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With little fanfare, vacuum excavation equipment has begun making an impact on the way outside electrical infrastructure is constructed and maintained.
Vacuum excavators are portable, dual-purpose machines that are both “soft” excavators able to dig small, precisely controlled holes using either high-pressure air or water, and powerful vacuum systems.
They are available in many sizes, ranging from compact contained models on small trailers to large truck-mounted systems. For electrical work, vacuum excavators are most often used on underground distribution and power line projects, street and highway lighting work, and installing underground cable for traffic signals and controls and communications systems. Specifically, they are used to:
o Excavate potholes to uncover existing buried pipe and cable to visually confirm their exact locations before beginning construction that requires excavation or directional drilling;
o Dig holes for utility and lighting poles in easements containing buried utilities;
o Dig short trenches in easements containing underground utilities and in areas too small to bring excavating equipment;
o Keep directional drilling job sites free of drilling fluids that flow from bore holes;
o Clean out manholes, vaults and duct banks;
o Vacuum cuttings made by pavement saws.
In addition, the versatile machines can tunnel under sidewalks or driveways, make small excavations inside buildings by running the air or water hose through a window or door, and the vacuum has been used to pull cable through conduit.
Safely locating buried utilities
Potholing is perhaps the most frequently used application for utility construction.
Soft excavation technology can dig around buried pipe or cable without the risk of damage inherent with backhoes, excavators or other mechanical tools and do it faster and with less mess than conventional excavation methods.
Depending on the machine used and soil conditions, a 12-inch-square, 5-foot-deep pothole can be completed in 20 minutes or less. Most models are capable of digging much deeper, but utility potholes seldom need to be more than 6 feet deep. The machine’s vacuum function picks up soil displaced during excavation for later use as fill or removal from the job site. Soft excavation causes less surface damage than mechanical excavation methods and reduces disruption of traffic and other surface activities, and the small excavation is easier, faster, and less expensive to fill and repair.
The excavating potential of vacuum excavators isn’t limited to potholes—they are effective whenever a small, precisely-controlled hole is needed for any purpose. As portable vacuums, the machines are suitable for about any type of cleanup operation.
What users say
PAR Electric Contractors Inc. has used vacuum excavation equipment for several years; the company owns vacuum excavators and subcontractors also use the equipment on PAR projects.
“We routinely use them for potholing and for clean up on directional drilling jobs,” said Richard Holbeck, PAR regional vice president, Rocky Mountain East. “They also are good for removing water from manholes.”
Holbeck said recently the equipment is being used more frequently to dig pole holes.
“Easements these days are so full of buried lines, it often is difficult to dig by conventional methods,” he added. “And we recently had a job where a sub used vacuum excavation to dig trench around switch cabinets because the area around them contained so many buried lines.”
In Kansas City, Mo., PAR general foreman Chris Saban’s personnel use company-owned vacuum excavator equipment for potholing.
“It is essential to accurately locate buried utilities before work begins, and there is so much stuff in the ground these days vacuum excavation is the best way to do it with a reduce risk of causing damage during the process,” Saban said.
In addition, Saban said vacuum excavation often is used to dig pole holes, again because it reduces the risk of damaging existing utilities when poles most be set in crowded easements. The equipment also is used as a vacuum to suck water out of duct banks after rains.
The City of Chicago’s Bureau of Electricity placed its first vacuum excavator in service last year on a large street lighting project.
Potholing is the primary reason for acquiring the unit, said Mike Quinlan, construction superintendent. On the lighting project, the machine was used to expose utilities before directional drilling crews installed duct for power cable to new light poles. The machine not only speeded locating work, but eliminated the danger of damaging utilities with hand tools.
“We also have started digging pole holes with the machine,” continued Quinlan.
“It is ideal for moving light poles imbedded in concrete. We dig around the pole, pull it out, quickly dig the new pole hole with the vacuum excavator, and set the pole in its new location.”
Two methods of excavating
Vacuum excavation equipment digs either with high-pressure water or air. Each has advantages and disadvantages and the most important decision in selecting vacuum excavation equipment can be its method of excavation.
Water excavators are less expensive to purchase than air units and because water has a higher density than air, the force of high-pressure water effectively displaces most types of soil. However, digging operations are limited to the available supply of water in the machine’s holding tank. Some equipment users say that the resulting wet spoil is more difficult to handle than dry material produced by air units. Improper operation of water systems also has the potential of damaging the utilities being uncovered. While compressible air flows around pipes or cables, non-compressible water, when used at high pressures, can cut through cable or damage plastic pipe.
The on-board compressors used by air excavators generate a limitless supply of high-pressure air, which penetrates porous soils, blowing particles apart. Spoil remains dry and is immediately reusable. But air excavation isn’t always effective in all soils, notably caliche and wet, heavy clays. Also, air excavation can create dust during the digging process. Air units are more expensive to purchase than water machines of comparable size, but proponents of air technology say faster digging capabilities and the easier-to-handle dry spoil translate into greater pothole production, and they emphasize that high-pressure air does not damage utilities. Manufacturers of air equipment also say that today’s air units work effectively in more types of soil than earlier models. Some air equipment also includes water excavation capabilities.
Cost and productivity aside, the potential for damage to buried lines with water excavation equipment is the most important issue.
“The primary concern of anyone buying a vacuum excavator should be whether it digs with high-pressure air or water,” said Roger Kirwan, vice president of marketing, Vacmasters division of Barone Inc. Vacmasters manufactures air and water units, but is primarily known for its air equipment.
Water, unlike air, he explained, is non-compressible and therefore has the capability of cutting utilities. “Because of this, we instruct operators of water units to reduce pressure to between 1,200 and 1,500 psi and wear insulating boots and gloves.”
Vacuum Source Inc. manufactures equipment that excavates with water.
Said President Bill Gilman: “We provide a patented, four-nozzle reduction tool which allows for non-destructive exposure of utilities. We recommend the use of this tool, except in extremely difficult soils where a water lance may be required, and we caution against the use of a water lance unless the buried service has been checked and verified for location and depth.”
Gilman added that his company’s testing has found that water pressures below 2,000 psi will not adversely effect buried cable.
“By using a combination of multiple nozzles and selected degree orifices,” he continued, “we can control the water pressure at the face of the excavation. The only time the operator will see the full force (3,000 to 4,000 psi) of the water pump at the face is when he is excavating with a single-point water lance, which, again we emphasize, requires knowing the location and depth of any utility within the digging area.”
Of course, there are other factors to consider in addition to whether a machine excavates with water or air.
Productiveness is related to a combination of factors, including horsepower and torque curve of the power unit, type of positive displacement pump or blower used to create vacuum, capacity of pump or blower, speed of air flow through the system, filtering of air flow, pressure and gallons per minute of water pump in the reduction phase, nozzle configuration of the excavation tool, and amount of water and speed of air at the reduction head. Other considerations for evaluating equipment include size and weight, capacity of the vacuum holding tank, weight of the unit when holding tank is full, capacity of water supply tank on water models, dust filtering systems on air units, and sound levels during operation.
Buyers have plenty of choices from more than 20 suppliers. The smallest and least expensive vacuum excavator is an air excavation unit that can be carried on a small cart and sells for less than $10,000. It is powered by a separate air compressor. Prices for self-contained models on skids or trailer packages begin at about $25,000 and range upward to more than $100,000 for large units installed on trucks. EC
GRIFFIN, a construction and tools writer from Oklahoma City, can be reached at 405.748.5256 or [email protected].