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.
