April 2008: Feature Story

Air vs. Hydro

Vacuum excavation technology has been around in some shape or form since the 1950s. Today, there are two main forms of vacuum excavation: air and hydro.

A variety of factors influence which method is right for a given situation.

Air vacuum excavators use compressed air to loosen the soil and positive displacement blowers to vacuum the spoils into a tank. Hydro vacuum excavators use high-pressure water to loosen soils, and the residual slurry spoils are easily extracted into the debris tank.

Consider the size and cost of a typical contractor test pit — most are 50 to 100 sq ft, whereas air vac test pits are 1 sq ft — not to mention that backhoes and excavators often require a lot of manpower, closure of streets and are not as environmentally friendly.

Air vacuum excavator systems are generally used in dry sands, gravel and other loose materials, using little if any water. Hydro vacuum excavator systems work better on the clays and heavy, dense soils typically found in the Southern and Midwestern states, although there are some very powerful air vacs now on the market as well as “combo” vacs that offer air and hydro from the same unit.

The biggest mechanical difference between the hydro and air is filtration. Hydro requires very little filtration as few airborne particles pass through the debris tank and into the blowers. Air vacuum excavator systems, however, have significant amounts of small particles traveling through the tank into the filter cartridges. All of the airborne particles need to be extracted from the airflow before going through the blower system. The buildup of this dust on the filter media will begin to starve the blower of air. This creates heat and potential for damage to the system if the airflow cannot be maintained. Purchase, rental and maintenance costs for air vacs are typically more than for hydro.

While hydro vacuum excavators may appear to be less complicated and less expensive, there are other factors to consider. Hydro vacuum excavations often require hundreds of gallons of water for a day’s work. This greatly increases the size and weight of the truck or trailer, which could be a problem when working on soft or sloped terrain or in a tight area. The spoils from hydro vacuum excavators cannot be placed back in the hole and the surface cannot be restored quickly. Most significant, if the soil is contaminated, the increased volume from water may increase the handling and disposal costs of the spoils.
Air vacuum excavators weigh far less, and the dry material can be replaced in the hole and compacted using tampers. Disposal issues of contaminated spoils are also mitigated as no water or chemicals have been introduced into the soil.

There are additional economical and safety advantages of air vs. water. Water is a non-compressible fluid, so it will try to cut whatever it encounters. Air is compressible, so if it hits something hard, it will compress and flow around it, avoiding any damage. Air is also non-conductive, so it is safer for operators.

After the utility is exposed, the following information is typically recorded: utility, material, size, depth, condition, location (X, Y, Z), orientation, roadway section materials and depths, soil type and water table.

Air vacuum excavation can also be used at proposed boring locations to excavate below the utility window, which is usually about 8 ft. This is very useful where the mandated setbacks to utilities cannot be maintained or the location of certain utilities is in question.

Vacuum excavation is especially useful in applications where, in the middle of a big site, there are hidden underground utilities. It’s also great for checking for environmental contamination without exposing a large area.

If vacuum excavation is the endoscopy of the SUE world, then Mother Earth is our patient. We should treat her — and her people — right.

Mike Twohig is a subsurface utility engineer with Woolpert Inc. based in Orlando, Fla.