Construction Technology of Floor Slab for Deep and Large Top–Down Excavation in Soft Clay

The construction quality and elevation of floor slab obtained using the top–down construction method in soft clay may be affected by soil uplift due to excavation unloading and recompression settlement due to the weight of concrete. Based on the Shanghai EXPO axis and underground complex project (Shanghai EXPO axis excavation), soil uplift is studied by conducting centrifugal model test and numerical simulation. The soil uplift process for soft clay in Shanghai EXPO Axis excavation is mainly concentrated during the 14 days after excavation, attaining approximate 84 % of the final values. The configuration of soil uplift is serrated. The maximum soil uplifts for excavation depths of −1.08, −9.10, −12.80, and −17.30 m (WSEY) are 85, 127, 143, and 153 mm respectively. On-site monitoring reveals that the floor slabs and beams are settling during concrete casting. The settlement values are related to concrete weight, bearing capacity of soils under the excavation surface, and stiffness of scaffold. Based on these findings, exposure time duration between soil excavation and concrete casting and reasonable pre-camber values of scaffold are proposed. The results of this paper demonstrate that the application of these values is effective in controlling construction quality and elevation of floor slab and beam.