"Using compacters we can achieve around 102 per cent MDD whereas the yellow book of the Indian Road Congress states that the desired density for soil is 95 to 98 per cent," says Joy Saxena, Group Chief Financial Officer, ERA Infra Engineering. Excerpts from the interview.

Why vibratory compactors over static ones and what is the vibration effect in compaction over the static loading?
The static compactor utilises static force which is simply the deadweight of the machine applying downward force on the soil surface, compressing the soil particles. The only way to change the effective compaction force is by adding or subtracting the weight of the machine. Static compaction is confined to upper soil layers and is limited to any appreciable depth. Kneading and pressure are two examples of static compaction. However, the vibratory compactor is based on vibratory force which uses a mechanism, usually engine-driven, to create a downward force in addition to the machine's static weight. Vibratory compactors deliver a rapid sequence of blows to the surface, thereby affecting the top as well as deeper layers. Vibration moves through the material, setting particles in motion and moving them closer together for the highest density possible. Moreover, it has been proved that the required density achieved by utilising the vibratory compactor is three times faster as compared to the static compactor.

To what extent do soil conditions determine the selection of compactors, and could you tell us the reasons?
Every soil type behaves differently with respect to maximum density and optimum moisture. Therefore, each soil type has its own unique requirement and controls, both in the field and for testing purposes. Soil types are commonly classified by grain size which is determined by passing the soil through a series of sieves to screen or separate the different grain sizes. Soil classification is categorised into 15 groups. Moreover, soil found in nature is almost always a combination of soil types. A well-graded soil consists of a wide range of particle sizes with the smaller particles filling voids between larger particles. The result is a dense structure that lends itself well to compaction. Hence, soil conditions prove to be the major factor behind selection of compactor.

Could you tell us about the density achievable from the range of compactors? What determines the maximum achievable density?
Using compacters, we can achieve around 102 per cent MDD whereas the yellow book of the Indian Road Congress states that the desired density for soil is 95 to 98 per cent. To achieve the desired density, the following features are required: perfect combination of amplitude and frequency, static linear and dynamic linear load of the machine. For soil, the following things need to be taken care of: type of the soil, distribution of particle size in the soil, layer thickness, fine and clay content in the soil, and optimum water content in soil.

What are the variants in compaction equipment?
As per the requirement of compaction following ranges are available: Soil compacter (4T to 27T) (for base, sub-base, WMM, GSB, DLC layer compaction of road, in backfill and dam compaction); Tandem rollers (1.6T to 17T) (for DBM & BC layer of road); and Pneumatic tyre rollers (12T to 27T) (for DBM & BC layer of road).

What is the importance of grading before compaction?
Grading provides a levelled ground enabling better compaction.

How do we determine the effectiveness of grading in various applications?
Effectiveness of grading majorly depends upon the slope and depth achieved.

What are the factors that affect grading?
Major factors affecting grading are moisture content of soil, type of soil, sand and clay content of soil, among others.

What are the selection criteria of these machines?
The selection criteria includes: market review of equipment, cost of equipment, nature of work, credibility of vendor, after sales service, cost of equipment spare parts, maintenance cost of equipment, etc.