Vacuum Excavation, or "potholing" is the process of using vacuum and sometimes an assisting process such as high pressure water jetting and high velocity air jetting to “suck” a hole in the ground. The hole size can be from 4 to 12 inches in diameter and can be from 1 to 20 feet deep.
This process replaces manual or machine digging when there is the possibility of damage to anything in the underground vicinity of the area being uncovered.
With the number of utilities being run underground and the use of plastic pipe and conduit instead of heavier steel the risk of damage once a line is found by either a shovel or a backhoe is greatly increased.
Vacuum excavation greatly reduces this risk and is referred to as a “non-intrusive” digging method. Once on top of a line or cable the vacuum tube simply cannot continue down and the line being sought is safely found.
This process is being used by gas companies, water and wastewater utilities, telephone companies, and underground contractors doing work for all of the above.
Vacuum excavation began with the use of large vacuum trucks, originally designed for sewer cleaning but which could be adapted for digging. Exactly when this began I’m not sure but it has been at least 25 years ago. These trucks were large and very costly but at the time they were all that was available.
The interest in the process of vacuum excavation was not what it is today because at the time we did not have as many delicate underground utilities such as fiberoptic cable and small plastic lines that we now have.
Since that time several companies introduced smaller, cheaper, but equally powerful machines to do this work. Not only were they more affordable, they could get into tighter spaces, could be operated with minimal training by normal work crews, and were generally better suited for this work.
The large sewer cleaning trucks relied on the combination of high pressure water jetting to turn ground into a slurry that was then sucked into a tank. It could never be immediately re-used as backfill. It had to be hauled off and new fill hauled in to fill the hole.
Today’s smaller vacuum excavators can work dry in almost anything but clay or very hard soils.
The machines we are discussing here can be as small as something mounted on top of a 55 gallon drum to a 5 or 6 cubic yard system mounted on a trailer or a truck bed. The larger units can incorporate their own water and air jetting systems and some even have multiple debris tanks for storing wet or dry debris separately.
The increased uses of directional boring machines to tunnel from point “A” to point “B” instead of open trench digging to lay lines greatly helped the development of the vacuum excavators. The process of directional boring produced a slurry of Bentonite and water which had to be sucked from the boring machines launching pit before it overflowed and ran into storm sewers or down a city curb. Vacuum systems to recover the drilling mud were not required to have the horsepower necessary to “dig” but merely to slurp up the slurry.
When accidental piercing of underground lines by the directional drills started to occur because the line was not where it was indicated to be the drillers started to uncover lines to confirm their presence and depth before drilling. This is where vacuum excavators came in handy. The holes only had to be large enough to see the line or cable.
About 10 years ago vacuum systems were introduced for the municipal water departments to clean out underground gate valve boxes and meter boxes. These were also less powerful than today’s machines and could not dig a hole. The utility departments also asked for more powerful machines so that they could be used to relocate a valve box or excavate down to find lines where work was to be done.