Once thought of as companion equipment for horizontal directional drills, vacuum excavation systems are now stand-alone equipment, as contractors continue to find new ways to use them. Since vacuum excavation is a soft excavation system mounted on a trailer or truck, contractors rapidly discovered the practicality of the systems in many applications for which an excavator would be too large or too invasive. “Compared to traditional excavation methods, excavation using pressurized water or air can be precise, providing minimal ground disturbance and virtually eliminating the risk of damage to existing utility lines,” says Deepesh Nayanar, product manager, Vactor.
Although vacuum excavators are still vital for removing slurry and mud produced from HDD applications, potholing and soft excavation are now their primary uses. Keyhole operations such as potholing require precision excavation – exactly where vacuum excavators excel, since they can remove spoil just where needed, without the soil disturbance created by a machine bucket. If you’re using your vacuum excavation system primarily with horizontal directional drilling, consider the practicality the system offers for locating underground utilities, cleaning out culverts and storm drains, exercising a water valve box or hydrant and even cleaning up after a natural disaster. Mike Moore, vice president, McLaughlin, says the applications for vacuum excavators are extensive. “There are few jobs I would steer clear of,” he says. “The only application I would recommend not using a vacuum excavation system for is a job involving hazardous materials.” Since vacuum excavation systems now address a broad range of tasks, the systems themselves have a broad range of sizes, options and prices.
Water vs. air
When using a vacuum excavation system, you have two main choices – water and air. Each type of system is different, and each offers pros and cons, depending on your job.
Water: These excavate with a flow of pressurized water. Since the water breaks up the material by force, water vacuum excavation systems excel in difficult material such as hard clay. Also, many systems either have a standard or optional water heater – particularly useful when working with frozen ground.
Water systems do have limitations, though. In many areas, you can’t put wet spoil back into the ground. Unless your system is equipped with an auxiliary air system to use in such situations, you must have a source of dry spoil, or you will have to wait for the wet spoil to dry before you can backfill. Also, consider the amount of time your job requires to complete. If it exceeds the capacity of your onboard tank, make sure a water supply is convenient. Nayanar says the latest models offer extended operating times, however. “The newest hydroexcavators provide up to seven hours continuous operation with the onboard water,” he says.
Use the proper system at the right pressure, says Moore. “Although the pressure of the vacuum is 3,000 psi, our system uses a 1,600-psi digging tool. If you keep the pressure in this area, you won’t damage utility lines.” Moore says to remember the flow rate is as important as pressure, though, and a water system is faster than air. Most water systems cut away soil with jets of between 4 and 20 gallons per minute, at rates of between 1,500 and 2,500 psi, depending on the pressure adjustment. Kelly Clark, chief operating officer of H2X, says 20-gpm pumps and 3,000 psi is the upper limit. “There is no need to have any higher psi rating on a hydroexcavator,” Clark says. “Some clients require us to turn the pumps down to 1,500 psi when working around soft plastic gas service lines.”
Air: These systems break up loose soil with a flow of high-pressure air, leaving the spoil dry. If you need to put the excavated spoil right back in the ground, an air system is a good option. Since the supply of air is unlimited, you can use the system for extended jobs without having to refill a tank. If the pressurized air comes into contact with a pipe or utility line, the airflow will simply be displaced around it, causing no damage to the utility.
Air systems will not be effective in every environment. If you work with hard or frozen soil regularly, you may prefer a water vacuum excavation system. Many manufacturers equip air vacuum excavation systems with auxiliary water systems specifically for this reason.
Options for different jobs
Auxiliary systems and water heaters are not the only options available for vacuum excavators. You can choose from a wide variety of accessory items, depending on your job, says Moore. “Deciding what an essential option is for you depends on the job. For example, the sewer jetter option is popular with contractors for performing cleanout before turning the sewer line over to a city,” he says. For most contractors, Moore recommends a hydraulic door with a flip switch on the outside to make cleanout easier. “The mechanism needs to be on the outside of the tank in case a hydraulic line ruptures. This option really helps employees stay cleaner on the job.” You can also equip truck-mounted systems with a telescoping boom with between 320- and 360-degree rotation for additional reach. If you need a greater payload capacity, some manufacturers offer supplemental tanks mounted on a tandem-axle chassis.
Vacuum excavators work on either a fan or positive displacement system. A fan system moves a large amount of material quickly. “With the fan system, the material mixes with the high-velocity air, creating almost an aerosol effect,” Moore says. “The positive displacement system is better – it creates a steady, solid airflow.” The fan system also has advantages, says Nayanar. “Some customers prefer fan air conveyance because it moves a large amount of air and excavates more rapidly than PD systems,” he says. “It can also be more forgiving for less-experienced operators.” According to Nayanar, however, customers excavating over longer distances and at greater depths may prefer positive displacement. Clark says although a positive displacement system can do any job a fan system can do, the reverse may not be true. “A PD on a truck-mounted system can excavate heavy materials and perform remote digs up to 400 feet from the truck.” Clark says fan machines, however, have small advantages over positive displacement – you can clean them out without damage to the fan itself in the event of debris carryover, you can move them easily from hole to hole and they don’t require filtration. “If you decide on PD units for your job, be sure the filtration system is adequate and easy to maintain,” Clark says.
Price points for vacuum excavation systems vary widely depending on size of the unit, manufacturer and options included, but a basic 100-gallon vacuum excavation system will cost about $15,000, while a 1,000-gallon system loaded with options will run about $90,000. The most popular size is the mid-range 500-gallon system.
The versatility and productivity of a truck-mounted unit makes them a more popular choice than the smaller trailer-mounted unit, Clark says. “A truck unit can do four to six times the work in a day a trailer unit can do, at only twice the daily rate. Because the truck unit has an extendable boom, performing deeper line spotting is easier.”