modified proctor density

huangyhg

modified proctor density
in soil specifications, i frequently see the following:
backfilled soil, soil under concrete slabs and structural soils and fills to be "compacted to 98% modified proctor density".
what is "modified proctor density"?
why 98%?
thank you!
proctor density is a density of a sample of soil after it has been compacted in a laboratory.
the astm designation for it (d1557 i think) calls for a soil sample to be sequentially placed in a small steel form (like a can with an open top) in layers.  a set hammer size is dropped from a set height for a set number of times for each layer.  after the "can" is full with the compacted soil, the soil is weighed and with the can volume a dry density is calculated.
this process is repeated at various moisture contents and the density is plotted on a graph (vertical axis = density and horiz. axis = moisture content).  the curve generally follows an upward hump with a peak point at a certain density and moisture.  this is termed the max. density or proctor density and it is associated with the optimum moisture content.
the standard proctor (astm d698)was developed some years ago and the hammer/height/drops was put together, supposedly, to mimic current excavation equipment at the time.
the modified proctor was developed (in the 1960's i think) with a heavier hammer/ higher drop, etc. to mimic the heavier machinery that had been developed.  
the % of proctor then is a way to specify and test how compact a material is.  the soil is tested in a lab, the proctor density is determined (say its 100 pcf) and in the field the contractor may be required to get to 95% of the proctor or 0.95 x 100 = 95 pcf.  this can be tested in the field.

the optimum moisture content is known so if the soil is too dry or too wet, it will be more difficult to get to the max. density.  so steps can be taken to dry out or wet the soil to help in the compaction efforts.
here is a one picture of jae's compactor.
the crank lifts the weight to a predetermined height, by means of a quarter circle, and let it drop. different test require different rotations.  to change the force on a sample you change the size.
you can take the sample and run all manner of test, compressive strength, shear strength, permeability, et all.
in australia we refer to it as maximum modified dry density, or mmdd for short.  specified values for mmdd range from 92% to 98% depending on the application.
this topic has been discussed in a number of other forum past threads.  if you do a search on "proctor" or "density" i think a lot of good discussions will pop out.