The Best Excavation Method: Hydro, Air or Both?

Today’s vacuum excavators offer a range of versatility to meet new demands

In the old days, installing underground utilities usually began with open cutting along roadways. Those days are long gone. Especially in developed urban areas, today’s underground rights-of-way are already congested with fiber, gas, electric, water and sewer lines and damaging one of these utilities can lead to costly fines and downtime.

This presents a challenge for horizontal directional drilling (HDD) projects to install new utilities and has led to an industrywide best practice of underground construction operators exposing all nearby utilities before boring.

The most effective and efficient way of doing this is through vacuum excavation. This process is quickly becoming the norm and, in many circumstances, required by contractors and municipalities. This is not a new process, but as vacuum excavators become more prevalent on jobsites, manufacturers are trying to meet the ever-changing needs of contractors and utility owners with the evolution of their machines.

Today, vacuum excavators can be customized with many debris and water tank combination options, utilizing either air or water, to fit the need of virtually any size job. With a range of options, it’s important for contractors to understand the different offerings to determine which unit will deliver the most value to their projects.

When choosing between hydro or air excavation, contractors should consider many factors. Depending on the soil conditions, disposal requirements in the region and some other variables, one method may be more suitable than the other.


Before diving into excavation methods, it’s important to understand whether the vacuum spoils will be classified as dry or wet. This is typically determined by the paint filter test, in which a tester places material in a paint filter and observes if any portion drops from the filter within five minutes. If material does fall and spoils fail the test, they’re considered wet spoils, which can be more costly to dispose of.

Regulations and disposal requirements vary from region to region, so it is important that contractors understand local requirements before undertaking any vacuum excavation job.


Hydro excavation uses pressurized water to do the dirty work. It is the most widely accepted form of soft excavation, because it can be used in a range of soil conditions, including tightly compacted and hard soil, cobble and clay.

Because hydro excavation requires operators to dispose of liquid spoils and replenish water sources while on a job, selecting the proper digging nozzle and following best practices for water conservation are of utmost importance. Contractors need to consider the distance to a spoils disposal site and associated costs.

However, pressurized water typically exposes utilities faster than air. Plus, the ability to conquer various soil conditions quickly and efficiently makes hydro excavation the preferred method for many contractors.


Air excavation allows operators to break up soil with compressed air and vacuum dry spoils, which can be reused onsite as backfill. This method works best on softer soils, such as topsoil, loamy sand and some clay formations.

Unlike hydro excavation, which requires access to water, air excavation keeps machines running and operators on the jobsite without having to make trips to acquire water or dispose of liquid spoils. Additionally, many operators are turning to air excavation on jobsites as restrictions on liquid spoils disposal tighten and certified disposal sites become more difficult to find.

Operators need to keep in mind the soil conditions of the jobsite before deciding to use air excavation exclusively.


Operators don’t have to choose between hydro or air excavation; they can use a combination of methods on jobs. For example, they can start excavating the ground surface with air and switch to hydro once they reach harder soil formations. The water will cut through the clay and be sucked into the spoils tank to mix with the dry spoils from the air excavation. In most cases, this combination of spoils will pass a filter test and be permissible for backfill.

Today, most equipment manufacturers design vacuum excavators with both air and hydro excavation capabilities. This gives operators the flexibility to adapt to changing jobsite conditions and effectively perform in a full range of soils.


Using both air and hydro excavation in compliance with best practices will also provide the lowest chance of damaging a utility. Operators can apply these best practices to limit unanticipated costs and maintain productivity on all excavation jobs:


For both air and hydro excavation, operators should constantly move the nozzle around within the excavation area. This prevents excessive pressure from consistently hitting a specific area, preventing damage and keeping excavation moving along. Hydro excavation operators should use a rotating nozzle, also known as an oscillating nozzle, to deliver a stream of circulating water rather than a direct spray.


Holding the nozzle too close to the utility increases the risk of damage. Operators should keep the nozzle six to eight inches from the utility. Additionally, the nozzle should never impact the soil or be used as a shovel or pry bar. While it may seem convenient, this can clog the nozzle and decrease efficiency. When using an air excavator, it’s even more important to avoid putting the nozzle in the ground, as cleaning dirt out of the nozzle can be more challenging with air excavators. If you’re struggling to expose utilities in hard soil or heavy clay, hot water heater packages are an option with most vacuum excavators. Using hot water can help break down clay without applying additional water pressure. However, operators should keep the temperature below 150 degrees Fahrenheit and reduce water pressure to avoid damaging utilities.


The recommended pressure for soft excavation is 3,000 psi. Although many vacuum excavators and nozzles offer higher psi capabilities, too much pressure can damage utilities. The pressure should be reduced even further if using heated water. Utility owners often have their own recommended water pressure, so operators should always check with owners for specific guidance.

Vacuum excavation can be a helpful asset on a range of underground construction projects. With new applications for vacuum excavation on the rise, the machines will only continue to grow in popularity. Understanding the different vacuum excavation methods and best practices will help operators ensure jobsite safety, improve efficiency and increase productivity on a variety of projects.

Chapman Hancock is Ditch Witch<sup>®</sup> product manager, vacuum excavation. For more information, visit

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