Going Under: Vacuum-excavation equipment

The vacuum excavator is a portable, dual-purpose machine. It is a “soft” excavator that can dig precisely controlled holes using either highly pressurized air or water, and it is a powerful mobile vacuum system for most types of job-site cleanup. This tool is used on projects to install underground power and telecommunications cable.


For crews involved in underground construction, the most common uses for vacuum excavators are digging potholes—exposing existing buried cable and pipe to visibly confirm their precise locations—and as a vacuum to remove drilling fluid that escaped from bore holes during horizontal directional drilling (HDD) operations.


Vacuum excavators are available in many sizes. Those used most often on utility projects are compact, trailer-mounted models that can be pulled by a pickup truck or larger systems mounted on dual-axle trailers.


Demand for vacuum excavators has grown as many project owners and some regulatory agencies require potholing any time a path of a bore trench closely parallels or crosses existing underground utilities.


Even though there has been little promotion of vacuum excavators to the electrical market, electrical contractors that install underground power and telecommunications cable recognize the value of these specialized machines.


Rosendin Electric Inc., based in San Jose, Calif., operates three trailer-mounted vacuum excavators.


“Most work is related to electrical construction, and potholing is the primary job for this equipment,” said Victor Mumford, fleet manager. “We dig potholes by soft excavation to be safe. For cleanup, we vacuum spoil from trenching, go behind rock wheels to clean up cuttings, to clean up pavement saw cuttings, and as a vacuum anytime we need a vacuum.”


Tri-County Electric Cooperative, a cooperative utility based in Hooker, Okla., recently put in service a truck-mounted, large vacuum excavator model.


“Being on a truck provides good mobility,” said Rick Wayman, manager of construction. “We do a lot of potholing with the machine. Many city areas we work in are full of utilities, and vacuum excavation can quickly make potholes without damaging the lines being located.


“We also are using it to dig piers for light poles,” Wayman said. “Locations where poles are being set also are in areas full of utilities, and without the new machine, they would have to be dug by hand—there are too many utilities to use mechanical equipment.”


A crew also recently used the vacuum excavator to dig a 30-foot-long trench.


“Soft excavation allowed us to dig the trench without damaging anything that was already in the ground. As a vacuum, we can see almost unlimited possibilities,” Wayman said.


Depending on soil conditions, a vacuum excavator can dig a 12-inch-square, 5-foot-deep pothole in 20 minutes or less without the risk of damaging the utilities being exposed, which is an inherent danger with mechanical equipment such as a backhoe or compact excavator.


The machine’s vacuum function picks up soil displaced during excavation for reuse as fill or for removal from the job site. Soft excavation causes less surface damage than mechanical-excavation methods and reduces disruption of traffic and other surface activities.


The Common Ground Alliance, a nonprofit, member-driven organization dedicated to establishing and promoting practices to prevent damage to the underground utility infrastructure, recognizes vacuum excavation as the preferred method of potholing.


Keeping construction sites free of drilling fluids also is being mandated in some areas, and the vacuum excavator is perfectly suited for that task. Like soil, the fluid is sucked up and deposited in the spoil tank for reuse as fill or for removal.


The choice is yours


When looking at vacuum excavators, many factors need to be considered.


Work site productivity with excavators depends on several factors: horsepower and torque curve of the power unit, type of positive displacement pump or blower used to create vacuum, capacity of pump or blower stated in cubic feet per minute, speed of air flow through the system, filtering of air flow, pressure and gallons per minute of water pump in the reduction phase, nozzle configuration of the excavation tool, and amount and pressure of water or velocity of air at the reduction head. Other considerations for evaluating equipment include size and weight, capacity of the vacuum holding tank, weight of the unit when the holding tank is full, capacity of water supply, capacity of spoil tanks, dust filtering systems on air units, and sound levels during operation.


A primary decision in purchasing vacuum excavator equipment is whether it uses water or air. Each option has its proponents, and manufacturers have persuasive sales messages for their particular products.


Ditch Witch manufactures water vacuum excavators and also provides trenchers, horizontal directional drills and other underground construction equipment.


“Pressurized water is more effective than air in most soil conditions,” said Jason Proctor, product manager, Ditch Witch. “Hydro excavation is effective in almost all soil types and is the most productive means of soft excavation. Water pressure is adjustable for exposing different types of buried utilities. It produces less flying debris than air excavation. Various nozzles are available to match different soil conditions, and heated water can excavate frozen ground and also can be used for cleaning grease and oil off equipment. Water pressure also can be used to blast paint off roadways.”


Be advised of the risk of personal injury and damage possible from improper operation of soft excavation with water.


About the question of potential damage by hydro excavation, Proctor said, “Water pressure is adjustable and can be set for lower pressure when digging in sensitive areas. Also, different nozzle types are available that perform appropriately for the intended task. For example, a rotating turbo nozzle distributes the water force in a conical shape versus a straight nozzle where the cutting force is directed in a single point contact. Experienced operators use different nozzles depending on soil conditions and the type of utility being excavated.”


The case for air excavation is presented by Trevor Connolly, vice president of Vacmasters, a leading manufacturer of air vacuum-excavation equipment.


“As a manufacturer of both types of systems, we encourage all of our potential customers to thoroughly study this issue and understand the economic and safety advantages offered by air systems,” Connolly said.


Connolly cited benefits of air excavation: air works much faster than water in most soils; the spoil stays dry for use as backfill; mud spoil disposal costs and problems are eliminated; the air won’t damage the road base; and air is much safer, with fewer injuries or damage claims.


Connolly discounts the claim that air excavation is effective only in certain soil conditions.


“To understand the real differences between the two methods of excavation, it is important to understand that it takes more horsepower to dig a hole with air than it does with water,” he said. “This is because air is a ‘soft’ or compressible gas and water is a ‘hard’ or noncompressible fluid. Air systems work by using the energy stored in compressed air to break apart the soil from within, while water works by cutting the soil and turning it into mud.


“One of the major problems with water is that it is very indiscriminate in terms of what it cuts—meaning it will cut through the soil as intended, but there is always the chance that it will also cut through the utility buried within the soil.


“On the other hand, higher horsepower air vacuum-excavation systems deliver air at supersonic speeds into the soil while the air is still in its compressed state. As the compressed air shoots into the tiny fissures and cracks found in all types of soil [wet or dry], it begins to expand and break apart that soil while it is simultaneously being vacuumed out.


“Overall excavation performance and speed depend on what size unit [is] being used. With an air system, smaller equipment with lower horsepower must work harder in more compact, damp clay, where larger, more powerful systems will be fine in these conditions. I point out smaller systems with less horsepower may struggle in more compact, damp, clay-like soils, whereas larger equipment will be just fine,” Connolly said.


Rent one today


Previously, vacuum excavators have not been readily available from equipment rental centers. However, that is changing.


United Rentals, the world’s largest equipment renter, currently has more than 130 vacuum excavators in its fleet, with 47 in the western United States.


“We have two models, both trailer-mounted, and both excavate with high-pressure water,” said Bob Krause, vice president of the Mountain West Region. “The larger model has [a] larger water supply and water tanks and is more powerful.”


United Rentals acquired its first vacuum excavators in 2000.


“We dipped our toe in the market to test demand,” Krause said. “In 2010, demand really started to grow.”


One driver for the growth has been construction of the public utilities, including power companies, municipalities, and the contractors who serve those markets.


“Our rental customers are using them to make excavations in rights-of-way that are already crowded with easements to access vaults, other underground infrastructure for repairs, and to uncover existing utilities [potholing] to verify their locations before construction that will displace the soil,” Krause said. “We’re finding that anyone who must dig in these conditions recognizes the benefits of soft excavation.”


Much of the work with the rented equipment relates to horizontal directional drilling where potholing often is required. On these sites, the vacuum capability is used to keep work areas free of drilling fluids that escape from bore holes. And as mobile vacuums, they can be used for virtually any type of jobsite cleanup, Krause said.


About the Author

Jeff Griffin

Freelance Writer

Jeff Griffin, a construction and tools writer from Oklahoma City, can be reached at up-front@cox.net.

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