Technology of Air-Vacuum Excavation

Benefits

Using an advanced air-jet to dig rather than conventional tools or water offers a number of advantages in vacuum excavation including:

• Air digging tools generate less worker fatigue / injuries than a pick or shovel.
• In most soils, excavating with air is much f aster than hand digging.
• A pick and shovel are not damage free tools; while air is non-damaging to all kinds of buried utilities.
• Air breaks soil into small pieces that are ideal for recompaction.
• Air excavated soil is more easily vacuumed at lower levels of suction.
• High pressure water jets are dangerous to operators since they can cut through clothes or work boots
• A high pressure water jet can also cut a plastic gas line or harm pipe coatings.
• Soil excavated with water becomes mud that is considered contaminated and becomes a disposal problem costing time and money.
• Water can accidentally undermine adjacent areas to the excavation.
• Compressed air comes free from the atmosphere on the job site.
• Mud just dumped in an open area dries to a hardened spill and doesn’t blend in.
• Water must be paid for and transported to digging locations requiring large tanks and bigger, heavier rigs
• Muddy conditions can undermine footing around job site Splashing water is messy to contain and wet clothes are objectionable to workers.
• Air is non-conductive when working potentially around buried electric utilities.
• Air digging gets the particles of soil already airborne to be entrained more easily by the suction air stream.

Supersonic Air-jet

The CEG patented supersonic nozzle turns compressed air into a high speed, laser-like jet moving at twice the speed of sound, Mach 2. All of the energy and momentum of this air moving at approximately 1200 mph is focused onto the soil dislodging it in a fraction of a second. Soil is an unconsolidated assemblage of solid particles that may or may not contain organic matter, the voids between the particles being occupied by air and/or water. The aggregate nature of soil aids the ability of the air to fracture it; while stronger materials and ones that are not porous like metal or plastic pipes or cables or even wooden tree roots are unaffected.

Although exhaust nozzles for rocket engines have been designed and built for many years, supersonic air jet excavation nozzles are different. CEG has carried out extensive research into the aerodynamic design of its earth excavation nozzles. CEG has developed its own nozzle design, which is protected by U.S. patents. Unlike propulsion nozzles, the energy to accelerate the air comes primarily from the release of its compression rather than from the combustion of a fuel. Because of their small size, particular attention must be paid to the effect of the boundary layer on the nozzle profile. Special tooling and computer-aided-machining is used to manufacture the nozzles. CEG continues to refine and improve its design through detailed mathematical modeling and laboratory experimentation.

CEG Supersonic Air-jet vs. Other Air Jets

Soil fractures from stress, i.e. force per unit area, on its surface. As shown below for the same pressure and flow, compressed air exiting from a pipe nipple, orifice, or improperly designed nozzle expands outward rapidly to 3 to 4 times the area versus the jet from the patented supersonic nozzle in the AIR- SPADE ®. The flow from these competitors can even go sub-sonic as indicated by the presence of a “Mach Disk,” which can be seen in the flow if the light is right.

In head to head tests, the AIR-SPADE ® dislodged harder clay soil and dug faster than other air digging tools. Shown below is a comparison test between the AIR-SPADE ® and two competitive air excavation tools. In the test, the first 3 inches of soil with a moisture content of 23% were excavated by each tool. The AIR-SPADE ® worked up to 50% faster in the harder soil. In fact, in one test the where the competitive tool could not finish digging its clay test plot, the AIR-SPADE ® did.

Relative time required to excavate 1 cubic foot.

Air Pressure Required

The standard AIR-SPADE ® nozzles all are designed to work at 90 psig corresponding to a Mach 2 jet. Increasing the air pressure above 90 psig on a properly designed supersonic nozzle does not lead to a proportional gain in excavation capability. For example, doubling the nozzle pressure to 180 psig increases the air jet force by only 10% and the exit momentum flux (stress seen by the soil) by 45%. Supplying higher pressure to a nozzle designed to work at 90 psig actually un-focuses the air jet degrading performance and consuming more air.

Most portable air compressors are designed to deliver air at about 90 to 100 psig to run standard air tools such as paving breakers. Industrial workers are familiar with working with compressed air at this pressure. Working with compressed air at elevated pressures is costly and difficult. To adiabatically compress 100 cubic feet of air per minute in a single stage takes theoretically 18 hp at sea level. Doubling the pressure to 200 psig increases the power needed by 40% and correspondingly increases fuel costs. Working at pressures above 150 psig requires fittings other than the Air King Universal normally supplied with rental air compressors. Air hoses must also be rated for the elevated pressure and are subject to greater working stress.

Performance vs. Soil Characteristics

Because of its unique, focused air-jet, the AIR-SPADE ® works in most soils, even hard clays. Cohesive soils can be classified and described by unconfined compressive strength as shown below. Tests have shown the AIR-SPADE ® to be effective in compacted soils with unconfined compressive strengths well above the values listed for hard clay.

OSHA Cohesive Soil Classifications

Consistency and Unconfined Compression Strength of Clays

Soil texture can also be classified on a triangular diagram that shows composition as a percentage of clay, silt, and sand. Shown below is the U.S. Department of Agriculture standard chart (left) with the OSHA soil classifications superimposed (right.)

Watering the work area ahead of time can be helpful sometimes. Watering reduces airborne dust if the soil is extremely dry. It also reduces the soil’s strength making the digging easier. Combined use of the AIR-SPADE ® with a low pressure water jet is effective even with extreme cases of highly compacted or sun-baked soils.

The AIR-SPADE ® in general will not cut through rock, since its unconfined compressive strength as shown below is much greater than for soil. In fact, soil results from the physical and chemical breakup of weathered rock. Shales, however, may be broken apart by the AIR-SPADE ® if the jet is directed between the laminations of the rock. Similarly, the AIR-SPADE ® will not dislodge hard frozen soil which may behave like pavement or concrete.

Unconfined Compressive Strength of Rock.

Proper Digging with the AIR-SPADE ®

The AIR-SPADE ® will dislodge up to several inches deep in a medium to stiff soil. High-speed movies show that an air-jet penetrates and dislodges the soil in a fraction of a second. Unless the soil is highly compacted, dwelling on the same spot is unnecessary and tends to increase spray. The AIR-SPADE ® can be moved over the soil surface at a rate of about 1 to 2 feet per second. When several inches of soil have been loosened, the soil should be removed to expose a fresh working face for the air jet.

Nozzle Size vs. Excavation Rate

As shown below nozzles are available that use from 15 to 225 scfm of compressed air. The amount of soil that can be dislodged in a given amount of time is roughly proportional to the amount of air used.

Excavation Rates (cu ft / min)

Compressor Size

Air compressors are sized by pressure and flow. In the US pressure is measured in pounds per square inch above atmosphere, psig. Flow is measured in cubic feet of air per minute, cfm. Generally all air compressors will produce at least 100 psig; while flows will vary from a few cfm for small electric piston units to hundreds of cfm for gas or diesel driven portable screw compressors. All AIR-SPADE ® nozzles are designed to operate best at a pressure of 90 psig, but vary in flow to match standard available compressors according to the table below. Note, a smaller nozzle may always be used on a larger compressor, but not the reverse. Trying to run a larger nozzle on a smaller compressor will result in significantly less than 90 psig being delivered and will noticeably diminish performance.

Recommended Compressor Size

Hose Size Required

Compressed air flowing through a hose experiences a drop in pressure from friction and constrictions. Friction loss is proportional to the length of the hose. The amount of air, its pressure, the hose inner diameter and its smoothness also determine the loss. The table below shows the pressure loss for 50 feet of common air hose with couplings as a function of size and nozzle flow, cfm, for air at a pressure of 90 psig. Generally, a 1-inch air hose is recommended for use with the AIR-SPADE ®.

Pressure Loss (psig) for 50 feet of common air hose

Pneumatic Transport of Soil

Pneumatic vacuum transport naturally matches air jet excavation as it:

• Is likewise non-contacting.
• Easily removes the material that is already dislodged and entrained into the air stream by the supersonic jets.
• Can be used in a small diameter, deep excavation (pot hole) where a hand shovel or a backhoe bucket cannot.
• Can convey the material dislodged from the surface to the desired remote location.
• Is powered by conventional rotary blowers, fans or injectors.
• Can be filtered to various degrees depending on the material being excavated and the site regulations.

The excavation and vacuum transport of soil has its own unique set of characteristics. Soil types vary widely in grain size, particle shape, packing, moisture content, grading, plasticity, organic matter content, etc. Soils, in general, are not the free flowing material most conventionally moved by pneumatic transport. Special care must be taken in the design of the vacuum transport system to avoid the persistent problem of cohesive material clogging. CEG personnel have carried out extensive experimentation to determine how to most effectively design the supersonic air jets and vacuum transport systems to work together in an efficient and synergistic manner.

Recognized by Brooklyn Union who developed perhaps the first air-vacuum excavator in the 1960’s, key to vacuum excavation is to provide an air stream of sufficient velocity to lift and carry the material of concern through the hose from the pickup point to the disposal point. The suction, i.e. inches of Hg, necessary to accomplish this is a consequence of several items including: an inlet or pick-up loss; an acceleration loss to bring the material from rest up to its transport velocity in the hose; local losses at bends and expansions/contractions; air friction in the hose; lift of the material against gravity; and a discharge loss. For the relatively short distances and small lifts in air-vacuum excavation, the air friction loss tends to dominate. Typically air-vacuum excavation can be readily done below 10 in Hg suction. Units that use water to excavate, however, often need higher heads to move the mud that the water creates.

Tables and graphs of transport velocity for various materials are readily available in the literature. For example, an air velocity of about 5000 feet per minute is sufficient for a granular soil weighing up to about 100 lbm/cu ft. A damp clay/sandy soil or wet sand/gravel weighing up to about 130 lbm/cu ft may need up to10,000 ft/min. Horizontal hose runs are actually need higher transport velocities than vertical.

Special Features of CEG Pneumatic Transport Systems

Telescoping Booms

To avoid the persistent problem of material clogging in suction hoses, CEG has designed and patented a unique telescoping digging boom. Employed on systems built for the Electric Power Research Institute, the Idaho National Engineering and Environmental Laboratory, and Tyndall Air Force Base, the straight tube eliminates hose bends and, hence. the places where damp material tends to accumulate and eventually plug. This is particularly important if moving large quantities of soil or soil that contains hazardous materials where operator interaction to clear a clogged hose is not readily possible. The boom on the Soft Trencher is 7 inches in minimum diameter (almost big enough to suck up a bowling ball), extends up to 22 feet, and slews +/- 25 o in all four directions.

Telescoping booms: Soft Trencher (left) has three steel tubing sections: INEEL (center) and Tyndall AFB (right) have two sections of fiberglass epoxy tubing.

Telescoping booms: Soft Trencher (left) has three steel tubing sections: INEEL (center) and Tyndall AFB (right) have two sections of fiberglass epoxy tubing.

Excavating Heads

Conventional vacuum excavation equipment uses an opened tube for suction and a separate soil reduction tool for digging. CEG has designed, built, tested, and supplied several combined digging and suction heads, which do both operations in a synergistic manner. The hardest thing for a vacuum system to do is to initially get loose material off of the surface. (This is why vacuum cleaners have brushes.) Using supersonic air jets to loosen the soil also gets the material airborne where the vacuum can entrain it. Having both functions together also allows one man to do the work rather than two. Excavation rates from 5 to over 15 cubic feet per minute, depending on soil type, have been achieved with the Soft Trencher. This is significantly faster than the average rate of 1 to 2 cubic feet per minute for most vacuum excavation equipment.

Combined air jet / suction heads for the Soft Trencher (left and center) cut disciplined straight trenches. Patented rotary excavation head (right) can bore a 12 to 18 inch diameter hole vertically downward.

Combined air jet / suction heads for the Soft Trencher (left and center) cut disciplined straight trenches. Patented rotary excavation head (right) can bore a 12 to 18 inch diameter hole vertically downward.

Remote Control Box

Control of the boom, machine motion, and all operating functions has been incorporated into a single, portable control box. Using 4 proportional joysticks, the operator can lift, slew, and extend/retract the boom, steer and drive the whole unit quickly or slowly as needed. The digging air jets and vacuum can be turned on/off with push buttons. The box on the Soft Trencher is on a 25-foot umbilical cable allowing the operator to walk around the excavation for his best viewpoint. The box design could easily upgrade to infrared or radio remote for radioactive or hazardous waste situations.

Flow through Material Handling

Conventional vacuum systems all draw the spoil into a holding tank. The size of this tank limits the throughput capacity of the system. Even though it can be substantial once it is filled, vacuuming must stop until it is taken away and dumped. CEG material handling systems use a patented, flow through approach. In a primary enclosure the material is separated from the air stream by expansion, impact, and turning. By using a discharge rotary valve, material is constantly released from the system. Just like conventional excavating, for larger jobs like trenching this allows the excavator to remain at work while dump trucks remove the spoil. Also, the physical size, weight, and cost of this primary separator is significantly less when compared to the holding tanks on large vacuum excavation trucks. The chassis to hold and transport is also correspondingly smaller. Dumping can also be a source of secondary dust emissions. In the radioactive soil vacuuming system for the Rocky Flats Environmental Technology Site Lift-Liner™ disposal sacks were loaded directly from the separator in a dustless manner.

Patented systems contain material separation chamber, discharge rotary valve, and filter chamber all in one unit. INEEL (left), Tyndall AFB (middle), and RFETS (right).

Patented systems contain material separation chamber, discharge rotary valve, and filter chamber all in one unit. INEEL (left), Tyndall AFB (middle), and RFETS (right). Filtration System

Conventional vacuum systems use various means to filter the discharge air including bags, cyclones, and cartridges. CEG uses a combination of compact cartridges with an automatic cleaning system. The unique Roto-Pulse™ cleaning system uses pulses of air disposed along the entire inside surface of the cartridge to temporarily create a back flow instead of the more conventional blast from a quick opening valve. If the job requires it, the cartridge material can be special such as Tetratex ® High Efficiency Expanded PTFE membrane on thermally fused polyester. This material is almost HEPA quality by itself with a manufacturer’s test of 99.978 to 99.986% efficient for a size range of 0.3 to 0.5 μm.

Compact Power Unit

CEG has done the engineering to integrate the three main components of an air-vacuum excavation system, prime mover, air compressor, and vacuum pump into a single compact unit. Gasoline or diesel engines or electric motor are available as prime movers. One engine adds simplicity as one maintenance/service point instead the multiple engines used by some competitors. One set of controls runs the unit. The components are mounted together on a skid base for portability. The unit can be mounted on a pick-up truck, trailer, utility type vehicle or tracked unit for unique off-road capability.

Equipment skids for SAFEX ® systems: Electric for INEEL (upper left); Diesel for Tyndall AFB (upper right): Gasoline on Mule™ (bottom left) and GATOR ® (bottom right).

Equipment skids for SAFEX ® systems: Electric for INEEL (upper left); Diesel for Tyndall AFB (upper right): Gasoline on Mule™ (bottom left) and GATOR ® (bottom right).

SAFEX ® systems: Commercial on truck (left) and trailer (right) with extra HEPA filter for Rocky Flats Environmental Technology Site.

SAFEX ® systems: Commercial on truck (left) and trailer (right) with extra HEPA filter for Rocky Flats Environmental Technology Site.