Evaluation of Mechanized Tunnels Lining Loads by Application of Physical Modelling

Summary : Urban development and rapid extension of cities have been accompanied by a considerable growth in mechanized tunneling. Commonly precast concrete segments are used as a tunnel lining which constitutes relatively considerable part of tunnelling cost. The Optimum design of lining that decreases tunnelling cost, especially in tunnels with large diameters and high lengths, needs an accurate evaluation of loads acting on the lining. Laboratory model tests conducted under gravity or in a centrifuge allow one to investigate the most relevant factors influencing the tunnel behavior. Testing results also provide valuable data for refining the chosen numerical model.
Because of its vantages into conventional excavation methods such as the New Austrian Tunneling Method (NATM), mechanized shield tunneling becomes the most important tunneling method in recent years, especially in urban areas.
The Optimum design of lining needs an accurate evaluation of loads acting on the lining. The various aspects of soft ground tunnelling have been studied with several methods such as empirical methods, numerical and physical modelling and Analysis of instrumented projects and field trials. Although advances in computational techniques have led to extensive numerical and analytical tunneling research being conducted, geotechnical engineering researchers depend heavily on physical modeling to understand different phenomena related to excavation of tunnels.
Methodology and Approaches : In this paper, the effects of surface buildings specifications, tunnel depth and surface surcharge on lining loads were studied. For this purpose a 1/40 small scale physical model is used. 1g models allow one to investigate complex systems in a controlled environment and are considered to be more economical compared to centrifuge or field investigations.Despite the limitation ofrealistically simulation of in situ stresses 1g models have long been used in soft ground tunnelling research. The geometry of tunnel, lining segments and the surrounding soil properties are adapted from the under construction of Tabriz urban railway line 2 project.
Results and Based on obtained results, as a general rule, the existence of surface buildings will cause the lining loads increase compared to the green-field condition. However, the influence value depends on the combination of geometrical and mechanical parameters of the tunnels, buildings and surrounding soil. For shallow tunnels these effects is greater and with increase of tunnel depth building effects decrease.