During compaction, the moisture content of the soil or material plays a very large role in the degree of compaction that can be achieved.

The work of hammering or rolling needs to overcome the internal frictional resistance and cohesive force between soil particles so that the soil particles can be displaced and approached each other. The internal friction resistance and cohesive force of soil increase with the density.

When the water content of the soil is small, the internal friction resistance between the soil particles is large. After compaction to a certain extent, a certain compaction work can no longer overcome the resistance of the soil, and the dry density obtained by compaction is small.

When the water content of the soil increases gradually, the water plays a lubricating role among the soil particles and reduces the internal frictional resistance of the soil. Therefore, a larger dry density can be obtained for the same compaction work. During this process, the volume of air per unit of soil gradually decreases, while the volume of solids and water gradually increases. When the soil moisture content continues to increase beyond a certain limit. Although the internal frictional resistance of soil is still decreasing, the volume of air per unit of soil has been reduced to a minimum, while the volume of water is increasing. Because water is incompressible. Therefore, under the same compaction work, the dry density of soil gradually decreases.

Such a close relationship between soil dry density and water content forms a hump-shaped compaction curve. therefore. Fine-grained seven and various pavement materials such as natural gravel soil, laterite gravel, graded crushed stone, graded gravel, lime stabilized soil and cement stabilized soil can only be compacted to the maximum dry density under certain water content conditions.

It is difficult to achieve a high degree of compaction by using a certain type of road roller to compact soil or graded aggregates with too low water content at the application site; Compaction is equally difficult. Therefore, in the case of special arid and special wet areas, when it is impossible or impossible to take appropriate measures, the requirements for compaction degree must not be reduced in actual construction.