Study On Deformation Characteristics And Control Measures Of Super-large-section Slate Tunnel

As with the rapid development of China’s transportation construction, super-large cross section tunnels and underground engineering have mushroomed in recent years, especially the passenger dedicated line. In order to overcome the aerodynamic problems caused by the high speed train on passenger dedicated line, super-large-section tunnels are adopted gradually. The clearance area of above ones is larger than100m2, and the excavation area is larger than150m2. Compared with previous tunnels, more strict and new requirements are put forward, mainly contained the mechanical behavior, the cross-section style, lining structure, construction methods, initial support structure mode and support parameters of super-large tunnels etc. The surrounding rock deformation characteristics are influenced by many factors, including geological conditions, design parameters, construction technology and management etc. which are differed with each other. Despite many researchers both at home and abroad, have devoted themselves to the research on the large-cross-section tunnel, there are still some shortcomings both in the theoretical and practical aspects, mainly as follows:(a) The definition of large deformation is not clear, also the cognition is different among the survey and design units and construction teams.(b) The diversity of research leads to the limitation of the research results.(c) It is badly lack of the comprehensive researches by application of systemic test methods. In practical cases, the phenomena that, the area which might occur large deformation has adopted the intensive supporting, whereas it does dot appear large deformation actually, may occur frequently. On the contrary, it may bring construction difficulty in some areas with no predicted large deformation.A large number of scholarly papers demonstrate that, the surrounding rock large deformation problem has become one of the most commonly experienced problems of underground engineering, especially during the constructed period and the control techniques in large-cross-section tunnels. And it is relatively short of relevant references.Relayed on the Shanghai-Kunming Passenger Dedicated Line Chang-Kun Hunan Section Ⅸ tender Manziwan Tunnel, the dissertation is originated in the Natural Science Foundation of Key Projects titled " the rheological deformation control mechanism research of surrounding rock and supporting system in soft slate super-large-section tunnel"(numbered2013CFA110), in Hubei Province. In combination with monographic research titled "the tunnel advance geological prediction and monitoring in Shanghai-Kunming Passenger Dedicated Line Chang-Kun Hunan Section", it carried out in-depth research on deformation characteristics and control measures of super-large-section slate tunnel. The dissertation deeply analyzed the engineering characteristics of slate. Under different construction methods and supporting styles, the mechanical properties and deformation characteristics and failure mechanism have been deeply researched by the application of theoretical analysis, field test (monitoring) and numerical simulation. Also, it emphasized on its influence factors and deformation mechanism. Additionally, the establishment of PSO-SVM model predicted the deformation effectively and was of great significance to the prevention and control. At the same time, the convergence-confinement method was employed to analyze initial supporting structure. To this end, in combination with corresponding theoretical analysis, the surrounding rock characteristic curve and supporting characteristics curves were drawn to evaluate its adaptation and applicability, and optimization supporting structure were put forward. Finally, on the basis of surrounding rock control principles and technical system, it proposed corresponding control measures, which can provide references for the design and construction of similar projects.Specific findings and research are highlighted below:1. Research on slate engineering properties by experiment(1) Through the field geological survey, data collection, and engineering geology report, the general geological situation of the region was elaborated. And the inventory of regional conditions, mainly including topography, strata lithology, geological structure, hydrogeology and geostress, was listed.(2) Through uniaxial and triaxial compression test, mechanical properties of slate under dry state of different structural plane angle were analyzed. It calculated that:(a) The value of slate poisson’s ratio ranged of0.15～0.25, with little relation of angle between structural plane and the loading direction.(b) Under the same confining pressure and different structural plane angle, the slate uniaxial compressive strength and elastic modulus varied from large to small, reducing to a certain value and then gradually increased. (c) Through fitting, it concluded the relationship between uniaxial compressive strength, σ, elastic modulus E, and the intersection angle of structural plane and loading direction.(3) Using the triaxial compression tests, mechanical properties of slate under dry state of different confining pressure were analyzed. It resulted that:(a) Within a certain range, the strain increased by linear along with the stress increasing. While the stress reached to its peak value, the specimen destructed suddenly. Then over the peak value, stress decreased sharply, while the lateral strain increased rapidly.(b) The test demonstrated that, the slate strength was mostly and largely influenced by confining pressure. Further, the compressive strength presented linear increasing tendency with the increasing of confining pressure.(4) Under dry station, the failure characteristics of slate were as follows:(a) Under uniaxial compression condition, shear failure and plane sliding failure mainly occurred.(b) When the dip angle of structural plane was vertical to axial force direction, the specimen mainly occurred shear failure under different confining pressure. Along with the increasing of confining pressure, the intersection angle between fracture surface and the maximum principal stress increased gradually, and the failure surface was more and more smooth.(5) With the confining pressure of lOMPa, mechanical properties of slate were analyzed. It showed that:(a) With the increase of immersion time, the compressive strength and elastic modulus were decreased, while the poisson’s ratio was increased.(b) After soaking, as for the stress-strain curve, before the peak point, it exhibited plastic yield section, while after the peak point, the strain softening occurred. Furthermore, the plastic yield before peak value and the strain softening were more obvious with the increase of bubble water time.(6) Based on fuzzy comprehensive evaluation method, Hoek Brown-strength criterion of disturbance factor was modified, and further to calculate mechanics parameters of slate. Through the comparative analysis of empirical method, Zhang Jianhai modified formula and sound wave test results, the disturbance coefficient value calculated by the proposed method based on fuzzy comprehensive evaluation method, was closed to the value by acoustic test. It indicated that the proposed method was rational. Moreover, it solved the high cost and difficulty in implementation by the acoustic test method, which can be reference for similar engineering applications.(7) Under the drying condition, the hierarchical load method was applied. Axial and lateral transient strain and creep strain occurred per level load. Among which, the instantaneous strain accounted for approximately80%of the whole deformation, and the axial strain was larger than lateral strain deformation, reaching to about2～3times.(8) The axial stress was crucial to creep deformation characteristics. When the axial stress was less than the yield value, the lateral strain and axial strain was linear increased. While when the axial stress was larger than the yield value, the lateral and axial strain changed by exponential relationship. (9) Under different moisture state, with the confining pressure of15MPa, the creep rule was similar to dry states. With the increase of moisture content, the axial and lateral deformation increased in turn.(10) On the basis of summarizing the typical creep model, modified Burgers model was selected to calculate creep parameters, laying foundation for the subsequent numerical analysis.2. Analysis on slate deformation characteristics(1)The tunnel monitoring measurement technology and the method of data processing of Manziwan Tunnel were introduced. Combined with monitoring measurement data, surface settlement and surrounding rock deformation law under different excavation methods were analyzed.(2) The surface transverse settlement obeyed normal distribution, character with large central value. The largest settlement appeared along the tunnel axis, and settlement on two sides decreased with the increasing distance, moreover, surface cumulative settlement decreased along with the increase of tunnel buried depth. When heading face reaching to the monitoring stations, the monitoring position was uplift or subsidence possibly. While the heading face crossing the monitoring position, due to the action of supporting ring closure, the surface settlement located in the distance of1to2times diameters, began to convergence.(3) Through the surrounding rock deformation-time characteristic curve, the deformation laws under different excavation methods along with the time were analyzed.(4) Via comparative analysis of fitting formula, exponential function was the best one for deformation characteristics. Further, the function, y=a(1-e-bt), was applied to crown settlement and upper bench convergence analysis, while for they=ae-b/t, it was more precious in lower bench convergence analysis.(5) As for radial release rate of deformation under three kinds of method, during an identical period, the crown was the smallest, and the largest one was the lower measured line. Furthermore, the release rate of each section reached to100percent after30days of its excavation.(6) Relayed on monitoring measurement data in Manziwan Tunnel and different class of surrounding rock, the relationship between the deformation and the distance from heading face were analyzed. It resulted that:(a) When deformation was stability, the vault, upper measured line and lower measured line were basically stable at the same time. And it recommended, as for the secondary lining opportunity, the distance to heading face was3D,2.6D and2AD (where D was the excavation diameter), for V, IVb and IVa level surrounding rock respectively.(b) The relationships between deformation and the distance to heading face were divided into three types,"steps" type,"parabola" shape and "factory" type.(7) The influence factors of surrounding rock deformation were analyzed. It indicated that it influenced by regional geological tectonic situation, the initial ground stress field, the nature property, ground water and tunnel buried depth, as well as the tunnel section shape and size and construction and so on.(8) The surrounding rock deformation mechanism and constitutive relation were studied. It demonstrated the bending deformation of plate and beam and opening and closed of structural plane were performed as deformation mechanism. As for constitutive relation, the layered constitutive equation was selected.3. Research on numerical simulation during construction period(1) It analyzed Mohr-Coulomb constitutive model of geotechnical material in FLAC, and the null element. Also it discussed the implementation of Burgers model in FLAC3D.(2) Through the establishment of three-dimensional numerical model, and with the emphasis on the displacement field, stress field and plastic zone of surrounding rock, spatial deformation law of surrounding rock are analyzed by different construction methods. Further, through the longitudinal displacement field analysis, it obtained the advanced deformation characteristics.(3) It deeply analyzed the creep effect and tunnel section size effect, through comparing analysis, it resulted that:(a) As for V level surrounding rock, creep effect played an obvious role on its deformation after excavation, and it should not be neglected. Whereas, in terms of IV level one, it can be neglected in practical case due to its tinny effects.(b) Based on numerical simulation on three typical commonly used high speed railway section size, it proved that, as for the surrounding rock deformation, size effect was obvious. Generically, deformation law would not change, while only the difference in its value. And the plastic area developed with the increasing of section size, but small section size can reduce the excavation disturbance area.4. Research on deformation prediction(l)The system principle of support vector machine (SVM) theory was analyzed, as well as the kernel function of support vector machine (SVM) and support vector machine (SVM) model parameters were elaborated presented.(2)In view of the value of penalty factor C and nuclear parameter σ, the particle swarm optimization algorithm was introduced to the SVM model, and the support vector machine (SVM) model of particle swarm optimization (PSO-SVM) was put forward. An inventory of the modeling steps and methods were listed accordingly.(3) With an emphasis analysis on the time-series forecasting results affected by penalty factor C and nuclear parameter σ, it verified the particle swarm optimization algorithm for model parameter was practical, rapid and accurate, and avoided subjectivity and blindness by human.(4) By the application of PSO-SVM model, it conducted deformation prediction of surface subsidence and crown settlement in Manziwan Tunnel. Through comparative analysis with conventional grey GM (1,1) model and curve fitting prediction results, it resulted; the PSO-SVM model was with highest prediction accuracy, and with superior prediction ability than other two methods.5. Research on surrounding rock deformation control technology(1) Through a systematical analysis on the convergence-confinement principle, it obtained the supporting opportunity and stiffness was of great importance on the surrounding rock stability of and supporting structure safety.(2) By the application of numerical simulation, the surrounding rock characteristic curves, including the V, IVa and IVb level, were drawn. As according to the in-suit condition, the stress releasing process during excavation period was carried out.(3) On the basis of convergence-confinement method, the most commonly used bolting and combined bolting support characteristic curves were obtained theoretically, as well as the primary support adaptability evaluation method was put forward.(4) In combination with primary support parameters in Manziwan Tunnel, its primary support characteristic curve was drawn. The primary support adaptability evaluation was conducted through the relationship between surrounding rock characteristic curve of and supporting characteristics curves.(5) The evaluation on primary support safety was operated through theoretical calculation and corresponding safety coefficient figures were drawn. It indicated that the steel and spray net support exhibited better safety performance and meet the security requirements in construction and operation in large-cross-section slate tunnel. Furthermore, the minimum safety coefficient of V, IVb and IVa level werel.24,1.37and1.39respectively, which appeared in the connected section of sidewall and inverted arch joint. Thus, much attention should be paid on the design and construction process.(6) Based on orthogonal design for the optimization of supporting opportunity and stiffness, a reasonable optimal indictor was selected and the combination was calculated. As for the V level surrounding rock, it showed that the optimize combination was:with75percent of stress releasing for supporting opportunity+35cm of spray thickness for primary support opportunity, and with96percent of stress releasing for supporting opportunity+50cm of spray thickness for secondary support, and the plane strain numerical simulation was performed as a basis for the validate results and its rationality.(7) The deformation control principle of and technology system were elaborated analysis, and in the view of construction methods and supporting measures, surrounding rock deformation control technology for large-cross-section slate tunnel were proposed.The innovations of the dissertation were as following:(1) The fuzzy comprehensive evaluation method was adopted to modify the Hoek Brown’s strength criterion of disturbance factor D value. It laid a theoretical foundation for ascertaining surrounding rock mechanics parameters, and possessed practical reference and application.(2) Through the three-dimensional numerical calculation results, mainly from three aspects of displacement field, stress field and plastic zone, spatial deformation law was analyzed under different construction methods. In special, with an emphasis on longitudinal displacement field analysis, the advance deformations under different construction methods were obtained. Also, deformation characteristics affected by creep property and size effect were analyzed by numerical simulation.(3) The support vector machine (SVM) was applied to predict surrounding rock deformation and the particle swarm optimization algorithm (PSO) was adopted to optimize the support vector machine (SVM) parameters. By comparative analysis with traditional prediction methods, PSO-SVM prediction model exhibited a rational and feasible performance.(4) The application of convergence-confinement method was conducted to evaluate the adopted supporting system. Additionally, optimization combination was put forward by orthogonal design for optimization of supporting opportunity and stiffness.