Ever stick your thumb across the end of a garden hose ad notice how the speed of the water increases? That’s called the Venturi Effect. When you restrict the volume of pressurized water passing through an opening, its velocity increases in proportion to the amount of restriction. Jet cutting uses this same principal to cut a tunnel through the earth for a trenchless installation. Water jetting is one more tool in the trenchless industry's toolbox.
Jet Cutting Basics
Instead of the low water pressure from a garden hose, though, jet cutting, also known as water jetting, uses a high-pressure pump to boost the volume of water, raise its pressure to hundreds of pounds-per-square-inch, and replaces your thumb with a special nozzle. The water jet creates a slurry of mud in the bore. The mud does two things: it consolidates the borehole walls and it creates the need to remove the muddy slurry from the bore.
The water jet's nozzle raises the pressure of the water flow, but as soon as the water leaves the nozzle, the pressure begins to drop and the diameter of the jetted water begins to expand. Both factors must be accounted for in planning a project that uses a water jet.
Vacuum excavator technology, which has been around since the mid-1960s and includes both a water jetting function as well as an industrial-strength Shop Vac-like suction/mud vacuuming function, provide an answer about removing the mud formed by the dirt that mixes with the high-speed water from the water jet. The water jet and the vacuum suction head can go downhole together, by design. As the nozzle forces its way into the bore (the water jet is only effective for a few feet at a time), the suction head vacuums the muddy water out of the borehole, leaving the muddy cake on the wall of the hole. (Also read "A Look at Hydro Vacuum Excavation.")
Water Works Better...
Just as trenchless installations have advantages over conventional trenching work, water jetting, as a form of trenchless technology, has some unique advantages over other trenchless methods. Water jetting might make a job-saving difference for projects in areas where a proliferation of underground utilities could mean damage claims resulting from soil expansion that cracks adjacent pipes or conduits.
This is because the vacuum excavator cuts a bore to the design specification with only minimal soil expansion and the soil is diluted into a muddy slurry and removed from the bore immediately (and because the water jet hasn't the pressure to cut through pipes or concrete, underground utility conduits).
Another benefit is that immediately adjacent utilities aren't an issue, public safety is a non-issue. No burst sewer lines or flooding streets. No exploding gas lines throwing manhole covers 20-feet into the air. Also included in the benefits package are all the benefits of a trenchless installation, including a lack of surface disturbance and a hindrance to the public.
The Disadvantages of Jet Cutting
Water jetting requires special training and the process can be time-consuming. The first part of that statement, training, is part of business-as-usual. The second; art, though, means it's not an option for jobs where contractual obligations impose rigorous-but-reasonable time limits. If your customer has to have it done by (whenever), you'll do what's needed to get the job done by that date, short of endangering your crew or the public. And that may be a selling point for municipalities or even private parties.
For example, if the most direct design path for the project is essentially a clear, straight 200-foot run – except for a 20-foot-wide hornet's nest of massed utilities a foot from where you'll make your final connection – water jetting under, around or through that hornet's nest makes sense. You won't be sued for damages to those utilities, nor will your customer, by irate utility customers whose homes were damaged or destroyed by a totally avoidable gas main explosion. (Read "The Science of Getting it Right: Locating Underground Utilities.")
That's a strong incentive for the thoughtful contractor to include the possibility of using a jet cutting system (for such an area, at least) in their estimate, bid and contract and for the thoughtful utilities board or commission to look beyond their budget. (Read more in "How Bid On That Trenchless City Project.")