In addition to a number of alternatives in size, you also have a choice between high-pressure air or high-pressure water (hydroexcavation).
It is generally accepted that hydroexcavation works most efficiently in a wider range of soil conditions than a high-pressure air unit. "The highly concentrated, high-pressure stream of water is very productive in cutting through the ground," says Smith. "As you work in multiple areas, you'll likely encounter a range of soils from clay to gumbo to loam to sand. A water-based system will allow you to perform in all of those locations."
Water also acts as a natural lubricant and controls static electricity. "Anytime air moves through the soil, it creates friction, which creates static," says Deepesh Nayanar, product manager for hydroexcavation at Vactor. "With an air system, there is potential for a problem with static electricity, especially in sand because of its conductive properties. Some companies will offer anti-static lances, but it's still an issue to consider."
Hydroexcavation has the upper hand in year-round applications, which is important in northern areas where the ground typically freezes. "We would typically heat the water to about 160° F to cut through frozen ground and also in hard clay," says Nayanar. "With air, you just can't do that."
Water also prevents the sandblasting effect of air to promote a clean, relatively dust-free working environment.
Benefits of air
Air is not without its proponents and its preferred applications.
"The biggest benefit of an air system is that you have dry spoil to put back into the pothole," says Savage. "In many cities and municipalities, there are restrictions for using wet spoils. That means the slurry created with a water system will have to be hauled away and dry spoils brought back."
That's because the property of the soil is changed with the introduction of water, says Nayanar. "You change the compaction ratio so wet soil isn't reusable until it's dry. You can't just dump wet spoils back into the hole."
Air also has the advantage in that it's always available. "You don't have to bring air with you to the jobsite," notes Fuller.
For both air- and water-based units, noise reduction has been furthered through the addition of soundproofing and silencers. This makes them much more conducive for working in residential neighborhoods, furthering their diversity.
"Although the vacuum excavator market basically came to a halt with the directional drill industry, it's become a much more stable market," says Savage. "What's kept it that way is the diversity of the vacuum and contractors finding other uses for them."