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Dewatering-induced ground settlement is an important issue, and there a cumulative effect in stratified soil settlement. Detailed research of the stratified soil settlement devotes to a better evaluation of ground settlement. To better investigate the dewatering-induced stratified settlement characteristics of the soil strata in Shanghai, an intensive field-monitoring program was carried out for the excavation of an underground metro station, which involved the dewatering of confined aquifers in urban areas. Analyses of monitoring results suggest that the groundwater tables dropped steadily. The ground settlement decreased with an increasing distance from the excavation, and the maximum ground settlement was less than the allowable value. The results for stratified settlement monitoring showed that the ground settlement was less than the settlement at the bottom of the soft soil clay layer, which indicates that the clay layer swelled during dewatering in the underlying soil layer. This special phenomenon, referred to as dewater-caused soil expansion, has not been reported in the existing literature. This project acts as a special case study and provides a basic insight into this phenomenon. [ABSTRACT FROM AUTHOR]

In this study, ground surface settlement induced by shield tunneling construction is investigated, where special strata composed of upper soil layers and lower rock layers are fully considered. The field data indicates that in soil-rock composite strata, the settlement trough along the transverse surface shows approximately normal distribution. The settlement that occurs during construction accounts for more than 60% of final settlement. The traditional Peck formula was modified to fit this composite strata by regression analysis of the measured data. The suggested values of modification factors α and β ranged from 0.6 to 1.2 and from 0.3 to 0.9, respectively. Moreover, the results of the numerical simulation show that the settlement pattern along the transverse surface in the soil-rock composite strata is significantly influenced by the hard-layer ratio m (hard-layer depth to excavation diameter). Surface settlement decreases, but trough width increases with as the hard-layer ratio increases. In particular, a roughly linear relationship between maximum surface settlement and hard-layer ratio is presented. A method of predicting settlement in shield tunneling construction in soil-rock composite strata is also proposed in this study. [ABSTRACT FROM AUTHOR]