It's not the type of project, but the type of soil and the application.
The standard Proctor was developed in the 1920's by R.R. Proctor. At that time, the compaction energy of the test was comparable to the capabilities of the compaction equipment that was available....mostly lighter static compaction. The test was used for most soil types from clayey sands to silty sands to clean sands. It was also used for graded aggregate base materials as long as the larger particle sizes were no more than about 3/4".
As equipment became better and compaction techniques became better, the compaction equipment was imparting more energy to the soil and getting higher compaction than the lab produced. Thus came the "modified" Proctor with higher input compaction energy to more closely match the better equipment.
The moisture density relationship (Proctor) can be used on a variety of soils and has applicability to most soils that would be acceptable for use under buildings or roadways. There are some exceptions where the Proctor does not give adequate information to use as a comparison test. This usually occurs in some clean, fine sands where the Proctor curve is too flat to have much relevance.
Unless you have clayey sands, clay or very silty sands, the modified Proctor is usually appropriate. It is appropriate for those soils as well; however, they are much more difficult to achieve proper compaction with the modified Proctor than the standard Proctor.
For what to use . . . . I typically (not always) use them as follows:
Standard Proctor:
Core and cohesive shells of dams
Landscaping
Subgrade and select subgrade of soils for roadways
engineered backfill not being used to support structures
engineered upfill (raising grade)not being used to support structures
Modified Proctor (also called the "hernia" test)
Engineeerd fill that will be used to support structures (under mat or spread foundations)
Roadway subbase and base course
I've had arguments before with junior engineers writing reports that I reviewed who used standard proctor and I, due to reasons, above insisted that modified be used. They wanted 100% standard proctor maximum dry density and I was willing to go 97% modified Proctor dry density. In our region they were about the equivalent with the fills we were using (not always the case though) - why did I want modified? I figured that when contractors saw the requirement "modified" they took it more seriously than they would if they saw "standard" - "heck you can do standard without any effort".
The big question though is what level of compaction you would use . . . For foundation support I would not use less than 98% modified and if footing 100%. For roads, you can look up your state's or province's road design manual. For earth core dams it is typically 95% standard. Landscaping areas I wouldn't go anything over 90 to 93% standard.
Also note that the Standard optimum moisture content will be higher than the Modified optimum moisture content. Higher moisture is desirable in some situations since it tends to decrease permeability and produce a more ductile fill. This is why it is often used for dams as BigH points out.
I consider them both equally approximate. I think it's misguided to think optimum moisture content is marked by a vertical line. For dams, hydraulic barriers and the like, I specify percent saturation, which is directly obtaied from the moisture-density relations.
I agree that airstrips and prominent structures are typically related to modifled proctor. That's because folks don't like specifying 100 percent of anything, so they go with 95 percent modified, which is (really) the same thing as 100 percent standard, notwithstanding the optimum moisture content matter. . .
You'll find that in the United States, the conventions are often regional.
