TBell47 - both D698 (Standard Proctor) and D1556 (Modified Proctor) are tests to determine a maximum dry density and optimum moisture contents.  Each test, though, uses a differnt energy in compacting the specimens in the mould.  Neither specifies 95%, or 98% or . . .  That is the specification to choose.  For instance, in road constrution, you might compact the embankment to 93% maximum dry density D-1556, the subgrade to 95% maximum dry density D-1556, the subbase and base courses to 98% maximum dry density.  For clayey fills consisting of expansive soils, you might actually specify 85% maximum dry density (MDD).  So the end result in specifying the desired relative compaction (the % of MDD)depends on your use, etc.  As for the selection of D698 or D1556 - this many times is a matter of choice.  Typically for granular fills I would use, 97% MDD D1556 = 100% MDD D698.  I will typically specify D698 for areal fills but D1556 for structural fills (say to support a footing).  Others, such as Focht3 might say differently due to norms in his area of work.  Please check out the many many many threads in the geotechnical or civil areas for, at times, heated discussions on this topic.  Do a keyword search.

BigH,

Thank you. Much clearer now.
I see I typo'd D-1556 in the body of the question.
Sorry for the delay in retrieving your response.
My computer at the office is being changed out for a newer model.

Many Thanks,
Thomas
TBell47

For anyone viewing this thread, remember that the difference between the testing methods is energy dependent. Very often it will be necessary to specify a standard proctor energy level reference wihin a large structural fill due to clearance issues (such as backfill around pipes or under haunches) and the use of smaller equipment.

TBell47...one quick clarification....it is ASTM D1557 on the modified proctor.

BigH has given a good practical approach to deciding applicability that is consistent with many Departments of Transportation requirements...embankment at 95% of standard proctor (ASTM D698, AASHTO T99), subgrade at 95% modified proctor (ASTM D1557, AASHTO T180) and stabilization/base materials at 98% modified.

The relationship between the maximum dry density as determined by the standard Proctor or the modified Proctor varies with material.  The selection of the method to use depends on the materials (generally, granular materials use the Modified Proctor), as well as the expected capability of the compaction equipment (based on the location, application, and accessibility of equipment).

Monahan (from NJIT) wrote a nice book on compaction - he explained, if I remember right, the modified proctor as the hernia test.  To add to Ron's comments, I would always specify modified under a foundation or when I wanted to ensure that the contractor took the compaction requirements seriously.  Standard is "too easy" to achieve that it may sometimes be taken as trivial.

Just 1 more thing to remember BigH. One difference that is often overlooked is the amount of moisture each method allows. That is a Modified Proctor will not allow as much moisture as using a Standard Proctor on the same material. Also a 95% Modified is close to being the same as 98% Standard.