What Is Subsurface Utility Engineering? A Practical Guide to SUE Quality Levels

Long before construction begins, a project team has to answer a deceptively difficult question: what utilities are already in the project area, and how reliable is the information about them?

Subsurface utility engineering—commonly called SUE—provides a structured way to investigate, document, and manage that uncertainty. It is not simply another name for utility locating, and it is not a single device or field technique. SUE brings together records research, surveying, geophysics, engineering judgment, utility coordination, and targeted exposure of underground facilities so project decisions can be based on an appropriate level of information.

SUE Is a Process, Not One Technology

The Federal Highway Administration describes SUE as a branch of engineering practice that manages risks associated with utility mapping, coordination, relocation design, condition assessment, cost estimating, data communication, and related work. The process can involve several professionals, including subsurface utility engineers, surveyors, geophysics specialists, utility owners, locators, designers, and contractors.

That team approach matters because no single source tells the whole story. Records may be incomplete. Surface features show only part of a system. Geophysical methods can help identify and trace utilities without excavation, while a test hole can provide detailed information at a particular point. SUE organizes those different sources rather than treating all utility information as equally reliable.

The Four SUE Quality Levels

FHWA and the American Society of Civil Engineers recognize four quality levels, progressing from Quality Level D to Quality Level A. A quality level describes how information was developed; it should not be read as a promise that one method is appropriate everywhere.

Quality Level D: Information from records

QL-D begins with existing sources such as utility records, maps, plans, permits, or recollections. This information can help a team understand what may be present and support early planning, but it has not yet been correlated with visible site evidence or field investigation.

Quality Level C: Surveyed visible features

QL-C adds a survey of visible utility features—such as manholes, valve boxes, hydrants, pedestals, and poles—and relates them to the record information. This step gives the project team field-observed reference points, while underground routing between those points may still be uncertain.

Quality Level B: Surface geophysical investigation

QL-B uses appropriate surface geophysical methods to identify the existence and approximate horizontal position of utilities within the defined project area. FHWA refers to this work as “designating.” The results are surveyed to project control so the information can inform design decisions.

The word appropriate is important. Electromagnetic locating, ground-penetrating radar, acoustic methods, and other tools have different strengths and limitations. Selecting and interpreting methods requires training and professional judgment.

Quality Level A: Verified location at a specific point

QL-A provides precise horizontal and vertical information by exposing or otherwise verifying a utility at a specific location and measuring it. Minimally intrusive methods, often vacuum excavation, may be used to protect the facility while allowing its depth, size, material, condition, and other characteristics to be documented.

QL-A is the highest quality level, but that does not mean every foot of every project needs to be exposed. The project owner and engineering team determine where higher-quality information is warranted, often where a proposed design and an existing utility may conflict.

Why Quality Levels Matter to Project Decisions

A utility line drawn on a plan can look definitive even when it came only from an old record. Quality-level designations help the people using that plan understand how the information was obtained. That makes it easier to decide where more investigation is needed before finalizing a foundation, drainage feature, roadway improvement, or other design element.

The current ASCE 38-22 standard provides guidance for investigating and documenting existing utilities in a standardized way. FHWA also emphasizes that the necessary quality level can vary across the same project. The goal is not to collect the maximum amount of data everywhere; it is to obtain suitable information where it can improve decisions and manage risk.

How SUE Differs from the 811 Process

SUE and the one-call process both support damage prevention, but they serve different purposes. SUE is generally used during planning and design to investigate and document utilities for a project. An 811 locate request is part of the required notification process before excavation, according to the laws and procedures that apply where the work occurs.

Using SUE during design does not replace contacting 811 before digging, and an 811 response does not automatically provide the engineering-quality information needed for design. Our overview of damage prevention explains how owners, excavators, locators, and one-call centers contribute to the larger process.

Recognizing the Full Utility Team

SUE makes visible how many specialties support a successful infrastructure project. Engineers define information needs and evaluate risk. Surveyors establish project control and document features. Geophysics professionals select and apply investigation methods. Utility owners provide records and operating knowledge. Locators interpret signals and field conditions. Contractors use the resulting information to plan and perform the work.

For a closer look at one of those roles, read What Does a Utility Locator Do?

The Safe Line USA was built to honor this wider community—the people who investigate, design, locate, build, maintain, and protect the systems others rely on. Our People Behind the Lines Tee reflects that shared identity.

Honor. Protect. Power America.