In dense urban areas, a new shield tunnel frequently crosses over in-service shield tunnels due to limited underground space. Construction of a new tunnel leads to relief of ground stress and soil ...displacement, which will inevitably result in a series of adverse impacts on the existing shield tunnels, such as tunnel heave, the dislocation between segmental rings, distortion of tunnel track. In this paper, a simplified analytical method was proposed to predict the responses of the in-service shield tunnel associated with overcrossing tunnelling in soft ground. The existing shield tunnel is treated as a Timoshenko beam. The tunnel-ground interaction is considered using the Pasternak foundation. The two-stage analysis method is used to divide the problem into two connected steps. First, overcrossing tunnelling-caused unloading loads imposing on the top surface of the existing tunnel are computed based on the Mindlin’s solution. Second, the tunnel deformation owing to the unloading loads is solved numerically by means of the finite difference method. The applicability of the proposed method is validated by two case histories in literature. The tunnel heaves predicted by the proposed method are in good agreement with the field measurements. According to the parametric analyses, it is found that when a new tunnel crosses over existing shield tunnel obliquely or parallelly, the induced tunnel heave is greater than that induced by perpendicularly crossing tunnelling. At given clearance between the new tunnel and the underlying existing shield tunnel, large-diameter tunnel excavation above causes greater tunnel heave and dislocation between adjacent rings than small-diameter tunnel. Improve the ground elastic modulus will effectively reduce tunnel heave when the ground elastic modulus is relatively low. To increase the equivalent shear stiffness will remarkably reduce dislocation between adjacent rings, however, its effects on reducing the tunnel heave is negligible.
Investigating the stress drop of abutment pressure is the key to a deep quantitative analysis of the discontinuous stress redistribution under mining. In the present study, uniaxial and triaxial ...compression tests are carried out separately to determine the bulk and shear moduli, the cohesion, and the internal friction angle of the coal samples. By extending the meaning of Mohr’s circle referring to yield stress instead of the maximum principal stress, a yield line is introduced to illustrate the stress drop of Mohr’s circle referring to yield stress instead of the maximum principal stress at the elastoplastic boundary. Furthermore, a theoretical solution of the stress drop as a function of the damage is proposed to investigate how the abutment pressure differs considering the yield line and failure line. In addition, applying the stress drop at the yield line in non-pillar mining, top coal mining, and protective coal mining shows that the damage has a nonlinearly positive influence on the stress drop. The results shows that the bulk modulus and internal friction angle have a more sensitive influence on the stress drop than do the shear modulus and cohesion. Finally, the stress drop is divided into a discontinuous stress drop at the yield line and a structural stress drop at the failure line. The stress drop is effective in describing the discontinuous stress redistribution and shows a clear difference in the movement direction of Mohr’s circle considering the unloading pressure.
Investigating the mechanical behavior of the excavation damage zone (EDZ) in jointed rocks is essential for underground openings. In this numerical study, the effects of joint surface roughness and ...orientational anisotropy on the size and shape of EDZ in jointed rocks were examined, based on a diversion tunnel excavated in columnar jointed rocks at the Baihetan Hydropower Station. The results showed that failure zones are usually at the two ends of the tunnel roof. Both open and shear zones are dependent on the JRC (Joint Roughness Coefficient) averages and standard deviations. With increasing standard deviations of JRC, the depths and areas of both open and shear zones generally increase. As the jointed rocks changing from anisotropic to isotropic, the shapes of EDZ are similar but rotate in jointed rocks. Under effects of water pressures, larger open zones appear in the lateral tunnel walls, and the depths of shear zones increase.
•Soil parameters of saturated Q3 loess are totally different from saturated soft soil.•Mohr-Coulomb model can better simulate saturated loess than Soft Soil model.•Grouting and eccentric effect of ...shield are more sensitive to surface settlement.•Lower limit of grout filling ratio and upper limit of eccentric ratio is 60% and 0.5%.•Bottom leakage of lining has the greater influenced on surface settlement trough.
During shield construction of Xi'an metro, saturated loess stratum with loose soil structure and poor engineering properties is often encountered, which is prone to cause excessive surface settlement (SS) and risk to the safety of upper infrastructures. Six factors that affect SS during shield construction and post-construction stages in Xi’an metro were focused, including shield tail grouting pressure ratio δ, eccentric over-excavation ratio α, groundwater decline, shield earth chamber pressure λ, shield tail grout filling ratio ξ, and leakage position of lining. According to Peck's formula, two characteristic parameters of SS were defined, i.e., maximum surface settlement (MSS) and settlement trough width (STW). Based on the Biot's theory, the computation of fluid–solid coupling during shield construction in saturated loess stratum was realized. Local sensitivity analysis indicates that SS of saturated loess stratum is more sensitive to shield tail grout filling ratio, eccentric over-excavation ratio, groundwater decline and grouting pressure ratio. In order to reduce the SS and its influence range, the following standards should be satisfied, i.e., ξ > 60%, α ≤ 0.5%, 1.0 ≤ δ ≤ 1.1 and 0.9 ≤ λ < 1.6. Moreover, in the post-construction stage, the closer the leakage position to the bottom of the tunnel, the greater the induced MSS and the wider the STW, and waterproofing measures for the lining near the bottom plate should be taken to ensure that the relative permeability coefficient of the lining β ≤ 0.001. The groundwater level should also be maintained in the overlying stratum.
A weak interlayer zone (WIZ) is a poor rock mass system with loose structure, weak mechanical properties, variable thickness, random distribution, strong extension, and high risk due to the shear ...motion of rock masses under the action of tectonism, bringing many stability problems and geological hazards, especially representing a potential threat to the overall stability of rock masses with WIZs in large underground cavern excavations. Focusing on the deformation and failure problems encountered in the process of excavation unloading, this research proposes comprehensive in situ observation schemes for rock masses with WIZs in large underground cavern on the basis of the collection of geological, construction, monitoring, and testing data. The schemes have been fully applied in two valuable project cases of an underground cavern group under construction in the southwest of China, including the plastic squeezing-out tensile failure and the structural stress-induced collapse of rock masses with WIZs. In this way, the development of rock mass failure, affected by the step-by-step excavations along the cavern’s axis and the subsequent excavation downward, could be observed thoroughly. Furthermore, this paper reveals the preliminary analyses of failure mechanism of rock masses with WIZs from several aspects, including rock mass structure, strength, high stress, ground water effects, and microfracture mechanisms. Finally, the failure particularities of rock masses with WIZs and rethink on prevention and control of failures are discussed. The research results could provide important guiding reference value for stability analysis, as well as for rethinking the excavation and support optimization of rock masses with WIZs in similar large underground cavern under high geostress.
It is essential to better understand the large deformation behavior of deep tunnels in weak rock mass to improve the long‐term stability of tunnel surrounding rock mass and design effective supports ...for squeezing tunnels. This paper introduces two representative large deformation engineering examples to study squeezing behavior. First, new algorithms of contact search, determination of actual contact, and calculation law of contact pairs are proposed for general‐purpose computing on graphics processing units (GPGPUs), by improving the original serial algorithms and developing a compute unified device architecture (CUDA) parallel program, to satisfy the large‐scale rock fracture process simulation. Then, tunnels excavation and rock reinforcement effect including the combination of U‐shaped steel and steel fabric, spray concrete (SMC), and grouting were modeled. The verification, done by comparing the field observation data and the FDEM simulation results of progressive fracture and the tunnel wall convergence displacement, shows that stress remarkable rebalancing after excavation because of high in situ stress, low rock mass strength, and rock mass long‐term strength reduction properties, thus, the rock mass fracture progressively and the rock blocks move continuously, leading to the rock mass volume increase significantly. The result demonstrates one of the root causes of squeezing. To effectively reduce the tunnel wall displacement and maintain tunnel stability, support with higher stiffness or multifactorial combined support methods and grouting with a better bonding effect should be used together. The findings provide insights into the progress of tunnel squeezing and the basis for support design in the squeezing tunnel.
Tunnelling in difficult and challenging conditions such as soft soils in urban areas is increasing. In this condition, it is important to minimise the possible negative effect of the tunnel ...excavation, such as settlement or, in the worst case, collapses. To achieve this result, earth pressure balance machines are commonly used. One of the key parameters that must be considered for an optimal management of the EPB-TBM excavation is soil conditioning since the excavated muck must properly transmit the pressure to the tunnel face. Soil conditioning is also necessary to reduce the effect of the problems, such as clogging in clay layers, that can occur during the excavation and that can affect the performance of the tools and of the entire tunnelling process. For this reason, in the last decade, much research has been carried out to understand how to deal with and reduce the effects of clogging and stickiness, using different conditioning additives. These studies have proposed several different test procedures to evaluate the effect of the conditioning on the adhesion of the soil on the metallic parts of the machines. The present research has been carried out with the aim of proposing a new approach and new devices to study clay conditioning with laboratory tests, and the results of many tests carried out with the proposed device are presented and discussed.
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