Types of Rollers
Rollers of different types are use for compaction of soils. The compaction depends upon the following factors : Contact pressure, Number of passes, Layer thickness, Speed of roller etc. Various types of roller which is use in construction sites are shown in below :
Smooth Wheel Rollers
A smooth wheel roller generally consists of three wheels; two large wheels in the rear and one small wheel in the front. A tandem type smooth wheel roller consists of only two drums; one in the rear and one in the front. The mass of a smooth wheel roller generally varies between 2 to 15 Mg. These rollers are operate by internal combustion engines.
Note : Some authors express 1000 kg mass as one tonne (1t). As tonne is not a standard SI unit, it is better to express 1000 kg as 1 Mg.
Smooth wheel rollers are useful for finishing operations after compaction of fills and for compacting granular base courses of highways. These are not effective for compaction of deep layers of soils, as the resulting compaction pressures include are low. Futher, these rollers also cause stratification in deep layers due to non uniform compaction. These rollers are generally use to ” seal ” the surface of the fill at the end of day’s work to provide a smooth surface to quickly drain off any rain water.
Pneumatic tyre rollers
Pneumatic tyre rollers use compress air to develop the require inflation pressure. The contact pressure depends upon the area of contact and the inflation pressure. The roller generally consists of 9 to 11 wheels fix on two axles, with the pneumatic tyres so space that a complete coverage is obtain with each pass of the roller. The rollers are available in a wide range of load sizes. The gross mass of the roller varies between 5 to 200 Mg. However, the rollers with mass more than 50 Mg are rarely use. The inflation pressure varies between 200 to 1000 kN/m2. The rollers are available as a self propel unit as well as towed unit.
The roller compacts the soil primarily by kneading action. These rollers are effective for compacting cohesive as well as cohesionless soils. These rollers are the best type of equipment for general use. Light rollers ( mass up to 20 Mg ) are effective for compacting soil layers of small thickness up to 15 cm, whereas heavy rollers are useful for layers of thickness up to 30 cm.
Sometimes, the rollers are design to produce a wobble effect, due to which a slightly weaving path is track. This improves the compaction of the soil. Pneumatic tyre rollers are generally provide with a weight box or ballast box. The box can be fill with ballast to increase the weight of the roller.
Sheep foot rollers
In ancient time before the advent of the rollers, it was usual practice to pass a flock of sheep on the newly form soil fill to cause its compaction. The same principle is use in the design of sheep foot rollers.
The sheep foot rollers consists of a hollow drum with a large number of small projections ( known as feet ) on its surface. These projections penetrate the soil layers during the rolling operations and cause compaction. The drums are mount on a steel frame. The drum can be fill with water or ballast to increase the mass. Sheep foot rollers are available both as a self propel unit and a towed unit. As rolling is done, most of the weight of the roller is impose through the projection on the soil. The contact pressure is generally between 700 to 4200 kN/m2. The roller may sink into the soil if the contact pressure is more than the bearing capacity of the soil.
The sheep foot rollers are ideally suit for compaction of cohesive soils. The rollers compact the soil by a combination of tamping and kneading action. When the roller is pass for the first time, the projections penetrate the soil layer and lower portion of the layer is compact. In successive passes, compaction is obtain in the middle and the top portion of the layer. This continually rising effect of the compaction is refer as walking out of the roller.
The depth of layer that can be compact depends upon the length of the projections and the weight of the roller. Small rollers can compact layers of 15 cm thickness, whereas heavy rollers can compact layers of 30 cm thickness. In general, the thickness of the layer compact is keep not more than 5 cm greater than the length of the projection.
In vibratory compactors, vibrations are include in the soil during compaction. The compactors are available in a variety of forms. When the vibrator is mount on a drum, it is refer as vibratory roller. These rollers are available both as pneumatic type and the smooth wheel type. In a smooth wheel type, a separate motor drives an arrangement of eccentric weights to create high frequency, low amplitude, up and down oscillations of the drum. These rollers are suitable for compacting granular soils, with no fines, in layers up tp 1 m thickness. However, if there is appreciable percentage of fines, the thickness has to be reduce. In a pneumatic tyre vibratory compactor, a separate vibrating unit is attach to the wheel axle. The ballast box is suspend separately from the axle so that it does not vibrate. These compactors are suitable for compacting granular soils with thickness of layer of about 30 cm.
Another form of a vibratory compactor is a vibrating plate compactor. In this system, there are a number of small plates, each plate is operate by a separate, vibrating plates is limit to small depths. Their main use is to compact granular base courses for highways and runways where the thickness of layers is small.
Vibratory compactors can compact the granular soils to a very high maximum dry density.
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