Study on the Deformation Characteristics of the Surrounding Rock and Concrete Support Parameter Design for Deep Tunnel Groups

During the long-term operation of tunnels, surrounding rock undergoes creep effects under environmental loads, resulting in changes in the aging evolution model of stress and deformation in surrounding rock and lining, which affects the long-term operational safety of the tunnel. Therefore, using the model test device for time-dependent characteristics of stress and deformation of weak surrounding rock and lining structure in operational tunnels, taking into account the influence of tunnel burial depth and lateral pressure coefficient of surrounding rock, a model test on time-dependent characteristics of stress and deformation in weak surrounding rock and lining structure was conducted, and the stress and deformation time-varying curves at key locations of surrounding rock and lining were obtained. The time characteristics of surrounding rock stress, the contact force between surrounding rock and lining, internal force, and displacement of lining structure were analyzed. Research findings indicate that the stress of surrounding rock, the internal force and displacement of lining structure, and the contact force between surrounding rock and lining all increase and tend to be stable over time under constant load. This implies that the stress and deformation of the surrounding rock and lining structure exhibit time-dependent changes. With changes in burial depth and lateral pressure coefficient, significant variations are observed in the various indicators of stress and deformation in the surrounding rock and lining structure, indicating both time-dependent and long-term characteristics in terms of stress and deformation. The research results provide basic data support for the study of the time-dependent characteristics of stress and deformation between weak surrounding rock and lining structures in operational tunnels and can provide theoretical and technical guidance for the long-term service status discrimination and disaster prevention and control of operational tunnels.

Mechanical behavior of multi-scale fiber-reinforced concrete for secondary tunnel lining: Field test and numerical simulation

Salt cavern gas storage (SCGS) is a key development direction for future energy storage. However, the stability of the surrounding rock in underground SCGS remains a challenging issue to be resolved. This study uses numerical simulation methods to analyze the stability of the surrounding rock in SCGS at different height-to-diameter ratios and burial depths during both solution mining and long-term operation. The research results show that: SCGS at the same burial depth, as the height-to-diameter ratio increases from 1.2 to 2.2, the maximum displacement of the surrounding rock decreases by 32.3% and the plastic zone area decreases by 54.1%. However, the density of the plastic zone and the volume shrinkage of SCGS rate increase. The optimal cavern shape lies between a height-to-diameter ratio of 1.2 and 1.5. At the same height-to-diameter ratio, the stability of the salt cavern decreases as burial depth increases: the maximum displacement of the surrounding rock, cavern shrinkage rate, and plastic zone area increase by 94.6%, 99.05%, and 78.61%, respectively. Therefore, within a reasonable burial depth range, a shallower burial depth is more favorable for the stability of the surrounding rock. The presence of interlayers reduces cavern displacement, plastic zone, and cavity volume shrinkage, thereby influencing the stability of the surrounding rock. Among them, the interlayer located at the cavern waist reduced the cavern shrinkage rate by 10% and the maximum displacement by 21.9%, exerting the greatest influence on the stability of the surrounding rock.

A shield tunnel is an assembly structure composed of connecting bolts and segments, generally considered to have good seismic performance. However, there is still a possibility of damage occurring in shield tunnels under strong seismic action. Therefore, a secondary lining can be applied on the inner side of the segment lining to improve the overall seismic performance of the shield tunnel. Using the Shiziyang Shield Tunnel as a case study, this paper employs numerical analysis to examine the seismic response characteristics of the shield tunnel with overlapped double-layer lining. Subsequently, it investigates the influence of segmental lining stiffness degradation and tunnel burial depth on the internal forces of the tunnel under seismic loads. The results indicate that under seismic loading, the stress in the segmental lining exceeds that in the secondary lining, with the maximum stress being three times higher. As the segmental lining stiffness decreases, the bending moment of the segmental lining decreases accordingly, while the secondary lining bending moment remains relatively constant. The bending moment of the segmental lining consistently surpasses that of the secondary lining. Furthermore, the variation in the axial force of the segmental lining is not significant, whereas the axial force in the secondary lining notably decreases. With increasing burial depth, the bending moment of the tunnel structure initially increases and then decreases. As the burial depth of the tunnel increases from 0.5D to 2D and 5.0D, the ratio of the maximum positive bending moment between the segmental lining and secondary lining first decreases and then increases, which are 7.56, 4.78, and 7.70, respectively. Similar patterns are also observed in axial forces. A burial depth of 2D is the critical depth between shallow and deep burial. When the tunnel is shallowly buried, the overlying strata have a significant impact on the seismic internal forces of the tunnel, which continue to increase with increasing burial depth. When the tunnel is deeply buried, it is subjected to the confining action of the strata, making it relatively safe, and the internal forces of the tunnel continue to decrease with increasing burial depth. Overall, under seismic loading, the segmental lining remains the primary load-bearing structure in a tunnel structure with double-layer lining.

Based on the Zaosheng No. 3 tunnel of the Yinchuan‐Xi’an high‐speed railway, the surrounding rock pressure, contact pressure of the primary support, and secondary lining and internal force of the secondary lining concrete are systematically tested using a vibrating wire sensor, and the correlation between the advance construction distance and the surrounding rock release rate is studied with finite element software. The results show that the pressure on the surrounding rock is low when the deeply buried soil tunnel is excavated and can be divided into three stages: rapid growth, slow growth, and flattening with time. It is more reasonable to calculate the surrounding rock pressure by using tunnel planning calculations. For the contact pressure, although the value of each measuring point in the inverted arch changes a little, the arch pressure obviously has the characteristics of rapid growth and a sharp rebound. Most of the test points of the second lining concrete show a compression state, which is far less than the ultimate compressive strength. At the same time, the initial support of the tunnel bears a large load, while the secondary lining bears a relatively small force, and the load sharing ratio of the two ranges between 0.1 and 0.7; with the progress of the excavation section, the surrounding rock deformation (deformation release rate) increases gradually. When the excavation face is close to the monitoring section, the deformation (deformation release rate) is the most severe. With the increase in the distance between the excavation section and the monitoring section, the deformation (deformation release rate) tends to be flat.

Underground gas storage is an important technical measure for future natural gas storage. The stability of the surrounding rock during excavation and under ultra-high gas storage pressure is the key to the stable operation of gas storage reservoirs. A numerical calculation model for different surrounding rock conditions, different depth-span ratios, and different buried depth conditions was conducted to study the stability of surrounding rock after large section underground gas storage excavation and under an ultra-high gas storage pressure of 20 MPa. The results show that after construction is completed, the deformation of the rock surrounding the cavern increases with a decrease in the surrounding rock grade, and the deformation of the rock surrounding the cavern increases as the burial depth increases. In addition, the maximum vertical deformation of the surrounding rock decreases with the increase in the depth-span ratio of the cavern, and the maximum horizontal displacement increases with the increase in the depth-to-span ratio. While operating at 20 MPa gas storage pressure, the displacement of the rock surrounding the chamber tends to increase with the decrease in the surrounding rock grade and the deformation of the surrounding rock of the chamber decreases as the burial depth increases. Furthermore, the vertical displacement of the rock surrounding the chamber decreases with the increase in the depth-span ratio, while the horizontal displacement of the surrounding rock increases with the increase in the depth-span ratio. Considering the stability of the surrounding rock during construction and operation, gas storage chambers should be built in areas with better conditions, such as Grade II and Grade III surrounding rocks within a burial depth range of 200 m. Moreover, the stability of the surrounding rocks is better when the chamber depth-span ratio is 2.5~3.0. These research results can provide a theoretical reference for the design of large underground gas storage structures.

16 - Design, construction and installation of support structures for offshore wind energy systems

In order to accurately obtain the characteristic parameters of the rock mass structural plane of the tunnel and provide effective basis for numerical calculation, based on tunnel of Guiguang railway, this paper measured the structural characteristics of typical tunnel side wall and excavating face by using Sirovision non-contact measurement technology of 3d photogrammetry and accomplished digital geological logging, information statistics of structural plane attitude, rock stability analysis. By using the discrete element software UDEC, the whole process of tunnel excavation, primary support and secondary lining are simulated. The study show that the surrounding rock deformation, internal force of primary support, axial force of anchor bolt will improve with the increase of fragmentation degrees of surrounding rock. The primary support is the determinant of constraint of surrounding rock deformation and bearing the load after tunnel excavation. Secondary support only as a permanent support structure, as a safety margin, the internal force is small compared with the primary support, but taking into account the decreasing of physical and mechanical parameters of the surrounding rock excavation, rheology of soil and reducing of mechanical properties of primary support material. The secondary lining should be calculated and designed as force structure.

Failure mechanism of surrounding rock with high stress and confined concrete support system

The coal resources in the coal-rich area of western China are mostly located in the late diagenetic Cretaceous and Jurassic strata. In this paper, a study on the support of soft rock roadways was carried out in the background of the soft rock track roadway in the Jiebangou coal mine. The field investigation showed that the surrounding rocks of the roadway were weak, soft, and broken, and the surrounding rocks were cemented, with the roadway local deformation exceeding 1 m. The borehole television results showed that the surrounding rocks were mainly weak sandy mudstone and yellow mudstone. The average uniaxial compressive strength of the surrounding rock was 15.49 MPa. The roadway is a shallow buried soft rock roadway; site investigation revealed that the original U-shaped steel shed had an extremely low resistance to slip, the filling body behind the U-shaped steel shed fell off, the interaction between the U-shaped steel shed and the surrounding rock was poor, the U-shaped steel shed could not provide sufficient timely support resistance, and the bearing capacity of the U-shaped steel shed was far from consideration. The floor was not effectively supported. The floor had different degrees of the bottom drum, and frequent undercover caused new stress disturbances, which loosened the bottom corners of both rock types and made the shed legs move continuously inward, reducing the bearing capacity and actual support resistance of the bracket. Numerical calculations were performed to study the deformation characteristics of the surrounding rock of the tunnel and the yielding damage characteristics of the brace. The results showed that the current U-shack support strength was insufficient, the two sides were deformed by 950 mm, the bottom of the roadway bulged by 540 mm, and the surrounding rock was mainly shear damaged. The fall of the filler behind the shed caused damage to the U-shaped steel shed spire. Through site investigation results and numerical calculations, the deformation and damage characteristics of the soft rock roadway and its damage causes were analyzed, and the support technology system of ‘strengthening support for weak structural parts’ was proposed. This improved the mechanical properties of the weak structural support body, the stress state of the local surrounding rock, and the bearing capacity of the support structure, and effectively controlled the deformation, damage, and instability of the surrounding rock of the roadway, and deformation, damage, and destabilization of the roadway, thereby achieving overall stability for the surrounding rock of the roadway.

Tunnels are generally designed for a sustained usage of 80 to 100 years, during which the safety of tunnel structures must be guaranteed. A common supporting form utilized in contemporary tunnel engineering is composite lining. To derive applicable parameters of the supporting form and therefore ensure the long‐term safety of the tunnel structure, it is imperative to determine the extra acting force exerted onto the composite lining by the creep of the rock surrounding the tunnel and to calculate the stress‐strain characteristics of composite lining. In the current study, this paper proposes an approach termed surrounding reinforcement, which is based on the homogenization method. Specifically, this paper defined the bolt force as the internal force of the surrounding rock, analyzed their viscoelastic‐plastic properties using the unified strength theory, and derived an equation for calculating the stress‐strain relationship of the composite lining. To further validate the method in tunnel structures, this paper applied the derived equation to a representative instance. The results of this paper show that the initial support force has also increased during the creep process of the surrounding rock, indicating that engineers should pay close attention to the coordination between the strength of initial support and the secondary lining and thus ensure an optimal distribution of the pressure from the surrounding rock when designing composite lining tunnel within weak strata. This paper proposes that the initial support not only would guarantee the tunnel safety during the construction stage but also could cooperate with the secondary lining to brace the stress caused by the creep, ensuring that the supporting structure stays stable across the whole period of tunnel operation. This paper provides an alternative to previous methods that is more comprehensive, with simpler calculations, and more applicable to the composite lining supporting design within weak strata.

The low frequency strong-dynamic disturbance is one of the sources of deformation and failure of surrounding rock in rockburst roadway, but its deformation and failure mechanism and spectrum characteristics still need further study. In this paper, we selected 4 levels and 5 factors with an orthogonal test approach to study the stability of surrounding rock under dynamic disturbance (stress–time curve), the 5 factors are rock level (uniaxial compressive strength σc), disturbance stress σd, frequency f, burial depth H and propagation direction θ), respectively. The orthogonal test result indicated that the pecking order of λ, an coefficient has a bearing on the plastic area, is as follows: A(σc), B(σd), E(θ), C(f), D(H). Based on the engineering background of rockburst roadway in Qianqiu coal mine in Yima mining group, the spectrum characteristics of rockburst vibration wave and the failure law of roadway surrounding rock were studied by similar simulation experiment method. The test results show that with the increase of roof pressure, the load of the roof-floor and two sides first increase, then decrease and tend to be stable. After the dynamic disturbance, the acceleration waveform of the roadway surrounding rock measurement points lasts for about 0–0.05 s, attenuates quickly, the coda develops well, the frequency range of disturbance wave is wider, and the amplitude is larger, which is mainly manifested by more high-frequency components. The surrounding rock of roadway cracks increase, develop, and large fissure were formed through the cracks, the roof and side rock strata of the roadway fall off, and the deformation and destruction of the roadway was severely damaged.

To explore the control technology on surrounding rock of gob-side entry retaining (GSER) below a goaf in a near distance coal seam (NDCS), research was conducted on the floor ruin range, the floor stress distribution features, the layout of the GSER below near distance goaf, the width of the roadside filling wall (RFW), and the control technology of the GSER surrounding rock below the near distance goaf after upper coal seam (UCS) mining. The results show that (1) the stress of the goaf floor has obvious regional features, being divided into stress high value zone (Zone A), stress extremely low zone (Zone B), stress rebound zone (Zone C), stress transition zone (Zone D), and stress recovery zone (Zone E) according to different stress states. The stress distribution features at different depths below the goaf floor in each zone also have differences. (2) Arranging the roadway in Zone A below a coal pillar, the roadway is at high stress levels, which is not conducive to the stability of the surrounding rock. Arranging the roadway in Zone B below the goaf floor, the bearing capacity of the surrounding rock itself is weak, making it difficult to control the surrounding rock. Arranging the roadway in Zone C, the mechanical properties of the surrounding rock are good, and the difficulty of controlling the surrounding rock is relatively low. Arranging the roadway in Zone D and Zone E, there is a relatively small degree of stress concentration in the roadway rib. (3) When the RFW width is 0.5–1.5 m, stress concentration is more pronounced on the solid coal rib, and the overlying rock pressure is mainly borne by the solid coal rib, with less stress on the RFW. When the RFW width is 2~3 m, the stress on the RFW is enhanced, and the bearing capacity is significantly increased compared to RFW of 0.5–1.5 m width. The RFW contributes to supporting the overlying rock layers. (4) A comprehensive control technology for GSER surrounding rock in lower coal seam (LCS) has been proposed, which includes the grouting modification of coal and rock mass on the GSER roof, establishing a composite anchoring structure formed by utilizing bolts (cables); the strong support roof and control floor by one beam + three columns, reinforcing the RFW utilizing tie rods pre-tightening; and the hydraulic prop protection RFW and bolts (cables) protection roof at roadside. This technology has been successfully applied in field practice.

Mechanical behavior of secondary lining vault void and mold grouting repair considering contact effect

ABSTRACT: A new orthogonal matrix analysis method is proposed firstly in this paper, which aims at providing a new idea for the stability analysis of surrounding rock. Based on the orthogonal test method, a three-level analysis model is established. In detail, the geological conditions and rock masses grade, the variation of different factors, the deformation and fracture field of surrounding rock is regarded as the factor layer, the level layer, and the index layer, respectively. This method not only inherits the advantages of orthogonal test, but also avoids the influence of artificial weighting by establishing the index layer. Then, based on the combined finite-discrete element method, the influence of lateral pressure ratio, vertical stress, and rock mass grade on the surrounding rock stability is carried out by three analysis methods. The results show that the orthogonal matrix analysis method can well reflect the influence of factors, which is consistent with the result of the range analysis method and variance analysis method. In addition, this method can also realize the quantitative sensitivity analysis of factors under different levels, which is essential for the surrounding rock support design. 1. INTRODUCTION As we all know, the stability of roadways is affected by many factors. So, the research on the stability of surrounding rock is still a challenge. It has an important reference value for roadway excavation and support. Currently, many experts and scholars have proposed different methods and models to evaluate the stability of surrounding rocks. These include experimental methods (Chen et al., 2021), numerical simulation (Jia et al., 2022), theoretical analysis (Zhang et al., 2022), etc. The laboratory experiment is the basic method that evaluates the stability of surrounding rock based on the test result of rock samples. The numerical simulation is the most common method, which includes continuum mechanics methods (Tian et al., 2021; Wang et al., 2022), discontinuous mechanics methods (Lisjak and Grasselli, 2014; Zhao et al., 2022), continuum and discontinuous combination method (Deng et al., 2020; Mahabadi et al., 2012). The theoretical analysis method is based on the analysis of the stress field and deformation field, which contains the longitudinal deformation profile (Sugimoto et al., 2019), ground reaction curves (Fang et al., 2013), the pressure arch (Kong et al., 2018) and the stand-up time (Nguyen and Nguyen, 2015).