Greenfield settlements due to tunnelling using tunnel boring machine (TBM) in layered soils: a parametric study

As India’s cities continue to increase in population, a large number of new metro lines are being built to accommodate the mobility of residents. For construction of these metro lines, underground tunnels are being constructed to alleviate traffic problems in congested areas such as Delhi, Chennai, Mumbai, etc., where land scarcity is a major issue. Although tunnel boring machines (TBMs) are used in the construction of metro tunnels, ground settlements on the surface are inevitable. Tunnelling induced ground settlements may have adverse impact on the structures that are above the ground level as well as adjacent to the excavations. In the present study, ground settlements caused by TBM tunnelling are determined in greenfield conditions, using a finite element method (FEM) based software, PLAXIS and parametric study is performed after validation of numerical model with empirical results. Ground settlements obtained using PLAXIS 2D are compared with those obtained using empirical methods as available in literature. This study captures the effect of governing tunnel and soil parameters such as diameter of tunnel (D), depth of the tunnel axis from ground surface (z), grout pressure (GP), undrained shear strength of soil (s_u) and soil modulus (E’_ref), covering the most practical range, on ground settlements under greenfield condition. The settlement results of this study are validated and compared closely with earlier experimental and observational findings that have been published in the literature. It is observed from the various parameters studied that they are having a major impact on the ground settlements during tunnel construction. Therefore, a tunnel designer should consider the effect of all the governing factors in design prior to the tunnel construction to keep the maximum surface settlements within the permissible limit set by the standards.