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Abstract Understanding the characteristics and distribution patterns of the initial geo-stress field in tunnels is of great significance for studying the problem of large deformation of tunnels under high geo-stress conditions. This article proposes a ground stress field inversion method and large deformation level determination based on the GS-XGBoost algorithm and the Haba Snow Mountain Tunnel of the Lixiang Railway. Firstly, the hydraulic fracturing method is used to conduct on-site testing of tunnel ground stress and obtain tunnel ground stress data. Then, a three-dimensional model of the Haba Snow Mountain Tunnel will be established, and it will be combined with the GS-XGBoost regression algorithm model to obtain the optimal boundary conditions of the model. Finally, the optimal boundary condition parameters are substituted into the three-dimensional finite-difference calculation model for stress calculation, and the distribution of the in-situ stress field of the entire calculation model is obtained. Finally, the level of large deformation of the Haba Snow Mountain Tunnel will be determined. The results show that the ground stress of the tunnel increases with the increase of burial depth, with the maximum horizontal principal stress of 38.03 MPa and the minimum horizontal principal stress of 26.07 MPa. The Haba Snow Mountain Tunnel has large deformation problems of levels I, II, III, and IV. Level III and IV large deformations are generally accompanied by higher ground stress (above 28 MPa) and smaller surrounding rock strength. The distribution of surrounding rock strength along the tunnel axis shows a clear "W" shape, opposite to the surface elevation "M" shape. It is inferred that the mountain may be affected by geological structures on both sides of the north and south, causing more severe compression of the tunnel surrounding rock at the peak.

As a unique geological hazard in a high-geo-stress environment, rockburst happens with strong suddenness, randomness, and destructiveness, but the mechanism of its occurrence in a deep-buried tunnel in a high-geo-stress environment needs further study. Based on the analyses of the stress field of the rockburst section of Ping'an Tunnel, which is over-long and deep-buried, the occurrence mechanism of rockburst is figured out. Furtherly, the intensity and location of rockburst are predicted by using the rockburst criterion. Results show that there exists large compressive stress at the side wall of the tunnel, which is the main cause of rockburst. Under a high-geo-stress condition, due to the existence of unfavorable factors like structural planes, high intensity rockburst is likely to happen in the middle of the tunnel face after the sudden release of original rock stress. Arranging stress-releasing holes in the rockburst section can effectively reduce the possibility of rockburst by releasing the original rock stress in advance. The research results can deepen the understanding of the mechanism of rockburst in a high-geo-stress environment and provide scientific basis for the prevention and control of rockburst in similar engineering projects. [ABSTRACT FROM AUTHOR]

Multi-cluster fracturing has emerged as an effective technique for enhancing the productivity of deep shale reservoirs. The presence of natural bedding planes in these reservoirs plays a significant role in shaping the evolution and development of multi-cluster hydraulic fractures. Therefore, conducting detailed research on the propagation mechanisms of multi-cluster hydraulic fractures in deep shale formations is crucial for optimizing reservoir transformation efficiency and achieving effective development outcomes. This study employs the finite discrete element method (FDEM) to construct a comprehensive three-dimensional simulation model of multi-cluster fracturing, considering the number of natural fractures present and the geo-mechanical characteristics of a target block. The propagation of hydraulic fractures is investigated in response to the number of natural fractures and the design of the multi-cluster fracturing operations. The simulation results show that, consistent with previous research on fracturing in shale oil and gas reservoirs, an increase in the number of fracturing clusters and natural fractures leads to a larger total area covered by artificial fractures and the development of more intricate fracture patterns. Furthermore, the present study highlights that an escalation in the number of fracturing clusters results in a notable reduction in the balanced expansion of the double wings of the main fracture within the reservoir. Instead, the effects of natural fractures, geo-stress, and other factors contribute to enhanced phenomena such as single-wing expansion, bifurcation, and the bending of different main fractures, facilitating the creation of complex artificial fracture networks. It is important to note that the presence of natural fractures can also significantly alter the failure mode of artificial fractures, potentially resulting in the formation of small opening shear fractures that necessitate careful evaluation of the overall renovation impact. Moreover, this study demonstrates that even in comparison to single-cluster fracturing, the presence of 40 natural main fractures in the region can lead to the development of multiple branching main fractures. This finding underscores the importance of considering natural fractures in deep reservoir fracturing operations. In conclusion, the findings of this study offer valuable insights for optimizing deep reservoir fracturing processes in scenarios where natural fractures play a vital role in shaping fracture development. [ABSTRACT FROM AUTHOR]

With the depletion of conventional reservoir development, reservoir fracturing under deep high geo-stress and high geo-stress difference conditions is receiving increasing attention. Deep reservoirs typically require multi-cluster fracturing to achieve efficient reservoir transformation and development. In this paper, considering the relevant geological parameters of a certain reservoir in the southwest, three-dimensional multi-cluster reservoir fracturing models were established based on cohesive element modeling. Then, the propagation law of artificial fractures in reservoirs under the influence of the different number of fracturing clusters, injection displacement, and Young's modulus in different regions of the 60 m fracturing well section is analyzed, and the quantitative law of parameters such as fracture length, maximum fracture width, injection point fracture width, fracture area, and tensile failure ratio during multi-cluster fracturing construction, as well as the propagation law of fracture morphology are revealed. The simulation results show that using multi-cluster fracturing can significantly improve the effectiveness of reservoir reconstruction, but as the number of fracturing clusters increases, it is also easy to form some small opening artificial fractures. These small opening artificial fractures may not be conducive to the transportation of proppants and fluids. During single cluster fracturing, the interface stiffness and rock Young's modulus have a significant impact on the propagation of artificial fractures in the reservoir. As the number of fracturing clusters increases, the competition between artificial main fractures expands significantly, which may reduce the impact of interface stiffness and rock Young's modulus. The fluid injection rate has a significant impact on reservoir fracturing, and in the same area, using high displacement injection can significantly increase the volume of reservoir reconstruction. This study can provide some reference for multi-cluster fracturing construction in deep reservoirs. [ABSTRACT FROM AUTHOR]

The Maoxian Tunnel in Sichuan, China has experienced significant deformation and supporting failure in the surrounding phyllite rock. To address this problem, on-site engineering geological investigation and testing methods were deployed in the present study to examine the characteristics and mechanism of deformation in the tunnel. Scanning electron microscopy, rock hydraulic tests, and uniaxial compressive tests were used to investigate the hydraulic and mechanical properties of the surrounding rock, and to examine the mechanisms underlying the significant deformation. The results suggest that the large deformation in Maoxian Tunnel is a result of shear expansion and progressive failure of the soft rock under high stress. To enhance the integrity and self-sustaining ability of the rock mass, it is necessary to promptly construct support structures and reinforce the surrounding rock. [ABSTRACT FROM AUTHOR]

In order to study the deformation law of soft rock tunnels within high geo-stress, the paper quantitatively analyzed the relationships between the deformation value with the compressive strength of rock mass, the integrity of surrounding rock, the geo-stress & depth of tunnel, the coefficient of lateral stress, the ground water, the value of basic quality[ BQ] and the stiffness of support of soft rock tunnel within high geo-stress based on the statistics of monitoring dada from 68 monitoring sections in 41 tunnels. The results indicated that the large deformation of surrounding rock in soft rock tunnel within high geo-stress was the outcome of the comprehensive action of geological conditions of surrounding rock, the design and construction parameters and other factors. Among them, the in-situ geo-stress, surrounding rock characteristics and supporting countermeasures had the significant influence. The results from the case statistics could preliminarily reveal the development level of the control technology of large deformation of soft rock traffic tunnel within high geo-stress in China. The deformation of soft rock tunnel within high geo-stress had noticeable time-space effect with the variation patterns from the rapid growth stage to the continuing growth stage and then the stabilizing stage at last with time. The deformation coordination coefficient was mostly affected by the excavation with large fluctuations in the early stage, but tended to be stable very soon after the inverted arch was constructed which also verified and emphasized the key role of the ring closure to the deformation control from another perspective. With the increase of large deformation grade, the proportion of the deformation after construction of inverted arch decreased. [ABSTRACT FROM AUTHOR]

As a unique geological hazard in a high-geo-stress environment, rockburst happens with strong suddenness, randomness, and destructiveness, but the mechanism of its occurrence in a deep-buried tunnel in a high-geo-stress environment needs further study. Based on the analyses of the stress field of the rockburst section of Ping’an Tunnel, which is over-long and deep-buried, the occurrence mechanism of rockburst is figured out. Furtherly, the intensity and location of rockburst are predicted by using the rockburst criterion. Results show that there exists large compressive stress at the side wall of the tunnel, which is the main cause of rockburst. Under a high-geo-stress condition, due to the existence of unfavorable factors like structural planes, high intensity rockburst is likely to happen in the middle of the tunnel face after the sudden release of original rock stress. Arranging stress-releasing holes in the rockburst section can effectively reduce the possibility of rockburst by releasing the original rock stress in advance. The research results can deepen the understanding of the mechanism of rockburst in a high-geo-stress environment and provide scientific basis for the prevention and control of rockburst in similar engineering projects.

Multi-cluster fracturing has emerged as an effective technique for enhancing the productivity of deep shale reservoirs. The presence of natural bedding planes in these reservoirs plays a significant role in shaping the evolution and development of multi-cluster hydraulic fractures. Therefore, conducting detailed research on the propagation mechanisms of multi-cluster hydraulic fractures in deep shale formations is crucial for optimizing reservoir transformation efficiency and achieving effective development outcomes. This study employs the finite discrete element method (FDEM) to construct a comprehensive three-dimensional simulation model of multi-cluster fracturing, considering the number of natural fractures present and the geo-mechanical characteristics of a target block. The propagation of hydraulic fractures is investigated in response to the number of natural fractures and the design of the multi-cluster fracturing operations. The simulation results show that, consistent with previous research on fracturing in shale oil and gas reservoirs, an increase in the number of fracturing clusters and natural fractures leads to a larger total area covered by artificial fractures and the development of more intricate fracture patterns. Furthermore, the present study highlights that an escalation in the number of fracturing clusters results in a notable reduction in the balanced expansion of the double wings of the main fracture within the reservoir. Instead, the effects of natural fractures, geo-stress, and other factors contribute to enhanced phenomena such as single-wing expansion, bifurcation, and the bending of different main fractures, facilitating the creation of complex artificial fracture networks. It is important to note that the presence of natural fractures can also significantly alter the failure mode of artificial fractures, potentially resulting in the formation of small opening shear fractures that necessitate careful evaluation of the overall renovation impact. Moreover, this study demonstrates that even in comparison to single-cluster fracturing, the presence of 40 natural main fractures in the region can lead to the development of multiple branching main fractures. This finding underscores the importance of considering natural fractures in deep reservoir fracturing operations. In conclusion, the findings of this study offer valuable insights for optimizing deep reservoir fracturing processes in scenarios where natural fractures play a vital role in shaping fracture development.

With the depletion of conventional reservoir development, reservoir fracturing under deep high geo-stress and high geo-stress difference conditions is receiving increasing attention. Deep reservoirs typically require multi-cluster fracturing to achieve efficient reservoir transformation and development. In this paper, considering the relevant geological parameters of a certain reservoir in the southwest, three-dimensional multi-cluster reservoir fracturing models were established based on cohesive element modeling. Then, the propagation law of artificial fractures in reservoirs under the influence of the different number of fracturing clusters, injection displacement, and Young’s modulus in different regions of the 60 m fracturing well section is analyzed, and the quantitative law of parameters such as fracture length, maximum fracture width, injection point fracture width, fracture area, and tensile failure ratio during multi-cluster fracturing construction, as well as the propagation law of fracture morphology are revealed. The simulation results show that using multi-cluster fracturing can significantly improve the effectiveness of reservoir reconstruction, but as the number of fracturing clusters increases, it is also easy to form some small opening artificial fractures. These small opening artificial fractures may not be conducive to the transportation of proppants and fluids. During single cluster fracturing, the interface stiffness and rock Young’s modulus have a significant impact on the propagation of artificial fractures in the reservoir. As the number of fracturing clusters increases, the competition between artificial main fractures expands significantly, which may reduce the impact of interface stiffness and rock Young’s modulus. The fluid injection rate has a significant impact on reservoir fracturing, and in the same area, using high displacement injection can significantly increase the volume of reservoir reconstruction. This study can provide some reference for multi-cluster fracturing construction in deep reservoirs.

The deformation characteristics and the timing for secondary lining support in high geo-stress soft rock tunnels have drawn significant attention. In carbonaceous shale formations, tunnel construction deformations are very pronounced under construction disturbances due to the development of joints, dense fractures, and poor interlayer bonding. With the Xishuangbanna tunnel as our research backdrop, this study meticulously analyzed the deformation patterns and characteristics inherent to high geo-stress tunnels constructed within carbonaceous schist formations. Employing a comprehensive approach involving full displacement analysis and on-site construction mechanics testing, we utilized the displacement release rate and structural safety factors as key indicators to determine the secondary lining supporting time. Employing this innovative approach, we successfully identified the ideal junctures for implementing secondary lining support in tunnels excavated through high geo-stress carbonaceous schist. The research findings indicate that the primary damage modes in high geo-stress carbonaceous schist tunnels are initial support failure and extensive early support deformation. These vulnerabilities are primarily attributed to weak and fragmented strata, elevated ground stress levels, and inadequate support strength. During the early stages of tunnel construction, substantial deformations are observed, exhibiting high rates of change. Horizontal convergence, notably, significantly surpasses the settlement at the tunnel's crown. When employing the three-bench method for construction, the deformation occurring before the excavation of the middle bench contributes the most to the total deformation monitored, whereas the deformation generated after the excavation of the inverted arch constitutes a minor proportion. The tunnel's crown and invert experience tension while the secondary lining undergoes compression. The internal forces are most significant at the tunnel's hance and knee, with the left tunnel knee being the weakest section of the secondary lining. The findings of our study are poised to guide the design and execution of tunnels constructed within high geo-stress carbonaceous schist formations. [ABSTRACT FROM AUTHOR]

Due to the extremely complex load distribution form and evolution law of soft-rock tunnels under high geo-stress, there has been a long-term lack of theoretical support for designing supporting structures and preventing large deformation disasters. Therefore, it is significant to explore the interaction between surrounding rock and supporting structures for soft-rock tunnels under a high geo-stress environment. Firstly, the evolution characteristics of the supporting structure load and surrounding rock damage were analyzed using the field measurement and the proposed composite viscoelastic-plastic creep damage model. Then, the load sharing and transfer mechanisms were discussed based on the decreasing evolution process of the bearing capacity after the primary support yields. Furthermore, a theoretical solution method that can reflect this load evolution mechanism was derived by synthesizing elastic–plastic constitutive, creep damage constitutive, and concrete damage constitutive models. The results showed that the load grows with the development of the surrounding rock damage. Hence the surrounding rock damage is an intrinsic factor leading to load growth. If a supporting component yields, the bearing mode will change from cooperative sharing to load transfer. In other words, part of the load originally acting on the yielded component will be transferred to the other unyielded components. During 55 days after the primary support yielded, a total load of 381 kPa was transferred to the secondary lining, accounting for 61.3% of the total load of the secondary lining. Calculation examples showed that the proposed theoretical solution method could reflect the progressive yield process of each supporting component and the load sharing and transfer processes. [ABSTRACT FROM AUTHOR]

In constructing high-geo-stress soft rock tunnels, the major deformation disaster of the surrounding rock has always been the main problem faced during construction. The research on the deformation and loose zone characteristics of large deformation tunnels has positive significance for the safe and rapid construction of tunnels. Therefore, based on the Yuntunpu large deformation tunnel, this article first analyzes the geological and deformation characteristics of the tunnel site area in response to the problem of high-geo-stress soft rock large deformation. Subsequently, on-site testing and analysis were conducted on the loose zone characteristics of four tunnel sections. Finally, based on the comprehensive analysis of tunnel deformation and loose zone characteristics, the causes of large deformation in the tunnel are analyzed. The results indicate that the large deformation characteristics of the Yuntunpu Tunnel are mainly manifested as a large initial deformation rate of the surrounding rock, a short self-stabilization time of the surrounding rock, a large cumulative deformation amount, and a long deformation duration. The Yuntunpu Tunnel is influenced by the grade and structure of the surrounding rock, with a loosening zone ranging from 12 to 14 m, and the wave velocity variation characteristics exhibited by different grades of surrounding rock vary greatly. Adopting collaborative active control of long and short anchor rods is recommended to limit the continued development of loose zones and the deformation of surrounding rocks. The large deformation of tunnels is mainly affected by high geo-stress, formation lithology, geological structure, engineering disturbance, and groundwater. Among them, high geo-stress and formation lithology are the decisive and important factors for the occurrence of major deformation disasters in the tunnel. [ABSTRACT FROM AUTHOR]

Copyright of Tunnel Construction / Suidao Jianshe (Zhong-Yingwen Ban) is the property of Tunnel Construction Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Tunnel Construction / Suidao Jianshe (Zhong-Yingwen Ban) is the property of Tunnel Construction Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Considering the influence of active faults on an in-situ stress field is of great significance in guiding the construction of tunnel engineering projects crossing active fault zones. Because the traditional multiple linear regression method violates the linear superposition principle concerning unified boundary conditions, a multiple linear regression method with improved boundary conditions for in-situ stress inversion was proposed. However, since the two multiple linear regression methods cannot restore the in-situ stress field, the nonlinear method was proposed to solve the nonlinear problem caused by the irregular ground surface. After obtaining the in-situ stress field of a tunnel in southwest China, the disturbance law of the strike-slip activity of the fault on the in-situ stress field was assessed. The results showed that the inversion accuracy from high to low was the nonlinear fitting method, the multiple linear regression method with improved boundary conditions, and the traditional multiple linear regression method. Their inversion errors were 14.7%, 18.4%, and 19.4%, respectively. The in-situ stress showed a slight upward trend near the fault area and a sudden drop while entering the fault, with an average decrease of 36.3%. Inside the fault, the orientation of the in-situ stress was deflected toward the fault tendency. For sudden strike-slip activity, the in-situ stress was manifested as the stress release outside the fault region with an average decrease of 8.4%. In comparison, the stress inside the fault region tended to increase with an average increase of 11.8%. For continuous strike-slip activity, the in-situ stress was basically unchanged. In addition, the strike-slip activity of the fault caused the in-situ stress to be deflected toward the strike-slip direction. Highlights: A multiple linear regression method with unified displacement boundary conditions is proposed to improve the in-situ stress inversion accuracy. The nonlinear problem caused by the irregular ground surface in in-situ stress inversion is assessed. The distribution characteristics of the in-situ stress field near the fault strata are analyzed. The disturbance law of the sudden strike-slip activity and continuous strike-slip activity on the in-situ stress field is investigated. [ABSTRACT FROM AUTHOR]

In constructing high-geo-stress soft rock tunnels, the major deformation disaster of the surrounding rock has always been the main problem faced during construction. The research on the deformation and loose zone characteristics of large deformation tunnels has positive significance for the safe and rapid construction of tunnels. Therefore, based on the Yuntunpu large deformation tunnel, this article first analyzes the geological and deformation characteristics of the tunnel site area in response to the problem of high-geo-stress soft rock large deformation. Subsequently, on-site testing and analysis were conducted on the loose zone characteristics of four tunnel sections. Finally, based on the comprehensive analysis of tunnel deformation and loose zone characteristics, the causes of large deformation in the tunnel are analyzed. The results indicate that the large deformation characteristics of the Yuntunpu Tunnel are mainly manifested as a large initial deformation rate of the surrounding rock, a short self-stabilization time of the surrounding rock, a large cumulative deformation amount, and a long deformation duration. The Yuntunpu Tunnel is influenced by the grade and structure of the surrounding rock, with a loosening zone ranging from 12 to 14 m, and the wave velocity variation characteristics exhibited by different grades of surrounding rock vary greatly. Adopting collaborative active control of long and short anchor rods is recommended to limit the continued development of loose zones and the deformation of surrounding rocks. The large deformation of tunnels is mainly affected by high geo-stress, formation lithology, geological structure, engineering disturbance, and groundwater. Among them, high geo-stress and formation lithology are the decisive and important factors for the occurrence of major deformation disasters in the tunnel.

The problem of large deformation for soft rock has always been difficult in tunnel construction under high geo-stress. Previous studies have mainly focused on the single stratum, while the soft-hard contact stratum is often encountered after tunnel excavation. There are limited studies on the large deformation caused by tunnels in soft-hard inclined contact strata. This paper investigated a case on asymmetric deformation of surrounding rock and unequal pressure behavior of supporting structure in Dongmachang I tunnel at the Chenghai active fault zone. Through the in-situ stress test and three-dimensional transformation calculation, we found that the tunnel site area was dominated by horizontal stress and was in extremely high geo-stress. Besides, the analysis of field monitoring and numerical simulation pointed out that maximum squeezing deformation and pressure occurred at the left spandrel of the tunnel, and the asymmetric deformation disaster of surrounding rock and the asymmetric compression failure of secondary lining were caused by dolomitic broken limestone in soft and hard inclined contact and high geo-stress. Notably, the minimum safety factor at the left spandrel was only 0.7, which clearly showed that the secondary lining had been in a failed state. Consistent with the actual damage result, this study vividly represents the damage to the secondary lining under the biased pressure on site. [ABSTRACT FROM AUTHOR]

This study aims to control the deformation of a large section of tunnel excavated in steeply inclined layered phyllite under high geo-stress. To this end, anisotropic mechanical properties, which are factors influencing the deformation and stress characteristics of the phyllitic structures, were studied by combining rock tests with field tests based on Yangjiaping Tunnel on the Chengdu-Lanzhou Railway, China. Moreover, measures to control the surrounding rock deformation are proposed. The main conclusions are as follows: mechanical properties of chlorite-phyllite present significant anisotropy and its strength and failure modes are related to the azimuth of the applied load. There are significant differences in the failure modes of the surrounding rock at different positions in the tunnel excavated in steeply inclined layered strata. The rock surrounding the sidewalls is dominated by tension and the bending failure of beddings, whilst the vault and inverted arch are characterised by shear failure. Furthermore, the surrounding rock deformation of the tunnel excavated in steeply inclined layered phyllite under high geo-stress is affected by factors such as tectonic stress, strata occurrence, a soft interlayer and construction disturbance. The surrounding rock deformation and stress of the support structures are distributed non-uniformly. Large displacements and deformations are found on sidewalls, where targeted control measures should be taken. The measures that can be taken include optimising the cross-sectional profile, strengthening support rigidity, simplifying the excavation steps, enhancing advanced pre-support measures, adjusting the angle at which anchor bolts are applied and using mechanised construction. They can timeously provide support to the surrounding rock and limit the development of plastic zones, which not only controls tunnel deformation, but also improves the progress of construction by around 50%. [ABSTRACT FROM AUTHOR]

The gas occurrence mechanism of deep coal seam is an important theoretical basis to guide the safe and efficient mining of deep coal and gas resources. Compared with the shallow coal seam, the occurrence environment of the deep coal seam has the characteristics of high stress, high temperature, high gas pressure and low permeability, which determines the unique characteristics of the deep gas occurrence and the technical problems faced by the deep resource output. We systematically reviewed the influence of stress, temperature, pressure and permeability on the gas occurrence laws of deep coal seam, pointed out the shortcomings of single physical field influence research, and made clear that the multi-field coupling environment is the objective condition for the study of gas mechanism of deep coal seam, and pointed out the direction for the study of gas occurrence mechanism of deep coal seam.

In order to explore the feasibility of rubber cement concrete supporting in soft rock roadway with high geo-stress, by using ABAQUS numerical simulation software, the supporting effect of rubber cement concrete in soft rock roadway with high geo-stress and the influence of elastic modulus and uniaxial compressive strength on the unloading effect are simulated and calculated. The results show that under the high geo-stress environment, the effect of applying rubber cement concrete between roadway floor and ordinary concrete layer is better. When the elastic modulus of rubber cement concrete is between 3 GPa and 4 GPa, and the uniaxial compressive strength is between 3 MPa and 5 MPa, the best pressure relief effect is obtained. Rubber cement concrete releases the stress in the surrounding rock by cooperating with the surrounding rock of the floor, thus reducing the load on the supporting structure.

Similar to the squeezing soft-rock tunnel, the surrounding rock pressure for a deep-buried hard-rock tunnel under high geo-stress also possesses certain long-term evolution characteristics, which have a significant impact on the safety performance of the supporting structures. This paper investigated the evolution characteristics and transfer mechanism of the surrounding rock pressure for a deep-buried hard-rock tunnel through the combinations of the field measurements and numerical simulation. Firstly, the triaxial compression and uniaxial creep tests for granite samples indicated that the hard-brittle rock could exhibit the rheological properties under high geo-stress. Secondly, the field measurements showed that the surrounding rock pressure continued to increase throughout a period of 1200 days. Thirdly, a new composite viscoelastic-plastic creep constitutive model was used to calculate the creep damage degree and scope of the surrounding rock. The results showed that the evolution characteristics of the surrounding rock pressure can be divided into three typical stages, i.e., phase I: rapid growth stage; phase I : decelerating growth stage; and phase III: basically stable stage. From the beginning of phase II to the end of phase III (about 1100 days), the pressure on the supporting structures increased by nearly 40% compared to phase I (about 50 ~ 150 days), and the average pressure-sharing proportions of the primary support and the secondary lining finally were stabilized at 0.6 and 0.4, respectively. The damage of surrounding rock was distributed mainly from the haunch to the vault and the bottom, with the degree and scope of the bottom being the most serious. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering Geology / Gongcheng Dizhi Xuebao is the property of Journal of Engineering Geology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Zonal disintegration of surrounding rock is characterized by the intermittent distribution of failure zones, which can be observed in deep underground roadways and tunnels under high-geo-stress confinement. These deep underground roadways can be quite hot and thus require extensive cooling throughout their construction and operation. However, the way in which cooling affects the zonal disintegration of deeply buried high-temperature roadways has rarely been reported. We therefore studied the formation mechanism of zonal disintegration at room temperature using the strength reduction method as part of a self-developed numerical code: RFPA. Subsequently, the effects of cooling and geo-stress on the development of zonal disintegration were disclosed by examining the spatial and temporal distributions of the temperature and stress of the surrounding rock. Numerical results indicated without the thermal stress effect, zonal disintegration is caused by macroscopic shear failure, and cooling transforms the zonal disintegration into a hybrid tensile-shear failure. Although the acoustic emissions quantified revealed that high geo-stress is the primary factor affects roadway instability, the failure area caused by cooling cannot be ignored when it comes to roadway support and stabilization because the time and mechanism that damage the surrounding rock are different from those that caused by geo-stress. [ABSTRACT FROM AUTHOR]

Large squeezing deformation has always been a critical concern in the construction of deep-buried tunnels in soft-weak rock masses. This paper describes a case study on the large deformation mechanism and supporting method of the Maoxian tunnel in Sichuan Province, China, which is located in the core area influenced by the 2008 Wenchuan earthquake and suffered severe large deformation in broken phyllite under high geo-stress. Through a survey on the geological features, the deformation mechanism of surrounding rock and the failure characteristics of supporting structures of the Maoxian tunnel in F1 fault zone were studied. It was found that the occurrence of large deformation was due to the combined action of the high geo-stress and poor self-stability of carbonaceous phyllite. In order to control the squeezing deformation, single and double primary support methods were adopted in succession. A comparative field test was conducted to study their supporting mechanism and mechanical behavior in terms of surrounding rock pressure, internal stress of the steel arch, and axial force and bending moment of the secondary lining. The results revealed that the single primary support method cannot ensure the long-term safety of the tunnel, since many cracks in concrete occurred after about 180 days. The double primary support method, however, was able to control the large deformation and rheological effects of broken phyllite under high geo-stress effectively. [ABSTRACT FROM AUTHOR]

Abstract The China Jinping Underground Laboratory phase II (CJPL-II) is currently the world's deepest laboratory with a maximum overburden over 2.4 km. For this project, a deeply-buried shaft was constructed on the floor of the existing large-span laboratories under conditions involving high geo-stress. To explore the failure characteristics encountered and optimize the excavation and support scheme of the shaft, an in situ microseismic (MS) monitoring experiment was carried out. Firstly, an appropriate monitoring scheme was developed for this special project (i.e. the details of the sensor array required, velocity model, and system protection). Secondly, the location accuracy of the monitoring system was analyzed. On this basis, the effect of excavation and support on the fracturing processes induced in the surrounding rock was analyzed by interpreting the MS information. Thirdly, the spatial relationship between the potential damage zones and direction of the maximum principal stress was validated by analyzing the spatiotemporal evolution of the MS activity with the help of numerical simulations. In addition, some suggestions for excavation and support schemes are provided based on the monitoring results. Hence, the MS monitoring scheme and results described in this paper constitute a reference for construction design and MS monitoring of similar projects. [ABSTRACT FROM AUTHOR]

Based on the analyses of data obtained from the underground powerhouse at Jinping I hydropower station, a comprehensive review of engineering rock mechanics practice in the underground powerhouse is first conducted. The distribution of strata, lithology, and initial geo-stress, the excavation process and corresponding rock mass support measures, the deformation and failure characteristics of the surrounding rock mass, the stress characteristics of anchorage structures in the cavern complex, and numerical simulations of surrounding rock mass stability and anchor support performance are presented. The results indicate that the underground powerhouse of Jinping I hydropower station is characterized by high to extremely high geo-stresses during rock excavation. Excessive surrounding rock mass deformation and high stress of anchorage structures, surrounding rock mass unloading damage, and local cracking failure of surrounding rock masses, etc., are mainly caused by rock mass excavation. Deformations of surrounding rock masses and stresses in anchorage structures here are larger than those found elsewhere: 20% of extensometers in the main powerhouse record more than 50 mm with the maximum at around 250 mm observed in the downstream sidewall of the transformer hall. There are about 25% of the anchor bolts having recorded stresses of more than 200 MPa. Jinping I hydropower plant is the first to have an underground powerhouse construction conducted in host rocks under extremely high geo-stress conditions, with the ratio of rock mass strength to geo-stress of less than 2.0. The results can provide a reference to underground powerhouse construction in similar geological conditions.

In a high geo-temperature environment, it is rarely reported that geo-temperature has been considered during a back analysis. This may cause the initial geo-stress field that is obtained by a back analysis to be wrong. In this study, according to the theory of elasticity, the theoretical solution of the hydraulic fracturing equation is obtained in a high geo-temperature environment. Since the vertical stress that is obtained by the hydraulic fracturing method is calculated using the density of overlying strata, this vertical stress lacks the thermal stress that is caused by geothermal gradients. Therefore, in a high geo-temperature environment, inverting the initial geo-stress field of rock masses directly using the stress that is measured by the hydraulic fracturing method can cause serious errors. We propose that the regression coefficient of a gravitational stress field should be set to one during a back analysis if stresses are measured by the hydraulic fracturing method, and this regression coefficient should not be equal to one if stresses are measured by overcoring methods. We also propose a workflow for the back analysis of the initial geo-stress field of rock masses that considers geo-temperature, and this workflow is applied to the Sangzhuling tunnel in China.

Rockbursts are one of the prominent problems faced by deep underground engineering. Not only do they affect the construction progress, but they also threaten the safety of construction personnel and equipment, and may even induce earthquakes. Therefore, the prediction of rockbursts has very important engineering significance for the excavation of deeply buried tunnels. In this paper, a new indicator for stability and optimization evaluation of hard, brittle surrounding rock under high geo-stresses, namely the minimum energy storage limit of surrounding rock induced by transient unloading, is proposed. In addition, the time for erecting support for tunnel excavation in the rockburst area and the impact of excavation dimensions on rockburst are investigated. The results show that transient unloading during the tunnel excavation process will reduce the energy storage limit of the rock mass. When the strain energy density of the local surrounding rock exceeds the minimum energy storage limit of the rock mass, the rock mass energy is suddenly released, and rockburst occurs. Rockburst is most likely to occur at 0.42−0.65 D away from the working face. The increasing length of a round adopted in high geo-stress areas will make the surrounding rock unstable and increase the probability of rockburst.

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The complex geological conditions of high arch dam sites could increase the difficulty in the design and construction processes, making the selection of the dam foundation surface a major subject in high dam engineering. This paper discusses some main problems in rock mass utilization for the foundation surface of high arch dams in medium or high geo-stress regions, including the rock mass classification and treatments for poor conditions. Poor conditions include the presence of large-scale discontinuities, asymmetry in the topography or geology and stress relaxation in high geo-stress zones. With respect to these main problems, six Chinese arch dams with heights of more than 200 m are selected to investigate the rock mass utilization. The special rock mass issues encountered by the foundations of the six dams and relevant treatment measures are discussed to describe the rock mass utilization characteristics for the arch dams. The key technical factors, such as excavation, reinforcement and structural optimization, are summarized to provide an effective reference for the rock mass utilization of high arch dam foundation. [ABSTRACT FROM AUTHOR]

High geo-stress and its engineering problems have severely affected the development of civil infrastructures in western China. The problems include high rock slope instabilities, rock burst, gas explosion and large-scale soft rock deformation in deep tunnels. This paper investigates the distribution of the high geo-stresses and the models of the stress concentration areas in the eastern margin of Qinghai-Tibet plateau so that a solid foundation can be formed to address the problems. The investigation is based on a comprehensive analysis of the previous research data of the eastern margin and uses remote sensing techniques, geophysics, geochemistry, and large scale geological surveying methods. The investigation has found that some special tectonic zones have high geo-stresses. The high geo-stresses are located at (1) the convergent boundary areas between two fault blocks with large strength differences, (2) the tectonic necks in front of active fault blocks, and (3) the intersection and/or termination areas of faults within the fault blocks. An example for (1) is the north Qilian high geo-stress area. Another example for (2) is the Minshan high geo-stress area in the northwest Sichuan. Furthermore, the investigation has summarized six basic models to characterize the high geo-stress concentration areas. The first one is the convergent stress concentration model at the boundary of two fault blocks. The other five stress concentration modes are oblique fissures or intersecting areas, areas without lower velocity layer in the crust, areas of compression induced tensile cracking, tectonic wedge areas, and tectonic neck areas, respectively. [ABSTRACT FROM AUTHOR]

The Jinping I Hydropower Station will generate some 3,600 MW. The site of the 305 m high arch dam is in a steep, 1,500–2,500 m deep valley on the Yalong River in Sichuan Province, South West China. The present geo-stresses in the marble bedrock are up to 40 MPa. Relaxation due to river incision has resulted in fractures opening up to 350 m from the rock face. The geo-stresses in the rock mass are significantly affected by the topography. A concentration of stress has developed at the foot of the slope and in the valley floor; it was estimated there would be 132 mm of relaxation/heave of the formation level. The paper discusses some of the engineering geological problems encountered as a result of the high geo-stresses and the mitigation measures undertaken. [ABSTRACT FROM AUTHOR]

• High geo-stress and soft rock mass are main reasons of large deformation in tunnel. • NPR anchor cable keeps constant resistance 350 kN and adapts to large deformation. • Energy absorbed by NPR anchor cables should been greater than releasing by rock mass. • The release of energy of the primary support ensures the safety of secondary lining. • NPR anchor cable can solve the large deformation and failure in soft rock tunnel. To solve the large deformation problem of thin-layered carbonaceous slate in Minxian Tunnel, the mechanical properties and large-deformation mechanism of carbonaceous slate are analyzed. A negative Poisson's ratio (NPR) anchor cable supporting solution was designed based on the principle of energy coupling support and field tests were carried out. Thin-layered low-strength jointed rock mass and extremely high geo-stress is found to be the main causes of large deformation in Minxian Tunnel. Dissolution of clay minerals in carbonaceous slate by fissure water accelerates softening of surrounding rocks, which is another reason for the large deformation. The adopted NPR anchor cable energy supporting system can maintain a constant support reaction force (350 ± 10kN) for the surrounding rock, releasing the deformation energy of the surrounding rock through conical body sliding. Evaluation study on NPR anchor cable support was performed including axial force of NPR anchor cable, convergence deformation of surrounding rock, pressure monitoring between steel arch with surrounding rock and pressure monitoring between primary support structure with secondary lining. Monitoring results demonstrate the effectiveness of the adopted NPR anchor cable (7.3 m and 12.3 m) supporting system in Minxian Tunnel. This report would be helpful for the construction of soft rock tunnel under high geo-stress conditions for the control of large deformation and failure. [ABSTRACT FROM AUTHOR]

Rockbursts are one of the prominent problems faced by deep underground engineering. Not only do they affect the construction progress, but they also threaten the safety of construction personnel and equipment, and may even induce earthquakes. Therefore, the prediction of rockbursts has very important engineering significance for the excavation of deeply buried tunnels. In this paper, a new indicator for stability and optimization evaluation of hard, brittle surrounding rock under high geo-stresses, namely the minimum energy storage limit of surrounding rock induced by transient unloading, is proposed. In addition, the time for erecting support for tunnel excavation in the rockburst area and the impact of excavation dimensions on rockburst are investigated. The results show that transient unloading during the tunnel excavation process will reduce the energy storage limit of the rock mass. When the strain energy density of the local surrounding rock exceeds the minimum energy storage limit of the rock mass, the rock mass energy is suddenly released, and rockburst occurs. Rockburst is most likely to occur at 0.42–0.65 D away from the working face. The increasing length of a round adopted in high geo-stress areas will make the surrounding rock unstable and increase the probability of rockburst. [ABSTRACT FROM AUTHOR]

In a high geo-temperature environment, it is rarely reported that geo-temperature has been considered during a back analysis. This may cause the initial geo-stress field that is obtained by a back analysis to be wrong. In this study, according to the theory of elasticity, the theoretical solution of the hydraulic fracturing equation is obtained in a high geo-temperature environment. Since the vertical stress that is obtained by the hydraulic fracturing method is calculated using the density of overlying strata, this vertical stress lacks the thermal stress that is caused by geothermal gradients. Therefore, in a high geo-temperature environment, inverting the initial geo-stress field of rock masses directly using the stress that is measured by the hydraulic fracturing method can cause serious errors. We propose that the regression coefficient of a gravitational stress field should be set to one during a back analysis if stresses are measured by the hydraulic fracturing method, and this regression coefficient should not be equal to one if stresses are measured by overcoring methods. We also propose a workflow for the back analysis of the initial geo-stress field of rock masses that considers geo-temperature, and this workflow is applied to the Sangzhuling tunnel in China. [ABSTRACT FROM AUTHOR]