Pillar uplift of a subway station caused by foundation pit excavation in deep soft soil area

The steel pillars are usually used as vertical supports to maintain the stability of the internal support in the braced excavation. As a case study, the pillar uplift in a subway station, with the excavation depth of 17.6 m, located in a thick soft layer of paleochannel with a total thickness up to 40 m in Shanghai are studied. When the excavation reaches to the bottom of pit, the monitored uplift δhv in the standard section is in the range of 60-80 mm, causing cracks in some reinforced concrete layer. The uplift process of steel pillars is roughly divided into three stabilization stages: the early low-speed stage (Fs <1.5), the acceleration stage (Fs <1.3), and stable stage in the slab construction period. There are significant positive correlation between the uplift amount of steel pillars δhv and the maximum lateral displacement of the diaphragm wall(δhhm). The lateral displacement of the deep soil at the bottom of the diaphragm wall causes a larger δhv/δhhm of the standard section in the late stages of excavation. The uplift amount of the steel pillar for the end well is within 30 mm and much smaller than that of the standard section. There is no obvious lateral displacement in the soil at the bottom of the wall, showing a significant spatial effect. In addition, the uplift amount of the steel pillar decreases along the excavation direction during the stepped excavation mode. It is necessary to pay more attention on the influence of excavation mode and pouring process of slab on the uplift of the pillars. © 2020, Science Press. All right reserved.