Earth, Wind, and Water

Hydro Excavation and Air Excavation Operations
Hydro excavation uses both positive pressure in the form of an applied force to a high-volume stream of water and negative pressure in the form of a vacuum applied to the pooled decant. The stream of water is applied with precision both in terms of targeting, water velocity, and water volume. The impact of the high-pressure stream of water breaks up soil, penetrating through its surface and dislodging soil and organic materials. The water used to perform the excavation settles in the excavation hole, accumulating and suspending the dislodged soil and other materials.

Once the water is deep enough (with enough depth to prevent cavitation of the vacuum pump) the vacuum truck’s extraction hose is inserted and begins drawing out the sediment-laden water. At this point, provided that the water stream’s flow rate can match the vacuum extraction rate, the vacuum truck can operate continuously. This results in a steady operation; however, the amount of water left in place can obscure observation of the excavation’s progress and still requires lowering the inlet elevation as the bottom of the excavation goes deeper.

In addition to mass excavation, hydro excavation can be used in an exploratory mode. This is referred to as “potholing” or “daylighting.” It is used to blast away the soil surrounding buried utilities and pipelines and expose them to observation from the surface. This can be used to expose cracks, leaks, and other pipe failures while facilitating repair operations. This is much safer than using backhoes or excavators to poke around in the soil. Equipment used in this fashion is being operated in a very inefficient manner. Instead of mechanical excavation equipment, direct manual labor can be used. Laborers armed with shovels and pickaxes can dig exploratory holes the old-school way. Needless to say, this is both extremely time consuming and very expensive given the low productivity of human diggers and the resultant labor costs. Hydro excavation is also much safer. Physical contact by machines or humans wielding shovels that can damage a buried utility is eliminated. Water flows around impervious solid objects like buried pipes, wearing away the surrounding soil and leaving an intact, undamaged pipe exposed. Given its ability to focus force in a tight area, hydro excavation can minimize overall soil disturbance by limiting it to a tightly controlled area and avoiding damage to surrounding foundations and pavements. This, in turn, reduces the cost and schedule required for post-construction site restoration as well as minimizes potential environmental impacts.

In addition to mass excavation and exploratory daylighting, there are slot excavations. These are narrow trenches of variable depth that are cut into the earth to allow the installation of utilities, cables, and pipelines. These narrow excavations can be cut with accuracy and precision with hydro excavation, utilizing a single jet of high-pressure water to cut through the soil like a knife. And the force, water volume, and flow velocity of this particular “knife” can be adjusted during the excavation to allow continuous variations in depth and width as the cutting continues. This method has several advantages over traditional mechanical trench excavation practices.

Mechanical methods are time-consuming and often require workers to enter a potentially dangerous trench. This, in turn, requires the use of expensive shields and boxes to protect workers from sidewall collapse. Furthermore, mechanical trenching can either produce a deep trench (with backhoe and excavator bucket) or narrow slit (with a Ditch Witch)—but it can’t produce a trench that is both narrow and deep. This ability to minimize depth also minimizes the amount of earth that has to be removed, which in turn minimizes costs. Backfilling is required in traditional trenching, which also adds to time and cost. Mechanical slot trenching can also damage any existing buried utilities within the trench or that cross it at some point. All of these problems are avoided with the use of hydro excavation.

And then there is the use of hydro excavation for the removal of debris. Debris consists of organic landscaping material and vegetation as well as inorganic construction and demolition debris. Hydro excavation provides a cost-effective alternative to burning, grinding, and chipping debris. Using its inherent precision and accuracy, hydro excavation can be used to remove debris without increasing the hazardous nature of a construction and demolition site and without damage to surrounding areas. Traditionally, backhoes and claws have been used for the removal of debris. The methods used were the same as traditional excavation efforts but with poorer results, as debris is far more difficult to work with than soil. Mass excavation of debris is wasteful in that it inevitably also removes non-debris materials.

While not suitable for the removal of large chunks of steel and concrete, hydro excavation can safely and efficiently remove soil from in and around structures with pressurized blasts of water. The resultant slurry is collected into one corner of the site where it is removed by a vacuum truck for eventual transferal to a large debris tank or holding pen. As with soil excavation, hydro excavation of debris is safer and more efficient and cost-effective than mechanical means.

Lastly, the controlled precision of hydro excavation can be used to blast vertical holes. These holes can be used for the installation of poles, sign posts, foundation pilings, etc. By varying the volume and pressure of the water jet, holes of various diameters and depths can be created. By precisely removing only the soil that is necessary, there is no over excavation—and therefore no need for the additional expense of backfilling and compacting the soil around the installed structure.

Air excavation operations tend to be more localized and used for the removal of sandy or loose soils. The air jet and the vacuum extractor move together over the soil surface at a height of about 2 feet and at a movement rate of about 2 feet per second. Dust generation is always an issue. Therefore, the contractor should lightly spray water on the working surface to minimize dust, but not wet the soil so much that it becomes cohesive and hard to remove. Using this method, air excavation can be expected to remove more than 2 inches of existing loose soil with each pass (hard soil excavation usually removes less than 2 inches per pass).

Like hydro excavation, air excavation provides significant advantages over mechanical digging. The process removes the danger of damage to existing utilities by the sharp edge of an excavator bucket. No direct labor is required for digging, saving costs and time. It can operate easily in smaller spaces and limited work areas. However, it is less effective in hardpan, tightly compacted structural fill soils or frozen soils. And the hole it creates is relatively small—12 inches by 12 inches being typical—but ideal for exposing utilities. Air excavation has a faster removal rate and saves time. For the amount of soil removed, air excavation can be cheaper than hydro excavation.

The Mechanics of Hydro Excavation, Air Excavation, and Vacuum Extraction—Comparisons to Traditional Excavation Methods
Now that we know how a hydro excavator and vacuum truck are used, how do they work? What are the internal mechanisms that provide the force for excavation and removal? In general, the process uses a combination of highly pressurized water and a vacuum. With these two sources of energy, hydro excavation can clear out structures and areas of debris. The pressurized water is blasted into the ground through the use of a handheld device. The slurry is then vacuumed away from the site by a debris hose and is transferred to a large debris tank.

Applied pressure and force can move mountains. In this case, the applied pressure and force of a water jet can move soil. The water is ejected from hoses at a rate of 5 to 15 gallons per minute (gpm) at 3,000–4,000 psi. The lower pressure is generally used except in especially tough or frozen soils. More precise excavation will utilize higher pressure with less water volume. Broader excavation may require the opposite, utilizing larger volumes of water at lower pressure. Assuming a resultant slurry that is roughly 50% solids content by weight (and having a specific gravity of 2.5), approximately 2.5 volume units of water will be required to excavate one volume unit of soil. Such large amounts of water cannot be practically supplied from a mobile storage tank. It is much better to tap into a local source of water (hydrant, surface pond, etc.).

The characteristics of the resultant soil slurry can vary greatly with each work site, soil type, depth of excavation, and amount of water used. A soil slurry with a solids content of 40% to 50% is not uncommon. With the weight of water being 62.4 pounds per cubic foot (pcf) (or 1,000 kg/m3), a typical soil could have a specific gravity of 2.5 and would have a density of 156.1 pcf (or 2,500 kg/m3). Assuming a concentration of solids by weight in the slurry of 50%, the resulting density of the slurry would be almost 90 pcf (or 1,429 kg/m3).

Removing the resultant slurry requires the application of a vacuum. There are two means to apply this vacuum, a powerful fan system or a positive displacement blower. A fan system can rapidly move large volumes of air. This results in faster excavations and better control of the removal process. Fans also tend to be lighter, less bulky, and cheaper than positive displacement. Positive displacement blowers can remove air from greater distances. This allows for the greater offset of the vacuum truck form the immediate work area and at greater depths.

Vacuum trucks can operate as far away as 400 feet from the excavation site. This greatly reduces traffic congestion at the excavation location and simplifies the requirements for choreographing the movement of the equipment. The hose utilized to extract the slurry is typically either 6 inches or 8 inches in diameter. The suction can be powerful enough to lift rocks that are larger than the hose diameter and weighing up to 100 pounds, then place them to one side by maneuvering the hose while the rock is stuck to its opening by negative pressure. The 6-inch tube can typically extract 3,000 cubic feet per minute (cfm) and the larger 8-inch tube can remove 5,500 cfm. However, the larger tube will need an engine with greater horsepower and have a higher fuel consumption rate.

Vacuum pumps suck up the slurry of wet excavated material at a rate of at least 1,000 cfm. The slurry is then deposited in tanks on the back of the vacuum trucks that can hold between 300 to 5,000 gallons (40 to 668 cubic feet) of slurry. At the high-end 5,000-gallon tank capacity, the slurry described above with its density of almost 90 pcf would result in a truckload of 60,000 pounds or 30 tons. At 50% solids content by weight, this is equivalent to 15 tons of soil.

A load cycle for excavating the slurry is primarily a result of the truck’s holding capacity and its suction rate. With 1 cubic foot equal to 7.48052 gallons, an extraction rate of 1,000 cfm would be roughly equivalent to almost 7,500 gpm. With a tank capacity of only 1,000 gallons, it would appear at first glance that loading a vacuum truck would be a relatively quick operation. But time is required to position both the truck and the inlet hose. Proper depth has to be maintained for the inlet to prevent pressure breakage and cavitation.

Dumping at the point of deposit is via a wide-opening door at the end of the tank. Dumping is typically accomplished with just gravity, though some systems employ applied pressure in order to “pressure out” the slurry quicker with a direct discharge. This can also be used if the disposal point is actually at a higher elevation than the discharge invert of the tanker truck. The time required to dump and discharge the slurry depends on the thickness and viscosity of the slurry.

Dumping is just the last step in the hauling cycle. This cycle includes truckload time, truck maneuver in load area (typically 0.6–0.8 minutes), and maneuver and dump time at dump point (typically 1.0–1.2 minutes). Adding these durations to the hauling time (while loaded) and the return time (while empty) gives the trucks total cycle time. The total capacity of the tanker truck divided by its total cycle time gives its overall productivity rate.

Off-site dumping is typically not required for air excavation. Once the air excavation process begins, the soil is broken up by the high-pressure air jet. As soon as the soil is loosened, it is extracted by the associated vacuum. As the hole advances and utilities are uncovered, the extracted soil accumulates in the excavator storage tanks. Afterward, the excavated soil is returned to the soil as backfill. This minimizes the disruption caused by the operation and removes the need for stockpiling or dumping soil.

An equivalent articulated truck used to haul off soil that has been dug up by a backhoe or excavator has similar bulk operational characteristics when it comes to loading and offloading. The 5,000-gallon capacity of a large vacuum truck is a volume equivalent to 668 cubic feet, which in turn is equal to almost 25 cubic yards and a projected load of 30 tons. This is in the same range as the struck capacity of a mid-range articulated truck such at the Caterpillar B series 735 off-road truck, which has a heaped volume capacity of 25.8 cubic yards and a maximum load of 36 tons (Source: Caterpillar Performance Handbook, 43rd Edition). One main difference is a vacuum truck can load itself, while an articulated truck is loaded by the excavator. This also reduces the truck’s overall cycle time.

In addition to being able to expose buried utilities in a safe, non-destructive manner, hydro excavation increases safety for the workers doing the excavating. Less labor is required, and workers rarely, if ever, have to enter the actual trench or pit that is being excavated. Minimizing the need for heavy equipment (and the movement of this equipment) greatly reduces the chances of a tragic accident occurring that would involve a nearby worker or pedestrian.

Major Suppliers
Since its founding in 1949, the Ditch Witch organization has grown from inventing the first mechanized, compact service-line trencher to being a one-stop source for underground construction equipment. Introduced in the early 2000s, Ditch Witch vacuum excavators help with cleanup and hydro- and air-excavation on job sites globally. The newest in the organization’s lineup of vacuum excavators are the low profile HX models. The HX units feature patent-pending designs for a wide range of applications. They can be used for compact urban projects as well as large-scale excavation, potholing, and slot-trenching and micro-trenching projects. This product line includes three models—the HX30, HX50, and HX75. Each features a cyclonic three-stage filtration system and debris tanks sized at 500 or 800 gallons as well as freshwater tanks of various sizes.

Ditch Witch machines have been used by utility and electric cooperatives to avoid buried utilities while digging. One example is the Tri-County Electric Cooperative (TCEC), a Touchstone Energy cooperative based in Hooker, OK, and serving approximately 23,000 residential and business meters in five states. TCEC recently acquired a Ditch Witch FXT50 truck-mounted vacuum excavator. Powered by a 49-horsepower (hp) diesel engine, it can use either high-pressure water or air excavation systems serviced by a 400-gallon water supply tank and 800-gallon spoil tank. “We are doing a lot of potholing with the FXT50,” says Rick Wayman, TCEC manager of construction. “Many areas we work in are full of city utilities, and vacuum excavation can quickly make potholes without damaging the lines being located.” TCEC is using the machine to dig piers for light poles, usually in areas already full of utilities. The use of this machine avoided the need for hand digging. Similar results were obtained when digging a 30-foot-long trench. “It was an area filled with utilities,” says Wayman. “Soft excavation allowed us to dig the trench without damaging anything that was already in the ground.”

McLaughlin Underground manufactures a wide range of trailer and truck mounted vacuum excavators under the Vermeer brand. The product line begins with the Vermeer V25 and V30 Gas series vacuums that are designed for fluid management and small site cleanup. These units come with a 3-inch suction hose available in tank sizes ranging from 100–1,200 gallons. A diesel
version of the VX30 is also available with a 300-, 500-, or 800-gallon spoil tanks. The VX30 diesel model features automotive style belt tensioners, quick grip tool storage, high clearance trailer with Dexter torsion axles, standard tool storage box, quick drain service points, as well as an operator friendly “No-Hassle” hose with Strong Arm option.

Next in the McLaughlin vacuum excavator lineup is the trailer-mounted VX50 and the truck/skid-mounted ECO50 Series. Equipped with a 49-hp Kubota Tier 4 diesel engine, the VX50 and ECO50 Series vacuum excavators have a 4-inch wide suction hose and 1,025-cfm blower. Contractors can choose between 500- and 800-gallon spoil tank as well as optional features including a hydraulic boom and hot box water heater.

In the 5-inch hose vacuum excavator category, McLaughlin has the recently introduced trailer-mounted VX75 heavy duty series, and ECO75 truck mounted vacuum excavators. A 74-hp Deutz Tier 4 Final diesel engine powers a 1,200-cfm vacuum blower and an 8.0-gpm water pump that is capable of producing 3,000 psi of operating pressure. These excavators are available with a 500-, 800-, or 1,200-gallon spoil tank. There is an optional 5-inch boom, hot box, core saw, sewer jetter, and air compressor available for these models.

McLaughlin offers a Mega Vac line for high-production potholing. The PTO-driven, Vermeer VXT truck series is a 6-inch system that comes in three standard spoil capacity options: the VXT6 (1,200 gallons), the VXT8 (1,600 gallons), and the VXT10 (2,000 gallons).

Exclusive to all vacuum excavators built by McLaughlin include a three-stage cyclonic filtration system allowing for both wet and dry excavation and a cam-over, full open, external hydraulic door that provides a 360-degree positive seal, even under reverse pressure and without additional clamping to keep fluids securely in the spoil tank. In addition, McLaughlin ECO Series excavators deliver better fuel economy and lower operating costs than similarly sized systems that utilize a PTO-driven system.