## 4 Conclusions  
We carried out physical mechanics and microscopic experimental tests on heavy metal-contaminated soil, explored the microstructural change mechanism of heavy metal-contaminated soil, and studied the change rule of engineering characteristics related to heavy metal-contaminated soil with different pollution types and different pollution concentrations after the invasion of heavy metal ions. The relevant results are as follows:  
1. With the increase in heavy metal concentration in contaminated soil, the properties of several soil samples changed. The increase in heavy metal ion concentration leads to the decrease in liquid limit, plastic index, and clay content. This indicates that, to a certain extent, the soil has lost some of its original physical properties, such as fluidity and plasticity, and the content of fine particles in the soil is also reduced.
2. According to the results of the resistivity test, it can be observed that the resistivity of heavy metal-contaminated soil shows a complex relationship with water content, porosity, and heavy metal concentration. With the increase in soil moisture content, the resistivity tends to decrease, because the addition of water changes the conductivity characteristics of the soil and makes the resistivity gradually stable. As the porosity increases, the resistivity tends to rise, because more pore space leads to a lengthening of the conductive path, thereby increasing the resistance. With the increase in heavy metal ion concentration, the resistivity drops sharply because heavy metal ions display electrical conductivity in soil.
3. Before and after soil is polluted by heavy metals, its pores, particles, and colloidal morphology will change. The fundamental reason for the change of permeability coefficient before and after pollution is that metal ions change the internal structure of soil, especially the size of pores.  
After the invasion of heavy metal ions into soil, a complex process of physical, chemical, and biological reactions occurs. The results of this study can provide a theoretical basis for  
Figure 13: Experimental results of microscopic characteristics of Zn- and Pb-contaminated soils with different concentrations.  
basic research on heavy metal-contaminated soil. Research focusing on the engineering properties of heavy metal-contaminated soil is still in the preliminary stages. A stable and mature theory on the soil joint action of the engineering properties of the soil, from a single-heavy-metal ion pollution to a variety of heavy metal ions, is yet to be formed and need to be further studied.  
Data curation, G.Z., T.L., H.L. and X.Y.; Formal analysis, G.Z., Z.W. and D.Y.; Funding acquisition, T.L. and Z.W.; Investigation, G.Z. and Z.W.; Methodology, X.H.; Project administration, T.L. and X.H.; Resources, T.L.; Supervision, X.H.; Validation, D.Y.; Visualization, T.L., H.L. and X.Y.; Writing--original draft, G.Z.; Writing--review and editing, T.L. and X.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China: U2006213; National Natural Science Foundation of China: 42277139.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the confidentiality of the subject research.  
The authors declare no conflicts of interest.