By Sara Heger, Ph.D.
Often your choice of equipment for installing an onsite system is predetermined by the equipment your company owns. It is important, however, to understand the limitations of different pieces of equipment and recognize when renting or leasing equipment will facilitate effective and efficient system installation.
It’s important to select the right piece of equipment for the job; the size of job and impact on the site must be considered. For example, it is often more cost-effective to rent a track hoe for a day than to dig for a week with a backhoe. Likewise, a skid-steer with tracks often has less compaction potential than a man's boot print.
The key issues for an installer to consider regarding equipment are:
- Equipment currently owned
- Range of site conditions where work will occur (reach of equipment, accessibility, wet soils, slopes, etc.)
- Application — what type of technologies you plan to install
- Tasks you plan to perform related to the types of technologies
- Speed required for installations
- Matching equipment to the size of the crew
- Ground pressure and compaction
If you are considering buying new equipment, most suppliers will let you demo a machine before you purchase it. You can then determine if this equipment is essential to your day-to-day operations or if a rental is a better option.
For efficient installations, the speed of the machine and the skill of the operator typically dictate the crew size. Another important variable that impacts the crew size is site constraints. If you only have a small space to operate the equipment, one ground laborer may be all that is needed to keep up with the equipment operator. Membership in a professional association offers good opportunities to discuss equipment choices and issues with other installers.
Ground pressure is one key issue to consider with the equipment you own or when you are making a purchase. Ground pressure is the pressure exerted on the ground by the tires or tracks of a motorized vehicle and is one measure of its potential mobility, especially over soft ground. Ground pressure is measured in pounds per square inch. It can be calculated with the formula: Loaded weight ÷ Ground contact area
Increasing the size of the contact area, or footprint, on the ground in relation to the weight decreases the ground pressure. The ground pressure of motorized vehicles is often compared to the ground pressure of a human foot, which can be 9 to 12 psi while walking. A wheeled all-terrain vehicle may have a ground pressure of 35 psi while a tracked all-terrain vehicle has 0.75 psi.
Ask the dealer about the ground pressure of a particular machine. The ground pressure of all equipment on the site must be considered during construction particularly those delivering the tank and materials, but also from other contractors who may be on the construction site. The property owner should also be advised that traffic over the system should be avoided over the long term to minimize damage to the system.
Another issue to consider is that a loaded bucket can shift the weight to the front of a piece of wheeled equipment and change the ground pressure of the machine.
Wheels vs. tracks
Wheeled equipment has greater ground pressure than tracked equipment, as shown in the figure, due to the reduced ground contact area and therefore should not be used in areas where compaction is a concern. Wheels do provide quicker movement and do not damage paved roads as much as tracks, but they provide less traction in muddy soils.
Tracked equipment has lower ground pressure due to the larger footprint of the tracks. Tracked equipment is more stable, particularly on steeper slopes, and can be driven over a small spoil pile, whereas the material has to be moved out of the way with wheeled equipment.
There are multiple types of tracks:
- Low ground pressure tracks versus standard tracks
- “True” tracks versus those added to wheeled equipment
- Steel versus rubber
LGP equipment causes less compaction than narrow/standard tracks due to their greater width. Tracks can be added to equipment that is wheeled, but these are not as effective as “true” tracked equipment. Steel tracks last longer and are therefore more economical particularly on large equipment, but rubber tracks have better traction and cause less damage to pavement and grass.
Whenever possible you want to avoid compaction of the soil treatment area and choosing the correct equipment is a great step to meeting this goal.