Web Exclusive: Va-Va-Voom Vacuum Excavation
by Greg Ehm
March 1, 2009
As the ground under our feet becomes more congested with utility lines, it’s vital to visually identify the location of these utilities (gas, fiber, water, telecommunications and sewer) to ensure the safety of your crew and protect your bottom line.
“Not taking the time to visually locate underground lines can have detrimental effects on the safety of your crew and could impact your company finances with heavy fines and repair expenses for hitting an existing underground line,” says Mike Moore, vice president of sales for McLaughlin, based in Greenville, South Carolina.
The nationwide One Call system has helped to make identifying infrastructure easier. However, the vast and complex array of underground lines means that One Call contractors’ marks may not be as accurate as underground projects may demand. This is where potholing comes in.
Potholing has been around for years, but many contractors still don’t take the time to visually identify underground lines before digging. Potholing is a technique to expose a utility prior to excavation and allows the contractor to know exactly where the utility is located, right to left, as well as depth. This information can help avoid construction damage to the utility and to the people around the utility.
According to Moore, the process can be done in different manners.
“One method is to use a shovel, pick or digging bar and carefully remove the soil until you find the lines,” he says. “The only problem is each has a cutting edge and can damage the line you are trying to identify. A more efficient and nondestructive method is to use vacuum excavation.”
Vacuum excavation gives a very clean hole; it’s quick, efficient and nondestructive to the utility exposed. Another benefit is that the process is clean and does not leave a pile of dirt in a homeowner’s yard.
Vacuum excavators are self-contained units that use pressurized air or water to displace soil and create dry or wet spoil. The displaced dry or wet spoil is removed from the area through a hose using high-velocity suction and stored in a holding tank on the vacuum. These vacuum excavators can be mounted to a trailer or the back of a truck and range in size from 100 to 1200 gallons of capacity.
Since vacuum excavators use low-pressure air or water to remove spoil, they can aid in potholing to identify existing utilities during underground construction projects. In fact, the air and water move around the existing utilities, giving the operator a clear view. Operators can select the amount of air or water pressure depending on the utility. A lower pressure of 1500 psi should be used for gas and fiber lines in order to not damage the line coating. A higher pressure can be used for water lines.
“Water is a more effective method for digging and works better in different types of soil,” Moore says. “It’s quicker than air in reducing the soil, but the downside is that you’re left with spoil that’s muddy. Air, on the other hand, is not as fast, but is neater. You’ve got dry material to put back into the hole, but not every soil works well with air.”
In the past, contractors had to choose between water or air units, but just last year a select few vacuum manufacturers introduced units that offer the choice of air and water in one machine. This allows the contractor to select the best excavation method depending on the ground conditions and the task at hand.
Before purchasing a vacuum, the size of the spoil tank is a consideration; how often do you want to go between emptying it? The larger the tank, the more potholes you can accomplish. Portability is also an issue; a smaller tank gets into the areas where the bigger units don’t reach as easily but a larger CFM unit has the ability to remove more material at a much higher rate of speed, which speeds up the vacuuming process.
Safety Considerations
On any jobsite, safety is paramount. Moore offers some tips to keep the site and your crew safe when potholing.
“First of all you want a clear work area, and no clutter,” he says. “Secondly, you need to be wearing dielectric boots and gloves. A vacuum excavation system is not going to damage the cable. But, if the cable already has damage to it, you’ll want to avoid any possible danger there. Eye protection is a must, as well as proper traffic control equipment when working along a street.”
Moore also stresses the importance of having a covered tip on the air knife or water lance . A sharp tip may slice or puncture a line.
Sidebar: Potholing at J.F. Electric
Ray Pour, with J.F. Electric in Edwardsville, Illinois, understands the importance of potholing. He oversees the overhead and underground transmission and distribution crews in the state of Missouri for the family-owned company.
“It’s becoming tougher to find an area where there isn’t something underground,” Pour says. “In subdivisions it’s common to find second and third generation electric and cable lines. Bore paths are more cluttered and companies are becoming more protective of their facilities.”
This makes Pour’s job of managing the installation of underground distribution lines tougher. Pour oversees a fleet of 16 horizontal directional drills (HDD) and five vacuum excavators spread all over the state. The vacuums work ahead of the drills to complete the potholing process since the company takes potholing seriously, as does the state of Missouri..
“In Missouri it’s a law to pothole if your bore will cross an existing facility,” he says. “But this is also a policy for our crews whether we are working in Missouri or Illinois.”
Missouri requires that the facility to be crossed must be visually located to determine the exact elevation. If the facility cannot be located, then the contractor can open the bore window by 2 feet on either side of the locate and excavate 2 feet (below the intended bore depth.
“If we pothole down 5 feet and cannot find the facility, the crew can bore at a depth of 3 feet, but must visually watch the head pass through the pothole to ensure the new facility does not interfere with an existing line,” Pour says.
Potholing is a very basic function, but a recent project in St. Charles, Missouri, required J.F. Electric to cross nine utilities. The challenge was that the nine lines were located underneath 9 inches of solid concrete. So in order to pothole, the crew working on the project brought in a core saw, and within 30 minutes two of the six plugs had been created and pulled allowing the potholing crew to do their job.
“It’s a tool [vacuum excavators] that is here to stay and it’s an integral part of our system,” Pour says.
Greg Ehm
Greg Ehm is a technical writer with Two Rivers Marketing in Des Moines, Iowa.
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