CIRCULAR TUNNEL STRUCTURAL ANALYSIS IN SOFT GROUND

Analytical bedded-beam spring model is most commonly adopted in the tunnel engineering practice due to its simplicity and promising outputs. Despite many well documented literatures exist concerning about the theories and fundamentals of various methods, there is a dearth of information on the modelling framework of a bedded-beam spring and many details are often overlooked in the modelling process. The main aim of this paper is to produce the modelling framework of the bedded-beam spring model in soft ground, for both direct and indirect approaches in considering the effects of segmental joints. Particular emphasis is devoted to the modelling techniques of each components of the structural model which in turn, is able to facilitate the modelling quality and process for the practicing engineers. Reviews on the technical papers of tunneling variables as well as parameters identification for the structural analysis have been first performed followed by the comparison of the consideration adopted in the modelling techniques. Analyses have been carried out to evaluate the structural forces on the tunnel lining and the obtained results are discussed in a comparison with the field measurements and design values of the actual tunneling projects. The results suggested that the direct approach always yields greater bending moments up to i) 26% for 6+1 ring configuration and ii) 21% for 5+1 ring configuration compared to that of the indirect approach. With the presence of more number of radial joints, the differences between the approaches have the tendency to be greater. With the validated models, parametric studies including tunnel lining flexibility ratio, compressibility ratio and the soil lateral earth pressure coefficient have been performed to provide better insight on the relative importance of each of the parameters. The results of the studies suggested that with the increase in flexibility ratio, the difference between the direct and indirect approach has shown to increase from 10% to 28%.  However, insignificant influence has been observed on the axial forces regardless of the joint consideration approach a well as the compressibility ratio.