What is a ballast shell: distinguishing features

On temporary railway tracks, in the absence of large and medium -grained sand, it is allowed to use fine -grained sand with a content of more than 90% of grains larger than 0.1 mm for ballasting, including at least 50% of grains larger than 0.25 mm. The ballast shell used on the railway construction of the south exceeds the sand in its qualities. The shell may contain an admixture of clean sand. The particle content of less than 0.1 mm is allowed in a shell no more than 10 %, including clay not more than 1 % by weight.