3D STABILITY ANALYSIS Of HETEROGENEOUS FACE EXCAVATION BY TBM- EPB MACHINE -CASE STUDY LINE 2 METRO OF MASHAD

Surface settlement is a disadvantage of construction of shallow tunnels in urban areas which leads to the damaging the existing structures and subsurface utilities. One of everlasting challenges for the tunnel engineering in urban areas is predicting considerable settlement during excavation in the heterogeneous soil.The face pressure is one of the most important parameters which affects surface settlement, which shall be calculated before arriving a certain section in order to apply to the face of tunnel by the operator. In the present paper, first, empirical and analytical methods for determination of face pressure by TBM excavation are studied. Then, by using Flac3D software, (a finite difference code), a fully aspect-oriented simulation of TBM-EPB is done, and as a result, the required face pressure is obtained.Excavation of Mashhad's metro project of line 2 is done by the EPB-TBM, and its operational data, such as surface settlement, face pressure, and tail void grouting pressure, are used. Finally, results of empirical, analytical, and numerical methods for face pressure calculation are compared with the real data.In this paper, a parameter, called ``allowable pressure'' (Pall), is introduced which shall be used in order to predict the optimum pressure required in the excavation progress. This parameter is defined based on the damage assessment for the adjacent structures. In this study, the Rankin risk assessment was used to determine the maximum surface settlement.The results illustrate that the empirical method (i.e., Peck relationship) does not satisfy the adequate accuracy, and this method is recommended for phase I design. On the other hand, the Broer method which considers the real condition, can predict the required face pressure with appropriate accuracy compared to the empirical method. Numerical simulation has high capability to calculate the face pressure in heterogeneous soil layers, so that the final result has about 6 % error.