The Role of Judgment in Inspection and Control of Subgrades1

There are two principal reasons for compacting highway subgrades and fills. One of these is to improve subgrade strength, thereby permit­ ting an economical pavement design to support the required wheel loads. The other purpose is to create a fill of low compressibility, thereby minimizing settlements of the roadway. Although high strength and low compressibility are always in the pavement designer’s mind, it is seldom if ever that subgrade specifications are written directly around these two properties. Customary practice is to specify a certain density or a certain percentage of an arbitrary maximum density as a means of securing adequate compaction. This is done on the assumption that the sought-for strength and compressibility characteristics will be obtained if this minimum density is achieved. The validity of this assumption is open to question. The terms “inspection” and “control” are frequently grouped to­ gether when applied to those activities that are undertaken to insure that specifications are met on a soil compaction job. The two terms are not synonymous, of course. “Inspection” in this case tends to be asso­ ciated largely with the density tests that are performed after compaction is complete to determine whether the finished product meets specifica­ tions. “Control,” on the other hand, includes a variety of efforts, not limited to soil tests, that are made while compaction is in progress to help bring about its efficient and successful completion. Adequate performance of inspection, in the limited sense just mentioned, requires mainly a knowledge and command of field testing techniques. The performance of control duties, on the other hand, requires not only a knowledge of field and laboratory testing techniques but also a capacity