transfer effect of water medium coupling is better.Compared with air uncoupling, molten tuff generates fewer cracks in water uncoupling conditions, only forming a crushing circle within a certain range near the center of the explosion source.

Hole Wall Pressure
The relationship curve between the peak pressure of the obtained borehole and the uncoupling coefficient can be obtained, as shown in figure 2. It can be seen that: 1) The compressive stress acting on the hole wall gradually decreases with the increase of the radial uncoupling coefficient.When the radial decoupling coefficient kd increases from 1.0 to 2.0, the compressive stress transmitted to the borehole wall significantly decreases, and then the stress decay rate slows down.It can be seen that the radial decoupling coefficient kd=2.0 is an important turning point, and special attention should be paid during design optimization.
2) Under the water coupling condition, the pressure acting on the hole wall is much greater than the air coupling condition under the corresponding uncoupling coefficient condition, and the time it takes to reach the peak stress is longer.Analysis suggests that due to the incompressibility, high density, and high flow viscosity of water, the expansion rate of detonation products in water is slower than in air.The explosion stress wave generated in the surrounding rock has high intensity, slow attenuation, and a longer action time, which means there is a high peak explosion pressure and a strong destructive effect on the rock.

Damage Analysis
The damage caused by rock blasting excavation mainly comes from blasting shock waves and stress waves.After the explosion of explosives, the pressure on the surrounding rock is applied to the blast hole wall in the form of dynamic loads.The rock mass undergoes destruction and damage under the immense pressure compression and the expansion of the explosive gas, and cracks continue to expand, forming crushing and fracture zones.Figure 3 shows the damage cloud map of the rock mass when the uncoupling medium is air and water, and the radial uncoupling coefficients kd are 1.0, 1.5, 2.0, 2.5, and 3.0.Every 10 on the circumference of the same gun center distance.Extract the damage values of the corresponding rock units, take the average value as the damage value generated by the explosive on the rock units at the center distance, change the size of the center distance under different radial uncoupling coefficients, and obtain the damage values at different center distances.Figure 4 shows the damage distribution curves of rock mass with radial uncoupling coefficients of 1.0, 1.5, 2.0, 2.5, and 3.0 when the medium is air and water, respectively.From the analysis in figures 3 and 4, it can be seen that when the coupling medium is air, the crushing area and crack area decrease with the increase of the uncoupling coefficient.As the distance between the blasting centers increases, the degree of blasting damage first slowly decreases, then rapidly decreases, and finally approaches a plateau.Under the condition of radial uncoupling coefficient kd=1.0, the radius of the crushing zone of the surrounding rock under air and water uncoupling conditions is 13 cm and 13.5 cm, respectively.When the medium is water, a larger crushing zone will be generated because the incompressibility of water leads to a higher intensity of the explosion stress wave, which completely damages the rock near the explosive.

Conclusion and Suggestions
1) In the case of radial uncoupling, as the uncoupling coefficient increases, the number of cracks in the fused tuff increases and the cracking becomes more pronounced in the case of air uncoupling.In the case of water uncoupling, fewer cracks are generated, and only a crushing circle is formed within a certain range near the center of the explosion source.
2) Reasonably increasing the radial decoupling coefficient can effectively reduce damage around the borehole.During the construction process, it is necessary to ensure sufficient blasting volume and reduce the damage of surrounding rock to surrounding rocks.For the formation of molten tuff, it is recommended to have a radial uncoupling coefficient kd=2.0 for the borehole during construction.

Figure 1 .
Figure 1.Cloud Chart of Effective Stress Distribution.

Figure 2 .
Figure 2. Relationship curve between peak stress and uncoupling coefficient.

Figure 3 .
Figure 3. Damage distribution cloud map.Introduce damage degree (D) to characterize the degree of damage to surrounding rock: crushing zone (D=1), crack zone (0<D<1), and elastic vibration zone (D=0).As shown in the figure, different

Figure 4 .
Figure 4. Distribution pattern of damage degree under different media.