When a project calls for compacting material in a trench, it's important to make sure you use the appropriate compactor for the conditions.
"Like any compaction job, the challenge starts with matching the right equipment with the material and size of the job," says Fred Paul, sales engineering manager, Wacker Neuson. "Trench compaction also offers an additional challenge of [ensuring] operator safety. The contractor needs to pay special attention to the width and depth of trench to determine if shoring is required."
"With shoring, a contractor can use any type of compaction available, since it is a safe environment for the operator," says Scott Bailey, North American sales manager, Weber Machine, Inc. But without shoring, the options become more limited.
Know your soil
According to Guy Boor, engineering product manager for compaction, Stone Construction Equipment, it's important for a contractor to have at least some understanding of the materials, moisture content and soil mechanics of the area to be compacted. "He will then understand the relationships involved to achieve compaction and can make an informed decision."
In the case of cohesive materials (e.g., clay or silt), rammers and sheepsfoot (padfoot) trench rollers are more effective due to their lower frequencies and high amplitudes. For granular materials (e.g., sand or gravel), vibratory plates and smooth drum rollers are the better choice due to their higher frequencies and lower amplitudes.
"If a contractor is compacting 'crush,' then the best option would be a vibratory plate compactor designed for the amount of material to be compacted," Bailey states. "A vibratory plate compacts mixed aggregate better than a trench roller due to its faster travel speed and larger surface area."
"The vibration action reduces frictional forces at the contact surfaces, allowing the particles to fall freely under their own weight," Paul adds. "Cohesive soils, made of silts and clays, are best compacted by the impact force of a rammer or sheepsfoot roller."
In most cases, notes Boor, contractors are compacting native material, which is usually made up primarily of clay. "Trench rollers work the best because of the drive wheels and the vibration," he says. "In clay material, a plate compactor will tend to bury itself."
The exception would be a boom-mounted plate attachment. "The boom-mounted option works because the operator is performing spot compaction and can pick the plate off the material and move it to another location," Bailey points out.
Match width for width
The dimensions of the work area will influence both the type and features of the compactor required.
"The width of the machine as compared to the trench is an obvious starting point," says Paul. For very narrow trenches, rammers (tampers) are the most common choice. "Rammers are ideal for utility and cable trenches as narrow as 4 in."
Several different foot sizes -- from as little as 4 in. to 13 in. or more -- are available to accommodate varying trench widths.
As the work area expands in size, however, plate compactors and trench rollers tend to be more productive. Walk-behind plate compactors come in working widths up to around 50 in., and are available in forward (single direction) or reversible (dual direction) configurations.
"The advantage of using a reversible plate in the trench is that it eliminates the need to actually turn the machine around. Simply changing the direction of the unit will make quick and easy work of trench compaction," Paul asserts. Many reversibles also come with extension plates to more easily match trench width.
Certain trench roller models can also be adjusted to accommodate narrower trenches. "BOMAG trench rollers offer two different working widths [in one machine] to be flexible for different trench width applications," says Albertus Krings, product manager. "By taking the drum extensions off, the working width can be changed from 34 to 24 in. in just minutes."