Shili Qiu

Case Histories of Four Extremely Intense Rockbursts in Deep Tunnels

In the process of excavating seven parallel tunnels at the Jinping II Hydropower Station, several extremely intense rockbursts occurred, killing and injuring construction workers and damaging several sets of equipment. Based on the characteristics and mechanisms of these rockbursts, four typical events were selected and their temporal and spatial characteristics were here described in detail. The geological conditions revealed after the rockbursts were surveyed carefully. The responses of support elements were also analyzed. The details documented in each case provide not only an important reference for understanding the development mechanisms of rockbursts but also a basis for the selection and development of rockburst prevention measures in deep hard rock tunnels.

Studies on the evolution process of rockbursts in deep tunnels

Abstract This paper focuses on the evolution processes of different types of rockbursts occurring in deep tunnels. A series of laboratory tests and in-situ monitoring in deep tunnels excavated by tunnel boring machine (TBM) and drill-and-blast (D&B) method have been conducted to understand the mechanisms and processes of the evolution of different types of rockbursts, including strain rockburst, strain-structure slip rockburst, immediate rockburst and time-delayed rockburst. Three different risk assessment methods are proposed to evaluate the intensity and potential failure depth of rockbursts. These methods can be applied before excavation and the results can be updated according to the real-time information during excavation. Two micro-seismicity based real-time warning systems have been established for predicting various intensities of rockbursts, such as slight, moderate, intensive and extremely intensive rockbursts. Meanwhile, the probability and intensity of the rockburst are also given. The strategy for excavation and support design has been suggested for various intensities of rockbursts before excavation. The strategy for dynamic control of the rockburst evolution process is also proposed according to the monitoring results. The methodology has been successfully applied to rockburst risk reduction for deep tunnels at Jinping II hydropower project. The results have illustrated the applicability of the proposed methodology and techniques concerning rockbursts.

An Experimental Study on the Pre-Peak Unloading Damage Evolution of Marble

The behavior of rock damage evolution under unloading conditions is of utmost importance for the analysis of the stress-induced failure of overstressed rock masses. In this paper, a new experimental approach, the incrementally cyclic loading–unloading pressure test (ICLUP test), is designed to quantify stress-induced micro-fracturing and fracturing under the condition of confining pressure reduction. The experimental results demonstrate that the pre-peak damage and deformation characteristics of marble specimens may be easily quantified by irreversible strains, and two damage stages, namely, the linear steady stage and the nonlinear unsteady stage, which are, respectively, represented as a linear steady rate and a nonlinear unsteady rate of damage evolution, occur along with the increase of unloading damage. The new model is proposed to describe the features of pre-peak unloading damage evolution, and the physical meanings and ranges of its material parameters are explained and analyzed. Furthermore, the evolution of volumetric dilation and elastic parameters which occurs along with the increase of unloading damage is revealed. Also discussed in this paper are the inhomogeneity and initial damage of specimens, as well as related research planned to be performed in the future.

Rockmass damage development following two extremely intense rockbursts in deep tunnels at Jinping II hydropower station, southwestern China

Two extremely intense rockbursts took place during the excavation of the drainage tunnel and headrace tunnel No. 4 at the Jinping II hydropower station in southwestern China. The geological conditions, damage, and failure of the surrounding rock mass at the sections where the rockbursts occurred were surveyed and are described here with a focus on the development of the damage to the rock mass and the mechanism by which it occurred. The numerical method based on the Cohesion Weakening and Friction Strengthening model and various indices, including the failure approaching index and the local energy release rate, were adapted for these purposes. The results shown here may help us understand the generation and process of development of rockbursts and evaluate the degree of failure in the rockmass. This information may be valuable in the study and design of rockburst-resistant measures in deep tunnels.

Rock mass damage induced by rockbursts occurring on tunnel floors: a case study of two tunnels at the Jinping II Hydropower Station

Rockbursts occurring on tunnel floors are a special form of failure encountered in tunnel projects, where the failure is characterized by uplift, fracturing, and severe shocks of the tunnel floor. Extremely intense rockbursts were encountered during the excavation of the headrace tunnels 2 and 4 at the Jinping II Hydropower Station. In this study, results of comprehensive analysis conducted using the combination of numerical methods, experiments, and onsite survey to analyze the occurrence and development process of the rockburst are presented. In addition, the degree and extension range of the surrounding rock mass damage were estimated using the failure approaching index and the local energy release rate. The results of the analysis presented in this study are expected to advance the existing knowledge of special rockburst forms and may also serve as a reference for the design of resistant measures for other similar rockbursts.

Layered fractures induced by principal stress axes rotation in hard rock during tunnelling

Abstract During tunnelling, layered fractures occurred in the deep tunnels at the Jinping II Hydropower Station. Using numerical simulation methods and the basic theories of fracture mechanics, the mechanism of rock crack propagation influenced by the intermediate principal stress, the stress path and the principal stress axes rotation was studied. In addition, based on the analysis of the stress path and principal stress rotation of the surrounding rock mass in the test branch tunnel F during the excavation process, the mechanism of formation of layered fractures is discussed.

Effect of precrack length and inclination on tensile failure behaviour of heterogeneous rocks

Abstract The failure behaviour of heterogeneous rock specimens with prefabricated single crack under direct tensile loading is investigated numerically by using an elastoplastic cellular automaton (EPCA2D) model. The response of tensile failure behaviour of heterogeneous rocks is investigated when different crack lengths and inclinations are used. The results confirm that the tensile strength of rock specimens presents a tendency of reduction with the increment of crack length at same fixed crack inclination. However, this reduction changes non‐monotonically due to the heterogeneity of rocks. Only the precrack is longer enough can it be the main factor of rock failure. With the same crack length, but different crack inclinations, the tensile strength increases with the increment of crack inclination. With the increment of the angle between precrack and tensile loading direction, the effect of the precrack on tensile failure behaviour of rock decreases.

In Situ Observation of Rock Spalling in the Deep Tunnels of the China Jinping Underground Laboratory (2400 m Depth)

To study rock spalling in deep tunnels at China Jinping Underground Laboratory Phase II (CJPL-II), photogrammetry method and digital borehole camera were used to quantify key features of rock spalling including orientation, thickness of slabs and the depth of spalling. The failure mechanism was analysed through scanning electron microscope and numerical simulation based on FLAC3D. Observation results clearly showed the process of rock spalling failure: a typical spalling pattern around D-shaped tunnels after top-heading and bottom bench were discovered. The orientation and thickness of the slabs were obtained. The slabs were parallel to the excavated surfaces of the tunnel and were related to the shape of the tunnel surface and orientation of the principal stress. The slabs were alternately thick and thin, and they gradually increased in thickness from the sidewall inwards. The form and mechanism of spalling at different locations in the tunnels, as influenced by stress state and excavation, were analysed. The result of this study was helpful to those rethinking the engineering design, including the excavation and support of tunnels, or caverns, at high risk of spalling.

Characteristic Stress Levels and Brittle Fracturing of Hard Rocks Subjected to True Triaxial Compression with Low Minimum Principal Stress

In this study, true triaxial compression tests were carried out on three types of hard rocks (i.e., granite, marble and sandstone) using rectangular prismatic specimens (50 × 50 × 100 mm3) with low minimum principal stress σ3, and various intermediate principal stresses σ2. The main purposes were to establish the relationship between the characteristic stress levels (i.e., crack initiation stress, crack damage stress and peak stress) and the corresponding principal stresses and to investigate the brittle fracturing process of hard rocks near excavation boundaries. The test results indicated that the stress–strain curves were primarily characterized by the linear-elastic–brittle behavior. The failure planes of the specimens in the tests were found to be adjacent to the σ3 loading surface, and almost parallel to the σ1–σ2 plane, which were analogous to the spalling of the surrounding rock. With the aid of scanning electron microscopy, it was shown that cleavage fractures accounted for the majority of the fracture morphology in the sandstone specimens. Two revised methods were developed to determine the crack initiation stress of hard rocks under true triaxial compression, and these characteristic stress levels could be appropriately fitted by utilizing both the parabolic and power functions. Although the power function achieved better fitting results, the parameters in the parabolic function could be associated with the tensile cracks induced during the loading process. The influence of intermediate principal stress on the strength, deformation and failure was significant. In addition, the brittle fracturing process could be illustrated by the crack-induced strains in three principal stress directions.