In structural design, the concrete or brick structure can't be in direct contact with the soil. The main reason is that there is always the possibility of cement slurry loss for materials with cement as raw material. When cement slurry is lost to the gravel cushion, because the strength of gravel itself is higher than that of soil, the strength will not decrease when it meets water, but will only increase when it meets cement slurry.

The difference between foundation cushion and foundation is that the foundation cushion is set below the foundation, and the top of the cushion is in contact with the bottom of the foundation.

Foundations are divided into brick foundations, concrete foundations and reinforced concrete foundations according to materials.

According to the shape, strip foundation, independent foundation, raft foundation, etc.

But the soil is not like this. When it meets water, it will liquefy and its strength will decrease. If the strength is reduced, the newly poured concrete will be deformed before hardening. Therefore, it is better to use gravel as cushion than to use soil as cushion directly. Similarly, cement mortar is used to build bricks, just like concrete.