# Dewatering-Induced Stratified Settlement around Deep Excavation: Physical Model Study
## 5 Discussion  
When pore water pressure decreases rapidly in the confined aquifer and if the overlying aquitard has excellent water barrier performance and almost no groundwater seepage occurs at the beginning, only vertical settlement and internal expansion will occur in the overlying strata, which are called \"coordinated deformation\" or \"reverse rebound\", under the constraints of its own structural stiffness and surrounding environment. Pore water pressure decreases first in the confined aquifer. Then, a hydraulic gradient occurs between the top of the confined aquifer and the bottom of the overlying aquitard. When the overlying aquitard reaches a certain degree of hydraulic permeability, vertical groundwater overflow will be driven by the hydraulic gradient from the underlying confined aquifer. With the increase in time, a hydraulic gradient forms in the aquitard and increases gradually, and its rising range is limited by the thickness and permeability of the overlying strata and the duration of dewatering. The soil shows compression under the drive of seepage within the range of hydraulic gradient. Meanwhile, the soil above it is still affected by the coordinated deformation and shows expansion (Figure 9).  
As shown in Figure 10a, when buildings are located around the deep excavation, the pile foundation of buildings settles along with the soil of the bearing layer, and the deformation of the strata within the pile length is limited under the influence of its frictional resistance. In the initial stage of deep excavation dewatering, pile foundations settle together with the confined aquifer. Furthermore, the soil around buildings shows larger expansion because of the overlying strata's coordinated deformation and the piles' frictional  
Figure 8: Groundwater level changes under the tunnel at JS–3.  
Figure 7: Earth pressure around the tunnel at JS–3.  
resistance. With the extension of time, groundwater overflow and compression gradually develop in the overlying aquitard, and the compression deformation of the soil is restricted by the frictional resistance of the piles again. Compared with open spaces, the final ground settlement around buildings is larger, and the rebound after dewatering is smaller. As for buildings, the uneven settlement of the bearing layer in the horizontal position leads to its tilt deformation (Figure 10b), and the friction resistance in different directions during two stages of deformation will produce complex additional stress on the piles and may cause damage. In the vertical position, the maximum uneven settlement between the foundation bearing layer and the ground reaches 4.95 mm at JM-2. This maximum level may cause fractures between the main structure of the buildings and the infrastructure or pipelines in their surroundings, which mostly occur in the surrounding parts of building groups.  
When a tunnel is located around, its structural integrity will restrict the deformation of the surrounding soil and itself. The ground settlement above it decreases to a certain range, and the earth pressure on the bottom and outer walls of the tunnel is also decreased. Given the uneven earth pressure around the tunnel, horizontal and torsional displacement may occur in the process of vertical displacement along with the soil (Figure 11), thereby threatening the sealing performance of the tunnel.  
Figure 10: Stratified settlement in strata around buildings: (**a**) Conceptual schema; (**b**) Buildings’ tilt in physical model.  
Figure 9: Conceptual schema of stratified settlement in strata.