During the excavation of a large number of deeply buried tunnels and mining projects, rockburst disasters occur frequently due to the complex geologic environment in deep underground, including high initial geostress, adverse tectonic actions, and excavation disturbance. Many rockbursts have been found to be induced by some small-scale structural planes in the area around the tunnels during the construction of Jinping II hydropower station. In order to study the influence mechanisms of the structural plane to rockbursts, the physical simulation tests of rockbursts under biaxial stress conditions are carried out using marble samples by considering different relative positions of the structural plane with tunnels, namely, in tunnel spandrel, in tunnel sidewall, and at the intersection with the tunnel. The digital image correlation (DIC) technique is used to trace the evolution of the deformation on the surface and the rockburst process of the marble sample. The results reveal that three types of rockbursts are identified, namely, fault-slip rockburst, split bulking rockburst, and shear rupture rockburst, and their evolution processes are reproduced. The presence of small-scale structural planes in the vicinity of deep tunnels could be one of the major influence factors in triggering rockbursts. The findings could provide helpful references for predicting the development process and the design of burst-resistant measures for this type of rockbursts.
As the foundation engineering construction and the resource development are progressing to greater depths, the frequency of rockburst increases remarkably. Rockburst disasters have been the significant threat to the safety construction of deep underground rock mass engineering. Besides rockbursts occurring easily in hard intact rock masses, small-scale structural planes have been found to play a significant role in inducing rockburst in deeply buried hard rock tunnels [
The 11.28 rockburst in drainage tunnel and exposed rigid structural plane [
In recent decades, although extensive research studies have been conducted to investigate the mechanism of rockbursts, the influence mechanism of structural plane on rockbursts is pretty rare and still unclear. Williams [
More than 20 rockburst cases influenced by structural planes are listed and the rockburst characteristics are presented during the excavation of Jinping II headrace tunnels [
In this paper, physical model experiments in laboratory are carried out for clear observation of the rockburst evolution process influenced by small-scale structural planes. The evolution processes are recorded simultaneously by the DIC technique. The structural-type rockburst caused by shearing is simulated by the means of UDEC software. A discussion of the results and the future research plan in dealing with the shortage of current models is presented.
Rock masses prone to rockbursts are typically stiff, strong, and brittle, with uniaxial compressive strength of 100–400 MPa with Young’s modulus > 20 GPa [
Basic mechanical parameters of the marble used in this study.
Type of material used | ||||
---|---|---|---|---|
Marble | 126.06 | 1.63 | 51.59 | 0.23 |
Three types of marble model samples. (a) The structural plane located in the tunnel spandrel. (b) The structural plane located in the tunnel sidewall. (c) The structural plane intersected with the tunnel.
The model tests were carried out on the Rock Biaxial Loading and Macro-Mesoscopic Measurement System at the Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. The experimental system is composed of biaxial loading system and deformation measurement system, as shown in Figure
The Rock Biaxial Loading and Macro-Mesoscopic Measurement System.
The axial stress versus axial strain curves of three types of model samples under biaxial pressure are illustrated in Figure
Curves of stress versus strain of three types of model samples: (a) model sample A; (b) model sample B; (c) model sample C (①, ②, ③, and ④ indicate the elastic deformation stage, stable crack propagation stage, unstable crack propagation stage, and postpeak stage) (after [
Table
The stress and strain threshold between different stages of stress versus strain curves for three types of model samples.
Type of model samples | ||||||
---|---|---|---|---|---|---|
Model sample A | 8.72 | 29.56 | 36.71 | 60.20 | 0.24 | 0.81 |
Model sample B | 12.61 | 37.53 | 46.27 | 76.93 | 0.27 | 0.81 |
Model sample C | 16.12 | 39.71 | 48.57 | 90.76 | 0.33 | 0.82 |
The shaded area below the stress-strain curve represents the strain energy stored during the loading process in the prepeak region, as shown in Figure
The rockburst evolution processes of three types of model samples are illustrated in Figure
The rockburst evolution processes of three types of model samples: (a) fault-slip rockburst, (b) buckling rockburst, and (c) shear rupture rockburst.
The evolution process of fault-slip rockburst experienced four main stages: quiet stage, crack initiation and propagation stage, shear slip stage, and rockburst stage, as illustrated in Figure
The buckling rockburst process also involves four stages: quiet stage, crack initiation and propagation stage, splitting and bending stage, and rockburst stage, as illustrated in Figure
Figure
According to the results of stress-strain curves and evolution processes, it can be concluded that the typical characteristics of structural-type rockbursts, including the location, time, intensity, and damage pit shape, are significantly affected by the control of stiff structural planes. Therefore, more attentions should be paid to the influences of the structural planes during the excavation of deep tunnels so as to prevent and mitigate rockburst damage.
As one of the most used optical and noncontact techniques for measuring material deformations, digital image correlation (DIC) is applied to trace the deformation and rockburst process of the model samples. A sequence of high-resolution images of the sample surface is captured by two high-speed cameras at a speed of five frames per second during the test. Then the displacement field or strain field of the sample surface can be obtained at any given moment by DIC analysis. In this paper, we emphasize on understanding the influence mechanism of the structural plane on fault-slip rockburst using the DIC technique. The shear strain fields of model sample A at different temporal steps are shown in Figure
The shear strain contours of model sample A at different test stages: (a) 83 sec. (b) 87 sec. (c) 299 sec. (d) 305 sec. (e) 307 sec. (f) 308 sec.
In order to reproduce the continuous-to-discontinuous process and further understand the fracture mechanism of the rockburst influenced by the structural plane, numerical analysis was carried out based on the 11.28 rockburst by using universal distinct element code (UDEC) software. For the sake of simplicity, a two-dimensional elastoplastic model was established in plane strain condition and the initial geostress condition is the same as that of the 11.28 rockburst in situ with reference to the study of Zhang et al. [
The evolution process of the 11.28 rockburst at different steps: (a) 150th step. (b) 1560th step. (c) 2590th step.
For simplicity, a biaxial model test was performed using small sized marble samples with dimensions of 150 mm × 150 mm × 30 mm, which is not exactly consistent with the site conditions. For further study, a true triaxial model test will be conducted with large-scale model samples, such as 500 mm × 500 mm × 500 mm. Furthermore, a smooth structural plane is used in this study, while some scholars reported that the asperity shear of the structural plane may cause fault slip [
In this study, biaxial model tests were undertaken using marble samples to investigate the influence mechanism of the structural plane on rockbursts by considering different locations of structural planes, i.e., in tunnel spandrel, in tunnel sidewall, and at the intersection with the tunnel. The failure characteristics and rockburst evolution process were traced by the digital image correlation technique. The UDEC software was used for better understanding the ejection process of the fractured rock masses during rockbursts influenced by the structural plane. The following conclusions are obtained: The stress-strain curves can be divided into four main regions: elastic deformation region, stable crack propagation region, unstable crack propagation region, and postpeak region. The stress drop in the postpeak region can release the stored strain energy, which may induce the rockburst damage. The evolution process of fault-slip rockburst (buckling rockburst or shear rupture rockburst) experienced four main stages: quiet stage, crack initiation and propagation stage, shear slip stage (splitting and bending stage or shear rupture stage), and rockburst stage, which is corresponding with the four regions of stress-strain curves. The shear moving process of the structural plane can be traced by the digital image correlation technique, which demonstrates that the shear failure of the structural plane is the main cause resulting in the fault-slip rockburst. The UDEC software can better simulate the process of the fault-slip rockburst and the results show a very good agreement with the actual 11.28 rockburst.
The data supporting the findings of this study are available within the article.
No potential conflicts of interest were reported by the authors.
The financial supports for this research, from the Key Development Program for Research of Shandong Province (Grant no. 2018GNC110023) and the National Natural Science Foundation of China (Grant no. 51574156), are gratefully acknowledged.