Hydro- and Air-Powered Vacuum Excavation

There are many uses for manual excavation and hydro or air excavation (frequently called “vac systems”). The main use in the underground utility industry is to expose existing utilities to keep workers safe while installing other utilities with boring or trenching methods. Other uses of these vac systems can be to clean out manholes, sewers, culverts, excavations full of water or mud; or to install posts or poles, tunnel under walls and sidewalks, slot trench short distances, pothole for engineering purposes, pre-design, and other uses.

Underground rights-of-way and utility easements are becoming increasingly crowded, especially with fiber optic buildouts from multiple vendors. Potholing of existing utility crossings and closely parallel installation is the only safe way to verify the location of these utilities in order to avoid them while installing new facilities in the ground. We have used these positive verification methods since our founding, saving us untold thousands of dollars in damages and crew downtime while repairs are made, far offsetting the costs of potholing. Plus, there is increased goodwill shown by municipalities and utility companies.

Potholing with shovels or backhoes to find existing utilities used to be the norm and is still an accepted practice. Manual excavation is labor intensive, time consuming, and leaves an area for landscape restoration reflecting the size of equipment used. Weather and soil conditions also play a large role in the damage to the surrounding area while excavating.

In the late 1960’s, the first hydro excavation system was produced. As designs and technology improved over time, air-vac and hydro excavation systems became more compact and powerful excavation systems. Today, they are both in widespread use, with hydro excavation being used more often. Both air and hydro are effective systems depending on soil content and the specific use of the systems. Both also have a high degree of safety concerning the existing utilities. The high pressure air or water pulverizes or liquifies the soil respectively while not damaging underground pipes or cables. These exposed utilities are then identified, measured for depth, and direction verified so installation of new utilities can be done safely.

Potholing of existing utilities can be done in almost any environment. Potholes are typically started with small holes at the surface, then enlarged and deepened as needed to find the existing utility. Some are quite large and deep before the successful completion of the hole. The size greatly depends on the accuracy of the initial One Call utility locate done to find the areas that need to be potholed. Potholing in grassy areas is the easiest. However, sidewalk, parking lot, and street potholes are also common. Concrete or asphalt potholes are more complicated. First, a core is cut in the concrete or asphalt, typically 6-12” in diameter, using a water-cooled, diamond- tipped core drill. These cores are saved for later restoration. The pothole is then made with the vac system to find the utility, which is then identified, measured, and directionally verified. When ready for restoration, most municipalities and building owners require flowable fill (thin concrete) to be poured back in these holes up to the depth of the core to adequately fill all the voids. The saved cores are then placed back in the hole and expanding grout is used to cement them permanently in place. Codes can differ for this procedure between municipalities.

Both hydro and air systems can typically be run by one operator. These units can each be skid-mounted for placement in truck beds, trailer-mounted for towing behind vehicles, or truck-mounted permanent complete systems. There are advantages to each of these units. Skidmounting makes the units portable between vehicles and trailers and easily removable for repairs. Trailer-mounting is useful because you can haul them with many vehicles, so the unit is not shut down for truck repairs. However, the added length of truck and trailer can make it more cumbersome in some situations. Truck-mounted units are more compact and have a shorter footprint for smaller space requirements (like city streets). The downside to these units is if the truck breaks down, the system is also out of service. Typically, the larger the unit, the more suction created and the easier it is to move more spoils into the tank. Air-vac excavation systems are especially effective in sandy or loose type soils, but are less effective in hard clay soils or frozen ground. These air-vac systems are comprised of a high-pressure air pump with a small hose and an air lance (wand) with a nozzle designed to cut and pulverize the soil as much as possible. The results are then sucked through a large hose, typically 3-12” in diameter, then into a large tank. After the pothole is complete and ready for restoration, a large door on the back of the tank can be opened and the tank tilted to dump the spoils back into the hole for compaction where this type of backfill is appropriate. If the spoils cannot be used for backfilling, they must be hauled away to a dump site.

Hydro excavation systems are more common due to the types of soil they can excavate. These “vac systems” are comprised of a water tank with a high-pressure water pump, a small hose, a wand, and a high-pressure tip to cut and liquify the soil. The results are then sucked through a large hose, typically 3-12” in diameter, through a filter system into a large tank. These spoils must be hauled off for disposal at a public or private dump site. Private sites can be expensive places to dump over time. Large pools, length of drying time, and the consistency of the hydro vac spoils are part of the reason for the high cost. In the Kansas City area, it can cost up to $90 to dump one tank load. At K & W Underground, we save money by dumping the spoils on a company land plot. After allowing the liquified mud to dry, it is pulverized and used for top soil and backfill when appropriate.

The more utilities placed in the ground, the more crowded this environment will become. In the future, these vac systems will be necessary to maintain the integrity of existing utilities and to protect the public.

(This article previously ran in the 2019 dp-PRO Summer Issue.)

Scott Plumb is Consulting & Design Engineer for K & W Underground Inc. He has been in the aerial and underground utility contracting business for over 38 years. He can be reached at splumb@kwunderground.com.