3.5-4.4m According to the website of Chengdu Municipal People's Government, the thickness of underground gravel in the main urban area of Chengdu is 3.5-4.4m.. Sand refers to a loose mixture of sand. Geologically, minerals or rock particles with a particle size of 0.074~2mm are called sand, and those with a particle size larger than 2mm are called gravel or breccia (the difference between them lies in their smoothness, see the entry conglomerate and sandstone for details).