Proctor density is the maximum dry density of soil determined in a laboratory test at optimal water content. It serves as a reference value for compaction control on the construction site: the required degree of compaction (e.g., 97% or 100%) is referenced to the Proctor density. A compaction degree of 97% means that the dry density achieved on-site is 97% of the Proctor density.
The Proctor test according to DIN 18127 works as follows: soil is placed in a cylinder and compacted with a standardized falling weight (2.5 kg, drop height 30 cm). By repeating with different water contents, a bell curve is obtained — its maximum is the Proctor density, with the corresponding water content being the optimal water content.
For the construction equipment operator, Proctor density is indirectly relevant: it determines how many roller passes are needed and at what soil water content compaction should be performed. Soil that is too dry cannot be compacted sufficiently, soil that is too wet becomes softened rather than compacted. The plate bearing test and the light falling weight test verify achievement of the target on-site.