Assessment of Ground Condition for a Mass Rapid Transit System in Singapore

摘要

Subsurface ground conditions will have a large influence on the design of underground structures suchrnas tunnelling and deep excavation works. Adequate site investigation information is very useful inrnunderstanding the ground conditions. For the on-going mass rail transit system Circle Line Stage 2 -rnContract 823 project, close to 90 exploratory boreholes were sunk to investigate the ground condition.rnThe project involves the construction of 3 underground MRT stations and twin bored tunnels ofrnapproximately 2.5 km long. The objectives of the site investigation were to obtain a betterrnappreciation of the ground subsurface profile along the route of the project and to ascertain designrnparameters. The results obtained from the site investigation are summarised in this paper.rnThe marine clay layer is the predominant soft soil layer found at the site. The maximum thicknessrnencountered in the site investigation is about 30 metres. One of the most interesting findings from thernsite investigation were evidence of possible occurrence of some previous earthslips of Old Alluviumrnhillocks that had taken place during the initial stage of the Holocene period, i.e. before the depositionrnof the Lower Marine Clay layer, and are now buried between Kallang Formation soils. Thesernearthslips had caused huge volume of soils displaced into adjacent valleys. Some of these "buriedrnvalleys" are now filled with intact Old Alluvium soils, which have undergone moderate degree ofrnweathering albeit the soil fabric of the parent material remains relatively undisturbed. Thisrnextraordinary finding is described in the paper.rnThe paper also presents evaluation of the characteristics and properties for some of the main soil typesrnthat are encountered in the project.