Standard Soil

• The type of soil and the moisture content determine what percentage of the soil is voids or air pockets. Tilled farm fields or gardens typically include about 25 percent air, 25 percent moisture and 50 percent soil particles. Clay has a lower percentage of voids than sand, because of the size of the particles. The smaller particles of the clay fit together closer than the larger particles of sand, for example. The relative compaction of soils containing organic material changes as the organic matter decays and disappears from the soil.
  
  

Testing

• The Standard Proctor test determines the best amount of soil moisture to achieve the maximum compaction. Dry a soil sample in an oven and add water equivalent to 14 percent of the weight of the soil sample. Compress the soil into a mold using a hammer. Weigh the soil sample to determine how much soil can be compressed into the mold. Perform the test at various moisture levels to determine what level of moisture allows the best compaction of the soil. The Standard Proctor test is commonly performed by an engineering lab.
  
  

Reaching Maximum Compaction

• Maximum compaction is not commonly achieved in field circumstances. Applying and maintaining the pressure needed to achieve maximum compaction is not practical for a road bed or building foundation.
  
  

Construction Standards

• Construction specifications commonly list a minimum level of compaction. For many road or street projects, the minimum level of compaction is 95 percent. Engineers take samples and monitor the project to confirm the materials are sufficiently compacted.