What causes the excessive metro tunnel settlement in soft deposits: learned from a detailed case with factor decomposition.

Widespread tunnel settlement has been observed in the Yangtze River Delta region of China, but its formation mechanism remains unknown. In this study, a settlement trough and its development processes over nearly 7 years were monitored in detail. Notably, this representative case covers the majority of factors that can contribute to tunnel settlement. Records of acting processes of these factors are so comprehensive that factor decomposition was conducted to analyze the quantitative contributions. Specifically, potential factors were classified into internal and external factors, where internal factors (self-consolidation induced by tunnel construction and cyclic loading from traffic) account for 12% of the maximum observed tunnel settlement. Among the external factors, adjacent pile construction and diaphragm wall construction, ground buildings surcharge, adjacent pit excavation, and basement construction accounted for 5%, no more than 10%, no more than 22%, and less than 1% of the maximum observed tunnel settlement, respectively. Adjacent dewatering activities, which accounts for 46 to 67% of settling, were determined to be the dominant factor. Additionally, it was further found that the impact of adjacent dewatering activities on tunnels cannot be reduced by specially designed cutoff diaphragm walls. Wall leakage and the “pit dewatering with adjacent barriers effect” were proved to be potential causing factors. In particular, this effect can be a factor causing significant water head drawdown and subsequent tunnel settlement along metro lines, where high-density buildings tend to develop. These findings were extended to the entire metro line to discuss the mechanism of widespread metro tunnel settlement in this region. [ABSTRACT FROM AUTHOR]