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Due to the weakness in mechanical properties of chlorite schist and the high in situ stress in Jinping II hydropower station, the rock mass surrounding the diversion tunnels located in chlorite schist was observed with extremely large deformations. This may significantly increase the risk of tunnel instability during excavation. In order to assess...
Context in source publication
Context 1
... maximum deformation of many sections after the primary support reached 0.5e0.7 m, which is 7.6%e10.6% of the radius of tunnel. Hoek and Brown (1980) and Barla (1995) have proposed assessment methods (see Table 4) of compressive deformation of rock mass based on empirical statistical data of large deformation of soft rocks. The method proposed by Hoek and Brown (1980) is adopted in the present study due to its advantage in classification. ...
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Citations
... The results of rock specimen tests show [52][53][54][55] that for mudstone, siltstone, argillite, and sandstone with a uniaxial strength of 25-40 MPa, the failure criterion for strain is about 0.02-0.03. Therefore, in the numerical simulation, the post-peak limit was accepted in the range of "−0.02"-"+0.02". ...
The severe floor heave in gob-side entry retaining is the major restriction factor of the wide application of pillarless mining thin coal seams. Reinforcement and stress-relief floor heave control methods are the most promising. However, in practice, floor restoration is widely used. Therefore, floor heave control technology in gob-side entry retaining needs to be improved. This study proposes anti-shear pile technology to control floor heave in gob-side entry retaining. The research was mainly carried out by numerical simulation. It was found that the transformation of high vertical stresses in the entry floor underneath the filling wall and coal seam body into horizontal stresses starts the floor heave process. The vertical dilatancy of rocks under the roadway span and their subsequent unloading lead to the delamination of the floor strata and uplift of the entry contour. In this paper, the best pile installation scheme was found. It is a 2pile 5+2 scheme with the installation of two piles, each 2 m long. After that, it was shown that filling piles are more than 3.3 times cheaper than comparable analogs, and pile installation is less labor-intensive. The implementation of the proposed floor heave control method leads to a reduction in heaving by 2.47 times.
... The main function of the support is to improve the stress state of the plastic fracture zone of the surrounding rock, control the roadway deformation and plastic fracture zone size, and ensure the functioning of the roadway over a reasonable service period (Yu et al. 2020). Previous studies have shown that a selfstable equilibrium arch can be formed owing to the effect of squeezing stress in the fracture zone of the surrounding rock, and roadway support should fully consider the self-stabilization balance time of the surrounding rock collapse (Cong et al. 2020;Yu et al. 2015;Zhou et al. 2014). ...
Roadway support can effectively improve the stability of roadway excavation and ensure the safety of underground mining. This study investigates the secondary support time and parameter optimization of combined support for a deep roadway in the stage of resource replacement in the Huize lead–zinc mine in Yunnan Province, China. The aim of this study is to increase the stability and safety of the roadway and decrease the cost of support. Research on support methods and failure modes has shown that under the action of high in-situ stress in deep mining, the surrounding rock of the roadway exhibits obvious rheological phenomena. The change in the radial displacement of the roadway is combined with creep tests of the main exposed surrounding rock to determine the secondary support time. Numerical simulations and orthogonal tests are utilized to optimize the support parameters in terms of the roof subsidence, floor heave displacement, side displacement, and plastic zone by analyzing the effects of the sprayed concrete thickness, bolt length, bolt row spacing, and bolt diameter on the support results. The proposed secondary support time and combined parameters can provide a reference for roadway support in similar strata.
... Cappa et al. (2006) reversely calculated the equivalent elastic modulus of the surrounding rock mass by measuring the fluid pressure and the normal displacement of a crack. Zhou et al. (2014) conducted triaxial compression experiments with direct shear experiments on rock samples from the Jinping II hydropower station to obtain shear strength parameters such as triaxial compressive strength and modulus of elasticity. Meng et al. (2016) quantitatively measured the loading elastic modulus and unloading elastic modulus of rock by uniaxial cyclic loading and unloading tests in the laboratory. ...
The mechanical parameters of the surrounding rock of a tunnel are the premise and foundation of the supporting design in soft rock tunnel engineering. To obtain the mechanical parameters of the surrounding rock accurately and quickly, a composite inversion model is proposed, and different models are used to invert different mechanical parameters. In this paper, first, the Latin hypercube and Monte Carlo sampling methods are used to construct the mechanical parameter samples from the surrounding rocks. The numerical simulation of the water diversion project in central Yunnan is carried out by using FLAC3D, and the corresponding displacement data are obtained, three sets of effective inversion datasets are formed along with the mechanical parameter samples. Second, the adjusted coefficient of determination and the symmetrical mean absolute percentage error are used as the performance evaluation indices. The Bayesian 10-fold cross-validation iteration is used to optimize the five regression model hyperparameters for the three sets of inversion parameters. The base model of the best performance is selected for inverting the friction angle (φ) and cohesion (c). For the deformation modulus (E m) inversion parameters, LIME interpretability is used to optimize the weights and integrate the five models. Finally, by developing a calculation tool for the elastic modulus of tunnel support, an orthogonal experimental design is applied based on the verified engineering numerical model. The results show that the average error of the inversion results of the mechanical parameters of the surrounding rock by the proposed composite model is 9.32%, the minimum error is only 0.81%, and the error is within 15%. The inversion trend is similar to the actual displacement height, and the optimized support scheme provides a reference for the engineering site.
... Figure 11 summarizes the test results during the first forward shearing for all the hydrate-bearing samples. The curves are quite similar to the stress-strain curve during triaxial compression tests on rock specimens with fractures [8,47]. The initiation of fracture accompanied by the sudden increase in horizontal displacement is reflected by the missing data circles on the stress curves, as the sheared halves of the sample slide so quickly that the data logger failed to record data. ...
Fractures and large shear deformation, aroused in gas hydrate reservoirs during gas production, may cause borehole collapse and submarine landslides. Currently, the behaviors of hydrate-bearing sediments undergoing long-distance shear displacement remain poorly understood. Here, a series of multi-reversal direct shear tests were performed on artificial samples of methane hydrate-bearing silty sand, aiming to reveal the strength and wear behaviors at the shear plane of hydrate reservoirs. Both peak and residual shear stress were extracted for hydrate-bearing samples. In addition to the primary fracture corresponding to the peak stress, the secondary fracture could occur due to the failure of asperities and depressions on the shear plane. Hydrate enhances strength mainly by increasing the apparent cohesion. The internal friction angle obtained from the peak strength of hydrate-bearing sediment is slightly higher than that derived from residual strength but closely approximates the internal friction angle obtained from hydrate-free strength. Dilatancy and wear are the two main factors controlling the internal friction angles of shear planes. As the shear distance increased, the hydrate-bearing sediment was worn with a stable reduction in volume. Normal stress aggravates wear and particle breakage for both hydrate-bearing and hydrate-free sediments. In contrast, hydrate has two opposed effects on wear: (1) Hydrate tends to increase roughness and thus enhance wear; and (2) hydrate cementation protects particles from wear. Under given normal stress, increasing hydrate saturation enhances strength and depresses wear significantly.
... According to the Equations (B.6) and (B.7), 2 ′ 0 equals to the sum of radial stress and tangential stress at node B. Thus, the evolution of the stress state at node B should be Line-1. Owing to the node being in the elastic state, the tangential stress can be calculated as Equation (2). In addition, the stress state moving along line-1 does not accumulate plastic shear strain. ...
... Unit Value [1] , [2] [MPa], this method will not affect the result accuracy. The correct solution of the HZ can be obtained by continuously circulating this calculation process. ...
... The strength parameter [2] is set to 2.0, 2.5, 3.0, 3.5, and 4.0 to study the impact of residual strength on ground reaction. Figure 11 shows the results of ground reactions. ...
Adopting the novel proposed unified strain‐hardening and strain‐softening (UHS) model and finite difference method, an efficient semi‐analytical solution is proposed to study the influence of the nonlinear behavior of rock on ground reaction. The proposed solution eliminates stress vibrations of the classical method that occurred in the strain‐hardening zone, because it judges the elastic or plastic state of the calculation nodes before calculating the ground reaction; compared with the iterative solution, the proposed solution avoids iteratively solving for the plastic internal variables at each calculated node, resulting in that its computational efficiency is improved by more than 10 times. The proposed solution was verified by comparing it with the iterative solution, and numerical solution. Moreover, the effects of initial yield strength, peak strength, residual strength, and supporting pressure on ground reaction after tunnel excavation are studied. The results show that increasing the initial yield strength and residual strength will obviously decrease the plastic zone (PZ) and sub‐zones. The peak strength of rock has limited influence on the radius of PZ, which mainly adjusts the ratio of each sub‐zone in PZ. In addition, increasing the support pressure is an effective means to control the radius of PZ and sub‐zones in deep buried tunnels.
... The kaolinite and illite in the internal components of the mudstone and sandy mudstone together determine the mechanical properties of the rocks. Therefore, they are soft rocks with poor cementation (Zhou et al., 2014). The mechanical properties of the main rock strata are obtained through rock mechanics experiments ( Figure 2B). ...
Introduction
Soft rock mining roadways are severely deformed and damaged during coal mining. Blindly increasing the support strength not only has little effect but also wastes material resources.
Methods
Maintaining the original support parameters, model experiments were conducted to investigate the mechanism of pressure relief protection of the front soft rock mining roadway by cutting the roof behind the longwall face. The roof-cutting height was 2.5 times the coal thickness, the angle was 10°, and the advance distance is 0.
Results
The study found that the abutment stress borne by the roof of the original roadway was transferred to the coal seams to be mined. The average stress of the coal seams increased by 10%, while the average stress of the surrounding rock in the front roadway decreased by 12.57%. The roof cutting weakened the influence of the overlying strata in the gob on the rear roadway. The stability of the rear roadway also weakened the traction effect on the front roadway. The vertical convergence of the front roadway decreased by 27.3%, and the deformation of the coal pillars decreased by 15.7%.
Discussion
The roof cutting reduced the stress of the front roadway to the peak failure stress, fundamentally weakening the main factor that induced the deformation of the front roadway. Numerical simulations were performed to research the deformation and stress distribution properties of the surrounding rock after roof cutting, and the model experimental results were validated. Finally, engineering recommendations are presented, which are expected to provide a reference for controlling the roadway stability of soft rock masses.
... ( Figure 3). Tis corresponds with the results of testing soft rock specimens in a volumetric feld [36][37][38] and under uniaxial compression [39,40]. According to the results of tests, it was found that for mudstone, siltstone, argillite, and sandstone with uniaxial strength of 25-40 MPa, the failure criteria for strain is about 0.02-0.03. ...
The floor heave is one of the key factors that can restrict high-efficiency and safety mining, especially in the deep roadways with soft rock. Considering the influences of rock fracturing over time on rock mass properties, a case study of the floor heave evolution and rock bolts reinforcement technology was performed in this paper. A numerical simulation was used to study the stress-strain state and displacement of surrounding rocks. It was found that significant floor heave caused by nonlinear deformation of laminated immediate floor under an increase in rock fracturing. The post-peak strain regions appear in the bottom corners of the roadway, after which strata in the immediate floor are destroyed one by one. The joint spacing of 0.45 m on the immediate floor is critical. At this step, post-peak strain regions merge in the central part of the roadway floor, which is the cause of uncontrolled floor heave. Rock bolts reinforcement was proposed to control the floor heave. Three floor support schemes with two types of support elements, different bolt orientations, and lengths of reinforcement were studied. The numerical simulation demonstrated that after reinforcement, post-peak plastic strain in the floor strata was reduced effectively. The optimal floor support scheme and depth of reinforcement were determined by the allowable floor heave. Ideally, the floor heaves could be reduced by rock bolts with a steel belt installed according to the support scheme III and reinforcement length of 2.0 m for outer bolts and 3.0 m for central bolts.
... According to the results of testing soft rocks specimen in a volumetric field [34][35][36] and under uniaxial compression [37][38][39][40][41], it was found that for mudstone, siltstone, argillite, shale, and sandstone with a strength of 25-40 MPa, the failure criteria for strain is about 0.03. The surfaces of elastic and plastic strains are shown in Figure 6. ...
A floor heave is usually a serious failure phenomenon in mine roadways, especially in the conditions of a wet soft rock. There are bright prospects for using a method of reinforcing for controlling a floor heave in mine roadways under a dramatic heave. In the current research, a floor heave mechanism in the wet soft rock of mine roadways was investigated while the moisture content of the rocks was increasing. That was done by performing numerical simulations in a finite element analysis software system Ansys and by reinforcing the floor in the shape of inverted arch. It was found that a decrease in the modulus of elasticity of rocks caused by saturation leads to a nonlinear increase in a floor heave. This can be explained by the fact that rocks of the floor become plastic strains. A stable correlation was established between water content and a floor heave of a wet rock. It was found that when the floor is reinforced, the proportion of plastic strains in the soil is significantly reduced. The growth of the modulus of elasticity of rocks in the reinforced zone, caused by reinforcing of the floor in the form of inverted arch, leads to a nonlinear decrease of a floor heave. The effective range of floor reinforcing was established. A reliable relation was established between the ratio of the modulus of elasticity of rocks of the reinforced zone to the modulus of elasticity of the surrounding rock and heaving of reinforced rocks. The obtained results can be used to predict the magnitude of heaving of rocks at their saturation, as well as after they are reinforced.
... Technical difficulties, delays, economic repercussions and health and safety risks have occurred at many engineering projects developed on complex formations (Goodman and Ahlgren, 2000;Lunardi et al., 2014;Medley, 2007Medley, , 2001. These difficulties have encouraged both private and public institutions to develop and fund several research projects all over the world during the 40 years (e.g., the Italian Research Council (C.N.R.) (D'Elia et al., 1998), the California Department of Water Resource's Division of Safety of Dams -DSOD, see (Lindquist, 1994a;Medley, 1994), the National Natural Science Foundation of China, see (Huang et al., 2021;Wang, 2014;Yang et al., 2019;Zhou et al., 2014), and the Alexander von Humboldt Foundation, see (Kahraman and Alber, 2008), to better understand the geotechnical behavior of heterogeneous formations with a block-in-matrix fabric. ...
The terms “bimrocks”, “bimsoils” and “soil-rock mixtures” indicate different and very common types of geological units with a block-in-matrix fabric that are also “geotechnically complex formations” and are characterized by an internal heterogeneity, and spatial variability of mechanical parameters and lithological compositions. Due to this internal complexity, the understanding of their geomechanical behavior presents a key challenge in geotechnical engineering. However, the lack of a standardized and clear terminology complicates the discrimination of different types of complex formations and their internal mechanical properties, which leads to inconsistency in the literature and research studies. This inconsistency causes misunderstandings, with possible practical implications for the characterization, analysis, design and construction of engineering works. By a combination of geological and geotechnical observations, we propose a new classification for geotechnically complex formations, with particular attention to those with a block-in-matrix internal fabric. Four properties are at the base of this new classification and have a primary role in controlling the geotechnical behavior of block-in-matrix units (bimunits): (i) the composition (i.e., lithology, degree of lithification/consolidation, nature, and rheology) of blocks and the matrix that affects the water sensitivity, (ii) the degree of internal anisotropy (DA) of the block-in-matrix fabric, (iii) the degree of stratal disruption and mixing, and (iv) the volumetric block proportion (VPB). As a result, we classified bimunits in those with “anisotropic”, “isotropic”, and “mixed” (i.e., different behavior depending on the DA of the matrix) textures and, each of these types, into block-in-matrix rocks and block-in-matrix soils (bimrocks and bimsoils in the following). According to the water sensitivity of the matrix, bimrocks are also differentiated into “hard” and “soft”. The novelty of the classification is that it is not limited to few types of geotechnically complex formations (e.g., flysch) but it can be easily applied to all field-based investigations of the different types of complex formations, regardless of their internal degree of stratal disruption, composition, and mechanical response to water sensitivity.
... The long-term behavior and stability of rock engineering structures such as tunnels have received considerable attention (Jia et al. 2020;Do et al. 2020;Sharifzadeh and Tarifard 2014;Pellet et al. 2009). A significant number of engineering applications and laboratory studies indicated the mechanical response related to weak and soft rock's time-dependent behavior is relatively complicated Yan et al. 2020;Török et al. 2019;Singh et al. 2018;Patil et al. 2018;Vlastelica et al. 2018;Lyu et al. 2017;Günther et al. 2015;Miščević and Vlastelica 2014;Zhou et al. 2014). One of the significant parameters affecting the underground spaces' longterm stability is weak rock mass creep response (Kovačević et al. 2021;Panthi and Shrestha 2018). ...
... It is in good agreement with previous works, where time-dependent changes in viscoelastic rock masses and the interaction of liners were studied (Do et al. 2020). The introduction of underground water strongly modifies the system's stability, especially when weak rocks are present (Zhou et al. 2014). In our study, the tunnel crown is stable in the case of rock mass creep behavior, but by introducing underground water and in the case of water table equals 40 m, the pressures created by axial force and bending moment are too considerable for the tunnel crown to sustain (Table 5 and Fig. 8). ...
The time-dependent stability of tunnels is an important and challenging topic, mainly when the tunnel is excavated in incompetent and weak rocks. The creep property of rock is one of the crucial mechanical properties of weak rock and the main factor affecting the long-term stability of rock masses. Also, water as an important environmental factor influences both the short-term and long-term behavior of rocks and is one of the causes of geotechnical engineering disasters, such as tunnel collapse, slope sliding, surface subsidence, etc. In this research, the effects of rock’s creep behavior and underground water on the long-term stability of the Shibli tunnels were analyzed. Geological maps and reports of Shibli tunnels show a highly jointed condition in the surrounding rocks which have been crushed by two orogenic stages. The Burger-creep visco-plastic model was used to simulate the tunnel host rock creep behavior. The model's parameters were adopted based on the displacement-based direct back analysis technique using a univariate optimization algorithm. In addition, the influence of underground water is assessed under the condition of the varying water table. Support capability diagrams were used to evaluate the loading created on the tunnel’s permanent lining due to the creep behavior of rock mass and underground water. This study suggests that the weak rock's creep behavior and underground water significantly affect the time-dependent stability of tunnels. Results show that the induced stresses due to the rock's creep behavior and underground water are more considerable in the tunnel spring-line. Also, the increasing 20 m in the water table approximately decreases ten years of tunnel lining stability time at the fault zone.
Article highlights
Rocks creep behavior and underground water significantly affect the time-dependent stability of tunnels in weak rocks.
Displacement-based direct back analysis using a univariate optimization algorithm was used to determine the CVISC model’s properties.
Increasing 20 m in the water table approximately decreases ten years of tunnel lining stability time at the fault zone.
