Lesson 4/Learning Event 5
FREQUENCY OF DENSITY CHECKS
If the density determined by the methods described above is equal to or greater than that required, then
rolling generally may be judged to be satisfactory and the placing of another lift may proceed. If the
density is lower than that required, additional rolling may be necessary or the moisture content may have
to be adjusted. If these measures fail, then the weight of the roller may have to be increased, the
thickness of lift reduced, or some other measure taken to obtain adequate compaction. Never overlook
the possibility that the soil which is being rolled in the field is not the same one which was tested in the
laboratory. Under normal field conditions the number of density and moisture checks required should
not be very great after the initial period of adjustment, assuming that the work is proceeding smoothly
and the same soils being compacted. If adequate densities are being obtained and the proper moisture
content is being maintained, the job of inspection may then be principally one of keeping to the set
number of passes and combinations of the rollers which will achieve the desired result. Where
conditions vary more, density and moisture checks may be needed more often for a fill of even moderate
length. The exact number of checks needed can only be determined by the engineer in charge of the job.
INFLUENCE OF MOISTURE CONTENT
For a given weight and given compaction effort, the moisture content determines the state at which
maximum dry unit weight (density) occurs. When moisture content is low, the soil is stiff and difficult
to compress; low values of dry unit weight and high values of air content are obtained. As the moisture
content is increased, the added water decreases surface tension and acts as a lubricant causing the soil to
soften and become more workable, resulting in a high unit weight and lower air content. The optimum
moisture content at which maximum dry density is attained is the moisture content at which the soil has
become sufficiently workable: that under compaction effort used, it has permitted the soil to become
packed so closely as to expel most of the air. As the moisture content is increased above the optimum,
the soil becomes increasingly more workable, but the increased moisture content and the remaining
unexpelled air fill the soil voids and prevent close consolidation.
97
Previous Page
