Abstract
The evaluation of the load acting on a tunnel support is of fundamental importance for the correct dimensioning of the structure when analytical calculation methods are used. The load acting on the support depends on various factors and for this reason its evaluation can appear somewhat complex. One way of evaluating the load acting on a support is to use the convergence-confinement method. This process involves intersecting the convergence-confinement curve with the support reaction line. However, in order to be able to adopt this technique, it is necessary to know the radial displacement of the tunnel wall at the point in which the support is to be installed. A technique for the evaluation of the load acting on a support is presented in this study. This technique is based on the convergence-confinement method and on the Vlachopoulos and Diederichs formulation for the estimation of the radial displacement of the tunnel wall at the point in which the support is to be installed. An iterative procedure has been introduced in order to obtain a final evaluation of the load acting on the support. Application of the procedure to over 1700 representative cases of the typical conditions that can be encountered during excavation of a tunnel in rock masses has made it possible to obtain graphs that illustrate the load acting on the support for variations of the in situ lithostatic stress. An analysis of the results has led to considerations on the factors that influence the load acting on the support and has made it possible to identify which factors are of greatest influence and those that can be considered negligible.
Highlights
In order to guarantee the stability of a tunnel, it is often necessary to introduce a temporary support structure during the excavation, close to the excavation face
In order to proceed with the correct evaluation of the load acting on the support with the convergence-confinement curve, it is necessary to know the radial displacements of the tunnel walls at the moment in which the support structure is installed
The prepared charts allow a quick estimation to be made of the load acting on the support structure, which can be useful in the preliminary dimensioning phase
Summary
In order to guarantee the stability of a tunnel, it is often necessary to introduce a temporary support structure during the excavation, close to the excavation face. The most common support dimensioning methods that are based on analytical approaches, such as the hyperstatic reaction method (Oreste, 2007; Do et al, 2014c) and the Einstein and Schwartz (1979) method, are very fast to use and allow parametric analyses, which are very useful in the preliminary design phases, to be developed These methods, because of the velocity that can be reached in the definition of the calculation model, in the development of the calculations and in the interpretation of the results, make it possible to develop probabilistic analysis (Oreste, 2005a), which are very useful in the geotechnics field, due to the elevated uncertainty that generally characterizes the ground parameters. The prepared charts allow a quick estimation to be made of the load acting on the support structure, which can be useful in the preliminary dimensioning phase
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