Granular soil is compacted with a standard amount of energy over a range
of moisture contents to identify the optimum moisture content for maximum
dry density.
Therefore, the objective of the moisture-density test
is to determine the maximum dry density and optimum moisture content for the
soil.
In practice, fills for highways and buildings
must be compacted to attain appropriate strength and minimize settlement.
The most common method of specifying compaction is to require a certain
percent of the maximum that can be attained in proctor compaction tests,
such as "95% of standard proctor."
The original moisture-density test was developed by R.R. Proctor and is commonly
referred to as the Standard Proctor Test, Proctor Test, or Standard Moisture-Density Test.
The Modified Proctor or Modified Moisture-Density Test is performed the same way
but in a larger mold with higher compactive energy. Each of the approximately 8 different
variations of standard and modified proctor se can produce different results, so contract
documents must specify which procedure is to be used.
Error considerations and analysis:
An error of 0.1 lb in the weight of soil in the mold would result in a 3
pcf error in the reported unit weight since the volume of the mold is 1/30
cu.ft. (0.1 lb / (1/30 cu.ft.) = 3 pcf).
The accuracy of the computed density was limited by the weight of soil
which was measured to the nearest 0.1 gm (0.002 lb or 4 significant figures
for the weights obtained). The nominal volume of the mold was 1/30 cu.ft.
The actual the computed volume of the mold based on measurements made to the
nearest 0.001 in. was __________. So the volume of the mold is known to 4
significant figures. Thus the wet unit weight could be determined to 4
significant figures. Weights for the water contents were made to 0.01 gm or
4 significant figures for weights greater than 10 gm. Thus, the water
content and dry density could be determined to 4 significant figures.
[However, if water contents were made with samples less than 10 gms, there
would only be three significant figures in the water content and dry
density.]
There are several additional sources of error
in this test:
- not trimming the soil even with the top of the mold, this is
aggravated by gravel which sticks up above the top of the mold;
- overfilling the mold on the last lift which would reduce the computed
wet density;
- non-uniform distribution of the blows;
- each lift not 1/3 of total mold height;
- non-uniform distribution of water in sample
|
