Silt three grades fine, medium and coarse 0.002 mm to 0.006 mm to 0.020 mm to 0.063 mm). In actuality, silt is chemically distinct from clay, and unlike clay, grains of silt are approximately the same size in all dimensions; furthermore, their size ranges overlap. Clays are formed from thin plate-shaped particles held together by electrostatic forces, so present a cohesion. Pure silts are not cohesive. According to the U.S. Department of Agriculture Soil Texture Classification system, the sand–silt distinction is made at the 0.05 mm particle size. The USDA system has been adopted by the Food and Agriculture Organization (FAO).
Internal erosion a larger cavity. The process continues until the formation of a sinkhole. It is possible for a cavity to not collapse; this will lead to backward erosion occurring. Soil contact erosion can occur between any granular layer and a finer soil such as in silt-gravel, and often results in a loss of stability, increases in pore pressure and clogging of the permeable layer. Experimental results show that close to the geometrical limit, the point at which the fine particles can "just pass" between the coarse particles(the filter criterion), erosion initiation and failure are much more likely. It is possible to