Even as kids, we instinctively understood this basic construction principle: Push a mound of wet sand into a plastic bucket, pack it down tight, pour the water out, flip it over and you create a stable foundation for a sandcastle. By removing the water and air, there are fewer voids and less distance between each grain of sand. The denser, compacted sand will support heavier loads than the original sand pile without shifting or collapsing, and the sandcastle is more durable. When choosing a walk-behind compactor for your project the same principle still applies. Knowing your soil type, the size of your job and any jobsite restrictions will help you choose the best walk-behind compactor for the job. A bonus: many of the same machines can do asphalt repair work.
What’s in your sandbox?
Assessing the types of soil on a project determines the amount and type of force needed to eliminate voids and achieve a specific density in the material. The American Society for Testing and Materials (ASTM) and the American Association of State Highway and Transportation Officials (AASHTO) classifies soil into five common types – gravel, course sand, fine sand, silt and clay. The characteristics of each soil determine the type of compaction equipment that will provide the best force required to achieve the desired density.
Particles of fine sand, course sand and gravel don’t naturally adhere together but they will crowd tightly together when air and water voids are removed. A vibratory force reduces the friction between the particles in granular soils and allows the grains to reposition themselves. The weight of the particles compounds as they settle into a tightly packed arrangement, reducing the number of voids and increasing the material’s density. Vibratory force is measured by the height of the vertical movement of the impact shoe in inches (amplitude) and the number of vibrations (frequency) per minute.
Cohesive soils like clay and silt contain minute, slippery plates that stick haphazardly together, trapping air between them. A strong impact force draws air and water out of the soil, forcing the plates together to collapse the voids and increase the soil’s density. The impact force is measured by the weight, drop distance and drop speed of the pounding element.
Mixed soils consisting of course grains and sticky plates require a steady force to move particles closer together and squeeze out air voids. Static compaction results from a direct weight resting on the material.
The moisture level in each soil also affects how the particles compact together. Dry granular particles will move around but won’t stick together when compacted. Particles that are too wet may stick together but as the water later evaporates it leaves air voids that reduce the compacted material’s density and strength. To determine if your site is ready for compaction, do a quick hand test. Grab a handful of soil and squeeze it together. When you open your hand, if the soil crumbles, it’s too dry. If the soil feels like plastic and you can squeeze moisture out of it, or it retains its shape when you drop it, it’s too wet to work. When the soil can be shaped with no excess moisture and breaks into several pieces when dropped, it’s ready to compact.
Tampers and rammers
Walk-behind compactors come in three common styles. Each type of machine is designed to work with specific materials and applications.
Tampers and rammers use impact force and are most effective on cohesive materials. The tamper’s portability lets the operator work close to obstructions and in restricted areas while still providing enough force to compact clay and silt soils. Usually weighing less than 175 pounds, rammers can deliver an impact force from 2,000 to over 3,500 pounds. Applications include cable and pipe installation, utility trench work and compacting footings. Tampers are available with two-or four-cycle engines and in gas and diesel versions. Wacker recently introduced a new carburetor on their BS-2i series two-cycle rammers that lets the operator purge air out of the carburetor and replace it with fuel for easier starting.
Walk-behind single-direction forward plate machines operate with vibratory force to compact thin layers of granular materials. Single plate machines work well on small jobs that have enough room to turn the compactor around, like compacting sand or gravel near a landscaping bed. Single plates can also work on patio blocks and paving stones when fitted with Vulcolan (polyurethane) mats to protect fragile materials. Some forward plates can do asphalt patching jobs using a water kit option. Stone Construction Equipment’s asphalt-specific forward plate Silver Fox model, for example, carries water tank and sprinkler system that coats the base of the plate with an even flow of water that keeps the mat cool and prevents material from sticking.
Reversible plate compactors run forward, backward and in place, eliminating the need for a turn-around area. Heavier than single plate compactors, reversible plates work best in thick and thin layers of granular materials and block paving and are the better choice for production work. Some reversibles, like Bomag’s new BPR 35-60 reversible, operate at higher vibration frequencies for improved performance on cohesive and mixed soils. Reversibles can backfill around foundations, compact base materials for driveways and sidewalks, and work close to curbs and retaining walls. Reversible plate compactors are especially effective for working in trenches and sites that don’t have room to maneuver. An operator can run the length of the trench in forward mode, then go back over the trench in reverse mode without repositioning the compactor.
Walk-behind drum rollers come in single- and double-drum configurations, employing static and vibratory forces. Single-drum walk-behind rollers work well for asphalt repair and maintenance, pothole patching and asphalt compaction in confined areas like walkways and driveways. In landscaping applications, single drum rollers can roll sod and golf course greens. Tandem-drum walk-behind rollers do construction backfills and trench work in granular soil, as well as shoulder work and asphalt repairs. Many tandem drum walk-behind rollers let the operator select the mode force – vibratory, static or a combination of both – to match the job’s application. When the front drum vibrates and the back drum is static, the compacted material will tend to pull under the drum. If the front drum is static and the back drum vibrates, the operator will be able to push the material ahead of the compactor. Walk-behind rollers, such as Wacker’s RD 7H-ES smooth double drum vibratory roller, compact asphalt and granular materials in narrow or restricted areas and still leave a smooth finish.
The operating weight of a walk-behind compactor isn’t the best measure of how much material a machine will compact. Compacting equipment is rated by its lift – the various layers of material that can be laid down or compressed. Choose a machine with a lift rating greater than the depth or number of layers of material you are compacting. For example, Bomag rates its compaction output on their BP8/34 walk-behind single direction vibratory plate compactor in cubic yards for soils and tons for asphalt. (See example above).
Stone Construction Equipment says most walk-behind compactors are preset to operate at their optimal vibration speed and amplitude. How an operator controls the machine’s work speed affects structural integrity and the visible surface of the project. If an operator passes over the job too slowly, the compactor’s impacts may be too far apart and cause warping or waving in the material. Passing over the material too quickly won’t deliver enough compacting force to reduce voids and create the desired density. Vibrating in place creates depressions on the surface of the material. Generally, soil compaction requires fewer passes than asphalt compaction.
Maintenance and operating safety
Doug Zoerb, marketing communications manager for Bomag Americas, says most walk-behind compactor’s components are maintenance free. “The vibration systems are sealed so most of the machine’s servicing involves taking care of the engine. Because these compactors work in dusty conditions, changing the oil and air filters is important,” Zoerb says. He also stresses keeping the safety guards on the compactors in place. “The guards keep rocks and dirt away from the V-belt and prevent damage to the belt.” Guards also help keep stray debris away from the operator.
Manufacturers are adding vibration isolation buffers on forward and reversible plates to reduce operator fatigue but operators should still wear vibration-dampening gloves with a gel palm insert to when working for long periods. Multiquip says their MVC-80 forward plate compactor’s vibrations have decreased 50-percent, when compared with previous models.
Multiquip attributes this drop in vibration to a new anti-vibration handle system that helps reduce operator fatigue. Bomag’s reversible plates offer an automatic shutdown system with reversing protection that immediately turns off the machine when the sensor switch is activated.
Unshored trenches and trenches in unstable soils should be compacted with remote control compaction equipment. Check OSHA’s trench safety regulations at www.osha.gov.
The Association of Equipment Manufacturers (www.aednet.org) offers a safety video and manual that presents safety tips for roller compactor operating, transporting and maintenance.