Chuanqing Zhang

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.

Evaluation Methodology of Brittleness of Rock Based on Post-Peak Stress–Strain Curves

Brittleness is an important characteristic of rocks, for it has a strong influence on the failure process no matter from perspective of facilitating rock breakage or controlling rock failure when rocks are being loaded. Various brittleness criteria have been proposed to describe rock brittleness. In this paper, the existing brittle indices are summarised and then analysed in terms of their applicability to describe rock brittleness. The analysis demonstrates that the widely used strength ratio or product (σc/σt, σc·σt) of rocks cannot describe rock brittleness properly and that most of the indices neglect the impact of the rock’s stress state on its brittleness. A new evaluation method that includes the degree of brittleness (Bd) and brittle failure intensity (Bf) is proposed based on the magnitude and velocity of the post-peak stress drop, which can be easily obtained from the conventional uniaxial and triaxial compression tests. The two indices can accurately account for the influence of the confining pressure on brittleness, and the applicability of the new evaluation method is verified by different experiments. The relationship between Bd and Bf is also discussed.

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.

Analysis of rockburst mechanisms induced by structural planes in deep tunnels

A rockburst is defined as damage to an excavation in a sudden or violent manner, frequently occurring during the excavation of civil engineering tunnels and deep-level mining. Small-scale structural planes in the vicinity of tunnels have been found to play an important role in controlling certain rockbursts in the deeply buried tunnels of the Jinping-II Hydropower Station. In order to study the mechanisms of bursts related to the structural plane, four typical examples were selected to illustrate the temporal and spatial characteristics of the burst and the exposed plane. Three types of rockbursts were classified based on these examples and on a preliminary analysis of the various mechanisms—namely, fault-slip burst, shear rupture burst, and buckling burst. Model experiments using cement mortar revealed three failure mechanisms of the structural plane under shear stress: slip dislocation of the asperities, tensile failure of the footwall and hanging wall initiated from the root of the asperities, and impact fracture of the front end of the hanging wall and back end of the footwall. These mechanisms are used to explain the development processes of fault-slip and shear rupture bursts. Analysis of the source mechanisms of buckling bursts caused by structural planes is also provided, and three proposed source mechanisms are put forth to illustrate the factors that may have triggered the buckling bursts: self-adjustment and accumulation, disturbance from machine or blasting excavation, and energy input from remote seismic sources. Issues that must be addressed in the future are outlined and discussed in the final section. The research results contribute to a mechanistic understanding of and control method for structure-type rockbursts in deep hard-rock tunnels.

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.

Experimental study of factors affecting fault slip rockbursts in deeply buried hard rock tunnels

As civil tunnelling and mining have progressed to ever greater depths, the magnitudes of the stresses resulting from these activities have also risen significantly, leading to increasingly frequent excavation-induced seismicity and rockbursts that pose a great threat to workers and equipment on site. Although considerable research effort has been devoted to understanding the factors that influence strain bursts, few studies have addressed the factors affecting fault slip rockbursts triggered by slip on discontinuities such as structural planes. Thus, in the present work, shear tests were performed under constant normal load (CNL) conditions on joints with rough surfaces and interlocked asperities to study their shear behaviour and acoustic emission characteristics. The effects of rock type, normal stress, surface morphology, infilling, and shear history on slip rockbursts were investigated. The test results indicated that slip bursts occur more easily in granite joints because of either sudden and violent post-peak stress drops or stress drops during stick slip. Static shear failure dominated in marble and cement mortar joints, except when the joint surface was extremely irregular, and rockbursts took place when asperities were sheared off or when tensile ruptures occurred on the joint. The value of the stress drop immediately after peak stress and the value of the average stress drop during stick slip of granite joints both increased with normal stress; thus, the probability and intensity of rockbursts rose with normal stress. The failure modes of the joints were strongly influenced by the normal stress level; fillings and previous shearings (i.e. a shear history) reduce the risk of rockbursts because they reduce the amount energy released.

Shear Behaviour and Acoustic Emission Characteristics of Different Joints Under Various Stress Levels

Rock masses are typically characterised by faults, joints, bedding planes and other planes of weakness, and the mechanical behaviours (such as shear strength, stiffness, deformation and permeability) of jointed rock masses strongly depend on the mechanical and geometric properties of discontinuities. Shear failure along weak joints is one of the main failure modes in rock slopes and underground excavations; thus, understanding and predicting the shearing behaviours of jointed rockmasses are important for the design and stability analysis of rock structures. Patton (1966) proposed a bilinear strength envelope that describes the shear strength of saw-tooth joints well. Ladanyi and Archambault (1969) developed a new model by identifying the areas on the joint surface where sliding and breaking of asperities are most likely to occur. Based on a series of shear tests conducted on natural rough joints, Barton and Choubey (1977) introduced an empirical model that includes three index parameters: the joint roughness coefficient, the joint wall compressive strength and the residual friction angle. Zhao (1997) modified Barton and Choubey’s criterion by introducing the joint-matching coefficient. With the development of optics and data processing technology in recent years, the surface morphology of joints can be quantitatively investigated, and some new empirical criteria have been proposed by considering threedimensional quantified surface roughness parameters (Grasselli 2006; Xia et al. 2014). Model materials (plaster, cement mortar) have mainly been used in previous studies to simulate rock joint, and the normal stresses applied were typically low as the burial depths of the engineering rock are generally shallow. Presently, the excavation depths of many tunnels extend beyond depths of 1000 or 2000 m, with high stress levels acting on the discontinuities. Therefore, it is important to understand the shear behaviour of joints under high normal stress. Real-time monitoring of joint shearing is an important issue that must be addressed to understand the evolution of the shearing process and its underlying failure mechanisms and predict imminent shear failure. The acoustic emission (AE) technique has been widely applied to monitor and predict the failure processes of rock materials, and few researchers have addressed the application of AEs for monitoring the shear behaviour of joints (Hong and Seokwon 2004; Moradian et al. 2010, 2012; Zhou et al. 2014; Meng et al. 2016). However, studies of the AE characteristics of joints during shear failure, especially for rough joints under high normal stress, remain scarce. In this study, shear tests under constant normal loading (CNL) on tensile joints in three different types of rock were carried out, the shear behaviours of the joints were studied, and the changes in the AEs that occurred during shear failure under different normal stresses were investigated. The strength characteristics were analysed, and the AEs were recorded and compared among the different types of joints and the different normal stresses. & Hui Zhou hzhou@whrsm.ac.cn

Experimental investigations on loading-rate dependency of compressive and tensile mechanical behaviour of hard rocks

Effects of loading rate on mechanical behaviour of brittle rocks are of great importance for stability assessment of underground structures. Brazilian splitting tension tests were performed under different loading rates ranging from 0.001 to 10 kN/s. The tensile strength increased with the increasing of the loading rate. There were clear transgranular microcracks in the specimen with loading rate of 10 kN/s, but intergranular microcracks in the one of 0.001 kN/s. Uniaxial compression tests were also performed under different loading rates. All the stress–strain curves under loading rates of axial stress ranging from 0.05 to 10 MPa/s show the typical mechanical responses of brittle rocks. The uniaxial compression strength and elastic modulus increased with the increasing of loading rate. The failure mechanism under uniaxial compression was similar to the Brazilian splitting tension tests. With the increasing of loading rate, intergranular microcracks became dominant instead of transgranular microcracks.