Standard Proctor, ASTM D698
ASTM D698-12(2021) uses:
- a 5.50 lbf (24.5 N) rammer
- dropped 12.0 in. (305 mm)
- a compactive effort of 12,400 ft-lbf/ft³ (600 kN-m/m³)
Modified Proctor, ASTM D1557
ASTM D1557-12(2021) uses:
- a 10.00 lbf (44.48 N) rammer
- dropped 18.00 in. (457.2 mm)
- a compactive effort of 56,000 ft-lbf/ft³ (2,700 kN-m/m³)
Both use a 4 in. or 6 in. mold. Modified is the heavier rammer, the higher drop and the larger effort on every figure.
The trap is swapping figures between the two tests, the listed common error. A 5.50 lbf rammer with an 18.00 in. drop, or a 10.00 lbf rammer with a 12.0 in. drop, belongs to neither. Keep the units straight too: 2,700 and 600 are the metric efforts in kN-m/m³, not ft-lbf/ft³.
What the compaction curve gives you
Both standards find the relationship between molding water content and dry unit weight. Plotting the results gives the compaction curve, which yields the maximum dry unit weight and the optimum water content.
The trap: the curve is built on dry unit weight, not wet, and it peaks at an optimum water content, not a maximum. Unconfined compressive strength is the Appendix A measure for classing soil as Type A, B or C, and maximum allowable slope and bench height come from Appendix B of the OSHA excavation standard. Neither is a Proctor result.
How much more effort is Modified?
Divide the published compactive efforts:
- 56,000 ÷ 12,400 = 4.52, about 4.5 times
The traps come from comparing only part of the test:
- 1.5 times is only the drop-height ratio, 18.00 ÷ 12.0
- 1.8 times is only the rammer-weight ratio, 10.00 ÷ 5.50
- 2.7 times is weight × drop, (10.00 × 18.00) ÷ (5.50 × 12.0) = 180 ÷ 66 = 2.73, which is not the ratio of the published compactive efforts
Use the published efforts, 56,000 and 12,400 ft-lbf/ft³, whenever the question asks how many times one effort is the other.