Agith G Antony finds out the technical aspects of compaction from Pradeep Kathuria, Head - Essar Equipment Bank, Essar Projects India.
“The demand for compactors and motor graders have gone up, therefore the return on investment on these equipment is high; however, this also depends on the utilisation factor, says Pradeep Kathuria, Head - Essar Equipment Bank, Essar Projects India. In an exclusive chat with Equipment India, Pradeep elaborates on the technical and practical aspects of using compactors for a variety of applications.
Speaking about the advantages of vibratory compactors over static ones and what is the vibration effect in compaction over the static loading Pradeep says, “In static compaction, it 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, whereas in vibratory compaction, it uses a mechanism, usually engine-driven, to create a downward force in addition to the machine's static weight. The vibrating mechanism is usually a rotating eccentric weight or piston/spring combination (in rammers). The compactors deliver a rapid sequence of blows (impacts) to the surface, thereby affecting the top and deeper layers. Vibration moves through the material, setting particles in motion and moving them closer together for the highest density possible. Based on the materials being compacted, a certain amount of force must be used to overcome the cohesive nature of particular particles.
What determines the maximum achievable density? Pradeep points out, “Vibratory compaction of soil is a complex process. More than 30 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 a 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 of the dirt or asphalt it is attempting to compact that determines the degree of compaction effort.”
“The characteristics of the material to be compacted play a part in the dynamics of compaction. Type, gradation, texture, initial density, moisture content, aggregate strength characteristics, layer thickness, subsoil base and its supporting capability all influence compaction. The sum effect of these properties is termed mass stiffness and damping,” he adds.
Pradeep further elaborates, “The design of the machine is also important to the dynamics of compaction. Influential factors include: frame size, overall weight, wheel base, ratio of machine weight supported over the front drum to rear drum or tires, and balance of machine weight from left to right of the machine. The list continues with factors like drum diameter, drum width, drum mass, shock isolators, eccentric weight mass, and the distance between the eccentric weight centre of gravity and drum axle. Even the weight of fuel and the operator have a bearing on the compactive performance of the roller. The manufacturer carefully considers all these factors when the machine is designed. And, of course, frequency, amplitude and working speed influence compactive effort. The operator can control these variables. What all these factors mean is that it's not always easy to set up a roller on a given job to achieve the best compaction results.” According to him, the objective in vibratory compaction is to find a point of maximum transmitted force into the material to be compacted. This occurs when the sum of all the components, material characteristics, roller characteristics, amplitude, frequency and speed, is contributing the most to the compactive effort.
An embankment is any fill whose top is higher than the adjoining surface. It could be a building site or a highway. In any job, the embankment is above the original ground.
Rock is increasingly used as embankment fill in highway construction. It is also used to a greater extent in dam, airport, building and harbour embankment construction. Shot rock often contains so many fines that considerable settling will occur if the fill is not compacted.
Rock fill is usually spread in 18” to 48” (457 to 1,219 mm) lifts. How the material is spread before compaction is vital. Tractor spreading in layers creates a uniform fill because the dozer blade does dome re-orienting of the rocks and the tracks perform some compaction. Therefore, a relatively dense and even surface is prepared for the compactor.
Heavy compaction forces are needed after spreading to relocate huge stones for density and stability. The largest smooth drum vibratory compactors are selected for this job. Even so, compactors are subject to great stresses on rock fill. The drum should be constructed of thick, high-grade steel. If there is a crushing effect on the surface material, the number of passes may have to be reduced. Or, if the machine is equipped with more than one amplitude, lower amplitude can be used to reduce surface material distortion.
Sand and gravel
Vibratory compaction with smooth drum machines is especially suitable and economical on sand and gravel. High densities can be achieved in few passes with the lift thickness determined by the size of the compactor. Free-draining sand and gravel that contains less than 10 per cent fines are easily compacted, especially when water saturated. When high density is required and the lifts are thick, water should be added. This water will drain out of the lift during the compaction process.
If the sand and gravel contains more than 10 per cent fines, the soil is no longer free draining and may become elastic when the water content is high. For this type of soil, there will be optimum moisture content at which maximum density can be reached. Drying of the wet soil may be necessary to reach the optimum moisture content.
Pradeep explains, “On poorly graded sand and gravel, it is difficult to achieve high density close to the surface of the fill. There is low shear strength in poorly graded soils and the top layer tends to rise up behind the drum. This is not a problem when multiple lifts are being compacted. The previous top layer will be compacted when the next layer is rolled. However, the difficulty of compacting the surface should be kept in mind when testing for density.”
Silts are non-plastic fines that are usually compacted with smooth drum vibratory rollers. They can be spread and rolled in thick lifts. Like all fine-grained soils, their compatibility is dependent on moisture. For best compaction results, the water content should not vary much from the optimum moisture content. If too much water is present, silts rapidly approach the fluid state and compaction is impossible. This means that the lifts may have to be aerated with discs, mixed with drier soil (an expensive procedure) or the borrow pit has to be better drained. Silty soils that also contain clay may have considerable cohesion. On these soils, padded drum, tamping foot or pneumatic rollers will give the best results.
Clays have plastic properties which means the compaction characteristics are highly dependent on moisture content. When the water content is low, clay becomes hard and firm. Above the optimum moisture content, clay becomes more and more plastic and difficult to compact. The main problem in clay compaction is very often the need to adjust the water content. The addition of water by using water trucks, discs or soil stabilisers is time-consuming. Water infiltration into the borrow pit may be a better alternative. Drying wet clay can be done only in warm and dry conditions, even using discs and soil stabilisers. Prolonged rolling with sheep foot rollers is sometimes done to lower the moisture content.
Even at the optimum moisture content, clay requires a higher compactive effort and a lower lift thickness compared to non-cohesive soils. Padded drum rollers work best because as the pads penetrate the soil, they break the natural cohesive bonds between the particles. Pneumatic tire compactors can be used on clays with a low to medium Plasticity Index.
On projects where high production is a requirement and clay is used as fill, good results can be obtained by using tamping foot compactors in conjunction with vibratory padded drum compactors. Tamping foot compactors equipped with dozer blades are efficient at spreading the fill and breaking large, hard lumps of clay often found in clay borrow material. These machines perform the first passes. Final density is reached by vibratory padded drum compactors.
Base and sub-base applications
Speaking about base and sub-base applications, Pradeep says, “Base and sub-bases are the layers constructed on top of an embankment on natural ground surface. They increase in strength as they near the finished surface. The materials used in these layers depend on the type of loads the road or building must support. Usually, very tight specifications are given for base and sub-base materials, for the thickness of the lift and for the required density.”
From an economic standpoint, it is preferable to use locally available soils. If these soils are suitable, they may be used without chemical treatment or additives. Proper compaction of these soils will substantially increase their load-bearing capacity and control other factors such as permeability, capillary action and shrink and swell. The choice of compaction equipment will depend on the type of soil. Generally, granular non-cohesive soils are specified as base and sub-base material. Smooth drum or pneumatic tire compactors are more often used in this application.
Mixing chemicals with native or imported soils can substantially improve the soil's stability and load-bearing characteristics. This is called soil stabilisation. After lime, cement, salt or asphaltic cement has been mixed into the soil; the soil should be compacted. The type of compactor used will depend on the soil's original, untreated characteristics. Where a large volume of cohesive soil is involved, a tamping foot roller may be more economical than a vibratory compactor. Smaller volumes may be compacted with a pneumatic roller.
Job specifications may call for well-graded crushed rock to be used as base and sub-base materials. By using crushed material, gradation can be controlled during the crushing process to match specifications. Crushed rock is generally easier to spread and compact than fine soils and the compaction results are more predictable. However, the expense of crushing and the often longer hauls to the project site offset these advantages. Crushed rock is usually hauled to the job in end dump trailers and dumped on the grade in front of a motor grader or spreading machine. The base material is then spread and shaped in lifts ranging from 6” to 10” (152 mm to 254 mm). After spreading, compaction is accomplished by smooth drum rollers (static or vibratory) or pneumatic tire compactors.
“The contracting fraternity can increase its productivity by adopting advanced equipment like vibratory rollers and graders.” —Pradeep Kathuria, Head - Essar Equipment Bank, Essar Projects India.