The article discusses a new mathematical method for comparing the consistency of two particle size distribution curves. The proposed method was based on the concept of the distance between two graining curves. In order to investigate whether the distances between the particle size distribution curves are statistically significant, it was proposed to use the statistical test modulus-chi. As an example, the compliance of three sieve curves taken from the earth dam in Pieczyska on the Brda River in Poland was examined. In this way, it was established from which point of the dam the soil was washed away. However, it should be remembered that the size of the soil grains built into the dam does not have to be identical to the grain size of the washed out soil, because the fine fractions will be washed away first, while the larger ones may remain in the body of the earth structure.