Time Dependent Analysis of Tunnels Using The Finite Element Method

Abstract

The analyses of tunnels in clay were carried out taking into consideration twogreat important and effective categories, the time independent and time dependent. Timeindependent behaviours are used to represent the excavation stage while time dependentanalyses are used to represent the behaviour of the unlined tunnel after a long time.The finite element analyses were carried out using the linear elastic model for theconcrete liner while elasto-plastic and modified Cam clay model for the soil. Theexcavation has been used together with transient effects through a fully coupled Biotformulation. All these models and the excavation technique together with Biotconsolidation are implemented into finite element computer program named "Modf-CRISP" developed for the purpose of these analyses.In this paper, the basic problem represents the proposed "Baghdad metro line"which consists of two routes of (32 km) long and 36 stations is analyzed. The tunnel iscircular in cross section with a (5.9) m outer diameter and (0.45) m of concrete liningthickness. Eight-node isoparametric elements are used to model the soil and concretelining. Four-node element is used for pore water pressure.Two kinds of analyses are carried out:a- Using elastic-plastic constitutive model for all soil layers.b- Using modified Cam clay model (MCC) for the first soil layer (upper layer) andelastic-plastic Mohr Coulomb for the other two layers.The movements of the soil around the tunnel at the end of excavation at typicalpoints (crown, spring line and the invert) are calculated. The results indicate that there isan inward movement at the crown and this movement is restricted to four and a halftunnel diameters. A limited movement can be noticed at spring line which reaches0.05% of tunnel diameter, while there is a heave at the region below the invert, whichreaches its maximum value of about 0.14% of the diameter and is also restricted to aregion extending to (1.5) diameters.It was found that the maximum consolidation settlement above the tunnel is aboutthree times that at the end of excavation. The settlement trough extends only to (5)tunnel diameters using consolidation analysis while it is extended to (12) tunneldiameters using undrained analysis.