# Dewatering-Induced Stratified Settlement around Deep Excavation: Physical Model Study
## 4 Results
### Stratified Settlement of Strata in Open Spaces  
Groundwater flows into the deep excavation driven by dewatering, thereby generating a funneled groundwater level around the deep excavation. However, given the existence of a diaphragm wall, the hydraulic connection of groundwater inside and outside the deep excavation is cut off within its depth range, and frictional water head loss is increased with the groundwater seepage path. In the graded dewatering stage, each time the groundwater level inside the deep excavation is low, the groundwater level changes in the confined aquifers reach stability after 1.5-3.5 days of dewatering, while reaching stability takes 35.0-40.0 days in aquitards (Figure 4). Combined with the strata deformation curve in Figure 5a, the strata deformation evidently falls behind the change in the groundwater level. In the confined aquifer, it reaches 80.0% of the total deformation on the 7 th-9 th day after the groundwater level has remained unchanged and achieves a stable state on the 20 th-25 th days. In aquitards, given the progressive vertical groundwater seepage and visco-elastic-plastic deformation, the stable state in the graded dewatering stage is reached at 35-40 days, and the groundwater level is recovered after 80 days. The time required for deformation stability and groundwater level is basically the same. Thus, the deformation of the clayey strata is mainly driven by groundwater seepage. Figure 4: Groundwater level changes in strata at JF–1 (15.0 m away from deep excavation). Figure 5: Stratified settlement in strata: (**a**) Stratified settlement with time in strata at JF–1 (15.0 m away from deep excavation); (**b**) Graded dewatering step I; (**c**) Graded dewatering step II; (**d**) Graded dewatering step III; (**e**) Groundwater level recovery; (**f**) Secondary dewatering. As shown in Figure 5a, for the entire strata, layers 1, 2, and 3 exhibit compression deformation. Meanwhile, layer 1 + 4 displays expansion deformation. In addition, the maximum stratified settlement appeared at the top of layer 5, and the settlement of layer 3 + 4 is approximately 60.0-80.0% of layer 5. As layer 5 (silty clay) has certain hydraulic permeability (\(K\) = 9.47 \(\times\) 10\({}^{-6}\) cm/s in prototype), the vertical groundwater overflow will be driven by the hydraulic gradient from the underlying confined aquifer, and compression occurs inside the aquitral. For layer 3 + 4 (mucky clay), given its low hydraulic conductivity (\(K\) = 4.18 \(\times\) 10\({}^{-6}\) cm/s) and far vertical distance from the confined aquifer, the expansion caused by coordinated deformation accounts for the main part. Thus, compression caused by dewatering and the expansion caused by coordinated deformation occurs simultaneously in the confined aquifer. As shown in Figure 5b-f, the stratified settlement curve of the strata is similar to a spoon in the vertical section. The maximum value appears 45.0 m away from the deep excavation (measuring point JF-2), and the value decreases with the increase in distance. The frictional resistance of the diaphragm wall has an intense constraint effect on the deformation of the surrounding strata, and the settlement in the ground and clayey strata around the deep excavation is small (measuring point JF-1). When dewatering stops, the groundwater is replenished to the confined aquifer from the lateral boundary, and the groundwater level recovers gradually until the initial value is reached. The frictional resistance of the diaphragm wall still exists, thereby causing the rebound value of the strata deformation at 15.0 m away from the deep excavation (measuring point JF-1) to be smaller than those of other measuring points. Furthermore, the final settlement has rebounded by 70.0-90.0% (Figure 5e) because the deformation of sandy strata (layers 2 and 3) is mainly elastic. However, for the clayey strata (layers 3 + 4 and 5), the deformation presents visco-elastic-plastic characteristics, and the final settlement has rebounded by 40.0-65.0%.