Calcium oxide is a common curing agent used for curing contaminated soils. It is often used with cement to adjust the pH value. Calcium carbonate and calcium hydroxide are common products of calcium oxide. The effects of three kinds of binder in curing Zn2+-contaminated red clay are compared. The Zn2+ concentrations were selected as 0.1 and 0.5%, and the curing agent was added separately. The values of the unconfined strength of soil samples were compared after the three curing agents were added to the soil alone. It was found that the strength of the specimen with the addition of calcium carbonate was lower than the strength of the contaminated soil without the addition of the curing agent, while the strength of calcium hydroxide and calcium oxide was enhanced with the addition of calcium hydroxide, in general, and the strength of calcium oxide was higher. When CaO is added, the strength of the sample with a 0.1% concentration of zinc-contaminated soil is greater than the strength of the sample with a 0.5% concentration. But when Ca(OH)2 is added, the strength of the sample with 0.1% zinc-contaminated soil is less than the strength of the sample with a 0.5% concentration. Ca(OH)2 provides enough OH-, and there are enough zinc ions to react with it in the high-concentration Zn2+-contaminated soil to form CaZn2(OH)6·2H2O, which enhances the strength of the soil. By comparing the pH values of calcium oxide and calcium hydroxide, the pH of the soil is about 12, and the pH of calcium carbonate is about 8.

Engineering practice and real-life cases show that the geological conditions of the Guilin overlying karst site are complex. In particular, the groundwater, which drives the accelerated formation of soil cavities, and the thickness of the overlying soil layer, which affects the speed of the groundwater subsidence process. Therefore, this paper is based on a physical model to evaluate the effects of groundwater level changes caused by different factors on the deformation of karst cover. The model tests are simulated for different cover thicknesses (6 cm, 9 cm, 12 cm, 15 cm, 18 cm) under rainfall and other recharge, cavity supply, and drainage conditions at the same density (1.40 g/cm3) and initial water content (30%), respectively. The results show that with the increase of rainfall and other recharge time, the basic change trend of different cover thicknesses is that the infiltration curve changes faster at the beginning and slows down at the end, but the thicker the cover, the slower the overall deformation; at a certain rate of cavity recharge and drainage, the thicker the cover, the smaller the deformation caused by the fluctuation of groundwater level. The cavity recharge makes the cover displacement obvious, in the order of 0.304 cm, 0.173 cm, 0.118 cm, 0.068 cm, and 0.056 cm. After the formation of the cavity, the rainfall, other recharge, and the cavity supply and drainage accelerate the destruction and deformation of the soil body and the upward development of the cavity. The research results provide theoretical support for the subsequent prevention and control of karst collapse in covered karst areas, and have certain practical engineering significance.

Artificial water pumping and rainfall will induce groundwater level fluctuations, leading to large-scale karst collapse. Therefore, the combination of physical model test and FLAC3D numerical simulation is adopted to simulate the soil cave collapse caused by three different initial water level fluctuations in this paper. The influence of various initial water level heights on the variation of water-gas pressure, soil pressure of the overlying soil layer, and deformation in the existing soil cave during the fluctuation was analyzed. The relationship between initial water level height, deformation and collapse of the cave, and water-gas pressure was established, and the action law of water level fluctuations on the collapse of soil cave was introduced. The results show the following: (1) The change of water-gas pressure positively correlates with the initial water level height, directly affecting the overlying soil layer deformation. (2) The change of soil pressure of overlying soil positively correlates with the change of water-gas pressure under different initial water level fluctuations. (3) The deformation and collapse difficulty of the soil cave negatively correlates with the initial water level. The higher the initial water level, the stronger the negative pressure absorption and positive pressure gas explosion effect on the overburden on the soil cave and the collapse will occur with fewer water level fluctuations. (4) The numerical simulation results are consistent with the laboratory model test. These laws elucidate the change of water-gas pressure in different initial water level fluctuations and reveal the collapse mechanism of water level fluctuations. It will provide essential theoretical support for further research on the action law of hydrodynamic factors on karst collapse and a scientific and reasonable basis for urban groundwater pumping control, karst and roadbed collapse prevention and prediction.

Curing agent is often used in the in situ remediation of industrial contaminated sites. The reaction between curing agent and contaminated soil has a great impact on the conductivity of soil. Quantitative analysis of the resistivity evolution of cured contaminated soil is an important prerequisite for the accurate evaluation of the in situ remediation effect of heavy metal contaminated sites. In this paper, the pH value, unconfined compressive strength, and resistivity evolution of Zn-contaminated red clay cured with new phosphate-based binder (KMP) at different curing ages were tested, and the internal relationship between the KMP content and the metal ion content and the pH value, unconfined compressive strength, and resistivity evolution were analyzed. The curing effect of KMP, Zn2+ concentration, and curing age on contaminated soil was further evaluated. The results show that the unconfined compressive strength of red clay is weakened with the increase of Zn2+ concentration. The curing effect of KMP on zinc-contaminated red clay is obvious. The pH control of KMP curing agent on zinc-contaminated red clay is effective, and the pH value within 10.5 can effectively stabilize metal ion leaching. There is a good linear relationship between KMP content and curing age and resistivity. Under uniaxial compression, the resistivity decreases with the increase of stress at different Zn2+ concentrations, and the minimum value of the resistivity corresponds to the peak value of uniaxial compression. It provides possibility for the application of resistivity method in the study of mechanical strength properties of red clay.