# Dewatering-Induced Stratified Settlement around Deep Excavation: Physical Model Study
## 2 Background  
Shanghai, which is located in the southeast of the Yangtze River Delta, has a relatively flat terrain with a ground elevation ranging from 2.2 to 4.8 m, except for scattered mounds with an elevation that does not exceed 100.0 m in the southwest. The estuary of the Yangtze River is located in the north of Shanghai, wherein shoals and sandbanks are formed under the combined action of rivers and sea tides, which then developed into sand islands. The Quaternary strata in Shanghai are formed under the action of river water and sea tide, and the deposition thickness is approximately 200.0-350.0 m. The area has an alternating strata structure of sand, silty, silty clay, clay, and muddy clay vertically, including 16 engineering geological strata (Table 1). According to the hydrological characteristics of the strata, the strata can be divided into one phreatic aquifer, one micro confined aquifer, five confined aquifers, and six aquitards [28], which are collectively called the MAMA by [29] or the multi-aquifer aquitard systems (MAAS) by [30]. This system in Shanghai is not complete or independent but is a part of the Yangtze River Delta groundwater system. At present, human engineering construction mainly focuses on layers 1 to 2, and only a few groundwater mining, artificial recharging, and geothermal resource developments come into contact with deep aquifers (Figure 1). Unlike the continuous deposition of layers 1 and 6 of clay in a normal sedimentary area, part of layers 1, 6, and 7 were eroded by the paleochannel of the Suzhou River, and redeposited layers 12, 13, and 14. Considering that the hydraulic conductivity of these three layers is better than those of layers 11 and 6, the groundwater level change in the first confined aquifer (layer 7) is transmitted to the overlying strata more easily. Hence, according to the distribution of paleochannel deposition and the contact relationship between layers 7 (first confined aquifer) and 6 (second confined aquifer), the strata in the coastal plain area of central Shanghai can be divided into four types: I. normal deposition area where layers 7 and 9 are connected; II. paleochannel deposition area where layers 7 and 9 are disconnected; III. normal deposition area where layers 7 and 9 are disconnected; and IV. paleochannel deposition area where layers c and 9 are disconnected [29]. In this study, the deepest underground excavations under construction for the Deep Drainage and Storage Pipeline Project under Suzhou Creek in Shanghai are selected as the engineering background. This project aims to solve the problem of waterlogging in Shanghai. It consists of nine deep excavations, a 15.3 km long main drainage tunnel, and other supporting facilities. The average buried depth of the tunnel is 60 m. Deep excavation in West Yunling Road is selected as the prototype of the physical model, and the complex environment around the deep excavation is simulated by hypothetical tunnels and buildings. To examine the law of stratified settlement in MAMA during deep excavation dewatering in Shanghai under the most unfavorable MAMA conditions, a typical area under the paleochannel deposition area where layers 7 and 9 are connected is selected as the geological background.