The overburden of shallow tunnels in urban areas is thin and the influence of the tunneling process such as displacements and subsidence due to creation of non-elastic zone in ground surface is very high. To control subsidence, the amount of minimum required support pressure at tunnel crown is determined based on the non-elastic zone characteristics such as thickness. However, in numerical modelling, the non-elastic zone thickness is obtained less than tunnel depth which is due to ignoring the influence of spatial variability of resistance properties to model tunnels in a spatially variable rock. Thus, in this paper, the influence of spatial variability of strength properties on the situation of non-elastic zone extension is investigated in the Finite Element Method (FEM) analysis of a shallow tunnel. The results show that considering the spatial variability concept changes the size and shape of the non-elastic zone in a more realistic way.

1 Introduction

Shallow Tunnels through variable media (rock or soil) in urban areas present significant design challenges (Zou et al. 2018). As the overburden of these tunnels is thin, the displacement analysis and the assessment of the non-elastic zone characteristics are very important, especially in areas where there are numerous high-rise buildings. Furthermore, the tunneling-induced ground displacement has a great influence on existing urban structures, and the evaluation of displacements (in the design stage) is a difficult subject, which many tunneling engineers have encountered. On the other hand, the excavation medium is characterized by a high degree of uncertainty due to the inherent spatial variability of soil or rock properties, the excavation depth, and the seepage forces (Massinas et al. 2015 & Xiang et al. 2013).

To control subsidence, the installed support system must carry the acting pressure due to overburden thickness. The amount of minimum required support pressure at tunnel crown depends on non-elastic characteristics of the zone surrounding the tunnel; named the Excavation Damaged Zone. Changes within EDZ, i.e. changes in the stress field and consequently changes in displacements field have significant effects on stability of the tunnel. Outside of EDZ, geomechanical properties of medium are slightly disturbed (Wang et al. 2018). Generally, according to Figure 1 and based on the results of physical modelling, to calculate the minimum required support pressure in shallow tunnels, the EDZ thickness is considered equal to the tunnel depth (Massinas et al. 2015, Xiang et al. 2013 & Wang et al. 2018). (In Figure 1, R0 is tunnel radius, H is tunnel depth, h is the distance between tunnel center and ground surface and D is tunnel diameter, thus, the EDZ thickness is equal to h-R0), however, in numerical modelling, the EDZ thickness of shallow tunnels is obtained less than tunnel depth. One of the most important reasons for this difference is to consider the same geomechanical properties for the entire environment and the concept spatial variability has not been considered. Spatial variability of rock or soil properties means that material properties are not the same within the profile (are not homogenous) and they change based on a parameter called spatial correlation length. Small value of spatial correlation length means that material properties are highly fluctuated, while large spatial correlation length means that material properties vary smoothly within the profile.

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