There are many types of compaction equipment designed for different applications. Basically, soil compaction is the conditioning of soil beneath or next to construction. In the case of utility construction, soil compaction provides a solid foundation around and above pipe and other utility lines where a trench is typically used. The most common machines to compact soil in this confined utility application are rammers and trench rollers, but there are a number of factors to consider when selecting the correct machine for the job.
Technically, soil compaction is the removal of air voids from the soil by applied force. The result is a denser soil with the ability to hold greater weight. The natural ability of soil to support a structure becomes altered during excavation. Its density changes and some soils become exposed. Mechanical soil compaction accelerates the natural process of the soil settling and, in some cases, makes construction possible on marginal building sites.
Proper soil identification is the starting point to determine which type of equipment is needed to get the best compaction results. Soil is classified in two main categories: granular and cohesive.
Granular soils consist mainly of sand and gravel. The particles are coarse and large enough to see with the naked eye. When granular soil is wet, it may be molded but will crumble easily. Granular soils are best compacted with vibration energy. The type of machine would be either a vibratory plate or vibratory roller. The vibration action reduces the frictional forces at the contact surfaces, allowing the particles to fall freely under their own weight. At the same time, as the soil particles are vibrating, they become momentarily separated from each other, allowing them to turn and twist until they can find a position that limits their movement.
Cohesive soils are made up of silts and clays. The particles are very small and feel smooth when rubbed between your fingers. When wet, cohesive soils are sticky and can be molded or rolled into almost any shape. When dry, this soil type tends to be very hard and difficult to crumble. Cohesive soils are best compacted by impact force. This is when a rammer or sheepsfoot trench roller is selected. The impact force of the rammer or sheepsfoot roller causes a shearing effect that squeezes out air voids and excess water from between the particles.
Along with the type of soil being compacted, machine selection can be dictated by jobsite specifications. The most common specification includes a percentage of proctor density, lift (soil depth/layers) and the number of passes made with the machine to achieve optimum compaction.
The machine should have enough power to compact the soil in the depth required. If the machine does not have enough power, the proper density will not be reached. If it has too much power, over compaction can occur. This is the result of excessive passes or too much power on a thin layer. How many passes? This varies with the situation, but the rule of thumb to attain 95 to 100 percent Standard Proctor density, which is a laboratory test used to determine the maximum soil density and load bearing capacity, is three passes with a rammer, four with a vibratory plate and five with a roller.
Of course, the actual jobsite condition is an important factor in selecting equipment. In general, vibratory plates are more efficient than rollers due to the large baseplate surface area in contact with the soil. Vibratory plates are also more maneuverable so they are ideal for confined areas. Rollers have the advantage in larger, open areas where there is plenty of room to maneuver. Because the roller has a fast travel speed, it will cover the area faster than a plate. The same logic is applied to rammers. The rammer should be used on cohesive soils in confined areas and the trench roller should be used in larger trenches and open areas.
Keeping an Eye on Safety
Safety on the jobsite is more than a trend, it is a reality — protecting your operators, protecting your jobsite and protecting your business is paramount to your success. One way to foster safety on the jobsite is through selecting the safest equipment available. For utility contractors, trench compactors are commonly used and are often controlled with a remote control.
There are two types of control signals: radio frequency and infrared. While radio frequency controls can be effective, this type of remote control does not require the machine to be within the line of sight of the operator, which can be dangerous because the operator could unknowingly move the machine behind a structure or other obstruction and put another worker in danger. Radio-controlled machines will even keep moving if the operator is distracted and turns away from the machine potentially putting him and other workers in harm’s way.
An alternative to radio frequency is infrared remote control, developed and patented by Wacker Neuson. This innovation offers safety advantages because the operator is required to maintain visual contact with the unit at all times in order to control it. Combined with a built-in safety zone that stops the machine should the operator come within about 6 ft of it, makes the infrared remote system an important risk management tool. Wacker Neuson also offers reversible plates with the same infrared remote control technology. Because infrared controls do not rely on radio frequency, the risk of interference with other equipment is also minimized.
Safety and Productivity Go Hand-in-Hand
The concept of safety is growing beyond the traditional concept of protecting the operator from injury-causing accidents. It is expanding into overall jobsite safety and protecting the operator from long-term exposures to noise, hand and arm vibration and exhaust. Regulations for personal protection, as well as engine emissions, are increasing. Savvy contractors know that effective machine selection includes safety that will enhance productivity and, for that reason, prefer to buy equipment that offers comprehensive operator and jobsite safety.
Fred Paul is a Sales Engineering Manager for Wacker Neuson Corp.