Subsurface Utility Engineering
Four separate quality levels of utility information are now generally recognized by various organizations. The Federal Highway Administration has taken the lead in promoting and using this concept. Other organizations such as the American Society of Civil Engineers
(ASCE), Federal Aviation Agency (FAA), Network Reliability Council, various state
DOTs, county governments, and so forth have also used this concept.
The generally accepted definitions are as follows.
Quality Level D (QL D): Information derived solely from existing records or verbal recollections.
Quality Level C (QL C): Information obtained by surveying and plotting visible above-ground utility features and by using professional judgment in correlating this information to Quality Level D information.
Quality Level B (QL B): Information obtained through the application of appropriate surface geophysical methods to identify the existence and approximate horizontal position of subsurface utilities. "Quality level B" data are reproducible by surface geophysics at any point of their depiction. This information is surveyed to applicable tolerances and reduced onto plan documents.
Quality Level A (QL A): Information obtained by the actual exposure (or verification of previously exposed and surveyed utilities) of subsurface utilities, using (typically) minimally intrusive excavation equipment to determine their precise horizontal and vertical positions, as well as their other utility attributes. This information is surveyed and reduced onto plan documents. Accuracy is typically set at 15mm vertical, and to applicable horizontal survey and mapping standards.
Federal Highway
Administration
The first phase of SUE - Quality Level B (QL B) - is the designation phase. Designating is the use of electronic locating equipment to determine the horizontal position of underground utilities. The information can be used in engineering design to determine the location of possible conflict points. The ability to make adjustments during the preliminary design phase can produce cost savings by eliminating utility relocation and construction delays. The information gathered in the designation phase still does not give the engineer the vertical location of the utility. The second phase -Quality Level A (QL A) - of this process is used when the vertical position of the utility is needed. Vacuum excavation is the use of non-destructive equipment at critical points along a subsurface utility’s path to determine the exact three-dimensional location of the utility. Vacuum excavation utilizes the kinetic energy in a high velocity air stream to penetrate, expand and break-up soil. The loosened chunks of soil and rocks are then removed from the hole through the use of a very powerful vacuum. In this way a hole is created that is typically 12" x 12" and as deep as required to expose the buried utility. The horizontal and vertical information gathered in this process provides the engineer with accurate positional data that can be used to make adjustments in design elevations or horizontal locations, avoiding the expense of relocating utilities. Contractors and directional drillers use this service to avoid damage to existing utilities when working in congested areas.
The Federal Highway Administration (FHWA) has been promoting the use of subsurface utility engineering (SUE) since 1987 as a means to save costs on highway construction projects. In 1996, the FHWA commissioned Purdue University to study the cost savings from a total of seventy-one projects (71) with total construction costs in excess of one billion dollars. These projects involved a mix of Interstate, Arterial, and Collector Roads in urban, suburban, and rural settings. A total of $4.62 in savings for every $1.00 spent on SUE was quantified. FHWA/ Purdue Study
Two very important reasons why utility professionals use vacuum excavation.
First, it automates the potholing process and greatly reduces the cost and time required to expose utilities. Secondly, it greatly reduces damage claims, as well as the risk of physical injuries that are the result of hitting and damaging a buried utility.
Vacuum excavation has become the preferred method of exposing utilities in order to verify their exact location and depth. There is less chance of accidentally damaging the utility. State of the Art Vacuum Excavation allows for fast potholing in all soil conditions and is the safest way of buried utility locating.
In many cases, records of locations and depths of existing utilities are either very poor or non-existent. Striking and damaging existing utilities has become an increasing problem during new utility installation and relocation projects. This can lead to costly repair, service outages, and in the worst cases, bodily harm. Vacuum excavation is a safe and efficient way to expose and locate buried utilities. The professionals involved in the vacuum excavation and utility location business provide the information needed to avoid design conflicts and provide a safer working environment for the utility installation contractors.
Exposing the utilities
The first step to exposing a utility is determining where to dig for it. GeoMasters may rely on electronic location equipment, ground penetrating radar, as-built plans, designation signs, or other evidence to determine the most likely location of the desired utility. From there, the vacuum excavation trucks can be brought in to "pothole" the surface. In this process, the high powered vacuum hose is used in conjunction with an air lance to remove the soil. The air lance releases pressurized air to break-up hard packed dirt. The vacuum hose removes the loosened soil and debris. This process is fast and safe for exposing any buried utility.
Locating or Marking the Utilities
GeoMasters offers several options when it comes to locating or marking the utilities. GeoMasters can field mark the utilities, map the utilities, or both. In field marking, GeoMasters can set stakes or iron rods at each location site. These stakes can be set directly above the utility running line, and the true elevation and/or depth of the utility can be marked in relation to the top of the stake.
GeoMasters is a licensed land survey consultant and can meet any surveying and mapping requirements. GeoMasters can plot the locations on existing project maps or create new project maps. Also, GeoMasters has the equipment and expertise for GIS data collection. If required, location data can be delivered in state plane coordinate, geodetic (latitude and longitude), or UTM coordinates, ready to be imported into a GIS system.
Covering the utility
Refilling the pothole is just as efficient as creating the pothole. The truck backs-up to the site and tilts its bin to backfill the hole. GeoMasters also keeps an air powered tamper to pack the soil to Department of Transportation standards and patch the asphalt or concrete as to leave the site safe for pedestrian and car traffic.
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