## 6 Conclusions  
Slurry penetration is an essential stage when used for balancing the earth and water pressure in underground and geotechnical engineering. In this study, experiments including slurry property tests and electrical resistivity of soil samples with different amounts of bentonite slurry were conducted to investigate the effects of the water environment, CMC additive, soil particle gradation, and electrode distance. The empirical formulas between electrical resistivity of slurry-soil mixtures and bentonite slurry concentrations were established. The main conclusions of this study are as follows:  
1. The slurry penetration observably increased the number of ions in saturated soil and decreased the electrical resistivity of soil. The relation between soil electrical  
Figure 15: Resistance of sand samples (0.5–2 mm) with electrode distance.  
resistivity and amount of slurry penetration can be expressed by the power function of slurry concentration. Additionally, the soil porosity and particle gradation also exerts influences on the soil electrical resistivity.
* Seawater and CMC decreased the electrical resistivity of bentonite slurry and slurry-soil mixture. Since the amount of free ions in seawater is much greater than that in slurry, the effect of bentonite slurry on the soil electrical resistivity can be ignored. Besides, the bentonite particles flocculate into aggregates rapidly in a seawater environment, which further promotes the decrease in soil resistivity. Therefore, the method of electrical resistivity measurement in this paper is not applicable in the offshore engineering.
* The magnitude of soil resistivity is a result of competition between the soil formation factor and electrolytes. Engineers cannot ignore the slurry-induced changes of soil structure when assessing the slurry penetration in practical engineering, otherwise the magnitude as well as the migration distance of the slurry will be overestimated.
* Contact resistance between electrode slices and soil samples decreased with the increase in slurry penetration. The variance of the contact resistances may bring about a strong deviation in the soil electrical resistivity. In order to obtain a stable and reliable value of soil electrical resistivity, multiple tests with different distances are recommended and the soil electrical resistivity can be determined by calculating the slope of fitting curves.  
In the actual project, formation is always heterogeneous and changeful. More studies concentrating on the electrical resistivity character of soil before and after slurry penetration are still necessary. The electric method applied on the excavation face of the slurry balance shield is a less-studied topic worldwide, but future studies that consider the electrical resistivity character of a soil-slurry mixture could provide knowledge in this area.  
Conceptualization, J.M.; methodology, B.W.; validation, D.Y., and D.J.; investigation, S.W.; resources, D.Y.; data curation, S.W.; writing--original draft preparation, J.M.; writing--review and editing, D.J.; visualization, J.M.; supervision, D.Y.; project administration, D.Y.; funding acquisition, D.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Joint Funds of the National Natural Science Foundation of China, grant number U1834208.  
Not applicable.  
This study was supported by the Joint Funds of the National Natural Science Foundation of China (Grant No. U1834208). Their support is gratefully acknowledged.  
The authors declare no conflict of interest.