8/6/2019 The Proctor Test Can Be Understood Easily

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The Proctor test can be understood easily. A soil sample gets taken in the field and dried in the lab. The dry

soil then gets divided into different piles, with each pile getting a different amount of water added. So at this

point, the lab technician has several soil samples at different moisture levels (say, 12%, 14%, 16%, 18%,

20%).

Each of these soil samples (for a standard Proctor test) is then pounded with a 5.5 pound weight dropping

on it 20 times from a height of 12 inches. Of course, the dropping of the weight compacts the soil. To

compact soil means to make it more dense. Density is defined as the weight divided by the volume. So each

of these compacted soil samples is tested for density. The results are graphed on a Proctor test density

chart. The figure below shows a typical set of data.

he results seen on the graph tend to correspond with common sense field observations. With too little

moisture, soil doesn¶t compact too well. By adding some moisture to the compaction process, the soil

certainly gets much more firm (i.e. doesn¶t deflect or pump under a heavy weight like a truck tire). If too

much moisture gets into the soil, as in the case after heavy rains, the soil will simply not compact well.

These common sense observations lead to the conclusion that there is an optimal amount of moisture in soil

for good compaction to take place. The example graph shows that optimal amount to be 14%. Reading

down the graph, the highest density obtained in this test is 118 pounds per cubic foot. So Figure 1 shows

the data from a standard Proctor test. But how does that data get used to determine allowable soil densities

and moisture contents?

Simple arithmetic. The highest point on that graph becomes the 100% density and the optimal moisture

level. The 95% density and the upper and lower moisture limits are calculated in the figure. The 95%

allowable density is 112 pounds per cubic foot and the upper and lower moisture levels are read off the

chart..

Just knowing that information about the test, the Construction Supervisor has some useful knowledge. Is it

possible to have compaction above 100%? Of course it is. Compacting a soil sample in the lab certainly

doesn¶t lead to the conclusion that the density achieved is the highest possible density.

An astute Construction Supervisor realizes that several factors could be in play here:

1.  The amount of force put on the soil in the field, under a heavy roller compacting machine, might be

more than was used in the laboratory.

2.  The soil tested in the laboratory is probably not identical to the soil being tested in the field (sincesoil by its nature is not completely uniform).

3.  There could be a mistake in the lab testing or in the field testing.

These conclusions flow from a simple understanding of the nature of soils and the Proctor testing. The

Construction Supervisor who understands these issues does his job better.

A bit more understanding of the Proctor test yields some additional information. Many specification books

require the soil to be compacted to 95% of modified Proctor testing (ASTM D 1557). This standard uses a 10

pound hammer and an 18´ drop. The modified Proctor soil testing is best suited for a heavily compacted

base course or the sub-grade for an airfield pavement.

8/6/2019 The Proctor Test Can Be Understood Easily

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Therefore the modified Proctor soil testing is generally too stringent a standard for earthwork associated

with low rise commercial, residential and industrial buildings. The use of this standard with 95% density

specified typically penalizes the project with delays since this density is difficult to achieve when soils are

wet.