This paper presents experimental study on rockbursts that occur in deep underground excavations. To begin with, the boundary conditions for excavation in deep underground engineering were analysed and elastic adaptive boundary is an effective way to minimize the boundary effect of geomechanical model test. Then, in order to simulate an elastic adaptive loading boundary, Belleville springs were used to establish this loading boundary. With the aforementioned experimental set-ups and fabrication of similarity models for test, the phenomena of strain mode rockbursts were satisfactorily reproduced in laboratory. The internal stress, strain, and convergences of the openings of the model were instrumented by subtly preembedded sensors and transducers. Test results showed that, with an initial state of high stress from both upper layers’ gravitational effects and in situ stress due to tectonic movements, the excavation brings a dramatic rise in the hoop stress and sharp drop in radial stress, which leads to the splitting failure of rock mass. Finally a rockburst occurred associated with the release of strain energy stored in highly stressed rock mass. In addition, the failure of the surrounding rock demonstrated an obvious hysteresis effect which supplies valuable guide and reference for tunnel support. Not only do these results provide a basis for further comprehensive experiments, but also the data can offer assisting aids for further theoretical study of rockbursts.
With the rapid development of construction and the overwhelming demands for resources, the development and use of underground spaces gradually reach into deeper areas, including mines, tunnels, and nuclear waste disposal sites. With incremental increases in the buried depth of projects, we find that some nonlinear deformations and failure phenomena occur which are different from those in shallow rock engineering. Also, they cannot be satisfactorily explained by the traditional theory of continuum mechanics. The special phenomena such as rockbursts, zonal disintegration, and anomalously low friction have sparked widespread concerns in the international rock mechanics engineering community. The study of these problems has become an important issue in the past decades [
Current theories, such as energy theory [
Many experiments have been performed all over the world including uniaxial tests [
This paper reproduces a strain rockburst phenomenon using tests of similarity models. First, to simulate the initial stress state in deep rock mass more accurately, we suggest an elastic adaptive boundary and Belleville springs were used to establish this loading boundary. Second, by accurately simulating the process of excavating the tunnel, a free surface caused by excavation is produced, and the stress is converted due to the unloading of surrounding rock. Thus a more realistic simulation of the generation process of in situ rockbursts is carried out. The aim is to reproduce the rockburst phenomenon using tests of similarity models. Then a variety of means are used to monitor the whole occurrence and the development process of rockbursts. This can provide a basis for an experiment and data to support the further study of the initiation mechanism of rockbursts.
A finite size model to simulate the excavation problem in an infinite medium is generally used in a geotechnical test model. Boundary conditions are essential for the success of the test. The excavation of the tunnel in practical engineering can be seen as creating a new opening with the radius
Equivalent spring model on the boundary of tunnel excavation.
The current boundary conditions can be classified into two kinds: one is the displacement boundary condition; the other is the stress boundary condition. The former boundary is rigid, and its displacement is even, but the stress is not uniform. For homogeneous materials it can provide satisfactory test results, but for discontinuous and heterogeneous material like a rock mass it can provide nonuniform and uncertain stress on the boundary, and thus the initial stress state of rock mass cannot be easily simulated. To ensure a uniform stress field and a more accurate simulation test boundary, the other boundary is of greater significance. As the buried depth of underground engineering increases, the stress becomes high enough so that the effect of boundary conditions on test results is very significant. In addition, deep rock masses have energetic properties [
A test apparatus for deep rock mass loading and unloading is used to simulate the initial in situ stress (Figure
Deep rock mass loading and unloading test apparatus.
Schematic diagram of Belleville spring loading system.
According to the theory, the equivalent stiffness of the loading device acting on the boundary should be equal to the rebound stiffness of the model. By considering the bearing capacity of the Belleville spring and stiffness requirements, we select the specifications of the Belleville spring to be
Belleville spring parameters.
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250 | 127 | 14 | 5.6 | 19.6 | 249 | 4.2 |
To summarize, the test apparatus provides the following conditions at the boundary of the model: (1) the boundary can quickly supply the model with a rebound energy caused by excavation and provide the model with a proper stiffness; (2) stress can be adjusted automatically with the stiffness
The transport tunnels of the Jinping II hydropower project are selected as the engineering prototype. This engineering project was located in the sloped terrain zone from the Tibetan Plateau to the Sichuan Basin of China. The two transport tunnels are about 17.5 km long and the cross-sectional sizes of the A and B tunnels are 5.5 × 4.5 m2 and 6 × 5 m2, respectively, with a line spacing of 35 m. Most of the tunnel’s buried depth is more than 1500 m. The lithology of the nature rock is mostly Triassic, Limestone, and maximum tunnel depth is 2375 m. Rockbursts did occur many times during construction.
Strain rockburst usually occurs in intact rock and is caused by deformation and failure of the rock mass [
Stress, strain, and temperature sensors measuring points.
At present the similarity criterion generally uses the elastic theory and dimensional analysis method. Specifically, the geometric similarity scale
Rockbursts generally occur in the hard and brittle rock. To simulate marble according to the similarity criteria, we adopt resin-based equivalent materials that are developed by Fan et al. [
The ratio of equivalent materials.
Components | Barite sand | Quartz sand | Barite powder | Rosin | Alcohol |
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Ratio | 45% | 5% | 50% | 0.5% | 4.5% |
Physical and mechanical parameters of the prototype and model.
Material types | Dry density |
Elastic modulus/MPa | Compressive strength/MPa | Tensile strength/MPa | Cohesion/MPa | Internal friction angle |
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Marble | 27.7 | 25300 | 89.2 | 4.95 | 4.24 | 53.1° |
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Equivalent materials | 27.7 | 369.6 | 1.1 | 0.07 | 0.06 | 50.7° |
Equivalent materials and mechanical experiments: (a) the mould; (b) sample of equivalent materials; (c) sample preparation; (d) maintenance of the sample; (e) uniaxial compressive test of sample; (f) shearing test of the sample.
The model uses the method of layered filling and compaction. To ensure a uniform stress field of the model at the boundary, flexible rubber is positioned at the contact areas between the model and test device. To decrease the friction effect, a layer of PTFE film is pasted on the other side of the flexible rubber (Figure
Fiber grating strain (FBG) sensor and pressure sensor embedded process: (a) layered filling of the model; (b) DZ-I pressure sensors; (c) FCC-Y FBG strain sensors.
We set the underground engineering construction to have a buried depth of 2375 m as an example. The vertical in situ stress is given by
To simulate the in situ stress, a pressure of 0.86 MPa needs to be applied to the model. The excavated tunnel is 70 cm long and 8 cm in diameter. According to the similarity criterion, each excavation length is 7 cm and it takes 5 minutes to finish the excavation. The next step of excavation is started 15 minutes later. The spiral drill pipe driven by the motor that simulates the excavation and unloading is adopted to create a new opening in the model. The internal stress, strain, and temperature are monitored during excavation process. Excavation equipment and internal stress and strain monitoring system are shown in Figure
The process of test: (a) the process of loading; (b) the drill and excavation process; (c) internal pressure monitoring system; (d) FBG strain sensor monitoring system.
The problem of an excavated circular tunnel can be simplified as a plane strain problem. The radial pressure points, which are 2 and 8 cm from the wall as calculated by elastic theory, reduce to
Hoop pressure change curve.
Radial pressure change curve.
Internal pressure of measuring points (front of tunnel face).
The measurement results of circumferential pressure during the excavation are shown in Figure
The pressure curve in the radial direction during the excavation is plotted in Figure
The pressure curve in front of the tunnel face during the excavation is shown in Figure
Two FBG strain sensors were embedded 2 cm from the wall along the radial and circumferential directions in the model. Figure
Internal strain change curve: (a) strain in radial direction; (b) strain in hoop direction.
At the beginning stage of excavation, the two measurement points are in a relatively stable state. With the onset of excavation and unloading, the pressure of the surrounding rock gradually adjusts. The wavelength of the sensor in the hoop direction gradually decreases beginning with the third step of excavation. This is because of the increase in hoop stress due to excavation. The wavelength decreases the most at the sixth step of excavation. The maximum strain in hoop direction reaches 1268
Based on the small angle principle (Figure
Displacement measuring principle of small angle method.
The displacement variations of the tunnel wall with time after excavation are shown in Figure
The convergence curve of tunnel wall after the excavation.
In the construction of Jinping II hydropower project, the surrounding rock shows obvious brittle failure characteristics, such as plate crack phenomena and rockbursts. The occurrences of these kinds of failure phenomena are obvious during the experiment. Polyaxial or true triaxial tests had been performed by Addis et al. [
The plate cracking phenomenon.
Rockbursts in model test.
From further analyses we can conclude that the stress level, under the deep rock mass environment, is usually very high. At the moment of creating a new opening, the increase in the hoop stress and the unloading of the radial stress result in a splitting tensile failure of the surrounding rock mass. Later, with further adjustment of the interior pressure and continual excavation and unloading, the surrounding rock gradually becomes damaged, and the peak of hoop stress transfers to the deep environment. After a certain period of time, the rocks surrounding the interior form a “support pressure zone.” In the “range of the support pressure zone,” the circumferential stress reaches the maximum value, while near the tunnel wall the surrounding rock loses a certain bearing capacity due to the gradual destruction. Thus, rockbursts suddenly occur 42 cm away from the model boundary. In the seventh step of excavation the pressure suddenly dropped to zero based on the measurement results, which indicated some damage was produced in the rock. However, rockbursts would not happen until a period of time after the excavation. With further adjustment of the surrounding rock pressure and potential energy concentration, rockbursts would occur suddenly. According to the ejected volume of equivalent material, this is a serious rockburst because the ejection volume is large. To sum up, a rockburst is a complex dynamic disaster phenomenon which occurs in the rock mass with high in situ stress induced by continual disturbances due to the excavation. The excavation brings a dramatic rise in the hoop stress and drop sharp in radial stress, which leads to the splitting failure of rock mass. Finally a strain model rockburst suddenly occurred associated with the release of strain energy stored in highly stressed rock mass.
Through a preliminary test of a model for rockbursts simulated by low-strength brittle equivalent materials we conclude the following.
(1) Based on elastic adaptive loading boundary, Belleville springs are introduced to the loading apparatus. It is a new loading boundary of the test model which can minimize the boundary effect of geomechanical model test. By adjusting the spring system, the stiffness of test apparatus can be adjusted conveniently. It can quickly supply the model with a rebound energy caused by excavation and provide the model with a proper stiffness. Besides, stress can be adjusted automatically at the boundary.
(2) Owing to the excavation and unloading, there is a stress distribution and energy-dispersion process in the surrounding rock. At a distance of 2 and 8 cm from the tunnel wall, the radial pressure reduces from the original values 0.96 and 0.92 MPa to 0.2 and 0.65 MPa, respectively, and the hoop pressure increases from 0.9 and 0.98 MPa to 1.47 and 1.4 MPa, respectively. The maximum values of the radial and hoop strain reach to 2528
(3) In the process of the test model, the brittle failure phenomenon of surrounding rock can be observed. The failure of the surrounding rock in deep underground openings demonstrated an obvious hysteresis effect. The mechanism of strain rockbursts can be interpreted as follows: with an initial state of high stress in rocks, the excavation brings a dramatic rise in the hoop stress and sharp drop in radial stress, which leads to the splitting failure of rock mass. Finally a strain model rockbursts occurred suddenly associated with the release of strain energy stored in highly stressed rock mass.
These results provide a basis for experiment and data to support the theoretical study of rockbursts. However, there is still some work to be done. The influence of stiffness of the test device on the destruction of the surrounding rock should be further investigated during excavation. To obtain more information of the dynamic mechanical parameters for rockbursts, the number of stress and strain measuring points should be increased.
The authors declare no competing interests.
This study was financially supported by the National Basic Research Program of China (973 Program: 2013CB036005), the National Key Scientific Instrument and Equipment Development Project (no. 51527810), and the State Key Laboratory of Coal Resources and Safe Mining, CUMT (no. 14KF02).