Damage evolution mechanism in production blasting excavation under different stress fields

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Abstract

Deep rock mass stores large amounts of energy. Energy accumulation, dissipation, and release during the deformation and failure of rock mass occur when the rock mass is excavated via the drilling and blasting method. According to the energy effects, failure pattern, and effect of the strain rate on the rock strength, a Johnson–Holmquist rock (JHR) model was established by combining the energy-based tensile–compressive damage model and the Johnson–Holmquist ceramic model based on the coupling method for tensile–compressive damage in a rheological-dynamical analogy model. To apply the same method to the damage evolution mechanism of deep rock mass, the JHR model was incorporated into the LS-DYNA software. Subsequently, the self-developed numerical model was employed to investigate the damage evolution in the production blasting excavation of stopping holes under different stress fields. According to the failure law, the challenges in rock-mass excavation were analysed. This study provides a basis for the analysis and research regarding the blasting design in deep rock-mass excavation.

Introduction

After deep tunnel excavations, strain energy accumulates in the surrounding rock. The energy is released or transferred in the surrounding rock mass when the accumulated strain energy exceeds the energy-storage capacity of the rock mass [41]. The energy release and transfer in rock mass exhibit both advantages and disadvantages for the excavation and construction of deep-buried tunnels. One disadvantage is that the energy change leads to rock deformation and failure, resulting in instability of the surrounding rock mass and a series of natural disasters. However, owing to the high energy-storage characteristics of the deep rock mass, after cut blasting, the strength of the rock mass is reduced under the action of excavation disturbance and unloading, creating favourable conditions for subsequent production blasting. Additionally, there is the advantage that rock excavation via the drilling and blasting (D&B) method becomes easier with rock deformation and failure. Engineering practices indicate that the adoption of the D&B method for the excavation of deep rock mass improves the rock fragmentation effect and increases the tunnelling efficiency even with the same blasting parameters [14], [16]. Following deep tunnel excavation, rock fragments are detached from the excavation face, providing a free face for subsequent blasting. Energy release and transfer are inevitable during the process. Therefore, it is rational to investigate the damage evolution mechanism of the deep rock mass via the D&B method from the viewpoint of energy.

Extant models that describe rock failure characteristics from the viewpoint of energy mainly include the continuous surface cap (CSCM) model [9], [22], rheological-dynamical analogy (RDA) damage model [7], and Rankine plastic fracture softening (RPFS) model [6]. The CSCM model considers the maximum principal strain as the criterion for material damage. The material failure under compression corresponds to ductile failure, and that under tension corresponds to brittle failure. In the RDA model, the material failure under compression corresponds to ductile failure and is caused by the increase in the plastic work due to plastic flow, wherein the tensile and compressive damages are coupled. In the RPFS model, material failure is related to the rate of strain-energy release. Failure occurs when the accumulated fracture strain energy reaches the critical rate of strain-energy release. However, the dominant factor is the material failure caused by the principal tensile stress [6].

Rock is a brittle material that exhibits high levels of strain, a high strain rate, and high pressure when it is subjected to a blasting load [8], [15], [21], [37], [38], [39], [40]. The mechanical responses of rock are considered when a dynamic constitutive model is selected for the numerical simulation of the rock failure process under a blasting load. Currently, the main constitutive models for rock dynamics include the Holmquist–Johnson–Cook (HJC) model [10], Johnson–Holmquist (JH) model series [11], and Riedel–Hiermaier–Thoma (RHT) model [19]. All three models consider the high strain, high strain rate, high nonlinear pressure, and damage softening due to material damage under impact loading. Their damage evolution equations have the same form; thus, they represent the material damage evolution process via the accumulative plastic strain and volumetric strain. However, the state equations that consider the nonlinear response of the material under impact loading exhibit different forms.

A deep rock mass is subjected to high in situ stress, which influences the damage to the rock mass under the action of a blasting load. According to previous studies, the damage decreases with the increasing in situ stress [1], [3], [31], [32], [33], [35]. Therefore, it is necessary to consider the effect of the in situ stress when researching the damage evolution mechanism of rock mass for deep-buried tunnels.

In this study, an energy-based damage constitutive model was adopted, and the high deformation, high strain rate, high in situ stress, high nonlinear pressure of rock under the blasting load, and role of energy in the excavation of deep-buried tunnels via the D&B method were comprehensively considered. The relevant model parameters were initially determined to employ the constitutive model in the numerical simulation of deep rock mass under a blasting load. Subsequently, the parameters were verified via laboratory blasting tests. The model was finally used to simulate the damage evolution mechanism of rock mass for deep-buried tunnels excavated via the D&B method.

Section snippets

Model description

To consider the effect of the material strength on the response characteristics of the strain rate and pressure, the yield strength surface, residual strength surface, and state equations in the Johnson–Holmquist II (JH2) model [4], [10], [11] were adopted for the material yielding surface in the Johnson–Holmquist–Rock (JHR) model [30]. According to the role of energy in deep rock excavation via the D&B method, compressive damage was described by the compressive damage in the RDA model [7],

Numerical model for production blasting

To accurately determine the maximum burden of the stopping hole, a ratio of Ls/B=1.2 between the hole spacing and the burden was adopted, as suggested by Per-Anders Persson [13]. The centre-to-centre distance of the stopping hole was 2.2 m, and the hole spacing was 0.7. Hence, the maximum burden was 0.5 m. The blasting design is shown in Fig. 8.

In the study, eight boreholes were arranged, with a centre-to-centre distance of 2.2 m. As shown in Fig. 9(a), the hydrostatic pressure was set as 0, 5,

Conclusions

The role of the energy in a rock mass was considered in the excavation of a deep-buried tunnel via production blasting. According to the type of rock damage, failure, and strain rate response of the rock mass under a blasting load, a JHR model was developed by coupling the energy-based compressive–tensile model and JH2 model. The JHR model was subsequently processed using the LS-DYNA software to develop a model that can be used to investigate the rock damage evolution mechanism of production

Acknowledgements

China Postdoctoral Science Foundation (2017M621867); the Fundamental Research Funds for the Central Universities (2015XKZD05); Jiangsu Youth Foundation Project (BK20180651); the Key Program of National Natural Science Foundation of China (51734009).

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