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Whether using pressurized water or air, vacuum excavators streamline the underground construction project.

By Carol Wasson

Belowground congestion is hardly out of sight and out of mind. Utility hits mean big costs and even bigger safety concerns. Enter vacuum excavators. Whether employing a highly pressurized flow of air or water, these precision excavators prevent dangerous utility damage while minimizing soil disturbance. As a result, the current market for these units is fairly solid across the US and Canada, say manufacturers.

Vacuum excavators gained popularity on utility construction sites during the late 1990s when horizontal directional drilling (HDD) crews found them ideal for digging potholes, confirming the location of buried utilities near planned bore paths, and vacuuming up drilling fluids escaping from pilot holes. Since then their applications have expanded into such projects as tower base excavation, culvert and drain cleanout, sign and pole installation, sewer jetting, digging under existing structures, slot trenching, water/flood extraction, sludge removal, and on and on into tasks limited only by the contractor’s imagination.

With so many uses for the versatile vacuum excavator, the method of using water or air has always been hotly debated. Which element will better tackle the majority of applications? Not surprisingly, manufacturers usually back the one that they offer, and contractors usually boast about the one that they bought—and there is nothing wrong with that. Each method has its strengths and weaknesses depending upon soil conditions particularly. The good news is that several recently launched units offer both air and water, so today’s contractors can have the best of both worlds.

With that said, let’s take a look at just what is said out in the marketplace regarding the uses of air and water in vacuum excavation.

Water Flow
Hydroexcavation models use a flow of pressurized water as a force to break apart the soil. Compared with air, water units work faster, say their manufacturers. In its white paper, Vactor Manufacturing Inc. points to several additional benefits. Water works in almost all soil conditions. It acts as its own lubricant, controls static electricity, avoids the sandblasting effect of air, and can be heated to excavate frozen ground, says the company.

Water does have its drawbacks. If the job requirements exceed the capacity of the onboard water tank, a water source must be readily available.

However, keep in mind that some of the newer hydroexcavators offer up to seven hours of continuous operation with onboard water. Next, one must consider that the displaced spoil is wet and must be hauled off and disposed of, or must be dried before being backfilled on the job site.

Some sources state that water can be potentially damaging at higher pressures, because it does not compress like air when it encounters an underground line. Vactor recommends using water pressures between 1,500 and 2,000 psi, as pressure higher than 2,500 psi is unsafe.

Operating at the proper water pressure virtually eliminates the chance of damaging line covers or casings, or of risking operator injury, says the company.

Vactor also stresses that in most cases, it’s best to use between 3 and 9 gallons per minute of water volume. “Contractors are paid to excavate the soil, not to dump debris, recover water or get more water. Using the proper water volume is cost-effective and saves time. Importantly, the resulting debris will then contain minimal water content, which may often allow spoil to be put back into the excavation,” says Deepesh Nayanar, product manager for Vactor Manufacturing, which recently introduced its HXX Prodigy, a model that digs with water or air.

Photo: Vacmaster

The Vacmasters Systems 4000 air-vacuum excavation unit can quickly expose buried utilities.

“Air vacuum systems are very limited in clay soils, and will do only about 70% of what water will do. Also, air units require large compressors, which make them very expensive to run. So one must take into consideration their additional costs and limitations,” says Steve Schoenberger, president of Vector Technologies Ltd., a company with nearly 35 years in the vacuum excavation industry. Vector manufactures the HDD Vacuum Excavation Series, which offers 200- to 2,000-gallon spoil tanks and up to 235 horsepower in both wet and dry applications.

“Both hydro and air can be beneficial as a soft excavation method; however, water can cut through dirt and debris much more effectively than air and greatly aids in the speed of excavation,” says Ditch Witch Vac Product Manager Greg Adkins. “Using straight air can be an advantage in less-compact soil conditions or where the reuse of dry spoil is a requirement,” he adds.

Adkins says that before deciding on a particular method, overall unit size is a contributing factor. “Air units can be larger in comparison to equivalent hydro units, so truck or trailer size may be an issue. Ditch Witch offers 300-, 500-, 800-, and 1,200-gallon spoil tanks on our FX30 and FX60 models to give our customers options in determining the right size unit for their application,”
he says.

Airflow
When choosing air versus water, contractors have the advantages of using the system for extended time periods without having to refill a tank, as well as the ability to place excavated soil right back into the ground. “This is a big issue, particularly in states such as California where wet spoils are considered to be hazardous waste. The time and cost involved in hauling and disposing large quantities of liquid waste is significant,” says Roger Kirwan, vice president of marketing for Vacmaster.

Kirwan cites a number of additional reasons why his company feels that air-vacuum excavation systems are “the preferred method” of exposing underground utilities. “Contractors never have to worry about damaging the road base with large volumes of water or spreading contaminants to surrounding soil and groundwater. Also, contractors can avoid the potential for damage to utilities caused by the use of water, which is a noncompressible fluid. There is also no potential for electrical shock to operators as a result of using an electrically conductive fluid,” he says.

Arizona-based Specialized Services Company (SSC), a family-owned-and-operated excavation company, recently used a Vacmasters System 4000 air-vacuum excavation system on the Phoenix Metro Light Rail project. The new light-rail line spans 20 miles and will ultimately have the capacity to transport the same number of people as a six-lane freeway—up to 15,000 people per hour. The job involved clearing a 30-foot-wide path through crowded city corridors where all buried utility lines must be located and moved without damage. Utilizing the System 4000, SCC was able to quickly and safely expose buried utilities along five light-rail line sections, while causing the least amount of disruption to city streets and allowing other contractors to speed up their work and avoid delays, reports the contractor.

Although the Vacmasters System 4000 features selectable air or water digging, the manufacturer says that the system offers contractors the ability to pothole using supersonic air in any kind of soil, no matter how hard, wet, or compact.

Utiliscope Corp., which manufactures the Utilivac VE-75 air-vacuum excavator, says that before the invention of its model, air-vacuum systems were much more expensive that water-jet based systems. Traditional air-vacuum systems need large, high-horsepower vacuum pumps, complex filtering, cyclonic separators, and a standalone air compressor to operate the air lance. The introduction of the VE-75 unit changed all that, says the company, as it operates from a single 185-cubic-feet-per-minute air compressor and doesn’t need a delicate engine-driven pump, filters, separators, or the large truck needed to haul them around. The Utilivac system is designed to provide airflow of 850 cubic feet per minute and a static lift of over 22-inches of mercury.

Elaborating on what it feels are additional weaknesses inherent in the wet systems, Utiliscope says that a limited onboard supply of water means limited digging time as 100 gallons of water only gives 25 minutes of dig time at the typical flow rate of 4 gallons per minute. Also, areas around the excavation become very messy and slippery, and in cold climates, antifreeze must be placed in the water tank to prevent freezing.

New Models Dig With Water or Air
McLaughlin Group Inc. has introduced the first trailer-mounted combination air/water vacuum excavator that provides contractors the advantage of both methods in one self-contained unit, says the company. A power pack added to the McLaughlin 500-, 800-, and 1,200-gallon (1893-L, 3028-L, and 4542-L) units make the air/water combination possible. In the past, if a contractor wanted a combination air/water unit, a separate air compressor would need to be placed on the vacuum unit, adding additional costs and requiring a larger truck or trailer.

Powered by an 85-horsepower turbocharged Kubota diesel engine, the 150-cubic-feet-per-minute rotary compressor and a 3,000-psi water system provide safe soil removal. Water pressure is reduced to 1,500 psi at the nozzle end of the reduction tool with a four-jet reduction tool or a “wobble” nozzle-digging tool that uses 1,500 psi of rotary pulse spray to loosen the soil. Both options create a clean 6-inch hole, says the company.

McLaughlin also uses an industry-exclusive, three-stage filtration system that allows for both wet and dry vacuum excavation. First, material enters the tank at 130 miles per hour, and the large soil particles lose velocity and fall out of the filtration system.

Next, the air moves to a prefilter cyclone where the remaining small particles drop into a collection tank. Finally, the prefiltered air enters a washable micron-poly filter that traps the remaining fine particles.

“It’s very important to have a good filtration system. If you don’t, it will cost you a tremendous amount of downtime,” says Mike Moore, vice president of McLaughlin. “If you have a high-maintenance air filter, you’re constantly cleaning it. If you’re running a baghouse-type filter, you’re constantly emptying and washing bags. If you’re running a cyclone, you get away from that high maintenance,” he says.

Moore says that the new combination air/water vacuum system is an answer to requests from both the US and European marketplaces. “We saw misapplications out there—contractors losing money by using wet systems in sandy soils that were better suited for air; or those burning up time by using air in heavy clay. If they had one or the other type unit, it seemed like they often ended up with the wrong tool for the job—so this new unit offers both options,” he says.

Another recently introduced model that digs with either water or air is the HXX Prodigy from Vactor Manufacturing. More compact than the full-sized HXX HydroExcavator, Vactor says that the HXX Prodigy is available at a price point that will appeal to municipalities and contractors who are just getting started in vacuum excavation or are ready to upgrade from a pull-behind to a truck.

Like the full-sized Vactor HXX, the Prodigy blasts away soil with jets of 10 to 20 gallons of water per minute, at rates from 1,500 to 2,500 psi, depending upon pump selection, pressure adjustment, and nozzle tip configuration. Its variable-pressure triplex pump allows the operator to adjust water pressure with the push of a button.

For applications where dry digging is preferred, the HXX Prodigy also comes with an optional air-excavation system. Specifically designed for mobile applications, the unit features an above-deck compressor system that does not need a holding tank to build pressure. This eliminates the wait time some units require for pressure to build up, says the company.

“This unit packs a lot of performance into a compact, maneuverable package,” says Nayanar. He also notes that its 600-gallon water tank allows for 3.5 hours of continuous operation between refills. The unit is also available with a choice of either fan or positive-displacement (PD) systems. Nayanar says that some customers prefer fan air conveyance because it moves a large amount of air and excavates more rapidly than PD systems. “It can also be more forgiving for less-experienced operators. On the other hand, customers who are excavating over longer distances and at greater depths may prefer positive displacement,” he says.

The HXX Prodigy can be equipped with a telescoping boom with a 320-degree rotation and a total reach of 16.5 feet to provide greater reach, while still excavating to the same depths.

Selecting the Right Unit
Vacuum excavators come in all sizes and options, so it’s important to select a unit that will best fit your intended use. “I encourage contractors to look for a unit with a good-quality vacuum pump and filtration system,” says Dave Gasmovic, president and chief executive officer of McLaughlin. “They should also select a tank that has the capacity to hold one-half day’s worth of spoil. This will reduce the number of trips needed to dump the holding tank. Finally, be sure to select a strong trailer frame that will support the weight of the unit and a full tank of spoil,” he says.

Gasmovic also says that there are a number of options to consider—such as controls that allow the contractor to reverse the flow of the vacuum to blow the spoil back into the hole; booms that support the weight of the hose, placing less effort on the operator; combo units that include a jetter to clean sewers and remove the resulting trash; and automatic tank cleanout systems and hydraulic systems that allow the contractor to run a concrete saw or breaker off the unit.

“The vacuum excavator has come of age,” says Gasmovic. “Contractors are discovering that these units reduce labor costs and speed up projects.”

So whether the unit of choice uses air, water, or both options, perhaps it’s high time to go with the flow.

Construction writer Carol Wasson owns JCL Marketing & Communications Inc.

GEC - May 2008

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