ABSTRACT
This paper presents some of the observations made from the monitoring records obtained during the excavation for constructing the tunnels and stations for the Singapore MRT North-East Line (NEL) contract 703. Various forms of excavations including open cut, cut-and-cover method and bored tunnelling were carried out for this project. The subsoil at the site is generally Old Alluvium underlying a relatively thin fill layer with some exceptions where relatively thin layer of Kallang formation is intersected between those layers.
Tunnelling was done totally within the Old Alluvium. The back calculated volume loss was about 0.5% based on the measured ground surface settlements. Open cut with a slope of 1(H): 1(V) was found generally safe in Old Alluvium even at shallow depths when adequate drainage was provided. The measurements during the braced excavation indicated that the strut forces and deformations were much lower than the anticipated values based on the analyses using drain parameters. This was probably because the Old Alluvium behaves more like an undrained material within the short construction period due to its low permeability. And it was also noted that the cohesion values at the site might be better than assumed values and the water table immediately behind the wall was generally much lower than the original ground level.
