Abstract:
The main objective of the research was to evaluate the prospect of tunnelling by Cut and Cover as well as New Austrian Tunnelling Method (NATM) through conventional and numerical study for underground metro rail system in Dhaka city. Mass Rapid Transit-4 (MRT-4) of Strategic Transport Plan, 2004 was selected in this study. Four locations namely Uttara, Mohakhali, Farmgate and near Dhaka University were selected along the MRT-4 route for development of sub-soil profile. Physical and strength properties of the sub-soil were determined from laboratory and field tests.
In this study, two-dimensional finite element analysis using an elasto-plastic constitutive model- Subloading tij model (Nakai and Hinokio, 2004) was performed. Subloading tij model requires only a few unified material parameters and can consider influence of intermediate principal stress on the deformation and strength of soils, influence of stress path on the direction of plastic flow and influence of density and/or confining pressure. The model parameters were obtained from triaxial and consolidation test results.
A conventional analysis was done for the retaining system of Cut and Cover excavation method. The constructions of tunnelling system for Cut and Cover method as well as NATM were simulated considering field scale. A comparison was made between results obtained from conventional analysis and numerical analysis for earth retaining structure of tunnel in the Cut and Cover method.
In conventional analysis for the greenfield condition, earth pressures along the proposed route found to vary between 50.76 and 56.12 kN/m2 for cut and cover excavation method. For the same case, the critical bending moments varied between 76.14 and 85.16 kN-m for the sheet pile wall. From numerical analysis, earth pressures on sheet pile wall were found as 138.32 kN/m2, 165.79 kN/m2, 66.51 kN/m2 and 72.01 kN/m2, respectively for greenfield condition, loading of nearby existing structure condition, greenfield with soil-water coupling condition and both loading of nearby existing structure and soil-water conditions. The maximum lateral displacements for the respective cases were 18.8mm, 36.4mm, 220mm and 340.8mm. In NATM, it was found that the surface settlement of tunnel ranged from 52.6 to 54.1mm with an affected zone of 55 to 65m along surface road way considering different loading conditions. It was also found that surface settlement occurred at the position of existing building was larger for shallow foundation than that for the pile foundation.
It was observed that the finite element analysis with Subloading tij model (Nakai and Hinokio, 2004) can simulate the interactions of soil-structure and soil-water as per practical situation. The analysis also provides more realistic results comparing the conventional analysis which is based on many simplifications and assumptions. From analysis, it was revealed that for open spaces like Tongi to Uttara along the MRT-4, Cut and Cover is more appropriate considering its simplicity in execution. On the other hand, NATM is preferable at flyover junction points in Cantonment and structurally obstructed places (Farmgate to Sayedabad).
