(This is a synopsis of a more detailed article that was published in the June 2015 issue of Trenchless Technology)
From water and electric to gas and communication cables, there are millions of miles of buried utilities located beneath the surface of the earth. Property damage, costly delays, service disruptions and injuries are all issues that can be avoided by determining where utilities lie before digging begins. Because each job requires its own independent level of investigation, owners, professional engineers and contractors need to determine if the situation requires One-Call or an additional quality level of Subsurface Utility Engineering (SUE).
One-Call was originally implemented after a number of construction-related activities caused damage to existing utilities. To minimize the occurrence of these costly instances, technicians representing utility companies are called out to a project site to mark the approximate location of existing utilities. This method is often effective if there is previous knowledge of the site conditions, especially in areas with a low possibility of utility population or in emergency situations where it is difficult to accommodate a more extensive approach. Although one-call is required prior to any excavation, an additional level of care is often needed to better avoid conflicts.
In 2002, the American Society of Civil Engineers published standard guidelines that define a level of care for professionals to follow when collecting and depicting existing utilities. Each of the four levels classifies the quality of data collected, and each builds upon the previous level.
Quality Level D is the most basic level, which involves obtaining information from existing records or oral recollections. This knowledge gives project managers and owners an understanding of the population of utilities within a specific area. While this information is useful when developing a strategy to move a project forward, the process of obtaining these records can often be tedious, and without verification, the utility layout is dependent on anecdotal data.
Quality Level C involves surveying above ground surface features and correlating that with Quality Level D data. This allows the engineer to get a better idea of the location of the utility that cannot be seen and discover errors in the record information. Nonetheless this method still has its limitations. Although the presence of a utility can be verified, this approach does not provide an actual physical location or account for unmarked and unrecorded utilities.
Quality Level B utilizes geophysical sensing technology, like ground penetrating radar or an electromagnetic unit to locate utilities and determine their approximate horizontal position. Adding to the data collected from Quality Levels C and D, located utilities are marked and mapped. While this method provides a high level of accuracy, it does not document and/or confirm a vertical component and therefore cannot be used solely to eliminate conflicts of utility crossings.
Quality Level A provides the highest level of accuracy by using nondestructive methods to physically locate a utility. Once exposed through vacuum excavation, technicians can document the horizontal and vertical location as well as other characteristics of the utility including condition, size and material. Although this technique presents the lowest amount of risk, digging a large number of holes can be a costly affair.
No matter the size of a project, be it a large construction job or small home repair, it is important to consider what utilities may lie underneath prior to digging. There are advantages and disadvantages related to each method, so depending on the project’s specific needs, engineers must consider various factors such as schedule, cost constraints, project life cycle and site history to determine which should be applied and when.
For more information on One-Call and each quality level of SUE and their place in the design and construction cycle, read our article, “The Why, When and Where of SUE vs. One-Call,” which was published in the Trenchless Technology magazine.