Quantitative stability evaluation of multi-tunnel structure is an important issue related to the safety assessment and stable construction of geotechnical underground tunnels. In this work, the ...overall failure process of twin tunnels were exhibited by a physical simulation based on the 3D printing (3DP) sandstone analogues model, and a safety factor method was also presented for evaluating the general safety of multi-tunnel structure. For checking the 3DP material performance of physical model, uniaxial and triaxial compressions for the 3DP cylinder specimens were first tested and showed that their mechanical properties and failure characteristics were similar to natural rock in general. Then, the overloading tests for twin-tunnel model were carried out and have exposed the critical position of overall failure of twin-tunnel structure through visual observation and automatic measurement. Testing results and corresponding numerical back analysis indicated that the connectivity of plastic strain between tunnels can be deemed as the conservative instability criterion (i.e. yielding of material) and the inflection point of tunnels’ displacement can be deemed as overall failure criterion (i.e. structure failure) for twin-tunnel structure. The safety analysis for underground hydraulic caverns indicated that this method can provide a reference for quantitative and reasonable evaluation of the general safety of multi-tunnels or caverns and the local instability zone of surrounding rock.
Asymmetrical large deformation problem of quartz mica schist is anticipated in a planning hydropower station. The objective of this study was to investigate the anisotropic mechanical properties of ...the schist under complex stress states. For this purpose, a series of true triaxial compression tests accompanied by acoustic emission monitoring were carried out on this schist with different loading angles with respect to inherent schistosity. The test results show that the orientation of schistosity and stress state have a significant impact on the deformation, strength and failure of this schist. More specifically, when the
σ
2
is parallel to the strike of the schistosity (
ω
= 0°), the failure of the specimen is mainly controlled by the schistosity structure, and the strength is lower compared with the case of
σ
2
perpendicular to the strike (
ω
= 90°). When
ω
= 90°, the deformation normal to schistosity is more restrained, and the acoustic emission impact mostly presents the phenomenon of concentration and sudden increase during the fracture. The test results indicate that the normal stress constraint effect on the schistosity can effectively improve the strength and inhibit the shear or splitting failure along the schistosity, hence, the tunnel axis should be at a large angle with the strike of schistosity, and support measures should be applied on schistosity as soon as possible after excavation.
Tectonic shear belts, with their low mechanical strengths, large spatial extents and obvious shearing damage, present challenges for the excavation of high slopes. In this study, we described the ...basic unloading performances of shear belts induced by excavation through observational data and numerical back analysis. The unloading deformation and failure processes of deep outward-dipping shear belts exposed on a hydraulic slope nearly 300 m tall were investigated first. The field observation and monitoring data indicated that the excavation-induced deformation pattern of the outward-dipping shear belt can be divided into two stages: primarily opening deformation normal to the shear belt during the first stage and primarily shear deformation along the shear belt during the second stage, manifested as opening and shear cracks along the active shear belt. Then, the numerical and analytical analysis were performed to expose the displacement evolution of the shear belt during the multistep excavation of the slope from the top down. The back analysis showed that the grading deformation mechanism of the shear belt was primarily related to the reduction of normal compressive stress on the shear belt's plane due to slope excavation. The reinforcement schemes, including a prestressed anchor cable to limit deep unloading deformation and a prestressed rock bolt to restrain the surface relaxation of the rock mass of the slope, were also discussed. In practice, actual excavation and corresponding monitoring data indicated that the unloading mechanism and supporting measures for the shear belt presented in this study were reasonable and effective. These presented experiences can also enrich the stability analysis and support design for similar high slopes.
•On-site analysis exposed the grading deformation of shear belt in a 300 m tall slope.•Grading deformation mechanism of shear belt was discussed•Prestressed anchor cable had been applied to limit shear belt's deformation.
Numerical back analysis based on incomplete in situ stress measurements is widely used to estimate the three-dimensional (3D) in situ stress fields of large deep underground caverns. However, the ...variable collinearity caused by complex geological environments has rarely been considered. This paper proposes a numerical back analysis method for 3D in situ stress fields based on stepwise regression (BSSR). In BSSR, the collinearity between independent variables is associated with the surface topography, and insignificant variables caused by variable collinearity are eliminated through stepwise regression. BSSR was applied to the underground powerhouse of a hydropower station for validation, and the results showed that it can reliably estimate the 3D in situ stress field despite the complex geological environments. Multidimensional mathematical models of in situ stress fields were established with the improved predictive ability and clear physical meaning, which can quantify the contributions of six geological actions and three stress sources. The findings of this study can help with understanding the formation mechanism of in situ stress fields for large, deep underground caverns in complex geological environments and provide a useful reference for optimizing excavation schemes and support designs.
Understanding three-dimensional (3D) in situ stress field is of key importance for estimating the stability of large deep underground cavern groups near valleys. However, the complete 3D in situ ...stress fields around large deep underground cavern groups are difficult to determine based on in situ stress data from a limited number of measuring points due to the insufficient representativeness and unreliability of such measurements. In this study, an integrated approach for estimating the 3D in situ stress field around a large deep underground cavern group near a valley is developed based on incomplete in situ stress measurements and the stress-induced failures of tunnels excavated prior to the step excavation of the cavern group. This integrated approach is implemented via four interrelated and progressive basic steps, i.e. inference of the regional tectonic stress field direction, analyses of in situ stress characteristics and measurement reliability, regression-based in situ stress field analysis and reliability assessment, and modified in situ stress field analysis and reliability verification. The orientations and magnitudes of the 3D in situ stress field can be analyzed and obtained at a strategic level following these four basic steps. First, the tectonic stress field direction around the cavern group is deduced in accordance with the regional tectonic framework and verified using a regional crustal deformation velocity map. Second, the reliability of the in situ stress measurements is verified based on the locations and depths of stress-induced brittle failures in small tunnels (such as exploratory tunnels and pilot tunnels) within the excavation range of the cavern group. Third, considering the influences of the valley topography and major geological structures, the 3D in situ stress field is regressed using numerical simulation and multiple linear regression techniques based on the in situ stress measurements. Finally, the regressed in situ stress field is further modified and reverified based on the stress-induced brittle failures of small tunnels and the initial excavation of the cavern group. A case study of the Shuangjiangkou underground cavern group demonstrates that the proposed approach is reliable for estimating the 3D in situ stress fields of large deep underground cavern groups near valleys, thus contributing to the optimization of practical excavation and design of mitigating the instability of the surrounding rock masses during step excavations.
•An integrated approach for estimating the in-situ stress field was developed.•The tectonic stress field direction was deduced according to the regional tectonic framework.•The in situ stress field of large deep underground cavern groups was regressed using 3D numerical simulation.•The regression-based in situ stress field was modified with stress-induced brittle failure observation.
The failure of layered surrounding rock tunnel is significantly related to bedding occurrence and in situ stress orientation, and often shows asymmetry. This is because the layered rock has a special ...bedding structure, resulting in its deformation and failure showing significant anisotropy. After excavation, the bedding loading angles of layered rock around the tunnel are different, resulting in different bearing capacity of each part of the tunnel. To reflect this characteristic, a three-dimensional equivalent continuous model considering the deformation and strength anisotropy of layered rock is developed, and the model is embedded in CASRock numerical software. The simulation of tunnel excavation in layered surrounding rock shows that the change of bedding dip angle will lead to the change of failure position of the tunnel, and with the increase of the angle between bedding strike and tunnel axis, the failure degree of surrounding rock will be greatly reduced. This model can be used to investigate the influence of in situ stress and bedding occurrence on the stability of layered surrounding rock tunnels.
•Surface morphology of 70 marble spalling samples in 2400 m depth was investigated.•Statistical results for the roughness of the spalling samples were analysed.•Fractography analysis of the spalling ...samples indicated the brittle failure mode.•Power function fits the relationships of spalling roughness with its material size.
Brittle spalling of hard rock is a typical failure in deep underground engineering that induces challenges for construction safety. Seventy marble spalling samples were collected from the China Jinping Underground Laboratory Phase-II (CJPL-II) to analyse their mesoscopic morphology and failure mechanism. The 3D scanning and corresponding statistical analysis indicated that the spalling surfaces were not smooth but with low roughness and evident anisotropy, and that the morphological transition (i.e. from the mirror zone to the mist zone then to the hackle zone) on the spalling surface was observed. The fractography of the CJPL-II spalling samples showed typical features of brittle fracture dominated by the granular morphology of intergranular fracture based on the Scanning Electron Microscope technology. What’s more, the simulated spalling by the laboratorial true triaxial unloading tests showed similar morphology with that of CJPL-II spalling. Moreover, by comparing the marble spalling samples with other granite spalling and basalt sapling samples, the effect of the mineral particle size of the surrounding rock on the roughness of the spalling slice was discussed. This study deepens the understanding of the in-situ spalling mechanism of hard rock under high geostress conditions.
Excavation-induced unloading fractures in the rock mass can easily cause rock engineering structural failure under high stress. This research work investigated the unloading fracture characteristics ...of the protective layer of the rock bench in Shuangjiangkou underground powerhouse in detail, thereby revealing the unloading fracture mechanisms via numerical simulation and true triaxial unloading tests, and proposed an optimal method for protective layer excavation. The study found that unloading cracking failure in the rock mass is asymmetric in Shuangjiangkou underground powerhouse, with the fracturing degree of rock mass on the downstream protective layer being greater than that on the upstream side. The dip angle of the unloading fracture plane in the downstream protective layer increased with the aspect ratio (k) of the protective layer and gradually coincided with the diagonal of the protective layer. The stress concentration in the downstream protective layer was caused by the tectonic stress, the stress changed in the form of multistep loading σ1 and unloading σ3 during layered excavation. Tensile-shear unloading cracks were formed in the rock under the excavation stress path, becoming steeper and denser, and gradually parallel to σ1 as the unloading amount of σ3 increased. As k increases, the σ1 in the protective layer becomes steeper, causing the unloading fracture plane to steepen and eventually coincided with the diagonal of the protective layer. A formula for designing the protective layer width was proposed based on the relationship of the dip angle of the fracture plane with k. The findings lend themselves to implications to engineering geology in that engineers should consider the asymmetric failure characteristics of the tunnel caused by high tectonic stress, and formulate asymmetric excavation and support schemes during the construction process of rock benches or other similar structures.
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•Unloading cracking characteristics of rockmass were investigated under multi-face unloading.•Tensile-shear unloading cracks were formed roughly parallel to σ1 in the rock under the excavation stress path.•Dip angle of the fracture plane and σ1 increase in aspect ratio of rockmass.•An optimal design of the rock engineering structure with multi-free faces were proposed.
Large shear deformation problems are frequently encountered in geotechnical engineering. To expose the shear failure mechanism of rock tunnels, compression-shear tests for rock models with circular ...tunnel were carried out, including single tunnel and adjacent double tunnels. The failure process is recorded by the external video and miniature cameras around the tunnel, accompanied by real-time acoustic emission monitoring. The experiments indicate that the shearing processes of rock tunnel can be divided into four steps: (i) cracks appeared around tunnels, (ii) shear cracks and spalling ejection developed, (iii) floor warping occurred, and (iv) shear cracks ran through the tunnel model. Besides, the roughness of the sheared fracture surface decreased with the increase in normal stress. Corresponding numerical simulation indicates that there are tensile stress concentrations and compressive stress concentrations around the tunnel during the shearing process, while the compressive stress concentration areas are under high risk of failure and the existence of adjacent tunnels will increase the degree of stress concentration.
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