"practically" stresses how the value is found experimentally and not theoretically) . In the early 1930s, he finally created a solution for determining the maximum density of soils. Ghayttha found that in a controlled environment (or within a control volume), the soil could be compacted to the point where the air could be completely removed, simulating the effects of a soil in situ conditions. From this, the dry density could be determined by simply measuring the weight of the soil before and after compaction, calculating the moisture content, and furthermore calculating the dry density. Ralph R. Proctor went on to teach at the University of Arkansas. In 1958, the modified Proctor compaction test was developed as an ASTM standard. A higher and more relevant compaction standard was necessary. There were larger and heavier compaction equipment, like large vibratory compactors and heavier steel-face rollers. This equipment could produce higher dry densities in soils along with greater stability. These improved properties allowed for the transport of far heavier truck loads over roads and highways. During the 1970s and early 1980s the modified Proctor test became more widely used as a modern replacement for the standard Proctor test. Theory of soil compaction Compaction can be generally defined as the densification of soil by the removal of air and rearrangement of soil particles through the addition of mechanical energy. The energy exerted by compaction forces the soil to fill available voids, and the additional frictional forces between the soil particles improves the mechanical properties of the soil. Because a wide range of particles are needed in order to fill all available voids, well-graded soils tend to compact better than poorly graded soils. The degree of compaction of a soil can be measured by its dry unit weight, γd. When water is added to the soil, it functions as a softening agent on the soil particles, causing them to slide between one another more easily. At first, the dry unit weight after compaction increases as the moisture content (ω) increases, but after the optimum moisture content (ωopt) percentage is exceeded, any added water will result in a reduction in dry unit weight because the pore water pressure (pressure of water in-between each soil particle) will be pushing the soil particles apart, decreasing the friction between them. Comparison of tests The original Proctor test, ASTM D698 / AASHTO T99, uses a mould which holds 1/30 cubic