Abstract
The convergence-confinement method, based on the analysis of the stress and strain state that develops in the rock around a circular tunnel, has been presented in this study with the two analytical solutions that have been most frequently used in recent years. Problem statement: Thanks to the increased power of electronic processors and the spread of numerical methods, the role of the convergence-confinement method had changed but nevertheless remains important in the geo mechanical design of tunnels. Approach: Apart from being a basic instrument for the preliminary design of support and reinforcement structures, it can be profitably used for the estimation of the loads that act on the support structures, of the thickness of the plastic zone at the tunnel boundary, of the expected convergences and it can also be used to obtain an estimation of the geo mechanical parameters of the rock mass, through back-analysis staring from the in situ measurements. Conclusion: This study illustrated the fields in which the method still presents important practical applications and also its greatest limits, in more detail.
Highlights
There are at present three different types of approach that are today used in tunnel design: The empirical, rational and observational approaches.The empirical approach is characterised by a set of design recommendations and intervention schemes that derive from experience and it very often refers to the geo mechanical characterisation of the rocks and soils involved in tunnel construction
The convergence-confinement method, based on the analysis of the stress and strain state that develops in the rock around a circular tunnel, has been presented in this study with the two analytical solutions that have been most frequently used in recent years
Approach: Apart from being a basic instrument for the preliminary design of support and reinforcement structures, it can be profitably used for the estimation of the loads that act on the support structures, of the thickness of the plastic zone at the tunnel boundary, of the expected convergences and it can be used to obtain an estimation of the geo mechanical parameters of the rock mass, through back-analysis staring from the in situ measurements
Summary
There are at present three different types of approach that are today used in tunnel design: The empirical, rational and observational approaches.The empirical approach is characterised by a set of design recommendations and intervention schemes that derive from experience and it very often refers to the geo mechanical characterisation of the rocks and soils involved in tunnel construction. Geo mechanical classifications are widely used, for example, in the excavation of tunnels Today this type of approach is only used in the preliminary design stages when it is necessary to have general indications that are able to orient the following detailed study in a more efficient manner. The rational approach uses analytical solutions, when they are available and numerical calculation methods to evaluate the stress and strain state in the rock and in the support and reinforcement structures. The purpose of this approach is to define the static conditions of the tunnel that is being examined and to measurement results and on the basis of the response that the surrounding medium supplies in answer to the construction of the tunnel. The use of sophisticated mathematical techniques, called backanalysis techniques, has permitted a more rational organisation of the information that is obtained from the monitoring of a work and the exploitation of the measurements carried out during the work construction to a greater extent
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