?The popular model being offered in India is the CS533E (11T Class) which is required to meet the National Highway Project requirements in India,? says Pankaj Dara, Associate Vice President - Construction, TIPL. In an exclusive interview with Equipment India, Pankaj elaborates on the industry trends. Excerpts from the interview.

Tell us about the range offered in compactors and its unique features.
Caterpillar has vibratory soil compactors in the operating weight range of 4.5T?18.5T with drum drive as a standard fitment on all models to increase the machine versatility. The drum drive model has separate hydraulic pumps for the axle drive and the drum drive which helps the machine in achieving high gradients wherever required. However, the popular model being offered in India is the CS533E (11T Class) which is required to meet the National Highway Project requirements in India.

To what extent do soil conditions determine the selection of compactors?
Many different factors influence the overall compaction effort. Vibratory compaction involves a drum which moves up and down (amplitude) rapidly (frequency) and moves forward (working speed) over non-homogeneous material. All components influencing compaction should be considered as a whole, not as separate entities. It is the combined characteristics of the compactor and the soil or asphalt it attempts to compact which determines the degree of compaction effort.

The characteristics of the material to be compacted play a part in the dynamics of compaction. Factors such as soil type, gradation, texture, initial density, moisture content, aggregate strength characteristics, layer thickness, sub soil base and its supporting capability all influence compaction. For example, granular soils tend to transmit vibratory forces well. Cohesive soils tend to have a damping effect on vibratory forces. Cohesive soil requires a kneading effect to drive out the excessive moisture from the soil. This is best achieved by a pad foot drum.

What about the density achievable from the range of your compactors? What determines the maximum achievable density?
Target densities are established by the specifying agency before the job begins, to ensure that the soil is adequately and consistently compacted. Generally, target density is set on the basis of either relative or absolute measures.

A relative measure may use a percentage of a laboratory standard. This is done through a standard AASHTO procedure or the modified AASHTO procedure depending upon the desired specification that needs to achieve on a specific project. The compaction energy in the modified AASHTO test is 4.5 times larger than the standard AASHTO test and is normally specified on projects where very high axle loads are anticipated. For a given soil sample, either the standard or modified AASHTO is performed five times. The same procedure is used each time the test is run but the moisture content varied for each.

The series is begun with the soil in the damp condition somewhat below the probable optimum moisture content. After the first sample is compacted in the mould, the wet weight is measured and the sample is placed in a drying oven. When the sample is completely dry, it is weighed again. The difference between the wet and dry weights yields the moisture content which is expressed as a percentage of the dry weight. A second sample with increased moisture content is compacted and the weighing and drying process is repeated. Additional samples with increasing moisture content are processed until the wet weight decreases or the soil becomes too wet to work. Dry density and moisture content values for each sample are then plotted and a smooth curve is formed. The highest point on the curve represents the maximum dry density and the optimum moisture content for that soil sample. Lab tests determine the moisture content at which maximum density can be attained.

Field target densities are specified as a certain per cent of the maximum laboratory dry density. Generally required field densities will be 95-98 per cent of the standard AASHTO for embankment and up to 100 per cent of modified AASHTO for roadway structures. Likewise, moisture content must be within a range of the laboratory determined optimum moisture content.