Expansive soil experiences periodic swelling and shrinkage during the alternate wet and dry environments, which will result in severe damage to the slope stability. In this study, a promising modifier OTAC-KCl is introduced, which has a good diffusivity and is soluble in water or other solvents easily. Firstly, a reasonable combination of ameliorant 0.3% STAC and 3% KCl is chosen referring to the free swell test. Then, the best curing period, 14 days, is gotten from UCS tests. The effect of wetting and drying cycles on engineering properties of expansive soil improved by OTAC-KCl admixtures after 14-day curing is also studied accordingly. Both treated and untreated expansive soil samples are prepared for the cyclic wetting-drying tests which mainly include cyclic swelling potential and cyclic strength tests. Experimental results show that the swelling potential of expansive soil samples stabilized with OTAC-KCl is suppressed efficiently, and the untreated soil specimens will collapse when immersed in water while the treated specimens keep in good conditions. Moreover, expansive soil samples modified with 0.3% OTAC + 3% KCl show enough durability on the swelling ability, shear strength, and unconfined compressive strength, which means, that both the physical and the mechanical properties of stabilized expansive soil have been improved effectively.

This study focuses on the settlement of buildings around the foundation pit caused by foundation pit dewatering. Within the study context, a test is conducted on the control of ground settlement during double‐layered foundation pit dewatering based on the seepage control‐recharge coupling effect. Moreover, a large indoor seepage test device is developed to accurately simulate in‐situ three‐dimensional seepage conditions of double‐layer foundations. The study results showed that compared to the antiseepage conditions, upon the additional recharge, the head of the upper row of the pressure measuring tubes outside the foundation pit is raised by nearly 10.7%, and the water level of the lower row of pressure measuring tubes is raised by about 3.1%. Additionally, the head of the outer side of each pressure measuring tube is higher than that of the inner side by about 2.75% on average, indicating that the recharge outside the pit can effectively increase the head height in the area above the soil layer at the bottom of the recharging well outside the foundation pit and reduce the settlement of the building outside the foundation pit. The numerical calculation of the settlement outside the pit using FLAC3D software further confirms the effect of the seepage control‐recharge coupling model on the ground settlement during foundation pit dewatering. The research results are important for the infiltration deformation and foundation settlement control in composite foundations precipitation.

The critical hydraulic gradient of cohesive soil is an important condition for judging soil piping. For force analysis of movable particles in pore channels of soil, this study proposes to consider the influence of surrounding particles on the drag force of movable particles by water flow. According to the principle of relative motion, considering the interaction force between moving objects in still water, the value of the drag force of water flow that is affected by surrounding particles is calculated, to derive the method of the critical hydraulic gradient. This calculation method is suitable for the results of previous piping tests, and the method is accurate and concise.

At present, in the design of geotechnical structures, the allowable bearing capacity is mostly designed according to the safety factor specified in the design specification. However, there are two problems that have not yet been clearly explained in theory: (1) why is the safety factor listed in the standard is suitable for the design of geotechnical structures and (2) can the safety factor selected according to the specification guarantee the long-term stability of soil mass. In view of the above problems, according to the statistical law in the rheological properties of soil, the relationship between the shear stress level of the soil and the stability of the soil is studied, and the stable shear stress level of the soil is obtained. The research results show that the stable shear stress level of soil is the corresponding shear stress level under static earth pressure. This conclusion not only verifies the correctness of the theoretical finding that “the earth pressure on the structure develops toward the static earth pressure, and whether the ultimate safety of the geotechnical structure is related to its design bearing capacity is greater than the static earth pressure” in the design code of the retaining structure but also clarifies the source of the safety factor. By calculating the design bearing capacity of the geotechnical structure according to the static earth pressure, the geotechnical structure that maintains long-term stability can be designed, which provides a reliable method for the design of the permanent geotechnical structure.

To obtain the applicable filter layer range of the silt sand around the dewatering well of a ship lock renovation project in Nanjing, several existing classic filter layer criteria are analyzed, and the concept of a progressively stable filter system is put forward accordingly. The self‐designed horizontal percolation filter is used to carry out the silt filter test, and the filter effect of these graded filter materials on the test silt filter is studied, and the mechanism of the progressively stable filter system is described. Tests show that when the ratio of the filter material to the characteristic particle size of the silt soil falls within the range of 4.3–7.7, the system maintains a stable percolation state. Through the particle tests of the base soil and the filter material after filtering, it is found that an effective filter system is that the filter layer induces soil’s self‐filtration, and they work together. No matter what filter material criterion is used to design the filter, as long as the gradual and stable filtration mechanism is formed in the filtration process, stable filtration can be ensured. The research on the progressively stable filter system will help to determine the gradation range of the filter material more reasonably in the engineering design, reduce the failure probability of the filter layer, and provide a certain reference for the design of the filter layer in similar projects in the future.