Deep underground cavity is often damaged under the combined actions of high excavating-induced local stresses and dynamic loading. The fracturing zone and failure type are much related to the initial geostress state. To investigate the influence of geostress orientation on fracture behaviours of underground cavity due to dynamic loading, implicit to explicit sequential solution method was performed in the numerical code to realize the calculation of geostress initialization and dynamic loading on deep underground cavity. The results indicate that when the geostress orientation is heterotropic to the roadway’s floor face (e.g., 30° or 60°), high stress and strain energy concentration are presented in the corner and the spandrel of the roadway, where V-shaped rock failure occurs with the release of massive energy in a very short time. When the geostress orientation is orthogonal to the roadway (e.g., 0° or 90°), the tangential stress and strain energy distribute symmetrically around the cavity. In this regard, the stored strain energy is released slowly under the dynamic loading, resulting in mainly parallel fracture along the roadway’s profile. Therefore, to minimize the damage extent of the surrounding rock, it is of great concern to design the best excavation location and direction of new-opened roadway based on the measuring data of

With the increase of excavated depth for underground rock engineering, numerous unconventional rock failures such as rock spalling [

Many studies have been devoted to investigating the rock failure mechanism of underground roadway, especially for the case when the roadway is situated in high geostress environment. Li et al. [

Meanwhile, underground rocks are always subject to complex stress fields, including gravity stress and tectonic stress [

In deep underground excavation, due to the combined actions of geostresses and dynamic loadings, the surrounding rock around work face is usually induced to fail with the ejection of rock pieces. The rock failure process is widely accepted as a typical dynamic instability and energy release process. This paper explores the dynamic failure response of deep underground roadway subjected to dynamic disturbance under various orientations of geostress around the roadway. The lateral stress and strain energy distribution at the roadway’s periphery are examined. Additionally, the strain energy releasing mechanism of the surrounding rock under dynamic loading is investigated, including its releasing magnitude and time. Finally, the influence of geostress orientation on the damage zone and fracturing pattern of the cavity was discussed.

The finite element method (FEM) program Ansys/Ls-dyna is widely used in nonlinear dynamical calculations to simulate sheet metal forming, bird strike and material failures, and so on. It is quite suitable for simulating rock failure due to large deformation and nonlinear dynamic loading. In this study, Ansys/Ls-dyna was explored to examine the dynamic behaviors of the underground opening subjected to dynamic loading.

A deep-buried roadway with straight-wall-top-arch cross section was considered in this case. The specific geometries and loading conditions of the numerical model are shown in Figure

Numerical model with specific geometries and loading conditions.

The calculating model was first prestressed by principal stresses, that is, the maximum principal stress ^{−4} s, of which the rising time is 1.0 × 10^{−4} s and the peak stress is 40 MPa. All the faces are defined as nonreflecting boundaries to exclude the reflected stress waves that may be generated at the model boundaries.

Triangle stress wave used as dynamic disturbance.

The dynamic response of deep-buried opening involves two loadings of static geostress and dynamic disturbance, which is a coupled static and dynamic loading problem. The problem should be divided into two steps for numerical calculation: the static stress state of overstressed opening should be solved at first, and then the dynamic loading process will be computed on the basis of the static results. For this problem, the code provides a good solution method, that is, implicit to explicit sequential solution method. First, the implicit module is used to calculate the initial static geostress state of the opening. The strains, the displacements, and the stresses obtained from implicit calculation are imported into the explicit module, which is accomplished by creating a database file that updates the geometry and the stress history of the explicit element so that it matches the implicit static solution [

Flow chart for implicit to explicit sequential solution process.

Lots of material models have been developed to simulate the damage or fracture process for rock or rocklike materials, such as the Johnson-Holmquist model, the continuous surface cap model (CSCM), and the brittle damage model that are available in Ls-dyna. The models are designed for special purpose to take into account the erosion, strain rate effect and cracking, and so forth. The Johnson-Holmquist model is advantageous for considering compression damage but does not consider tensile damage as extensive [

In this paper, the brittle damage model was employed to simulate the dynamic fracturing characters of the roadway under high geostress and dynamic loading. The model is designed primarily for concrete though it can be applied to a wide variety of brittle materials [

To validate the capability of the brittle damage model for simulating the fracturing behavior of rock, uniaxial compression tests (UCT) were conducted to compare the results from numerical simulation. The specimens for experiments were extracted from local surrounding rock buried in 500 m depth of Linglong gold mine where a violent bulking rockburst occurred in January, 2013 [

Then, a meshed cylinder entity with the same size as the rock sample was built in the code. Uniaxial compression simulations were conducted when the boundary conditions and the loading scheme were the same with experimental conditions. The stress strain relation curves from simulation and experiment were compared once the numerical calculation was finished. Differences between the two curves were step-by-step narrowed by modifying the input parameters during the repeated simulation process. The experimental stress strain curve was compared with the simulated results, as shown in Figure

Input parameters for the rock model.

Parameters | Value |
---|---|

Density, kg·m^{−3} |
2650 |

Young’s modulus, GPa | 45.2 |

Poisson’s ratio | 0.24 |

Uniaxial compression strength, MPa | 158.4 |

Tensile limit, MPa | 5 |

Shear limit, MPa | 35 |

Fracture toughness, MPa·m^{1/2} |
1.8 |

Shear retention | 0.01 |

Comparison of the experimental stress strain relation with numerical result.

The validated rock material was used in the numerical code to investigate the energy and failure characteristics of the cavity in this section. First, the distributions of lateral stress and strain energy density were calculated when the underground roadway was excavated. Then, the strain energy evolution of surrounding rock due to dynamic loading was analyzed, and the fracturing behaviors of the opening were simulated under different geostress states.

When the underground roadway is created, the geostresses near the cavity workface will change: the tangential stress of the surrounding rock increases while the radial stress decreases gradually to zero at the opening’s surface along the radial direction from outside rock mass. The tangential stress distribution at the periphery of the opening is different due to different initial stress states. Now that the closed-form solution of tangential stress for a tunnel with straight-wall-top-arch cross section is not straightforward derived, thus, the tangential stress around the roadway was calculated and plotted in the numerical program. Figure

Tangential stress distributions under four different geostress orientations (unit: MPa).

It can be seen from Figure

Based on the energy theory, the failure process of rock is actually a process of strain energy accumulation, dissipation, and release inside the rock. The excavating-induced stresses and the strain energy inside the surrounding rock keep changing continuously as the workface of the cavity moves forward. Some places of the rock mass accumulate energy, while others release energy. The rock will be induced to break once a certain failure criterion is satisfied. To express the energy characteristics of deep surrounding rock, strain energy density (SED) is used in this paper which is written as

SED distributions under four different orientation angles of the maximum principal stress with the floor face (unit: kJ/m^{3}).

From Figure ^{3} and 83.6 kJ/m^{3} when ^{3} and 177.4 kJ/m^{3}, respectively. This is because when the direction of maximum principal stress is parallel or vertical to the floor face (e.g.,

However, when the direction of maximum principal stress is heterotropic to the floor face (e.g.,

To further examine the strain energy releasing response of the opening subjected to dynamic disturbance, four special elements which are located at roof, left sidewall, right sidewall, and floor, as marked in Figure

SED evolutional curves for the four locations under various geostress orientations.

When ^{−3} s, indicating that the roof surrounding rock fails fiercely due to the dynamic loading, whereas little influences on the change of strain energy are observed at the opening’s sidewalls and floor under the action of dynamic stress wave.

When ^{−3} s, indicating a violent dynamic failure occurs in the rock mass. But there is little change on SED at the sidewalls and floor for this case.

For the case of ^{−3} s.

When ^{3} to 0 after 4.81 × 10^{−3} s. On the contrary, there is little strain energy accumulated in the roof and floor, where spalling failure is induced subjected to the dynamic disturbance.

In terms of the energy releasing time, it can be observed that the strain energy releasing time for the roof when

To understand how the geostress orientation influences the fracture zone around the opening under dynamic loading, further simulations were extended in the numerical code. The plastic region and fracture pattern are presented in Figure

The plastic region and fracture pattern of the roadway under various geostress orientations.

From Figure

When the angle is 30° or 60°, the damage zone and extent are much severer, presenting V-shaped breakout in high accumulated energy places. In this occasion, the excavating-induced stresses distributed around the surroundings are nonuniform, resulting in high stress concentration in the corner and spandrel of the roadway. The highly stressed surrounding rock will be largely fractured when experiencing dynamic loading. When the principal stress direction is vertical or parallel to the floor face, that is, when

An implicit to explicit sequential solution method was employed in Ansys/Ls-dyna to realize the calculation of dynamic loading on highly stressed underground roadway. A validated material model was used to explore the fracture response of deep-buried roadway due to static geostress and dynamic loading, especially to investigate how the geostress orientation influences the strain energy release evolution and failure pattern of the underground roadway subjected to dynamic loading. The numerical results indicate that when the geostress orientation angle

The authors declare that there is no conflict of interests regarding the publication of this paper.

This work was financially supported by the National Natural Science Foundation of China (41272304, 11472311), the Fundamental Research Funds for the Central Universities of Central South University (2014zzts057), and the Chinese Scholarship Council (CSC).