1994 Schlumberger lecture award paper

Rockbursts and the instability of rock masses

Rock mass dynamics disasters caused by excavations and mining occur frequently in deep mines. In order to establish a theoretical system and control technologies for such disasters, we first classify and define dynamic disasters, such as rock bursts, coal bursts, mine pressure bumps, and mine earthquakes. According to the occurrence mechanism of different types of dynamic disasters, we establish a compensation control theory based on excavation and mining effects. On the basis, we propose three key technologies: high prestress compensation technology for the roadway, pressure relief technology using directional roof cutting, and the goaf filling technology using broken rock dilation. These three technologies constitute the compensation control method for dynamic disasters in deep mines. Finally, this method was successfully applied in a deep coal mine with high stress, with monitored results suggesting its rationality. This work provides a new concept and control method for the prevention of rock dynamic disasters in deep mines.

The evaluation index and classification criterion of coal bursting liability are often used to evaluate hard rock rockburst proneness. In view of the great difference between the engineering conditions of embedding rocks (hard) and coal seam (softer), the evaluation results of rockburst proneness of hard rock are not accord with the actual situation. Aiming at this problem, rockburst tendency indices were studied by laboratory test in this paper. Based on the analysis of the data, it is concluded that the peak-strength strain energy storage index $W_{ET}^P$ and residual elastic energy index $A_{EF}$ can reflect the energy and rockburst proneness of hard rock from the elastic energy storage and residual elastic energy. By designing the cyclic loading and unloading test and the uniaxial compression test of rock, the loading rates of the above two kinds of energy indices were determined, and the determination and calculation process of the above two kinds of energy indices were clarified. The experimental results show that the linear elastic energy storage law obtained by cyclic loading and unloading of one sample is not very different from that obtained by multiple samples, both methods can be used for measurement. The peak-strength elastic energy density, pre-peak total input energy density and post-peak failure energy density of rock increase slightly with the strain rate, and the degree of rock fragmentation also has the same trend. The energy index of rock cannot fully reflect the rockburst proneness of rock. Based on the test process and results, in the case of hard rocks, 0.3–1 mm/min could be used for displacement control mode, and 50–140 kN/min should be used for load control mode. Extensometer or instrument sensor can be the strain acquisition methods.

Rockburst has become one of the most serious threats to the safety of workers, equipment, and excavations in deep underground engineering. However, the prevention of rockburst risk remains an unsolved problem. The slot-cut method was put forward, aiming at the problem of strain burst for tunnel projects. The mechanism and method of slot-cut prevention of delayed rockburst were discussed by laboratory tests, theoretical analysis, and numerical simulation. The mechanical responses, failure patterns and failure indexes of granite specimens were analysed. The rockburst process and deformation evolution law of test specimens with different crack parameters were studied in great detail. Based on the experimental results, a comprehensive discrimination of rockburst proneness was explored. The results demonstrate that the peak stress and damage index of pre-cracked granite initially decreases and subsequently increases with crack angle, and the pre-peak and post-peak elastic modulus with different crack numbers decreases linearly, indicating that a change in the crack number significantly affects the bearing capacity of granite. The failure mode of the granite specimen belongs to the tension-shear composite type. Meanwhile, it was found that both the brittleness index and ejection energy with different cracks show an U-shaped for the pre-cracked granites, confirming that the crack angle of 60° has been found to be the threshold for rockburst proneness. Ultimately, the combination of the rockburst energy index describing rock ejection qualitatively and the optimized residual energy index considering elastic energy at post-peak stage quantitatively provides a judgment criteria to comprehensively characterize the rockburst proneness of granite by the slit-cut method. When compared with original index, the results of the comprehensive discrimination exhibited good accuracy. Therefore, the experimental phenomenon and judgment criteria prove that the slit-cut technology can be well applied to the rockburst prevention.

Rockburst is becoming a huge challenge for the utilization of deep underground space. Extensive efforts have been devoted to investigating the rockburst behavior and mechanism experimentally, theoretically, and numerically. The aim of this review is to discuss the novel development and the state-of-the-art in experimental techniques, theories, and numerical approaches proposed for rockburst. The definition and classification of rockburst are first summarized with an in-depth comparison among them. Then, the available laboratory experimental technologies for rockburst are reviewed in terms of indirect and direct approaches, with the highlight of monitoring technologies and data analysis methods. Some key rockburst influencing factors (i.e. size and shape, rock types, stress state, water content, and temperature) are analyzed and discussed based on collected data. After that, rockburst theories and mechanisms are discussed and evaluated, as well as the microscopic observation. The simulation approaches of rockburst are also summarized with the highlight of optional novel numerical methods. The accuracy, stability, and reliability of different experimental, theoretical and numerical approaches are also compared and assessed in each part. Finally, a summary and some aspects of prospective research are presented.

Sound signals of rock cracking can be used as indicators of rock failure processes in deep rock engineering. Compression tests, including a cyclic uniaxial compression test and noncyclic compression tests (e.g., uniaxial, biaxial and rockburst tests), were conducted using a true triaxial test system to investigate the correlation between sound characteristics and damage in granite cracking processes. The sound signals of granite cracking were monitored during the tests. The Kaiser effect and Felicity effect of sound were found in the cyclic uniaxial compression test. The Felicity ratio (FR) decreases continuously with increasing rock damage, which indicates that sound signals can be used to generally evaluate rock damage. In addition, the zero-crossing rate (ZCR), a characteristic index of sound signals, was proven to be applicable to quantitatively detect damage processes and construct damage constitutive models in granite. Furthermore, a novel sound-based index, namely the total energy of local sound (TELS), was proposed to evaluate the cracking intensity for granite. Experimental results show that sound signals are useful indicators for the quantitative detection of damage processes in hard rock under high stress conditions.

To estimate the energy storage and release performances of rock pillars in high stress and gain insights into the prevention and control of rockburst hazards from an energy aspect, several series of single-cyclic loading-unloading uniaxial compression tests with different stress levels were conducted on five types of rocks sampled in standard cylinders and prisms. The concepts of relative energy storage potential and ultimate energy storage potential were proposed to evaluate the energy storage performance of the two shaped specimens, and the energy release potential was determined using the residual elastic strain energy index. The results showed that, under similar loading conditions, the relative energy storage potential of rocks was insensitive to the specimen shape, whereas the ultimate energy storage potential and energy release potential were sensitive to both the specimen shape and rock type. Based on the lab results, a theoretical approach of assessing the rockburst (or instability) risk of isolated rock pillars subjected to high geostress was illustrated by estimating its current energy state and ultimate energy storage potential. Importantly, the residual elastic energy of an isolated pillar can also be estimated based on the lab results, according to which the number of supporting devices such as the energy-absorbing bolts needed for scientific and sufficient support of pillar rockburst was quantitatively estimated. This study provides a new method for estimating the desired number of energy-absorbing bolts during rockburst support in engineering practice.