## 1 Introduction  
Bentonite slurry has been widely employed in underground and geotechnical engineering such as slurry pressure balance (SPB) shield tunneling, horizon directional drilling (HDD), diaphragm wall, and cast-in-place pile, etc. [1, 2, 3]. Fresh slurry is pumped to the front of the work face and the muck is taken out by slurry. Additionally, the bentonite slurry can serve as the medium to support the side wall of the bore hole [4]. The effect of support action depends on the formation and quality of the filter cake [5]. The process of slurry penetration has attracted extensive attention because of its great importance in the formation of filter calves [6, 7]. However, the observation of slurry penetration is very challenging during the soil-slurry interaction.  
Because of the advantages of non-invasion, high-efficiency, and low cost, the electrical characterization of soil water content [8, 9], underground solute transport [8, 10, 11], and soil liquefaction [12], etc. has been given more and more attention. For example, the electrical resistivity is generally used to characterize the degree of saturation, pollutant concentration, and porosity of soil. [PERSON] and [PERSON] [13] observed that the electrical resistivity and salinity for the soil water content ranging from saturation to \(-3\) kPa water potential had a linear relationship between each other. [PERSON] and [PERSON] [14] estimated the volumetric water content through pore solution resistivity, indicating that the electrical resistivity decreases when the water conductivity increases with a given water content. Additionally, different ions present in the solution (H\({}^{+}\), OH\({}^{-}\), SO\({}_{4}^{2-}\), Na\({}^{+}\), Cl\({}^{-}\), etc.)affect the conductivity because of differences in ion mobility. This explains why soil solutions at the same concentration, but exhibiting different ionic compositions, may have different electrical conductivities. Other researchers have also studied salt water intrusion in coastal areas by measuring the electrical resistivity [15; 16; 17]. The same principles apply to slurry penetration into sand. The value of soil electrical resistivity as an indicator can reflect the state of slurry penetration at some point during the soil-slurry interaction. However, none of the literature has reported on the electrical resistivity characteristics of soil-slurry mixtures, which are important for the quantitative observation of slurry penetration. The penetration velocity of slurry in sand is far slower than the electric current. Therefore, a group of static electrical resistivity of soil-slurry mixture can replace the electrical resistances of sand at a certain moment during slurry penetration. The relationship can be established between electrical resistances and slurry concentration in a given soil. We can learn the slurry concentration within soil pores indirectly by this relationship during further slurry penetration experiments.  
This paper aims to characterize the soil-slurry mixtures using the electrical resistivity method. The influences of water environment, CMC (carboxymethyl cellulose) additive, soil gradation, and electrode distance on electrical resistivity characteristics of soil samples with different slurry concentrations were obtained. The results can provide significant references for the application of the electrical resistivity method in the research of slurry penetration.