The physical and mechanical properties of rocks can be reduced significantly by an acidic environment, resulting in engineering weaknesses, such as building foundation instability, landslides, etc. In order to investigate the mechanical properties of rocks after hydrochemical erosion, a chemical damage constitutive model was established and used to analyze chemical damage variables and energy transformation. It is assumed that the strength of the rock elements obeyed Weibull distribution, considering the nonuniformity of rock. The chemical damage variable was proposed according to the load-bearing volume changes in the rock under water–rock chemical interactions. The chemical damage constitutive model was derived from coupling the mechanical damage under the external load and the chemical damage under hydrochemical erosion. In order to verify the accuracy of the model, semi-immersion experiments and uniaxial compression experiments of black sandy dolomite were carried out with different iron ion concentrations. Compared with the experimental data, the chemical damage constitutive model proposed could predict the stress–strain relationship reasonably well after water–rock interaction. The effects of water–rock interaction on the rock were a decrease in peak stress and an increase in peak strain. The peak strain increased by 4.96–29.58%, and the deterioration rate of peak strength was 0.19–4.18%. The energy transformation of the deterioration process was analyzed, and the results showed that the decrease in releasable elastic energy, U_e, is converted into dissipated energy, U_d, after hydrochemical erosion.

The research on the deformation of soil mass and ground connection walls is not sufficiently thorough due to the huge risk of deep excavation in soft soil areas. In this paper, finite element software is used to numerically simulate a symmetrical foundation pit in Suzhou, and the reliability is verified by on-site measured data. The purpose of this study is to investigate the deformation mechanism of the enclosure structure and surrounding soil during the excavation of soft soil foundation pits, and to carry out sensitivity analysis. The results show that the maximum subsidence of the surface is 21.25 mm, the maximum horizontal displacement of the underground diaphragm wall is 9.45 mm, and the maximum uplift of the pit bottom is 21.46 mm. By changing the soil layer properties (the elastic modulus, cohesion, and internal friction angle) and the insertion ratio of the diaphragm wall, the maximum horizontal displacement of the diaphragm wall is more easily affected than the maximum settlement of the surface. Based on different research results, the maximum land subsidence and maximum horizontal displacement of the support structure are 0.313–0.060% and 0.070–0.250% of the maximum excavation depth, respectively. Finally, the simulation may have some applicability to other foundation pit excavations.

Foundation pit excavation is common in urban construction, while safety evaluation is always significant in every specified project. The soil material properties, groundwater level, excavation method, supporting structure, monitoring points’ arrangement, and so on distinguish from one site from another. Thus, many studies have looked into the safety and reliability of designated projects. This paper was based on the co-construction underground tunnel project of a deep foundation pit excavation in Suzhou, China. This paper aimed to perform a safety evaluation on this foundation pit by means of numerical simulation for parameter influence analysis, as well as scientific comparison with in-site monitoring data. To minimize the energy consumption and contribute to the carbon neutrality, a brief energy consumption analysis was also conducted. The results indicated that the maximum deformation of the foundation pit bottom is 4.5 cm and the deformation of the foundation pit is within the allowable range. The maximum horizontal displacement of each excavation is approximately at 10 m to 12 m of the diaphragm wall and the largest deformation is 28 mm. The maximum ground settlement is less than 16 mm, which confirmed the safety during excavation. It is ideal that the above deformation law will provide a reference for similar projects. Furthermore, this research simulated and monitored the whole cycle of foundation pit excavation, and contributes to savings in energy consumption and limiting of carbon emissions.

In geological engineering, a series of safety problems caused by expansive mudstone are common, such as slope instability and roadbed up-arch. In this paper, the mineral composition of mudstones in the Xining area was analyzed by X-ray diffraction (XRD), and the microstructural and morphological changes of mudstones after water absorption were observed by scanning electron microscopy (SEM) test to analyze the internal factors and microstructural evolution patterns of water absorption and swelling of mudstones. Based on the microstructural units, the mudstones were defined into two categories, one is N-type mudstone with flat sheet-like stromatolite units, and the other is SN-type mudstone with more clastic particle units. Water absorption experiments were conducted on the rock samples to study the microstructure of these two types of mudstones under different water absorption conditions. The pore characteristics of the mudstones were analyzed by using Image-Pro Plus to reveal the water absorption mechanism. The results show that the pore area of N-type mudstone is smaller, as well as the distribution of pore diameter. The pore area of N-type mudstone develops rapidly, in the early stage of water absorption, lots of pores are produced, and the pore area of SN-type mudstone shows an overall decreasing trend. The pore area and the number of SN-type mudstones are at a low level after full water absorption. Under the condition of full immersion, water enters the pores rapidly and soluble salts are dissolved in large quantities. The change of water absorption rate of mudstone with time can be divided into the stage of sudden increase, decrease and stability of water absorption rate. Then, based on the stress theory, the relationship between the macroscopic expansion process and the microstructure of mudstone was analyzed. Finally, the energy basis of mudstone water absorption is discussed. In the swelling of mudstone, the energy gradually turns into swelling strain energy.

Rural domestic sewage has become very serious, while only 17.4% of it is treated centrally or partially, which results in environmental pollution and waste of water resources. To solve this problem, a wastewater treatment system by modular coupling is proposed, and the processing module includes two or three of multi-stage ecological pond, multi-stage drop aeration ecological ditch and multi-stage drop dam. The analysis of three examples shows that the system has a good treatment effect on chemical oxygen demand (COD), total nitrogen (TN), ammonium nitrogen (NH4 +-N) and total phosphorus (TP), and the removal ratios are 90.22%, 82.07%, 91.60%, 81.48%, respectively, when the flow rate is 3 m3/h. After 28 days of continuous operation, the treatment capacity decreases by about 10%, indicating that the system has good long-term treatment performance. The removal ratio decreases slightly under different flow rates, which shows its great adaptability in different hydraulic conditions. In practical application, the system can select different coupling systems according to rural terrain and wastewater quality, and it is suitable for the treatment of rural domestic sewage and agricultural non-point source sewage and the repairment of urban sewage and lake eutrophic water body.