Deformation and Stress Analyses of the Ground During the Construction of Multipurpose Road Under Karnaphuli River

Abstract

Underground tunneling is an excellent option to accommodate the increasing amount of traffic in the densely populated areas like Bangladesh. However, these tunneling constructions certainly induce ground movement to a certain scale. Even though developing new excavation technologies has opened the opportunity to construct tunnels in challenging ground conditions, the ground response due to these mechanized excavations is still attracts the attentions of the researchers. In particular for multilayered soft ground conditions like those found in Bangladesh, the ground surface movement brought on by such tunneling works is highly unpredictable. In order to gain a basic understanding of the ground's behavior, finite element analysis is a useful tool. In the course of this study, a finite element analysis was performed by making use of the Plaxis 2D program in order to examine a particular stretch of the multi-lane road tunnel that was constructed underground in Chattogram, Bangladesh which is also the first underground tunnel in Bangladesh. The tunnel is located at the sea entrance of River Karnaphuli of Chattogram suburb and its west coast starting point is connected with costal road. The geological condition of the area is consisting of alternating layers of cohesive soil layer and sandy soil layer mostly. Even though the tunneling depths is variable along the route, however, to simulate the most critical situation of the construction, maximum soil overburden depth of 35 m for the twin tunnel has been chosen for this analysis. To understand the soil structure interaction phenomena, the constitutive models that were utilized in the study are as follows : Subloading-tij model, Mohr –coulomb model(MC), & Hardening Soil model(HS). The finite element analysis has been conducted to investigate the following three scenarios, in the first place, the response of the ground before the tunnel was installed, in the second place, the response of the ground after the first tunnel was constructed, and in the third place, the response of the ground after the second tunnel was constructed. Mostly drained analysis is done for the excavation works. After that, each and every one of the finite element results are checked with the empirical and analytical findings. In addition to this, the failure mode of the tunnel lining structure has been studied, taking into account the load impacts of the soil in the surrounding area. When it came to the case of loading, factors including hydrostatic pressure ,soil pressure and the weight of the tunnel itself were taken into account. All the analysis were done in plastic condition. The Subloading-tij results reveal that the settlements of the ground surface caused by tunnel construction are greatest for the ground surface directly above the tunnel crown and significantly reduces respectively on both sides of the tunnel. The MC and HS models both seem to have the same settlement troughs in their results. Volume loss for shield tunneling is considered from 0.5% to 1.0% which is applied as contraction in Plaxis 2D. The settlement results were further compared with the onsite settlement data. A comparative results graphs have been plotted to understand the effect of using different soil models for the excavation works. As maximum deformation occurred in Subloading-tij model, higher displacement in the lining produces less bending moments. Similarly, with less deformation, hardening soil model shows higher bending moment than subloading-tij Consolidation analysis has been conducted for all three models in Plaxis. It has been discovered that, after the construction of both tunnels is finished, consolidation period is takes 15-24 days to finish. The effect on tunnel lining has also been studied at the end of the research. As Subloading-tij and Mohr-colulomb soil models are generating maximum soil stress, the forces (the moment, axial and shear forces) are found higher in these two models than the hardening soil models.