When tunneling in a hard and brittle rock mass within a tectonic stress zone, dynamic failure of rock mass-rockburst may occur. Considering the occurrence of rockburst is generally induced by a sudden release of storage elastic energy, a numerical analysis based on the geotechnical conditions of the headrace tunnels of the Neelum–Jhelum hydroelectric project was carried out to investigate the variations of the storage elastic energy of surrounding rock mass during excavation in the tectonic stress zone. As expected, the numerical results show that the storage elastic energy concentration zones is elliptical around the tunnel due to the influence of the tectonic stress field and that the long axis of the ellipse is perpendicular to the orientation of the maximum principal stress of the tectonic stress. Furthermore, the calculated storage energy concentration zone is consistent with the locations of blasting overbreak in the tunnel. Rockburst predictions were carried out using the strength-stress ratio and energy criteria to identify the applicability of the criteria in a tectonic stress zone. The comparisons between the predictions and the field observations show that the strength-stress ratio criteria based on the uniaxial tests do not consider the influence of the tectonic stress on the strength of the rock. These criteria overpredict the extent of the blasting pits in the tectonic stress zone. However, the energy criteria based on the energy conversion of unloading confining pressure tests are able to reflect the influence of the tectonic stress, and the prediction results are more close to the field observations.
In recent years, in the “One Belt One Road” countries, many long and deep buried tunnels have been constructed for hydropower and transportation purposes. During the excavation of these tunnels, tectonic stress is often encountered. For example, in the headrace tunnels of the Neelum–Jhelum (N-J) hydroelectric project in Pakistan, the measured maximum principal in situ stress was greater than 100 MPa. Such in situ stress causes a number of rock mechanics problems, such as squeezing and rockburst. As the rockburst occurs instantaneously and often without any identifiable precursor, the rockburst does not only cause severe damage to the underground structures and equipment but also threaten human life. At the N-J hydroelectric project, severe rockburst in the tunnel caused the loss of several lives and extensive damage to the TBM machine.
Strain rockbursts in deep buried underground hard rock are generally characterized by a sudden release of storage elastic energy in a volume of highly stressed rock. Thus, many earlier studies focused on the variation of the storage energy in the rock mass during excavation. Cook et al. [
Many studies were carried out on predicting the rockburst to avoid the hazards of rockburst. These studies mainly used two approaches. The first is based on field tests, such as, the microseismic monitoring, which were successfully used in many deep buried tunnels [
In this study, based on the geotechnical conditions of the headrace tunnel of the N-J hydroelectric project, a numerical analysis has been carried out to investigate the variation and distribution of the storage elastic energy in the rock mass during the tunnel excavation in the tectonic stress zone. Furthermore, the rockburst predictions have been compared with the field observations, and the applicability of different rockburst criteria to the tectonic stress zone has been investigated.
The N-J hydroelectric project is located in the Muzaffarabad district of Azad Jammu and Kashmir, Pakistan. Two 8.53 m diameter headrace tunnels, namely, Tunnel 696 and Tunnel 697, were excavated by the TBM method for N-J hydroelectric project. The length of each tunnel is over 11 km, and 70% of the tunnel is under more than 1000 m overburden (Figure
Tunnel profile with the range of rock cover.
The rock mass surrounding the tunnel belongs primarily to Murree Formation, which comprises alternating beds of sandstone, siltstone, mudstone, and shale. The sandstone is well cemented, fine to medium grained, and the main compositions of the sandstone are shown in Table
Composition of sandstone in the N-J headrace tunnel.
Quartz (%) | Potassium feldspar (%) | Albite (%) | Calcite (%) | Dolomite (%) | Hematite (%) | Clay mineral (%) |
---|---|---|---|---|---|---|
38.9 | 0.4 | 3.7 | 33.8 | 6.6 | 1.2 | 15.4 |
During the excavation of the Tunnel 696, hundreds of weak and medium rockbursts occurred in the sandstone since the buried depth reached and exceeded 800 m, and a severe rockburst occurred at chainage 09 + 700∼800. This severe rockburst caused loss of several lives and extensive damage to the TBM machine. The tunnel profile after the severe rockburst is shown in Figure
Measured blasting pits profile in Tunnel 696. (a) Longitudinal section and (b) cross section of the tunnel.
The N-J project is located in the Himalayas Mountain, and local faults have been developed along the TBM tunnel. The geological investigation has shown that severe rockburst occurred in the vicinity of a local reverse fault. Stress measurement was carried out to investigate the in situ stress of this area. Three overcoring HI (hollow inclusion) tests were conducted in the borehole at chainages 9 + 860, 10 + 938, and 13 + 834 in Tunnel 696 (Figure
Core discing in the borehole at chainage 9 + 860.
Rock core of HI cells: (a) chainage 10 + 938 and (b) chainage 13 + 834.
Stress test results at chainage 10 + 938 and 13 + 834.
Chainage | Buried depth | Tests no | Depth in borehole (m) |
|
|
|
---|---|---|---|---|---|---|
13 + 834 | 800 | 1 | 23.0 | −50.7/282/−14 | −24.3/206/43 | −19.5/358/44 |
2 | 24.3 | −54.9/287/−30 | −27.7/204/13 | −21.2/315/57 | ||
3 | 25.9 | −50.7/281/−39 | −21.3/210/22 | −15.9/322/43 | ||
|
||||||
10 + 938 | 1010 | 1 | 19.1 | −102.9/292/33 | −40.6/193/14 | −25.1/84/54 |
2 | 20.0 | −107.3/338/21 | −40.9/119/64 | −32.7/243/13 | ||
3 | 21.0 | −102.9/312/32 | −48.4/195/47 | −24.4/70/37 |
Plunge (dip) from horizontal positive down and trend (azimuth) positive clockwise from north.
Orientation of principle stresses at chainage 10 + 938 in the lower hemisphere stereonet (
The occurrence of rockburst is induced by a sudden release of storage elastic energy in the surrounding rock. In order to investigate the influence of tectonic stress on the distribution and magnitude of storage elastic energy, a numerical analysis has been carried out based on the geotechnical conditions in the severe rockburst zone in Tunnel 696.
Using a finite element method, a numerical model has been developed as shown in Figure
Numerical model of the severe rockburst zone in Tunnel 696.
Mechanical parameters of sandstone and shotcrete.
Materials | Young’s modulus (GPa) | Poisson’s ration | Cohesive strength (MPa) | Friction angle (°) |
---|---|---|---|---|
Sandstone | 20 | 0.25 | 4.3 | 42 |
Shotcrete | 25 | 0.2 | — | — |
Mean values of stress components in the local coordinate system (measurement results at chainage 10 + 938).
|
|
|
|
|
|
---|---|---|---|---|---|
−83.8 | −44.7 | −47.8 | −2.0 | −26.0 | 12.7 |
Stress positive-negative regularity follows linear elastic mechanics. Positive normal stress is tensile stress, and negative is compressive stress. The direction of local coordinate is shown in Figure
Using the established finite element model to carry out a numerical analysis, the procedure is as follows: Step 1. Apply the in situ stress. Step 2. Excavate the headrace tunnel step by step, and the excavation length of each step is 5 m.
After the excavation of the tunnel, the distribution of the storage elastic energy density of the rock mass at typical cross section is shown in Figure
Distribution of the storage elastic energy density of the rock mass after excavation (unit: J/m3).
The orientation of the principal stresses of the tectonic stress is shown in Figure
Variation of the storage elastic energy during tunnel excavation. (a) Location of the measurement line. (b) Variation of the storage elastic energy of rock mass on the measurement line.
The variation of the magnitude of storage elastic energy at the tunnel vault is shown in Figure
Prediction results of distribution and extent of sever rockburst using strength-stress ratio criteria.
Predictions of severe rockburst potential in Tunnel 696 were carried out to investigate the applicability of the strength-stress ratio and energy rockburst criteria in a tectonic stress zone, and the predictions results have been compared with the field observations of the blasting pits.
The commonly used strength-stress ratio criteria are shown in Table
Commonly used strength-stress ratio criteria.
Criteria | Proposer | Formula | Rockburst grade | |||
---|---|---|---|---|---|---|
No | Light | Moderate | Severe | |||
1 | [ |
|
>7.0 | 4.0∼7.0 | 2.0∼4.0 | <2.0 |
2 | [ |
|
>10.0 | 10.0∼5.0 | 5.0∼2.5 | <2.5 |
3 | [ |
|
0.34 | 0.42 | 0.56 | 0.70 |
4 | [ |
≤0.20 | 0.20∼0.30 | 0.30∼0.55 | ≥0.55 |
Based on the numerical results of stress distribution of surrounding rock after excavation, the predictions of the distribution and extent of severe rockburst used the Criteria 3 and 4 are shown in Figure
The commonly used energy criteria include the linear elastic energy, strain energy storage coefficient, and rockburst energy coefficient. They are based on the energy analysis of uniaxial loading or unloading tests and do not consider the influences of tectonic stress. In this study, the energy criterion based on the unloading confining pressure tests has been used [
To identify the maximum storage elastic energy of sandstone in Tunnel 696, a series of unloading confining pressure tests have been carried out. The cylindrical samples (100 mm long and 50 mm in diameter) for the unloading tests were machined from a large lump of rock mass, and the specimen is shown in Figure
Rock specimens for unloading confining pressure tests.
The typical stress-strain curve of the sandstone during the unloading tests is shown in Figure
Typical stress-strain curve of sandstone during unloading confining pressure tests.
Generally, the specimen is in the elastic stage during the application of the hydrostatic pressure, and the storage elastic strain energy
During the application of the initial axial stress, the axial stress is lower than 70% of triaxial strength of the specimen, and the specimen is in the elastic range. Hence, the storage elastic energy
During the unloading process, the storage elastic energy
According to the above analysis, the storage elastic energy of the specimen can be determined from the elastic parameters
Results of unloading confining pressure tests of sandstone.
Initial confining pressure (MPa) | Tests no. | Young’s modulus (GPa) | Poisson’s ration | Peak deviatoric stress (MPa) | Maximum storage elastic energy (kJm−3) |
---|---|---|---|---|---|
35 | 1 | 40.2 | 0.26 | 250 | 820.0 |
2 | 35.9 | 0.22 | 245 | 866.5 | |
3 | 38.3 | 0.24 | 246 | 833.4 | |
|
|||||
45 | 4 | 34.2 | 0.23 | 269 | 1114.3 |
5 | 40.3 | 0.24 | 282 | 1053.1 | |
6 | 51.7 | 0.27 | 283 | 898.0 | |
|
|||||
55 | 7 | 32.1 | 0.22 | 267 | 1363.9 |
8 | 32.8 | 0.24 | 286 | 1445.8 | |
9 | 32.7 | 0.23 | 285 | 1434.0 | |
|
|||||
65 | 10 | 52.4 | 0.28 | 336 | 1667.5 |
11 | 29.1 | 0.21 | 329 | 1909.8 | |
12 | 30.8 | 0.24 | 301 | 1788.8 | |
|
|||||
75 | 13 | 28.0 | 0.23 | 338 | 2233.2 |
14 | 35.0 | 0.25 | 357 | 2097.9 | |
15 | 38.3 | 0.25 | 353 | 1801.8 |
To investigate the influences of the initial confining pressure on the maximum storage elastic energy of the rock, the variations of the maximum storage elastic energy with the initial confining pressure are shown in Figure
Variations of the maximum storage elastic energy of sandstone with initial confining pressure.
As the correlation coefficient is greater than 0.95, this means that the exponential model is a good fit between
According to the analysis on the variations of the storage elastic energy during the excavation of the tunnel, the concentration of the storage elastic energy around the tunnel is induced by the loading at the maximum principal stress orientation and unloading at the minimum principal stress orientation. Considering the loading path of the unloading confining pressure tests, the minimum principal stress of the in situ stress can be treated as the initial confining pressure, and the maximum storage elastic energy of a rock mass can be calculated using equation (
According to the energy criterion developed by Chen et al. [
Prediction results of distribution and extent of sever rockburst using energy criteria.
In this study, by applying a numerical model to the geotechnical conditions in the headrace tunnels of N-J hydroelectric project, a numerical analysis has been carried out to investigate the variations of the storage elastic energy of a rock mass in the tunnel at the tectonic stress zone. The numerical results show that the line between storage energy concentration zones is elliptical around the tunnel due to the influence of the tectonic stress. The long axis of the ellipse is perpendicular to the orientation of the maximum principal stress of the tectonic stress. Furthermore, the calculated storage energy concentration zone is consistent with the locations of the blasting pits at the tunnel vault in the field. Besides, in a tectonic stress zone, the initial storage elastic energy induced by the tectonic stress is up to 30% of the total storage elastic energy after excavation. This means that the tectonic stress clearly increases the risk of rockburst.
To identify the applicability of rockburst criteria to a tectonic stress zone, different types of criteria have been used to predict rockburst in Tunnel 696. The comparisons between the predictions and the field observations show that the strength-stress ratio criteria overpredict the extent of the blasting pits in the tectonic stress zone. The overprediction is mainly due to the strength of the rock used in the strength-stress ratio criteria is obtained from the laboratory axial tests, and the influence of the in situ stress on the strength of the rock is not considered. The energy criteria based on the energy conversion of unloading confining pressure tests are able to reflect the influence of the tectonic stress (i.e., the initial confining pressure), and the prediction results are close to the field observations, indicating that the energy criteria are more applicable in a tectonic stress zone.
All data included in this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the support of the Youth Innovation Promotion Association CAS.