| Summary: | 博士 === 國立臺灣大學 === 土木工程學研究所 === 91 === Squeezing phenomenon, the time-dependent large deformation during tunneling, has become one of the major obstacles for tunnel construction with high overburden or under adverse geological condition. Lager tunnel deformation in squeezing may cause the insufficient tunnel clearance, the support failure, or the raveling or collapse of the tunnel, result in equipments damaged, constructing time procrastinated and project cost increased. In the near feature in Taiwan there are numerous of tunnels will be proposed, designed and constructed to complete the highway net and meet the requirement of infrastructure. Among which, the geologic condition of some tunnels may be worse than before, and the overburden will be higher. The squeezing problems during tunneling are inevitable. Up to now the squeezing behavior of rock tunnel and its affecting factors are still not understood comprehensively, the complete stress-strain curves for a rock mass under various stress situation are lack of a fitting constitutive law, the squeezing model falls short and the tunneling approaches lack systematic analysis and evaluation on their effects against squeezing ground. Consequently, focusing on jointed rock masses, the research explores into tunnel squeezing behaviors thoroughly by ways of: (1) study on the squeezing tunnels to probe into the associated mechanism and affecting factors, (2) building up of a constitutive law for jointed rock masses, (3) establishment of a numerical model for tunnel squeezing analysis, i.e. the squeezing model and relevant parameter studies, (4) application of the squeezing model to tunnel project and evaluation on the performance of various excavation and support methods.
At the first, study on the squeezing tunnels to probe into the associated mechanism and affect factors, this research founds a data base for squeezing tunnel by putting their basic information in order, including rock type, rock mass classes, overburden or in-situ stresses, cross section, excavation and support methods, construction records, monitoring records, hazard or anomalous situation and countermeasure etc. This data base is contributive to investigate the failure patterns of surrounding rock of squeezing tunnel and the characteristics of deformation, also conductive to probe into the associated mechanism for tunnel squeezing and provided an excellent reference resource for squeezing ground tunneling. It finds that squeezing is stemmed from four failure patterns of surrounding rock, those are complete shear failure, local shearing failure, buckling failure, and, splitting, shearing and sliding. Squeezing phenomenon may happen as if the excavation and supports of tunnel can not suppress the deformation in time. The complete shearing failure and the local shearing failure result from the nonlinear constitutive relationship between stress and strain of rock masses. The buckling failure results from the nonlinear unstable compatibility of rock masses. The splitting, shearing and sliding failure results from the nonlinear unstable boundary condition of bedding and joint surface of rock masses. The magnitude of squeezing deformation depends on the rock type and its characteristics, discontinuities, competency factor and, the excavation and supports of the tunnel.
On the second, the building up of a constitutive law for jointed rock masses, this research takes account of the discontinuity effects on the rock mass behavior. Based on the concept of equivalent continuum, a three-dimensional, nonlinear constitutive law integrates the mechanical behavior of intact rock and discontinuities into rock mass have been established through representative volume element. The strain-softening characteristic of intact rock and the sheared deformation related properties of joint such as the shear strength and shear stiffness have been taken into account. The constitutive law is capable of modeling the complete stress-stress curves of rock masses composed of various intact rock properties and any number of joint sets with arbitrary orientations, spacing and mechanical behaviors. The validity and applicability of the proposed nonlinear constitutive law have been tested and verified by comparing the predicted results with relevant existing model predictions and laboratory simulation outcomes tested by Yang(1992).
The results of parameters study show that the number of joint sets and their attitudes play key roles on the complete stress-strain curves of rock masses. The isotropic complete stress-strain curves of intact rock material become highly anisotropic as the joint sets number is less than four. The difference of strength and secant deformation modulus of rock masses can reach 10 times with various joints attitudes. The complete stress-strain curves get back to isotropic if the presence of joint sets is greater than four. The effect of joint properties on the deformation of a rock mass is not so significant if the failure of rock mass is controlled by joint sliding. However, the influence of joint properties are of moment if the joint sliding presence in a rock mass. The anisotropic behavior is observed at this situation, and, among the several considered parameters of joint, the basic friction angle is more important.
The stress states also affect the complete stress-strain curves of rock masses, and the influence is swayed by the relativity between the direction of principal stresses and attitudes of discontinuities. The proposed nonlinear constitutive law is able to account the effect of middle principal stress. The calculated results show that the effect of middle principal stress on strength and secant deformation modulus of rock masses is of important as joint strike is perpendicular to the middle principal stress and parallel to the minor principal stress.
Next, establishment of the squeezing numerical model and relevant parameter studies, this research builds up a numerical model for tunnel squeezing analysis based on the proposed nonlinear constitutive law for jointed rock masses. Two procedures, “the elastic-plastic deformation analysis” and “the time-dependent deformation simulation” are conducted to analyze the squeezing deformation of tunnel surrounding rock for plane strain condition. The squeezing numerical model has been verified by comparing its calculated results with existing analytical solutions and numerical results for circular tunnel excavation, simplified uniaxial compressive test, and Aydan et al’s(1996) calculated outcomes. It is proved that the proposed squeezing numerical model can simulate not only the anisotropic complete stress-strain behaviors of tunnel surrounding rock, but also the time-dependent deformation.
Tunnel squeezing analysis results indicate that the presence of discontinuities are of great moment on the behavior of tunnel surrounding rock. The characteristic of stresses variation around a tunnel, i.e. unloading in radial direction and loading in tangential direction, may cause the intact rock failure. Besides, the strength of joint in the surrounding rock where parallel the joint will reduce due as the reduction of normal force. This may bring about another failure pattern, i.e. the joint sliding. In case of many sets of joint presence, the failure zone will be superposed and result in significant increasing of tunnel convergence. The intact rock and joints usually exhibit strain-softening post-peak behavior and their mechanical properties are of time-dependent, i.e. the lower creep threshold. As the excavation sequence and supporting system during tunneling can not well control the deformation of surrounding rock in time, the squeezing phenomenon may initiate.
Finally, application of the squeezing model to tunnel project and evaluation on the performance of various excavation and support methods, the research applies the squeezing numerical model to simulate a squeezing phenomenon in New Kuanyin tunnel, North-Link railway. The non-uniform deformation of surrounding rock caused by the presence of discontinuity is well described. The time-dependent deformation is also predicted effectively. It is shown that the prediction results of the proposed model are better than the other existing methods, such as the mechanical properties of rock masses evaluated on the basis of geological strength index(GSI) suggested by Hoek et al(1998).
This research utilizes the proposed squeezing numerical model to evaluate the performances of various excavation and support methods. The factors of three-dimensional effects near the excavation face, excavation sequence and support installation time during tunneling are taken into account in the analysis. The support systems analyzed include not only traditional bolts, steel ribs and shotcrete, but also some newly developed support methods such as yielding bolts, bearing bolts, adjustable ribs and slot shotcrete. The results of the numerical analysis indicate that step excavation, among which the inner pilot tunnel method might be a better way for tunnel excavation in squeezing ground, can minimize the plastic deformation of the surrounding rock. The support stress can be reduced by the passive support approaches and the performances of various support systems are also presented. Finally, based on the results of this research, some effective strategies for the excavation and support of tunneling in squeezing ground are recommended.
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