The necessary compaction of subgrades of roads, earth fills, and embankments may be obtained by
mechanical means. The equipment that are normally used for compaction consists of
- Smooth wheel rollers
- Rubber tired rollers
- Sheepsfoot rollers
- Vibratory rollers
Laboratory tests on the soil to be used for construction in the field indicate the maximum dry
density that can be reached and the corresponding optimum moisture content under specified
methods of compaction. The field compaction method should be so adjusted as to translate laboratory condition into practice as far as possible. The two important factors that are necessary to achieve the objectives in the field are
- The adjustment of the natural moisture content in the soil to the value at which the field
compaction is most effective.
- The provision of compacting equipment suitable for the work at the site.
The equipment used for compaction are briefly described below:
Smooth Wheel Roller
There are two types of smooth wheel rollers. One type has two large wheels, one in the rear and a
similar single drum in the front. This type is generally used for compacting base courses. The
equipment weighs from 50 to 125 kN (Fig.21.9). The other type is the tandem roller normally used
for compacting paving mixtures. This roller has large single drums in the front and rear and the
weights of the rollers range from 10 to 200 kN.
Rubber Tired Roller
The maximum weight of this roller may reach 2000 kN. The smaller rollers usually have 9 to 11
tires on two axles with the tires spaced so that a complete coverage is obtained with each pass. The
tire loads of the smaller roller are in the range of 7.5 kN and the tire pressures in the order of
200 kN/m2. The larger rollers have tire loads ranging from 100 to 500 kN per tire, and tire pressures
range from 400 to 1000 kN/m2.
Sheepsfoot rollers are available in drum widths ranging from 120 to 180 cm and in drum diameters
ranging from 90 to 180 cm. Projections like a sheepsfoot are fixed on the drums. The lengths of
these projections range from 17.5 cm to 23 cm. The contact area of the tamping foot ranges from 35
to 56 sq. cm. The loaded weight per drum ranges from about 30 kN for the smaller sizes to 130 kN
for the larger sizes (Fig. 21.10).
The weights of vibratory rollers range from 120 to 300 kN. In some units vibration is produced by
weights placed eccentrically on a rotating shaft in such a manner that the forces produced by the
rotating weights are essentially in a vertical direction. Vibratory rollers are effective for compacting