BasePDH | Course 022 | 2 Professional Development Hours

Depicting Subsurface Utilities in Highway Design Plans

1. The Quality Level System: What a Depiction Can Assert

Utility data reach a highway plan set from many sources of very different reliability, and the common failure is that the plan does not say which is which. Data collected through records research, field reconnaissance, surveys, and designating and locating efforts without a utility quality level are routinely depicted in design plans as though they possess equivalent certainty [1]. The decisions made against that depiction include utility conflict evaluations, choices among protection, avoidance, and relocation, and highway, roadway, and structural design decisions, each with budget and schedule effects downstream [1].

The American Society of Civil Engineers created ASCE 38 to integrate utility data from multiple sources, with the emphasis on using utility quality levels to communicate the uncertainty in location data [1]. The standard offered suggestions on what utility data visualizations might look like but was deliberately not prescriptive about them, for two stated reasons: CAD and depiction software were not standardized at the time, and conflict avoidance with utilities was in a nascent state [1]. No single nationwide depiction standard has emerged since, and each agency maintains its own manual and symbology [1]. The current edition of the standard is ASCE/UESI/CI 38-22, and its quality level definitions, quoted in the glossary of NCHRP Research Report 1180, are the framework this course is built on [1].

1.1 The data sources, and what each can support

The guide identifies 11 sources used to identify and document existing utilities: utility owner records; DOT or agency records, including permits and project as-builts; the historical knowledge and verbal accounts of utility staff; One Call markings; field survey and field edits; utility field investigations, meaning visual field indications observed and transferred to plans; geophysics such as ground penetrating radar and electromagnetic locators; satellite imagery; AI-assembled mapping; exposure of the utility itself; and utility subject-matter expertise applied to complete the picture from the other 10 [1].

These sources are grouped by the quality level they support, with one governing qualifier stated before the groupings: the sources achieve those quality levels only when judged to do so within a sealed SUE investigation [1]. The same sources remain useful in non-SUE investigations; they simply carry no quality level [1].

Utility owner records, agency records and as-builts, verbal recollections, and One Call mark-outs support Quality Level D, the most basic level of information for determining existence and location [1]. Satellite imagery and AI-assembled sources also sit at QL D, for a stated reason that generalizes: they are external third-party sources, and a utility investigation professional must judge the accuracy of the information [1]. Field survey and utility field investigations support Quality Level C: visible structures such as manholes, vaults, valve boxes, cleanouts, and riser poles are surveyed, and record data are then interpreted and placed between or proximate to the surveyed structures [1]. QL C is slightly more certain than QL D because at least some visible structures are surveyed; QL D has no reliable fixed points at all [1]. Geophysics supports Quality Level B through designating, the application of surface geophysical methods to determine the existence and horizontal position of subsurface utilities within the study limits [1]. Nondestructive exposure supports Quality Level A through locating: the utility is exposed and its location, type, size, condition, material, and other characteristics are measured directly [1].

The guide singles out two of these sources for skepticism, because both are routinely treated as better than they are.

One Call markings are not a design-phase location source. The service operates under a state's underground utility damage prevention law, and its statutory purpose is preventing damage during excavation, not supporting preliminary design [1]. It is already overburdened meeting construction demands, and few examples exist of One Call markings being complete or reliable [1]. Public locating for design carries risk in part because of locator inexperience, high field staff turnover, high request volume, and the absence of any accountability to mark utilities comprehensively or accurately when the markings are used for design [1]. The standard itself settles the ceiling: One Call and private-locate markings are named inputs to QL D, so nothing built on them alone rises above the bottom of the scale [1]. Relying on them also produces a false sense that an adequate utility investigation was performed, and the project proceeds on a minimal approach with the responsibility for utility data quietly shifted to a third party [1].

Utility owner records cannot be relied on to be comprehensive. The notion that utilities know the locations and attributes of their own facilities is valid in principle and not a reality in practice [1]. Employee turnover is high, as-builts recording the details of each installation are rarely retrieved, maintenance updates are rarely obtained, and companies are sold and acquired, mixing personnel and infrastructure records [1]. The guide is blunt: it is rare that a utility has a full and reliable record of its infrastructure, and without federal or state laws requiring accurate records, that is unlikely to change [1]. Previous project plans inherit the same problem. Their depictions were often uncertain to begin with and were not updated with relocations, so the guide's rule is that information from previous plans is a baseline to start from and nothing more [1].

1.2 The four quality levels, and exactly what each asserts

ASCE 38-22 defines a utility quality level as "the value, assigned by the Professional, of a Utility Segment or subsurface Utility Feature that identifies the relative (nonquantifiable) uncertainty of a Utility Segment's or subsurface Utility Feature's existence and actual location to that of its documented location" (ASCE 38-22, quoted in [1]). The word nonquantifiable is in the standard's own definition. A quality level is not an error bar on the utility's position. The numeric tolerances that follow are accuracies of the tie to the project survey datum, and reading them as accuracies of the pipe's true position is a misreading the definitions invite.

Quality Level A is assigned to the portion of a utility segment or feature that is directly exposed and measured in x, y, and z, with its location and dimensions tied to the project survey datum at an accuracy of 0.1 ft (30 mm) vertical and 0.2 ft (60 mm) horizontal, applied to the measurements of the outside limits of the exposed portion (ASCE 38-22, quoted in [1]). The tolerance covers only what was exposed. The definition also carries a verification step: where the data come from a test hole observation, professional judgment is used to assert that the exposed infrastructure is indeed the sought target (ASCE 38-22, quoted in [1]). Exposing a pipe is not QL A until someone with the license to say so judges it to be the right pipe.