Behaviour of a diaphragm wall with top down construction method

The construction of the Mass Rapid Transit system in Singapore involved the construction of 15 underground and 26 above ground stations. The construction of the Newton underground station using the diaphragm walling technique and top-down excavation method is described. The 800mm thick diaphragm wall has an average height of 25m and penetrates through thick layers of weak deposits of fill, marine clay and fluvial materials into decomposed granite. Back-analyses were carried out using the finite element method in which the soil is modelled using a non-linear, hyperbolic stress-strain relationship, and comparisons made with field instrumented data. The analyses indicate that cracking in the concrete diaphragm wall is likely to have occurred because of high bending stresses. The studies also indicate that the behaviour of the wall is influenced significantly by the strength properties of the soft marine clay and decomposed granite deposits.