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Results
A Numerical Analysis of the Effects of Ground Properties on Immediate Ring Closure in Mountain Tunneling
Kawata, K. (Public Works Research Institute) | Awaji, D. (Public Works Research Institute) | Isago, N. (Public Works Research Institute) | Kusaka, A. (Public Works Research Institute) | Mashimo, H. (National Institute for Land and Infrastructure Management)
Abstract Immediate ring closure, based on full face excavation with small bench-cut, is achieved by installing an inverted arch at a short distance from the excavation face to stabilize the tunnel structure in poor geological conditions. However, the relationship between the effects of immediate ring closure and ground conditions is not fully understood. Our case study of past construction data shows that the technique has been used in conditions of deep overburden, low competence factor, and stratified rocks with strong cohesion and low internal friction angle compared with massive rocks. In this study, a three-dimensional numerical analysis was carried out to examine the influence of ground conditions in a construction with immediate ring closure. Cohesion, internal friction angle, and closure length (CL) from face to inverted arch were varied while keeping the competence factor of ground constant at a small value. As a result, immediate ring closure had the potential to constrain the convergence of tunnel cross section and to reduce ground-loosening. In addition, the shortening of CL had a large effect on constraining the convergence of tunnel cross section and reducing the ground loosening area because the inverted arch could bear a large axial force and generate large inner pressure acting on the ground. The results also showed that the instability of the tunnel structure was more likely to occur in stratified rocks because a larger plastic zone formed in stratified rocks than in massive rocks for the same competence factor. However, the effect of the plastic zone reduction by the shortening of CL was more prominent in stratified rocks than in massive rocks. Therefore, immediate ring closure is potentially effective, especially in stratified rocks.
A Simple Method for the Extracting Coefficients of Viscoelastic Behavior from Field Measurements in Mountain Tunneling
Awaji, D. (Public Works Research Institute) | Isago, N. (Public Works Research Institute) | Kusaka, A. (Public Works Research Institute) | Kawata, K. (Public Works Research Institute) | Mashimo, H. (National Institute for Land and Infrastructure Management)
In such cases, to ensure the safety and stability of the tunnel during construction, it is important to evaluate the creep phase, which is generally divided into three phases: the transient creep (wherein the displacement rate gradually decreases), the steady-state creep (wherein the displacement rate is constant), and the accelerating creep (wherein the displacement rate gradually increases and rocks are soon to be collapsed). For reasonable design of the support specification and tunnel excavation method, it is necessary to apply a rapid and easy method for extracting the ground properties associated with time dependency from field data, as measured during the daily management of tunnel construction. However, it is generally difficult to quantitatively evaluate time dependency from field data as it includes the influences of both time dependent behavior and stress release due to the advancement of tunnel excavation. Furthermore, a long measurement period is required to recognize the time-dependent behavior from only one section. This study aims to propose a simple and rapid method to detect the time-dependent deformation from field measurements, and to extract therefrom the coefficients of the ground properties related to transient creep. To isolate time-dependent behavior from field measurements including the influences of the stress release associated with the tunnel excavation, horizontal displacement rates are adopted as an identification factor while the tunnel excavation is stopped. An exact solution to express the transient creep behavior in horizontal displacement rates of a circular tunnel on the basis of the viscoelastic theory is derived. Each parameter related to time dependency in this model optimized to minimize the error between the exact solution and field measurements. The validity of this method is also discussed by applying it to some field measurement data.
- Geology > Rock Type (0.48)
- Geology > Geological Subdiscipline > Geomechanics (0.32)