From the magazine

A quick guide to vac excavation

Vacuum excavation systems have been around since the 1950s, and were originally used to clean septic tanks, car wash pits and to remove slurry from horizontal directional drilling (HDD) projects. Now contractors are discovering that these machines have a wide range of uses on site, from potholing for utilities, to cleaning valve boxes and digging post holes.

Applications

A wide array of attachments can expand a vac excavator’s applications. Different size-reduction tools allow cleaning of smaller water valve boxes and catch basins. A valve exerciser attachment, which mounts to the vacuum trailer, can save crews countless hours of exercising valves from one stationary location, eliminating the old-fashioned hand-cranking method.

Some contractors use these units to remove grain and coals trapped in the corners of barges, clean gutters, and remove the chips from stump removal projects. An expanding application for vacuum excavators is its use to excavate pot holes for road guard rails and traffic signs, as well as small excavation projects. If a larger hole is needed, the worker can repeat the process in spots. Using a vacuum helps eliminate the chance of damaging an existing underground line and requires less labour and time than using some other types of equipment.

Digging applications

Typically, vacuum excavators are thought of as a “÷soft’ digging tool that displaces soil using either pressurised water or pressurised air, depending on the make and model, to make precisely controlled excavations.

Portable vacuum excavation equipment such as suction excavators can quickly dig small, deep precisely-controlled holes to uncover buried utilities. Soft excavation technology can dig around buried pipe or cable. Operators can select the amount of air or water pressure, depending on the utility. A lower pressure should be used for gas and fibre lines in order to avoid damaging the line coating, while a higher pressure can be used for water lines.

Trailer-mounted vacuum excavators are well-suited for these applications because they are more portable than older units. Contractors can choose a 380 litre unit that fits into the bed of a 1 tonne truck or trailer units of up to 4,542 litres. These units also have the power to move displaced soil up to 60 m from the source and only one person is required to operate the unit.

Potholing

Potholing “Ó the process of digging a hole to locate utilities “Ó can be a disaster if you cut through utility lines or leave a mess. For years, the process of uncovering buried utilities to confirm their location required manual shovelling. The process was slow and labour-intensive. It was also risky because even the most experienced workers can make mistakes. To speed the process, workers began using backhoes. In many cases though, instead of saving time backhoes only shortened the time it took to cut through an electric cable or water line.

Potholing is advised anytime utilities lie beneath ground that must be excavated. Mechanical digging over utility markers is officially prohibited in many places.

Workers utilise a hand-held wand connected to their portable vacuum excavator’s water supply and use it to control a stream of water to displace soil. The unit’s vacuum component then sucks up the displaced soil and stores it in a holding tank. Later, the soil can go back in the hole or be removed from the job site.

Depending on soil conditions, a vacuum excavator can complete a 30 cm2 by 1.5 m deep pothole in less than 30 minutes. Vacuum excavators can dig deeper than 2 m, but utility potholes seldom need to be deeper.

Potholing: correct operations

Having the vacuum excavator perform at the correct speed is important. About 200 km per hour (130 miles per hour) is the correct air velocity to move a material that has mud, dirt and gravel in it. Use a method to apply the water to the ground – a reduction tool with four nozzles on it is recommended – with a water system of 3,000 – 4,000 psi.

If using the special reduction tool, it is recommended that the psi is reduced to a safe level where damage to the cable is unlikely.

The reduction tool is placed over the spot to be excavated, and the water trigger is keyed, which starts the flow of water into the reduction tool. The soil that is being loosened is vacuumed up along the way. The process is continued until you hit the utility, when the operator will start feeling resistance from the tool.

A reduction tool that has two nozzles can cross-cut the soil. The procedure leaves about a 6 inch hole. When it gets to a cable or pipe, the water exposes not only the top, but the entire cable. The vacuum pulls out all the water and mud, leaving the operator with a clean, dry hole. If the operator needs a larger hole, they can repeat the process in spots.

Potholing: soil considerations

Clay soil is difficult, but not impossible. Clay soils don’t work well with air systems. One option is adding detergent and polymers to clay soil to keep it from sticking.

HDD operations

Vacuum excavators are ideal for HDD operations, particularly for vacuuming up drilling fluids that escape from pilot holes during drilling and back-reaming. Compact, manoeuvrable vacuum excavators can do the job faster and more efficiently than other equipment types.

Similar to potholing, during the HDD process the unit’s vacuum removes spoil to a self-contained storage tank for reuse or removal and disposal from a project site. Although most drilling fluid additives are not harmful to the environment, project owners and municipalities usually require drilling fluids to be safely disposed of off-site.

Benefits of utilising a vacuum excavator

  • Reduce time and costs of potholing
  • Limitless versatility for site clean-up
  • Reduce OH&S risks associated with digging around live utilities
  • Can excavate in tight places, such as easements crowded with buried pipe and cable, where larger equipment can’t
  • Make easy work of digging holes to set utility and light poles, as well as sign posts
  • Can make small holes (eg for pipe and pipe-joint repairs, to plug unused pipelines, to clear the way to cut-off service lines, and to attach anodes for cathodic corrosion protection systems, etc) that are easier and less expensive and time-consuming to fill and repair than those made bigger digging machines.

Myth busted!

When considering a vacuum system, most people think that the larger the vacuum, the better. But that’s not true. Normally the machine runs with approximately 40 cm (15 inches) of mercury; bigger might be better if the operator is trying to lift a full column of water. In excavation, a solid column is not being lifted, so the secret is velocity, not inches of mercury.

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