If only fine sand or stone powder is used (the part with a particle size less than 0.075 mm does not exceed 90% of the total weight), gravel or pebbles with a total weight of not less than 30% shall be evenly mixed. The maximum particle size of sandstone should not be greater than 50 mm.
Extended data
The main function of sand cushion:
1, improve the bearing capacity of shallow foundation below the foundation bottom. The shear failure in the foundation starts from the bottom corner of the foundation and gradually develops in depth with the increase of the foundation pressure. Therefore, the bearing capacity can be improved by replacing the soft soil that may be sheared in the shallow layer below the basement surface with the cushion material with greater strength.
2. Reduce settlement. Generally speaking, the shallow settlement under the foundation accounts for a large proportion. Theoretically, the settlement caused by lateral deformation of soil is also a shallow part, accounting for a large proportion. Using cushion material instead of soft soil layer can greatly reduce the settlement of this part;
3、? Accelerate foundation drainage consolidation. Using sand and gravel as cushion material, because of its large permeable layer, is a good drainage surface after foundation compression, which can quickly dissipate the pore water pressure under the foundation, avoid the plastic damage of foundation soil, accelerate the consolidation of soft soil under the cushion and improve its strength.
Application scope of sand and gravel cushion: it is suitable for the treatment of soft and permeable cohesive soil layer within 3m; The thickness of cushion is generally 0.5 ~ 2.5m If it exceeds 3m, it will consume labor and materials, which is difficult to construct and has high economic cost. Not less than 0.5m.
Baidu encyclopedia-gravel cushion