Influence of large excavation on existing tunnel

Abstract

Singapore is a densely developed country with limited space. Inevitably, infrastructure development will often times be of close proximity to existing buildings and structures. Excavation is one of the major construction activities carried out as Singapore moves towards maximising underground space development. Thus, damage to surrounding structures and utilities caused by ground movements from excavations must be closely monitored.

This project focuses on the influence of a large excavation on a nearby existing tunnel in Bukit Timah Granite residual soils. The main parameters investigated are the effects of retaining wall stiffness, excavation with berms, tunnel distance, permeability of soils/rocks, increasing thickness of the GVI residual soil layer and the use of recharge wells. The effects of each parameter are evaluated based on the magnitude of the induced tunnel horizontal and vertical movements.

For this project, a total of 12 plane strain finite element analyses were carried out using the finite element Geotechnical Software Plaxis2D. The finite element results indicate that a stiffer retaining wall will generally result in smaller tunnel movements. The analyses also indicated that the use of earth berms reduced the tunnel movements significantly especially for horizontal movements. The tunnel movements reduce gradually with increasing distance of the tunnel away from the excavation site. Both the permeability and thickness of the GVI layer were found to significantly affect the tunnel movements. Lastly, recharge wells were found to be effective in reducing tunnel movements.