Feature: Surveying the Underground
Professional Surveyor Magazine - September 2012
An experienced SUE-services business owner describes the practice of subsurface utility surveying and mapping and addresses special concerns for surveyors.
By Michael T. Maguire, MA, LS
How do you measure and map what you cannot see? Underground utilities reveal little of their true form and extent from sparse surface clues and often-questionable records. Legacy tools and methods have left surveyors with few choices but extrapolation from records or costly and disruptive excavation to precisely locate and describe these features. However, underground utility mapping has recently evolved with new solutions and standards that include tools and methods that literally “energize” the utilities being located.
Suppose that you have just completed the boundary and topographic survey of a project site. How do you respond to your client’s request for a reliable depiction of the location and nature of all of the existing underground utilities on the property and in the adjoining streets? Many surveyors will inform the client (whether owner, developer, or site-design professional) that it can’t be done reliably.
You can, of course, survey and show all of the surface features that are commonly associated with underground utilities. There may be a wide variety of them, including manholes, inlets, lampholes, cleanouts, pedestals, transformers, hydrants, meters, and valves. Using the surface features and any record drawings that are available, along with your experience in design or a review of construction documents, you could develop a representation of the underground conditions at the site. If you have located “811” or “One Call” paint marks on the ground, you might be tempted to try to correlate them with the surface- and record-utility information, as well. But, what can you say with confidence to your client about the completeness or reliability of what you have shown?
Until a few years ago, there were few commonly accepted methods to communicate the sources and dependability of existing utility lines shown on a survey or other project plan. This article describes readily available alternative tools and methods that can be used to serve your client and help you facilitate communication about existing utilities to all members of the design and construction team on a project. These alternatives are contrasted with some of the legacy tools and methods that have been used to “locate and extrapolate” underground features.
ASCE 38-02 and SUE
In 2002, the American Society of Civil Engineers (ASCE
) published the consensus standard known as CI/ASCE 38-02 Standard Guideline for the Collection and Depiction of Existing Subsurface Utility Data (ASCE 38-02). Since then, there has been growing acceptance of the Standard and the concepts described by it from the surveying, engineering, and construction professions. In fact, the U.S. Department of Transportation Federal Highway Administration characterizes it as an important “standard of care” guideline in a recent brochure available on its website: www.fhwa.dot.gov/design/sue/suebrochure.dfm
. Many surveyors, however, have not yet been introduced to ASCE 38-02.
My introduction to the Standard began in 2003 after I was employed by a firm that practiced something called Subsurface Utility Engineering (SUE). As was described during my interview process, SUE included both field and office activities that first focused on the research, detection, and mapping of existing subsurface utilities and then, using the resulting data, managed the risks associated with underground utilities. As defined in the Standard Guideline, SUE “involves managing certain risks associated with utility mapping at appropriate quality levels, utility coordination, utility relocation design and coordination, utility condition assessment, communication of utility data to concerned parties, utility relocation cost estimates, implementation of utility accommodation policies and utility design.” While the first applications of SUE were on highway and other transportation projects, I could see immediately that this process had many applications for site-specific, land-development projects as well.
Quality Levels of Representation
In 2005, my business partner John Berrettini and I started Accurate Infrastructure Data, Inc
. (A/I/DATA) to offer SUE services to engineers, surveyors, and other design professionals as well as land developers, owners, and contractors. From the beginning, we conformed our services to ASCE 38-02 and the framework it provides for characterizing the quality of utility data we provided our clients. Among the essential elements of the Standard Guideline are the defined steps that should be taken to gather the data needed to properly represent existing utility systems. There are four quality levels (QL) of utility representation that are identified in SUE practice. Like a report card in school, each has an associated letter grade from A to D.
QL D depictions of subsurface utilities are taken from existing record information only, but may include oral recollections of utility lines as well. The compiled utility locations may be sufficient for preliminary project assessment work but probably not much else.
QL C data consists of field-surveyed surface features for all types of utilities, combined with the available record information to improve the spatial quality of the depicted utilities. Surveyors and engineers have traditionally combined records with field survey data to represent underground conditions. In the past it was probably the best that could be done.
QL B utility representations are much more reliable because utility positions are “obtained through the application of appropriate surface geophysical methods to determine the existence and approximate horizontal position” of each utility system that is being mapped, according to ASCE 38-02. This is the most reliable way to depict subsurface utilities short of digging them up, which brings us to QL A data.
A-level data is based on direct survey-grade measurements of the horizontal and vertical position of a utility that is exposed to the light of day, either through excavation (“potholing”) or direct measurement and “as-builting” at the time the utility is being installed.
How We Do It
As a SUE firm, A/I/DATA is most commonly engaged in providing QL A and B data to our clients. On a typical project, we are first asked to investigate and map at QL B some or all of the existing utility systems through a particular area of interest. We begin with a thorough investigation of all available utility record information to develop an inventory of all of the systems we have been asked to map. After we study the records, we proceed to the field with our “surface geophysical” detection equipment (Figure 1
), primarily a variety of electro-magnetic pipe and cable locators of various frequencies and power ratings.
During the course of our work, we energize the utilities that will carry an electronic signal and then trace the course of each from a surface feature through the area of investigation. The detected positions are marked on the ground with pin flags or paint. As an added measure, our crews conduct a sweep of the project site as we broadcast a detectable signal in an effort to energize otherwise unknown utilities. If a conductor is energized, we will trace its alignment (hopefully) to a surface feature that will allow us to identify the conductor as a specific utility type. Ground-penetrating radar (GPR, Figure 2
) may be used to investigate non-conductive utilities if the site and soil conditions are suitable. We then survey the locations of the markings and surface features using a total station or RTK GPS.
We download and process the coded field data in the office and produce preliminary utility mapping.
When we have great confidence in the detected positions of the underground utilities, the data is indicated at QL B. Sometimes, however, the field data is not sufficient to persuade us that the positions are “reproducible by surface geophysics at any point of their depiction” as required by ASCE 38-02. In those cases we will report the depictions as C or D, depending on the existence of associated surface features. The analysis and comparison of record information to field data is a critical step that must be taken before the appropriate quality level can be assigned to each run of utilities through a project site.
Some of A/I/DATA’s clients are surveyors or engineering companies with surveyors on staff, and they locate and map the gravity-dependent storm and sanitary sewer systems within the
project area. From time to time they will need to confirm the alignment of the pipe connections between structures, and we may be asked to help. We may use a self-contained signal transmitter that we attach to a line or push-rod and then trace its location as we move it through the pipeline from structure to structure. A more common method involves the use of a plumber’s snake that can be pushed through a pipe and energized and traced to determine the position of the pipe. The finished mapping of all of the utility systems in our scope is produced in a CAD environment
) and registered in the project datum selected by the client.
Occasionally, clients with in-house surveying capabilities will request that we limit our services to records research, field investigation, marking, and field sketch documentation of our work. We understand their desire to provide cost-effective services by conducting their own surveys, but we believe that they and their clients will realize the maximum value from a subsurface investigation if the SUE firm provides the complete QL B (as defined in ASCE Standard 38-02) depiction of the utilities.
That means not only investigating and marking the utilities but also surveying, mapping, and then resolving differences between the designated utilities, the utility records, and the surveyed features.
It is important to note that “paint on the ground” is an indication of the location of the underground utility based on the detected signal return from a buried conductor. It is not necessarily the best representation of the location of a utility line.
A/I/DATA’s survey teams have utility experience that adds another level of quality assurance to the field-locating activities. The office processing, mapping, and assessment by our utility engineers or surveyors continues the quality-controlled depiction of utilities before the product is released to the client. Many of our engineering customers have found that it’s easier to use the data we investigate when our fully developed CAD mapping is merged with their files.
On many projects, the data we provide is incorporated directly into the project design base map. After design development, the feasibility of the proposed improvements may need to be confirmed by determining the precise horizontal and vertical position of one or more existing utility lines at a particular crossing point. This calls for QL A data that is most commonly acquired through a process known as air-vacuum excavation (Figure 4
). This method employs a high-pressure stream of compressed air to break down soil into small pieces and then a vacuum system to remove the material from a small cut in the ground surface. The excavation is known as a test hole or “pot hole,” and it is usually about one square foot in size.
After the utility is exposed in the test hole, its horizontal position and elevation can be directly measured and documented. Other information, such as ground and utility condition, can also be recorded. Elevation data (Figure 5
) must be accurate to 15 mm in order to satisfy ASCE 38-02. This accuracy assures a designer that data is reliable, but it is important to remember that this is point-specific data only that often cannot be used beyond the actual point of excavation and measurement.
The test-hole data is used to confirm the QL B horizontal position of utilities and is the only fully reliable method of determining the true position of a buried utility line or structure. Experience and knowledge gained by SUE investigators through the comparison of results of excavation with the expected positions of designated utilities cannot be duplicated through any other means.
Test-hole excavation is also an invaluable method for learning how complex the underground can be and how remote sensing through electromagnetic or other means can provide less-than-completely accurate results. Those other means include ground-penetrating radar (GPR), a popular technology for subsurface investigation today that must be used with great caution in many applications. It is only through the exposure of a utility to the light of day that one can confirm the results of and gain certainty about otherwise inaccessible underground utility locations.
Special SUE Issues for Surveyors
Land surveyors often confront two important questions about subsurface utility. How do you handle a request to satisfy the location of utilities—Optional Item 11 (b) from Table A for an ALTA/ACSM Land Title Survey? Also, how much faith can I put in the paint marks that “811” or the “One Call” service has put down on and around my survey site?
Regarding ALTA requirements, ALTA seems to contemplate both records collection and utility detection and marking but doesn’t specifically recognize ASCE 38-02 methods as a possible “reference as to the source of information.” If the next revision of the ALTA/ACSM standards were to include ASCE 38-02 by reference, that would provide a clear way to characterize both the source and relative reliability of the underground utility data presented. While the note in the ALTA/ACSM standard correctly identifies the fact that only an excavated or exposed location of a subsurface utility can be exactly shown on the survey, the use of SUE methods and quality level distinctions on the survey could be used to address the needs of Item 11 (b).
The question of the reliability of “One Call” or “811” markings is often raised in discussions with other surveyors and engineers. It is important to note that the system of notification of public utility owners prior to construction activities is a “call before you dig,” not primarily a “call before you design” system. This system is just one part of the damage-prevention practices for excavators. The markings placed prior to excavation are intended more to warn that a utility exists than to define an exact position. A tolerance zone that consists of the width of a marked utility plus 18” on each side, in most jurisdictions, is indicated by the paint marks. Excavation within the tolerance zone must protect any underground facility by hand digging, vacuum excavation, or other “safe” digging methods.
This focus on damage prevention needs to be understood by any surveyor who uses excavation period markings as the basis for subsurface utility mapping. In my home state of Maryland, at least, the 811/One Call Notification system is concerned only with public utilities, while leaving private utilities unmarked.
In addition, even though there have been “designer” tickets available since the Maryland One Call laws were changed in 2010, the data provided through them is for “informational purposes only,” and utility owners “may not be held liable for any inaccurate information” provided. In other words, use at your own risk! The essential distinction to be made between One Call marks and a mapped representation of subsurface utilities based on ASCE 38-02 is that the purpose of SUE activities is to inform the design process, not provide safe-digging practice for the excavation period.
This is a cursory overview of the practice of Subsurface Utility Engineering as it applies to data collection and mapping of subsurface utilities for the land surveyor. In my experience, a large segment of the surveying community has not been introduced to SUE practices, but perhaps now they can respond to their clients’ requests for subsurface utility mapping by letting them know that there is a way to provide the data in a reliable and professional way.
Michael T. Maguire, MA, LS
, is president of Accurate Infrastructure Data, Inc. (www.aidatainc.com) in Baltimore, Maryland.
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