Long-term settlement of metro viaduct piers: A case study in Shanghai soft soil

Abstract

This study aims to investigate the long-term settlement behavior of metro viaduct piers in soft soil in Shanghai. Field monitoring was conducted for eight years, from the official opening until 2021. The results indicate that the settlement rate of the majority of piers was higher in the initial years and gradually reduced since 2017. Up to 2021, the maximum cumulative settlement St was 15.9 mm, which accounted for 0.8 times the limit of St (Ut). Seven settlement troughs were observed during the monitoring period. Muddy soil and nearby rivers significantly influenced the long-term pier settlements. St decreased proportionally with the thickness of the muddy soil. Piers located near the river experienced higher settlements than those located farther away, with a maximum settlement difference of 0.28–0.42 Ut. The influence of the river on St extended up to a range of 3–4 times the width of the river (Wr). Additionally, there were noticeable differences in settlement between piers constructed with tubular piles and those constructed with bored piles, particularly during the first two years after completion. Under similar conditions, the St of piers with tubular piles were 0.28 Ut (5.5 mm) larger than those of piers with bored piles on average. In addition, the slope k between the excavation-induced pier settlement and the ratio of the excavation depth to the distance was closely related to the type of retaining structure. The ratio of k values for the pit wall soil reinforcement, row piles and diaphragm wall was approximately 4:2:1. To prevent excessive pier settlement under similar geological conditions, it is recommended to employ a more rigid retaining structure and avoid using a soil-reinforced enclosure for pit walls.