Research on damage evolution mechanism of layered rock mass under blasting load

doi:10.21203/rs.3.rs-4631673/v1

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Research on damage evolution mechanism of layered rock mass under blasting load

https://doi.org/10.21203/rs.3.rs-4631673/v1

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Rock mass consisted of lots of discontinuities, such as faults, joints, etc, and the layered joints are a common kind of rock mass structure. The joints affected the stress wave propagation, and blasting is one economical and efficient rock breaking method for rock mass engineering. So, rock mass fragmentation effect and construction progress are affected by these layered joints. The numerical studies were carried out to analyze the damage evolution process of intact rock and rock mass with layered joints subjected to blasting load based on the RHT model in the LS-DYNA software, and the effect of the location of initiation points, fracture distribution on dynamic damage evolution of the rock mass was discussed. It is useful to know the damage mechanism of the rock mass with layered joints and to improve the effect of rock mass fragmentation by blasting. These results have very important theoretical significance and application value for optimization of the blasting construction technology.

blasting

simulation

rock mass

layered joint

initiation point

damage evolution

Rock mass is a common geological body in engineering construction. In order to achieve effective excavation and construction of rock mass engineering, drilling and blasting method is used in rock mass engineering construction because of its speed, convenience, and unrestricted geological conditions. When drilling and blasting method is used, it is necessary to realize the crushing of rock mass and the excavation and shaping of reserved rock mass. Therefore, it is necessary to study the damage to rock mass under blasting load, so as to ensure the safety and efficiency of blasting engineering. At present, the damage effect of blasting on rock mass is studied.

The rock-breaking process of blasting is the result of the explosion stress wave and the expansion of the blasting gas. Banadaki (2010) carried out a series of laboratory-scale blast to analyze dynamic fracture patterns of Laurentian and Barre granites. Xie et al. (2017) investigated the resistance of the in-situ stress to blasting waves and damage is determined using the RHT model, and analyzed damage evolution process of rock mass under compressive stress waves, reflected tensile stress waves, and superimposed stress waves. Yang et al. (2019) investigated the interaction mechanism between blast stress waves and cracks using the transmitted explosion dynamic caustics optical experiment system. Mahdi et al. (2020) conducted research and review on the existing models for accurate prediction explosion-induced crushed and cracked zones. Qiu et al. (2021) proposed a modified mixed-mode caustics interpretation on crack-wave interaction. Li et al. (2023) investigated the radial fracturing of rock masses under combined in-situ stress and blasting load using experimental, theoretical and numerical method.

Rock mass with layered joints is a commonly encountered geological structure in rock blasting excavation projects (Li et al., 2003; Lin et al., 2020; Huang et al., 2023). This structure increased the number of the discontinuities, meanwhile the presence of these discontinuities affected on the stress wave propagation, which made the damage evolution mechanism of rock mass with layered joints under blasting load different from that of the intact rock. Therefore, it is necessary to study the damage evolution mechanism of rock mass with layered joints under blasting load.

In the early 1990s, Rossmanith (1993) studied the effect of stress waves on the strength of the jointed rock by combining the constitutive model of viscoelastic interface materials with the damage law of continuous medium, and established a model for the development of rock joint damage. Wu et al. (2001) used the theory of elasticity and stress wave propagation to theoretically analyze the effect of rock structural plane on the results of smooth blasting, and pointed out that the angle between the structural plane of the smooth blasting effect and the blasting-induced fracture plane is an important factor in controlling the blasting effect. Li et al. (2010, 2015) derived a wave propagation equation and established an improved equivalent viscoelastic medium method to determine the property of layered rock masses, and then discusse effects of some parameters of layered rock masses on wave propagation. Li et al. (2015) taken into account the propagation law of stress waves, analyzed the explosive stress wave propagation from a microscopic view. Fan et al. (2020) concluded that the propagation direction induced difference of transmission coefficient (PDIDTC) commonly exists in the complex rock mass when the wave impedance on two sides of the joint are different, and find that the PDIDTC of particle velocity and stress are not influenced by the frequency, joint stiffness and waveform, but are influenced by the wave impedance ratio significantly.

In addition to conducting theoretical research, experimental research was also conducted to investigate the influence of joints on wave propagation in rock masses. Chen et al. (2015) performed experimental investigation on the propagation of stress waves in rock masses with rough joint surfaces, and the results showed that the contact surface and stiffness of the joints have an impact on wave propagation. Zou et al. (2019) studied the seismic response on obliquely incident wave propagating in jointed rock masses, found that reflected and transmitted coefficients were affected by the incident angle of the waves, and analyzed the normal and tangential dynamic characteristics of the joints. Li et al. (2023) studied the propagation of the blasting stress wave in jointed rock by the method of laboratory explosion tests, and found that the first peak strains in surrounding rock masses and adjoining structures were tremendously influenced by the joint orientation.

From the above research, it can be seen that joints have a significant impact on the propagation of stress waves in rock masses, and inevitably have a significant impact on the blasting effect in rock blasting engineering. In this paper, the blasting damage process of rock masses in layered rock mass with joints or without joints was studied based the RHT model in LS-DYNA software, and the damage evolution process of rock masses under different initiation methods was analyzed, the results would provide reference for blasting excavation design in layered rock masses.

2.1 Numerical model

A numerical model has a length of 30 m and a height of 25 m, the explosive was located in the center, the diameter of blasting hole was 40 mm, which is the same as the diameter of the charge. The length of the charge was 2 m, and the length of the blocked section of blasting hole was 1 m. The upper part of this model was the critical surface, the left part, the right part and the lower part of this numerical model were the bedrock, and the all boundaries were the non-reflective boundary, the model is shown in Fig. 1.

[Fig. 1 Diagram of the blasting hole]

2.2 Constitutive model

Since RHT model can better characterize the effect of strain rate, damage and brittle-ductile deformation on strength of rock under fiercely dynamic load. The P-α porous medium model is used to describe the nonlinear response of rock under high pressure. it is widely used to analyze the damage and failure characteristics of rock under explosive explosion. Therefore, the RHT constitutive model is adopted to analyze and calculate the numerical model (Banadaki, 2010; Xie et al., 2017). The material chosen is marble, and its basic parameters are shown in Table 1.

Table 1

Input parameters for rock (from Dehghan Banadaki and Xie ( 2010, 2017)).
Parameter	Value	Parameter	Value
Mass density RO (Kg/m³)	2660	Poisson's ratio	0.19
Elastic shear modulus(GPa)	21.9	Relative tensile strength FT*	0.04
Compressive strength (MPa)	167.8	Relative shear strength FS*	0.21

[Table 1 Input parameters for rock (from Dehghan Banadaki and Xie ( 2010, 2017))]

In order to characterize the high-pressure state of explosive after explosion, JWL equation of state is adopted, which can predict the explosive explosion response in a large pressure range, it is widely used in numerical simulation. The model parameters are shown in Table 2.

Table 2

Parameters of the explosive material and JWL equation of state (from Li et al.( 2011))
\({\rho }_{\text{e}} (\text{k}\text{g}/{\text{m}}^{3})\)	\(\text{V}\text{o}\text{D} (\text{m}/\text{s})\)	\({P}_{\text{C}\text{J}}\) \(\left(\text{G}\text{P}\text{a}\right)\)	\(A\) \(\left(\text{G}\text{P}\text{a}\right)\)	\(B\) \(\left(\text{G}\text{P}\text{a}\right)\)	R₁	R₂	\({\omega }\)	\({E}_{0}\) \(\left(\text{G}\text{P}\text{a}\right)\)
1,300	4,000	5.2	214.4	0.182	4.2	0.9	0.15	4.192

[Table 2 Parameters of the explosive material and JWL equation of state (from Li et al.( 2011))]

2.3 Initiation method

Blasting in rock mass is a complex dynamic process. The key to study the damage evolution process of rock mass under blasting load is to find the relationship between the damage variable and the density of microcracks within the rock. The two-dimensional model of plane stress is widely adopted to simulate the dynamic response of rock mass blasting, and these results are more intuitive, and the grid deformation is used to describe the stress state, and the calculation is fast and stable.

Some researchers showed that the location of the initiation point on the blast vibration has a significant impact. The initiation point of bottom detonation is located at the bottom of the hole, the initiation point of midpoint detonation is located in the middle of the hole, and the initiation point of top detonation is located at the mouth of the hole. From the simulation results, we know that the location of the initiation point determined the propagation direction of the blast wave, which caused different blast vibration responses (Gao et al., 2011). In this study, the bottom detonation, midpoint detonation and top detonation were simulated, and analyzed the effect of location of the initiation point on damage evolution process and the impact damage range. The initiation locations are shown in Fig. 2.

[Fig. 2 Three kinds of initiation point locations :(a)bottom initiation;(b)middle initiation;(c)top initiation]

2.4 Result analysis

2.4.1 Bottom detonation

When the charge was detonated at the bottom of the blasting hole, the produced detonation wave propagated from the bottom to top along the blasthole, the stress wave front formed by detonation wave propagated rapidly to the other end of the charge as a cone. Since the shock wave generated by the post detonation would strengthen the stress field that had been formed due to the earlier detonation, resulting in a stress superposition effect on the upper rock mass (Ding et al., 2021), and then high stress zone and high the energy zone formed, so that the damage range of the rock in the direction of stress wave propagation was relatively large, as shown in Fig. 3(a-b).

With the propagation and superposition of stress wave, the fierce reflected wave was produced when the stress wave encountered the free surface, so the damage to rock at the top is mainly tensile damage due to tensile stress caused by the reflected wave. As seen from Fig. 3(c-e), although the stress level at the top is slightly higher than that at the bottom, the damage extent at the top is larger. That is conducive to broke the rock near the blasthole. For the rock at the bottom of the hole, the damage range is smaller than that of the blasting at the top.

Below the blasting hole, the stress wave propagation attenuation was fast, so compression and shear damage to the rock is less. However, due to the low tensile strength of rock, a large range of tensile damage would still occur below the detonation point, as shown in Fig. 3(f). The detonation at the bottom of the blasting hole is characterized by a high residual ridge after the explosion.

[Fig. 3 Schematic diagram of cross-section damage at the bottom of the blasting hole]

2.4.2 Midpoint detonation

The rock damage mode by midpoint detonation was obviously different. The shock wave and stress wave propagate from the middle to top and to bottom in two directions of blasthole. As the subsequent explosives ware continuously detonated, the corresponding stress waves would also be superimposed in two different directions, which caused the formation of two high energy regions at the upper and bottom of the blasthole, respectively. Eventually, the damage range at the mouth and bottom of the blasthole was large and the shape of their damage areas was similar as shown in Fig. 4. The results showed that the midpoint detonation method is beneficial to the fragmentation of the rock at the mouth and bottom of the blasting hole (Fan et al., 2023).

[Fig. 4 Schematic diagram of cross-section damage at the middle of the blasting hole]

2.4.3 Top detonation

When the charge was detonated at the top of the blasting hole, the shock wave and stress wave propagated from top to bottom, and a high energy region appeared at the bottom of this blasthole. From the beginning of the damage evolution process (Fig. 5a) to the end (Fig. 5e), the damage area gradually formed the inverted funnel shape, and the rock at the bottom of blasthole was subjected to a stronger collapsing load, while the compression damage around the hole was obviously less, mainly shear damage and tensile damage, which was prone to large rock masses (Leng et al., 2021). Using this detonation method, the damage at the hole mouth was not sufficient, and there was a great possibility of leaving large rock masses. Since the damage to rock at the bottom of the hole was too large, which was not easy to handle, adding engineering safety hazards.

[Fig. 5 Schematic diagram of cross-section damage at the top of the blasting hole]

3.1 Numerical model

The numerical simulation parameters of layered rock mass blasting are set with a blasthole diameter of 40mm, a charge length of 2m, and a blocked section length of 1m. Considering the influence of the layered structure on damage evolution process of the rock mass, the layered joints with an inclination angle of 45° was set. The width of the joints is equal and the interval between the two layers is equal. The parameters of rocks, gun mud, and explosives are the same as the above numerical model with intact rock.

3.2 Numerical result

3.2.1 Top detonation

When the charge was detonated at the top of the blasting hole (Fig. 6), the blasting high-energy zone and high stress zone appeared at the bottom of the hole, and when the stress wave arrived the joint, part of that was reflected and absorbed, so that the energy of the stress wave attenuate rapidly, and the damage parallel to the joint appeared on the side facing the wavefront. When the stress wave propagated from the bottom of the blasthole, the reflected tensile wave was generated along the wavefront of the joint surface and propagated downward along the joint inclined plane, inducing the damage and failure depth of the rock mass greater than that of the intact rock mass. However, the damage range in the horizontal direction smaller than that of the intact rock because the joint absorbed part energies of the stress wave, the damage was mainly distributed in the layer rock where the blast hole was located and the layer rock near the blasthole.

[Fig. 6 The damage evolution of rock mass with layered joint under blasting load at top detonation]

3.2.2 Midpoint Detonation

When the charge was detonated at the middle of the blasthole (Fig. 7), the shock wave and stress wave ware from the initiation point to top or bottom transmitted simultaneously. There was stress and energy reinforcement in the hole mouth and hole bottom. The depth of damage to rock mass below the blasthole was greater than that of the intact rock, and the damage along the joint was exacerbated because of the rock mass near the joint surface subjected to the reflected tensile wave, the damage range in the rock at the hole mouth was greater than that of rock mass with detonation at the top of the blasthole.

[Fig. 7 The damage evolution of rock mass with layered joint under blasting load at midpoint detonation]

3.2.3 Bottom detonation

When the charge was detonated at the bottom of the blasting hole (Fig. 8), the damage range of the rock mass at the bottom of the blasthole was smaller than that of the rock mass at the top of blasthole. When the shock wave and stress wave transmitting upward resulted in the stress wave superposition at the upper rock mass, and then high stress and high energy zone was produced, so that the damage extent to rock mass at the top of the blasthole was larger than that of the rock mass at the bottom of the blasthole. For the section near the mouth of the blasthole, the damage range of rock mass become bigger than that of the rock mass using detonation at the top of the blasthole. For the section near the bottom of the hole, the damage range of rock mass was smaller than that of the rock mass using midpoint detonation in the blasthole, especially, the damage in the layer rock closed to the blasthole was much smaller.

[Fig. 8 The damage evolution of rock mass with layered joint under blasting load at bottom detonation]

4.1 The effect of location of initiation points

Figure 9 shows that the damage to rock mass without layered was subjected to blasting load by different location of initiation point. Figure 9a is top detonation, Fig. 9b is midpoint detonation, and Fig. 9c is bottom detonation. In the three kinds of blasting methods, the top detonation has the most serious damage to rock mass at the borehole bottom, which is easy to form the overbreak phenomenon, causing inconvenience to the later engineering treatment of rock mass at the bottom of borehole. Although both the bottom and the middle initiation have a good fragmentation effect on the upper part of the borehole, the damage effect to the rock mass in the middle of borehole is poor bucause the stress wave propagated upwards and downwards along two different direction of borehole. The initiation in the bottom has a good effect on the fragmentation to rock mass. This blasting method makes full use of the energy generated by the blasting to break the upper rock mass and obtain a better blasting effect.

As for the upper initiation, the high stress concentration and high energy zone appear at the bottom, induced the larger damage extent at the bottom, which is conducive to improve the fragmentation of the rock mass at the bottom. For the middle initiation, the high stress concentration area and high energy area occur at the upper and bottom, so that the fragmentation of the upper and bottom rock masses is enhanced, and the damage and destruction to the rock masses around the borehole are more uniform. For the bottom initiation, the high stress concentration and high energy area are concentrated at the top, which is conducive to boosting the fragmentation effect of the orifice of borehole and better forming the throwing crater.

[Fig. 9 Different location of initiation point without layered joint]

4.2 The effect of joint distribution

Figure 10 shows that the damage to rock mass with layered was subjected to blasting load by different location of initiation point. Figure 10a is top detonation, Fig. 10b is midpoint detonation, and Fig. 10c is bottom detonation. When there are layered joints, due to different wave impedances, multiple reflections and transmissions will occur on the upstream and downstream surfaces of multiple layered joints, changing the propagation path of the stress wave, causing the energy carried by the stress wave to dissipate, thus leading to the asymmetry of the damage distribution. In the layered rock near the blasting source, the damage to the rock mass at the bottom of the borehole is mainly concentrated in the rock mass less than 90⁰ directions of the vertical layered fractures.

In the direction parallel to the layered fracture, due to the reflected tensile wave generated by the reflection of stress wave and the stress concentration at the discontinuity surface, the rock mass corresponding to the upstream surface of the fracture has a strengthening effect on the rock mass fragmentation, and a large number of damage cracks are distributed. In the normal direction of the layered joints, due to the transmission and reflection of the stress wave, the energy is dissipated, and the shielding effect of the layered joints on the damage is shown on the downstream surface of the fracture (Li et al., 2019). This phenomenon has a very important application value in the protection engineering. It can make full use of the shielding effect of the layered joints on the stress wave to reduce the damage effect of external blasting waves on the protection structure, so as to ensure the safety and stability of the protection structure.

In layered jointed rock mass, in addition to changing the asymmetry of damage distribution due to the influence of layered joints, it is also affected by the initiation method, which makes the stress waves superpose in different directions along the borehole, thus affecting the distribution of high stress and high energy zone in the rock mass around the borehole. Moving up along the initiation position, the damage at the bottom of the borehole is more serious. With the upward movement of the initiation position, both the bottom and the middle initiation have a good fragmentation effect on the upper part of the borehole, while the bottom initiation has a relatively uniform damage and destruction effect on the rock mass around the borehole. It shows that in the actual blasting project, when encountering layered jointed rock mass for blasting, the location relationship between the blasting hole layout and the jointed structure, as well as the reasonable blasting method should be considered, so as to obtain better rock breaking effect.

[Fig. 10 Different location of initiation point with layered joint]

In this paper, through theoretical analysis and numerical simulation, the comparative analysis of different locations of initiation point, the joints on the damage extent of the rock masses, the following conclusions are drawn:

The nature of the effect of the initiation point location on the rock-breaking efficiency is that the initiation methods influenced the stress concentration and high energy distribution of rock mass under the blasting, The superposition among the stress waves in the blasthole has a certain impact on the damage range of the rock mass.
Top detonation has the most serious damage to rock mass at the borehole bottom, the damage effect to the rock mass in the middle of borehole is poor using middle initiation method, the initiation in the bottom has a good effect on the fragmentation to rock mass.
Due to the existence of joints, the rock damage under blasting load shows a non-symmetrical distribution. A series of complex transmission and reflection processes occur when the stress wave propagated to the upstream surface of the multiple layered joints, which intensifies the rock damage on the upstream surface, the damage to rock mass on downstream surfaces of multiple layered joints is obviously weakened.The joints have a certain shielding effect on the blasting stress wave propogation for the rock on the back side of the joints.

Author Contribution

Xie li-Xiang and Zhang jia-haowrote the main manuscript text and Yang dong-yu ,Wu lin-jun,Chen hong-yun prepared figures 1-10. All authors reviewed the manuscript

Acknowledgement

This work was supported by the Program of National Natural Science Foundation of China (52074262); Fundamental Research Funds for the Central Universities(2020QN06); State Key Laboratory of Precision Blasting and Hubei Key Laboratory of Blasting Engineering, Jianghan University(PBSKL2023A7); Undergraduate Training Program for Innovation and Entrepreneurship China University of Mining and Techonology(202310290022Z).

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No competing interests reported.

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Research on damage evolution mechanism of layered rock mass under blasting load

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Abstract

Figures

1. Introduction

2. Damage evolution of the intact rock

2.1 Numerical model

2.2 Constitutive model

2.3 Initiation method

2.4 Result analysis

2.4.1 Bottom detonation

2.4.2 Midpoint detonation

2.4.3 Top detonation

3. Damage evolution process of layered rock masses

3.1 Numerical model

3.2 Numerical result

3.2.1 Top detonation

3.2.2 Midpoint Detonation

3.2.3 Bottom detonation

4. Result and discussion

4.1 The effect of location of initiation points

4.2 The effect of joint distribution

5. Conclusion

Declarations

Author Contribution

Acknowledgement

References

Additional Declarations

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