According to the "Standard for Basic Terms of Geotechnical Engineering" (GB/T50279-98), the degree of saturation is defined as the ratio of the volume of pore water to the volume of pores in soil, expressing the extent to which water fills the voids; it can be calculated from the soil density, specific gravity of soil particles, and water content of the soil.

The strength indices of cohesive soil that we generally use are all obtained under saturated soil conditions, so whether the soil is saturated and what the degree of saturation is become particularly important.

For sand and silt, sampling is difficult; during sampling, due to disturbance and water separation, density and water content are hard to measure accurately. Whether they are saturated (which involves liquefaction assessment) has a clear boundary above and below the groundwater level, and is easy for survey personnel to determine. However, for cohesive soil, soil below the groundwater level is not necessarily saturated, because soil consists of three phases; coarse-grained soil has somewhat better permeability, and the air in the soil escapes relatively easily, whereas cohesive soil, due to its structure, causes part of the air to be trapped in the voids.

From the conversion formula for the degree of saturation: Sr=W*ds/e In actual experiments, when measuring water content, the oven-drying method is commonly used, which will drive off part of the weakly bound water, causing the measured W value to be too large, and thus the calculated degree of saturation to be too large. Sometimes the calculated degree of saturation is greater than 1, and this may be one of the factors involved.

In the registered examination, questions about saturated soil are often encountered; the degree of saturation is not given, and only a statement is made about "below the groundwater level" or "saturated soil," which raises many questions about what value to take for the degree of saturation.

The "Standard for Geotechnical Testing Methods" GB/T50123-1999, in the section "Specimen Preparation and Saturation," clearly gives the saturation criterion as [degree of saturation not less than 95%] (for soil with particle size less than 60mm).In some logging tutorials, sandy soil is classified according to the degree of saturation Sr: slightly moist (Sr≤50%), moist (50%<Sr≤80%), saturated (Sr>80%); however, this is not advocated by the geotechnical investigation codes. Precisely because of the difficulty in determining the degree of saturation, the codes use moisture content and liquidity index to express the moisture state of silt and cohesive soil, respectively. My understanding is that saturated soil is soil in which the pores within the soil body are basically filled with water.Considering practical application, the boundary for whether coarse-grained soil is saturated is the groundwater level; the boundary for whether cohesive soil is saturated is the capillary saturation zone; for soil under these two conditions, the degree of saturation is uniformly taken as 100%.