Study on Cumulative Damage Law of Surrounding Rock Induced by Blasting during Tunnel Construction

The tunnel drilling and blasting method inevitably causes damage to the rock mass during the blasting process. This article takes a certain tunnel as the research object, proposes to simulate the process of continuous blasting and repeated blasting in each section of the tunnel, and quantifies the cumulative damage range of surrounding rock under each working condition. The results indicate that: (1) it is advantageous to use equivalent loads to simulate blasting loads when calculating large-scale blasting damage simulations; (2) The damage to surrounding rock caused by single cycle excavation of tunnels is caused by the superposition of multiple stages of delayed blasting, mainly influenced by adjacent auxiliary equipment.


Introduction
During excavation using the drilling and blasting method, the energy generated by explosive blasting not only completes the crushing and throwing of the rock mass, but also causes damage to the rock mass, leading to deterioration of the mechanical properties, strength reduction, and integrity of the rock mass, thereby affecting construction safety.Blasting damage has always been a research hotspot in this field [1,2], and scholars have conducted many studies on rock damage induced by single hole charging and single stage blasting [3][4][5].The cumulative damage of rock mass is ultimately a problem of the duration of rock mass dynamic response.The millisecond millisecond blasting of tunnel drilling and blasting method is a segmented blasting process from the center of the face to the outside, which is a cumulative evolution process of damage.Jiling et al. [6] used numerical simulation and on-site testing methods to analyze the vibration response and damage effects caused by single cycle cutting blasting within the footage; Yang Jianhua et al. [7] used a statistical damage evolution model to simulate the cumulative damage evolution law of rock mass under repeated blasting loads during the full section millisecond blasting process of a circular tunnel with single cycle footage.In practical engineering, the blasting operation is a push type repetitive blasting, which causes a larger cumulative damage range and degree compared to single cycle footage [8].Based on the above ideas, Zhang Guohua et al. [9] used on-site acoustic monitoring to study the cumulative damage range of tunnel surrounding rock caused by the construction of double-sided heading method; Yan Changbin [10] used a large amount of on-site acoustic testing data to obtain a nonlinear increasing relationship between the cumulative damage of rock blasting and the number of blasting times; Yang Shengquan et al. [11] elaborated on the dynamic load accumulation effect of discontinuous surfaces such as joints and fissures in rock mass through onsite blasting seismic wave analysis.In summary, currently, research on rock mass damage induced by blasting construction mainly uses on-site monitoring methods to measure the cumulative damage of surrounding rock, which requires a large amount of work and lacks quantification of the cumulative damage value caused by explosive blasting in each section of the tunnel to the surrounding rock.
This article adopts numerical simulation research methods to propose the ability to simulate the process of successive and repeated blasting of explosives in each section of the tunnel, and quantifies the cumulative damage range caused by each working condition to the surrounding rock.

Application Method of Blasting Load
Numerical simulation is an important means of studying engineering blasting theory and technology.Currently, there are four main ways to simulate blasting loads: (1) directly simulate the explosive detonation process using the JWL state equation provided in the finite element software LS-DYNA [12].The JWL state equation contains many parameters to be calibrated and requires the establishment of refined blast holes and explosive elements, making modeling complex and time-consuming; (2) Establish a refined borehole model and simulate it by applying pressure and time history loads on the borehole wall [13].This method also has the characteristics of high modeling difficulty and low computational efficiency; (3) According to the attenuation law of seismic waves, the actual monitored vibration velocity, acceleration, and other parameters are regressed and analyzed to be reduced to explosive loads applied to the surrounding rock [9].This method is greatly influenced by rock mass structure and geological conditions, and the reduced explosion load may differ significantly from the actual situation.Currently, its application is relatively limited; (4) After the reduction of explosive load, it is equivalently applied to the boundary of the crushing zone and crack zone formed around the blasting hole during blasting, or directly applied to the excavation contour surface, that is, the explosive load is equivalently applied to the surface determined by the line connecting the center of the same section of the blasting hole and the axis of the blasting hole [7].This method does not require the establishment of a blast hole structure, has a relatively simple model, high computational efficiency, and can effectively simulate the dynamic response of rock mass under group hole blasting conditions in tunnel engineering.It is currently a widely used method [14,15].

Equivalent Blasting Load
For the cylindrical charging conditions of tunnel excavation, the gas pressure acting on the blast hole wall: In the equation, PH is the gas pressure on the wave front; ρe is the density of the explosive; D is the detonation velocity of the explosive;γ is the isentropic index, approximately taken γ=3.0.The impact pressure on the rock mass of the blast hole wall is: In the formula, P0 is the hole wall pressure; lc and lb are the length of the charging section and the depth of the blast hole, dc are db respectively the diameter of the charge and the diameter of the blast hole; n is the pressure increase coefficient when the detonation gas product expands and impacts the borehole wall, taking n =10.
For tunnel cut hole blasting, it is believed that the envelope line of the broken area formed by each cut hole is the excavation surface of the cut hole blasting; For the blasting of tunnel collapse holes, buffer holes, and smooth blasting holes, it is considered that the center line of the blast holes in this circle is the blasting excavation surface [16], as shown in figure 1.In the formula, PE is the hole wall pressure; S is the distance between the centers of adjacent holes.

Material Constituents and Parameters
The specific parameters of the rock, explosives, and air models used in this article can be found in reference [17].

Calculation Model
Establish a numerical model based on an actual tunnel project.The tunnel mainly passes through Class III weakly weathered fused tuff, with a cross-sectional area of 114 m2.It is excavated using full section millisecond blasting, with detonator segment numbers MS1, MS3, MS5, MS7, MS9, and MS11.The specific blasting parameter design is shown in table 1.In the calculation model, the blasting load is simplified as an equivalent load applied to the excavation surface for simulation, and the equivalent load calculation is determined by equation (3).Treat each millisecond delay blasting as an equivalent load loading, and use restart numerical simulation technology to conduct numerical simulation calculations of repeated blasting of surrounding rock at a three-dimensional scale.As shown in figure 2, the size of the model is 12m × 16m × 20m, the tunnel excavation section is 4m, and a total of 6 loads of A → F are continuously applied to different excavation surfaces within the same cyclic footage.

Analysis of Cumulative Damage Characteristics of Single Cycle Footage Surrounding Rock
Under the action of millisecond millisecond delay blasting on the entire section, the damage range and degree of the surrounding rock have a significant radial expansion process, and the accumulation of damage in the single cycle footage of the tunnel mainly occurs in the current working condition section.
Using the tunnel arch on the intersection line between the tunnel face and the tunnel wall in the numerical model (with a height of 1.5m) as the research reference point, the cumulative damage characteristics of the rock mass subjected to segmented blasting at this particle point are monitored.At the same time, the range of 0.2-1 damage values in the radial direction far from the reference point is considered as the excavation damage range of the rock mass under this working condition [17].Figure 3 shows the damage evolution process of the entire segmented blasting process.Analysis shows that: 1) The damage to surrounding rock caused by single cycle excavation of tunnels is not the result of single stage explosive blasting, but rather the superposition of multiple stage explosive delayed blasting; 2) The cumulative damage superposition effect at the tunnel arch waist is more evident during the blasting of the auxiliary eye MS7, MS9, and MS11 sections, with cumulative damage values of 0.29, 0.42, and 0.45 at the monitoring points, respectively; 3) The radial excavation damage range of the surrounding rock induced by single cycle excavation of the tunnel is 1.20m, mainly affected by the blasting of the auxiliary eye MS7 and MS9 sections, and the damage range reaches 0.40m and 1.20m respectively during the blasting of the auxiliary eye MS7 and MS9 sections.

Conclusion
Based on the research results of this article, it can be seen that this article proposes to simulate the process of continuous blasting in each section of the tunnel, and quantifies the cumulative damage range of surrounding rock under each working condition.The results indicate that: 1) It is advantageous to use equivalent loads to simulate blasting loads when calculating large-scale blasting damage simulations; 2) The damage to surrounding rock caused by single cycle excavation of tunnels is caused by the superposition of multiple stages of delayed blasting, mainly influenced by adjacent auxiliary equipment.

Figure 1 .
Figure 1.Excavation surface of blasting equivalent load.

Figure 2 .
Figure 2. Numerical conditions of repeated blasting of surrounding roc.

Figure 3 .
Figure 3. Cumulative damage law of rock mass in single-cycle footage and staged blasting.

Table 1 .
Parameters of blasting design scheme.