# Dewatering-Induced Stratified Settlement around Deep Excavation: Physical Model Study
###### Abstract  
The multi-aquifer and multi-aquitard system (MAMA) is a typical geological structure in deltas. Thus, the risks and challenges to settlement control and environmental protection are increased when demand for underground space extends to deeper strata. In this study, dewatering-induced stratified settlement in MAMA is divided into three stages according to whether the overlying aquitard is coupled with groundwater seepage. Subsequently, large physical model tests were carried out. Seepage and compression in the overlying strata come after the compression in the confined aquifer and the coordinated deformation in the overlying strata. The soil is compressed under the seepage drive within the hydraulic gradient range, while the soil above it is still affected by coordinated deformation and shows expansion. Dewatering-induced uneven settlement will cause damage to existing foundations and underground structures. Large-scale and uninterrupted excavation and dewatering are the main reasons for the continuous development of land subsidence. Although artificial groundwater recharging can reduce the settlement of the existing building, underground structure, and surrounding strata, a reasonable space arrangement is needed. multi-aquifer and multi-aquitard system; stratified settlement; foundation pit dewatering; physical model test; coordinated deformation +
Footnote †: journal: Applied sciences  
fissures [14]. Large-scale and uninterrupted deep excavation and dewatering have been recognized as the main causes of urban land subsidence [15; 16]. [PERSON] et al. [17] first discovered the coordinated deformation phenomenon of strata during the settlement monitoring of the deep excavation dewatering in Yishan Road Station of Shanghai Metro Line 9 and reproduced it in physical model tests [18]. [PERSON] [19] and [PERSON] et al. [20] constructed 3D mathematical models to describe this phenomenon in multi-layer strata. As a kind of intuitive and generalized method, the physical model test can reproduce the real environment by similarity theory [21] in a laboratory to eliminate the interference factors and analyze the stress and strain law of the research object under specific loading and boundary conditions. Among the physical model tests, the 1D physical model test, which is mainly used to study stress and strain response law in vertical soil under a complete lateral confinement condition, is the simplest. By analyzing the deformation rule of the combined strata under the condition of pumping water in the 1D physical model, deformation lags behind groundwater variation, and vertical uneven settlement in strata was observed [22; 23]. After repeated groundwater-level fluctuations, the compression of pores and the collapse of a card-house structure prolonged the consolidation time and tended to stabilize plastic and creep deformation; moreover, the overall performance is elastic deformation [24; 25]. Restricted by the boundary limited in the 1D model, scholars began to explore the deformation characteristics of strata in space and time by using a 2D or 3D physical model test. [PERSON] et al. [18; 26] reproduced the phenomenon of coordinated deformation and considered that the expansion of overlying strata is caused by coordination deformation, while compression is caused by overflow and consolidation. According to [PERSON] et al. [27], groundwater withdrawal changed the subsidence distribution, which developed at a building load period during the dewatering of the friction force from piles. These piles reduced the subsidence of buildings and the surrounding area yet increased the differential subsidence based on the physical model test of the variation in the groundwater level with the high-rise building group. For deep excavation dewatering, the deformation of strata caused by the decrease in pore water pressure and the increase in effective stress is more complicated.