drums on vibratory rollers vary from 3 to 5 feet indiameter and 4 to 8 feet in width.The engine, providing power for propulsion, alsopowers the hydraulically driven vibrating unit.Vibrations are generated by a rotating eccentric weightinside the drum, the speed of which determines thefrequency, or vibrations per minute, of the drum. Theweight and distance from the shaft of the eccentricdetermine the amplitude (amount) of the impact force.Both the frequency and amplitude of vibrations arecontrolled independently of roller travel and enginespeed.The vibration frequency of rollers used for com-paction is generally between 2,000 to 3,000 vibrationsper minute (vpm), depending on the model andmanufacturer. Some models provide only one or twospecific frequency settings; while others may provide afull range of frequencies within certain limits; forinstance, 1,800 to 2,400 vpm.Vibratory rollers achieve compaction through acombination of three factors: (1) weight, (2) impactforces (roller vibration), and (3) vibration response inthe soil.WeightWeight is the natural force in compaction of soils.Vibrating rollers amplify their static weight throughvibration to increase the overall dynamic weight.Impact ForcesThe impact forces are those generated by vibrationof the compaction drum. They are regulated bycontrolling the frequency and amplitude of thevibration. The amount of impact force required to obtainoptimum density depends on the type of material beingcompacted. The impact forces also vary with thediameter of the drum and the width and the ratio of theroller static weight and dynamic (impact) force.Vibration ResponseThe vibration response in the soil or material is theresult of the way the forces are exerted upon it by thevibratory roller. As with other types of rollers, thematerial will compact easily or with difficulty,depending on its moisture content, cohesioncharacteristic, particle shape and texture, andconfinement; for example, sandy soil requires morevibration and less impact force (amplitude). However, asoil with higher clay content requires more amplitudethan vibration because of the kneading action necessaryto compact the clay. Vibratory rollers exert repetitivedynamic force on the material, rather than the staticforce used by other rollers.The frequency and roller speed should be matched,so there will be at least 10 downward impacts per footof travel of the roller. The speed of the roller increasesfor a given frequency of vibration, and the spacing ofthe impacts grows farther apart.When using vibratory equipment, keep in mind thatthe energy imparted by the vibratory wheel must beabsorbed in the material being compacted. Controllingthe amplitude permits the operator to vary the forcedeveloped from the wheel and, therefore, the energyimparted to the material. A change in the lift thicknessand material gradation content may require adjustmentin the amplitudes being used.NOTE: It is important that the roller vibrates onlywhen it is moving. If vibration continues while the rolleris standing still or changing direction, the vibratingdrum will leave an indentation in the material at thestopping point.Most modern rollers have automatic cutoffs forvibration when the roller stops moving.The rollers used in the Naval Construction Force(NCF) are equipped with two interchangeable drums.One is known as the sheepsfoot, as shown in figure11-40; and the other is known as a smooth drum, asshown in figure 11-41.Sheepsfoot DrumThe sheepsfoot drum is used for compacting heavylifts of 6 to 12 inches thick. As consecutive passes aremade, the drum will start to walk out of the ground asthe penetration of the sheepsfoots decrease. Theserollers should only be used for initial compaction,because the footprints they leave will not allow excesswater to drain.These rollers concentrate the static and dynamicweight on the relatively small contact area of thesheepsfoots. This force is exerted through the one rowof feet in contact with the ground. With all the rollerweight concentrated on this row of sheepsfoots, theyexert more than 22,000 pounds of force.Smooth DrumIn most heavy fills, a smooth drum roller is workedbehind the sheepsfoot drum and grader. With thinner11-20
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