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.

The drawdown outside of a deep foundation pit has to be controlled during excavation. However, the vertical curtain cannot cutoff a deep and thick confined aquifer during deep excavation. In this study, a microbial-induced carbonate precipitation (MICP) horizontal seepage reducing body (HSRB) was proposed to control drawdown combined with a partially penetrating curtain. MICP HSRB is formed by using the seepage field generated by the recharge wells to drive the migration of a Sporosarcina pasteurii solution, stationary solution, and cementation solution into the deep confined aquifer. The migration of each solution was numerically simulated to study the HSRB formation process. The influence of different factors on solute migration was studied. The results show that the solutes in the fixed fluid and cementation fluid can reach the area under the driving of the seepage field, which proves that MICP HSRB can be formed. The calcium ions and urea in the cementation solution are more likely to migrate to the designated area than the bacterial solution. Increasing the injection rate of bacterial solution and adding recharge wells both made the bacterial solution migrate more quickly to the designated area. In the case of multiple grouting, the solute migration in the later stage will be hindered by the plugging of pores caused by calcium carbonate generated in the earlier stage. Therefore, different grouting methods need to be designed to drive the seepage field so that the solute injected in the later stage can continue to migrate. The MICP HSRB grouting technology can be used in foundation pit dewatering, providing reference for similar engineering.

Using a soil improvement biotechnology based on microbial-induced carbonate precipitation (MICP) to form a horizontal water barrier in foundation pit dewatering can not only achieve a good dewatering control effect, but also control the cost and obtain good economic and environmental benefits. In this study, taking the Shanghai layer ⑨ sand as the research object, the solution injected into the layer ⑨ sand by MICP grouting technology includes bacterial solution (Sporosarcina pasteurii) and cementation solution (urea and CaCl₂ solution). The migration mechanism of the bacterial solution and cementation solution in groundwater was analyzed through batch experiment and column test, and the mathematical model of migration was established. The dynamic adsorption characteristics of the S. pasteurii, Ca²⁺, and urea were obtained by dynamic adsorption test. Through isothermal adsorption test, the Freundlich isotherm was used to describe the isothermal adsorption of S. pasteurii onto fine sand, and the partition coefficient of the S. pasteurii was found to be 1.5 × 10⁻⁷ L/mg. The Langmuir isotherm can be used truly reflect the isothermal adsorption of Ca²⁺ onto fine sand. The distribution coefficient of the Ca²⁺ was 7 × 10⁻⁸ L/mg. The maximum adsorption capacity of the solute was 2404.8 mg/kg. The adsorption capacity of the urea was very small. According to the sand column test, the vertical dispersion was 11.5 cm. Due to the obvious size effect of dispersion, the sensitivity of dispersion in actual working conditions should be analyzed in a subsequent numerical simulation. The determined solute transport parameters provide support for the further study of the numerical simulation of the solute transport process of the HSRB formed by MICP grouting technology and provide a basis for further field application.

In ancient riverbed areas, the hydro-geological conditions are extremely complex because of the cutting of ancient river channels during the sedimentary process. How to lower groundwater level in water-riched gravel-confined aquifer during deep excavation is vital for underground engineering. Groundwater flow patterns had to be understood during foundation pit dewatering. This paper presents a field case study conducted at the deep foundation pit of the Qianjiang Century City station on Hangzhou Metro Line 6, which is notable for its 52 m deep unclosed waterproof curtain. A total of 34 pumping wells were installed within the pit. During the tests, one well was subjected to a pumping well, while the others served as observation wells. The research included two sets of multi-depth pumping tests, which differed in terms of their filter lengths, aimed at investigating the flow pattern around pump wells and the roots of diaphragm walls. The study found that the use of longer filters, higher pump rates, and filters placed nearer to aquifer roofs enhances dewatering efficiency and minimizes impact on the surrounding geological environment. This paper introduces a novel concept known as the diaphragm wall–pumping well effect, which regulates the water head outside the pit and the subsidence, thereby optimizing the drawdown of the deep foundation pit with an unclosed waterproof curtain. The findings were applied in the foundation pit dewatering of Qianjiang Century City station, and the drawdown in and outside the pit was effectively controlled.