"The vibrational impulses that the machine creates penetrate the soil and get the particles moving," Conrardy adds. "This releases the friction where the particles make contact. And after the machine passes, the gravity will cause the particles to settle into a denser condition."
Since particles in cohesive soils are flatter, with water and air between them, they require low-frequency, high-amplitude impact energy for compaction, says Peter Price at Bomag Americas Inc. Rammers provide such energy as they jump up and down on the soil, removing the voids between the particles. In contrast, plates use high-frequency vibration to agitate particles so they settle under their own weight.
Until recently, there has been no one machine that could do it all, but that is changing. "Contractors want one machine to do all jobs," says Price. Consequently, Bomag has developed its Dash 4 Series reversible plate, which can compact a wider range of materials, such as granular soils with some cohesive content. "You can't take our plate and use it on clay, but you can run it on native backfill, which is a mix of cohesive and granular."
Don't overdo it
Like many things, soil compaction is an area where you can get too much of a good thing. Soil can become over compacted, which threatens to undermine its bearing capacity.
"Continued compaction can cause soil to break down and cause segregation in soil mixes," says Fabian Salinas at Dynapac. "This leads to weaknesses in the compaction layers of sub-bases."
Conrardy agrees, adding, "Soil can only absorb a certain amount of energy over a given period of time. If too much energy is applied, the soil can shift and move, breaking down the compaction previously accomplished. The result can be fracturing or breaking up the soil particles so that the soil composition actually changes. The changed material actually has a lower density because the new particles are smaller."
Price says over compaction happens more than most people realize. A good remedy is to take note of the machine's behavior during operation and to follow the manufacturer's recommendations. "If the [rammer] jumps erratically, get off the material because that means the energy is going into the ground and coming back off the material and into the machine," he advises.
Wenzel explains, "During compaction of loose fill (soil), the machine's energy is directed and consumed by the soil. Once the soil particles are densely packed, this energy will be reflected back to the machine and the operator instead of being consumed in the soil." Consequently, the most practical and obvious sign of over compaction is the reaction of the machine being used.
As a general guide, experts advise limiting passes over the material to three times for a rammer and four times for a plate. Or better yet, simply match the machine to the depth of soil being placed. "Most manufacturers will rate their machines according to the depth of soil, or lift, it can compact," says Conrardy. "This rating is usually on a layer of well-graded sand and gravel material, which is a common backfill for footings, foundations and areas where higher densities are required."
He adds, "As a rule of thumb, take the maximum compaction depth and divide it by three. If this number is greater than the layer being placed, over compaction is a possibility." For example, if you have a compactor rated at a 24-in. depth and a 6-in. layer is being placed, you risk over compaction using this machine.
Determining the degree of compaction for soils on a particular project, and whether the results meet required specs, is normally the responsibility of a soils engineer. To help the testing process progress more quickly, Weber has introduced the Compatrol system, the first continuous compaction control system for reversible soil compactors.
With the Compatrol system, the operator is made immediately aware of any potential problem that might develop and affect results, such as over compaction, under compaction or situations where the soil is not compactible. The system is based on a frequency band analysis and consists of a display and acceleration sensor. The technology measures the acceleration increase and decrease of the soil compactor's base plate; compares the measured values with the registered soil characteristics; and translates the results into "electric voltage." The displayed compaction result is easy to understand.