Non-Destructive Exposure to Underground Utilities
Even though exposure of buried utilities has been used in the United Statesfor more than 40 years, it is just beginning to be realized as beneficial whenworking near underground lines.
The intent is to focus on not merely theexposure of these utilities, but specifically non-destructive exposure.
Vacuum excavation is a twofold process that consists of reduction andremoval. Reduction can be accomplished using a variety of methods: air,mechanical means or water (referred to as hydro-excavation). The intent of thisinitial phase of the operation is to reduce or fracture the soil into smallparticles that can be carried from the excavation by a high volume of air or bymechanical means.
Pneumatic reduction is a two-man operation. One memberof the crew uses a high-pressure air lance to break the soil into small pieces,while the second individual vacuums the spoil into the collection tank.
A backhoe, trencher or the shovel method accomplishes mechanicalreduction. This is the slowest and most unsafe method of the three. Thepossibility of damaging a utility, an injury to an employee and the amount ofrestoration at the site, make this method the least desirable.
Excavation by the use of high-pressure water, hydro-excavation, isperhaps the fastest method in most types of soils. In some sugar sandconditions, the use of water may not be required at all or used only whenhorizons or layers of clay or cobble are encountered.
Since we havealready touched on the method used in pneumatic and mechanical reduction, let usturn our attention to the two methods that can be employed in hydro-excavation:water lance and non-destructive reduction tool.
All contractors are requiredby law to contact their local one-call center prior to any excavation beingperformed. The one-call center will pass this information to the appropriatemember, whose responsibility is to locate the utilities in proximity to theexcavation site. These locates are usually performed by a one-call contractor.Check your state laws for one-call requirements.
The one-call contractorwill locate and give the line of the various utilities in the area either withflags or paint of different colors that designate a specific service(yellow-gas, blue-potable water, red-electric, orange-telecommunications, etc.).
While the one-call contractor will give the line of the utility, he/she willnot give depth. In addition, there is also a safe zone from the locate mark inwhich the buried utility can be found. This safe zone varies by state law andrequires non-destructive excavation to expose the utility.
It isrecommended to use a digital locator to verify the one-call mark and determinethe depth of the service before beginning the excavation process. This reducesthe time spent in uncovering the service for visual inspection. Furthermore, itwill give the vacuum excavation operator increased knowledge of exactly whereand how deep beneath the surface the utility is located.
Onceverification of the one-call marking has been accomplished, actual exposure ofthe utility can begin.
In hydro-excavation, there are two methods used toexpose the service. The more dangerous of the two is the use of a water lanceand a vacuum tool. A water lance produces a single conduit for the water toreach the face of the excavation. Therefore, if the unit’s water pump is either3,000 or 4,000 psi, that is exactly the amount of the force of water that willbe experienced at the nozzle that is performing the excavation. Using 3,000 to4,000 psi water supply is not doing non-destructive exposure.
As thewater lance makes a series of holes in the soil to super saturate the ground,the vacuum tool operator is responsible for using the tool to vacuum the spoilinto the collection tank. Not only is this method a two-person operation, but itis also an extremely messy undertaking — one that runs the risk of severelydamaging any buried utility that it may come in contact.
Experience hasshown that pressures in excess of 2,000 psi will damage utilities in a shortperiod of time. Furthermore, unless the operator has previously verified thelocation of the utility with a digital locator capable of showing calculateddepth, the possibility of utility damage is quite real.
The second method of exposure uses a special reduction tool. Unfortunately,some of the specialized tools on the market are far from non-destructive.
In hydro-excavation, water is the medium that produces the breakdown ofthe soil so it can be removed to the collection tank. The intent of aspecialized, non-destructive reduction tool is to use multiple nozzles or arotating nozzle to limit the pressure of the water at the excavation face.
These nozzles would normally be contained in a manifold with varyingangles to remove core and cut gauge or peripheral clearance simultaneously.These nozzles would ideally be recessed inside the cone of the reduction tool.Therefore, the only thing to come in contact with the buried service would bethe water and not sharp steel, which can cut the sheathing of the service or inthe event of a steel gas line create a sparking effect that could result in apossible explosion.
Furthermore, it is not so much the pressure of thewater at the face of the excavation but rather the output in gallons per minute(gpm) that assists in the excavation process.
A system is currentlyavailable that addresses all of the major concerns associated with thisnon-destructive exposure process and incorporates the following:
•Recessed multiple nozzles (peripheral and core cutting) Pat. 6,470,605B1
•Reasonable pressures at the face — Below 2,000 psi
• High rate of water flow— 5.6 gpm
• Trigger on reduction tool to control output of water — Pat.6,470,605B1
• Cone-shaped reduction head to eliminate splash back of water —Pat. 6,470,605B1
• Relief holes in reduction tool head to allow make-up airto enter the excavation — Pat. 6,470,605B1
• Low idle down control once watertrigger is released — Pat. 6,470,605B1
Even with all the industry safeguardsput in place by the manufacturers, the final decision to conform to and makeproper use of these techniques is ultimately left to the operator in the field.
Some have suggested regulations on the amount of water pressure used inthe system, while others have alluded to having maximum water pressure writteninto the contract to promote safe hydro-excavation. While these are worthwhilegestures, they would be extremely difficult to enforce under actual fieldconditions.
Soils are not the same in all areas. Working in previouslyexcavated material is preferable to virgin soil. Sandy soil is better than clay,clay is better than cobble, cobble is better than rock.
Since thecontractor is not paid for his/her utility exposure efforts and wants tocomplete the task as quickly as possible from a revenue standpoint, what is toprevent the operator from cutting corners by not verifying all of the one-callmarks, misusing a water lance or turning up the pressure on the water pump ifhe/she is dealing with adverse soil conditions?
Finally, in today’senvironmentally conscious world, the need to vacuum mud, bentonite, wet and dryspoil will continue to grow in favor with the owners of projects. Furthermore,with safety and damage prevention a growing concern for both government agenciesand utility owners, non-destructive exposure of utilities in proximity to anexcavation site prior to the service installation will continue to be mandated.
Our task is to continue to improve techniques that will promote safehydro-excavation. Most important, however, is to provide instruction to crews inthe field on the proper use of these techniques and the possible consequences ofusing them improperly. For ultimately these are the people that can and willmake or break the cycle of safe and damage free excavation.
Bill Gilman is director of marketing at McLaughlin Mfg., which isheadquartered in Greenville, S.C.