Subsurface Utility Engineering
The inability to obtain reliable underground utility information has long been a troublesome problem for highway designers in the United States. Many States utilize consultants to identify the quality of subsurface utility information needed for highway plans, and to acquire and manage that level of information during the development of highway projects. This engineering practice is known as Subsurface Utility Engineering (SUE).
Subsurface Utility Engineering: An Introduction
SUE is an engineering practice that has evolved considerably over the past few decades. The SUE process combines civil engineering, surveying, and geophysics. It utilizes several technologies, including vacuum excavation and surface geophysics. Its use has become a routine requirement on highway projects in many states.
Since 1991, the FHWA has been encouraging the use of SUE on Federal-aid and Federal Lands Highway projects as an integral part of the preliminary engineering process. Costs for SUE services are eligible for Federal participation.
The proper and successful use of SUE benefits both highway agencies and the impacted utilities in the following ways:
- Unnecessary utility relocations are avoided. Accurate utility information is available to the highway designers early enough in the development of a project to design around many potential conflicts. This significantly:
- Reduces costly relocations normally necessitated by highway construction projects.
- Reduces delays to the project caused by waiting for utility work to be completed so highway construction can begin.
- Unexpected conflicts with utilities are eliminated. The exact location of virtually all utilities can be determined and accurately shown on the construction plans. As a result:
- Delays caused by redesign when construction cannot follow the original design due to utility conflicts are reduced.
- Construction delays caused by cutting, damaging, or discovering unidentified utility lines are reduced.
- Contractor claims for delays resulting from unexpected encounters with utilities are reduced.
- Safety is enhanced. When excavation or grading work can be shifted away from existing utilities, there is less possibility of damage to a utility that might result in personal injury, property damage, and releases of product into the environment.
SUE Standards and Definitions
The American Society of Civil Engineers' (ASCE) Standard Guideline for the Collection and Depiction of Existing Subsurface Utility Data was published and distributed in 2003. The ASCE standard makes it very clear that SUE is a process, not a technology. It defines SUE as a branch of engineering practice that 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.
These activities, combined with traditional records research and site surveys, and utilizing new technologies such as surface geophysical methods and non-destructive vacuum excavation, provide "quality levels" of information.
SUE Quality Levels
Quality levels may be thought of as degrees of risk, or how much information is really needed to adequately design and construct a highway project. Highway plans typically contain disclaimers as to the accuracy of the utility information. The use of quality levels allows project owners to decide what quality level of information they want to apply to their risk management challenge and to certify on project plans that a certain level of accuracy and comprehensiveness has been provided.
There are four recognized quality levels of underground utility information ranging from Quality Level (QL) D (the lowest level) to Quality Level A (the highest level).
The highest level of accuracy and comprehensiveness is generally not needed at every point along a utility's path, only where conflicts with highway design features are most likely to occur. Hence, lesser levels of information may be appropriate at points where fewer conflicts or no conflicts are expected.
Each of the four quality levels is described as follows:
- Quality Level D. QL-D is the most basic level of information for utility locations. It comes solely from existing utility records or verbal recollections, both typically unreliable sources. It may provide an overall "feel" for the congestion of utilities, but is often highly limited in terms of comprehensiveness and accuracy. QL-D is useful primarily for project planning and route selection activities.
- Quality Level C. QL-C is probably the most commonly used level of information. It involves surveying visible utility facilities (e.g., manholes, valve boxes, etc.) and correlating this information with existing utility records (QL-D information). When using this information, it is not unusual to find that many underground utilities have been either omitted or erroneously plotted. Its usefulness, therefore, is primarily on rural projects where utilities are not prevalent, or are not too expensive to repair or relocate.
- Quality Level B. QL-B involves the application of appropriate surface geophysical methods to determine the existence and horizontal position of virtually all utilities within the project limits. This activity is called "designating". The information obtained in this manner is surveyed to project control. It addresses problems caused by inaccurate utility records, abandoned or unrecorded facilities, and lost references. The proper selection and application of surface geophysical techniques for achieving QL-B data is critical. Information provided by QL-B can enable the accomplishment of preliminary engineering goals. Decisions regarding location of storm drainage systems, footers, foundations and other design features can be made to successfully avoid conflicts with existing utilities. Slight adjustments in design can produce substantial cost savings by eliminating utility relocations.
- Quality Level A. QL-A, also known as "locating", is the highest level of accuracy presently available and involves the full use of the subsurface utility engineering services. It provides information for the precise plan and profile mapping of underground utilities through the nondestructive exposure of underground utilities, and also provides the type, size, condition, material and other characteristics of underground features.
How SUE Works in Project Development
The highway agency assumes responsibility for taking appropriate actions to consider and deal with utility risks. On many small projects, where few subsurface utilities are present, and/or where information about subsurface utilities is believed to be generally accurate and comprehensive, this will only involve making a conscious decision to proceed with the project using readily available information. On larger, more complex projects, the state highway agency may elect to employ the services of a SUE provider to obtain expert advice and to use available technologies to provide better information.
The engineer, when involved, will advise the highway agency of utility risks and recommend an appropriate quality level of utility data as described above for a given project area at the appropriate time within the project planning and design process. The highway agency will then specify to the engineer the desired quality level of utility data.
The engineer will furnish the desired utility quality level to the highway agency in accordance with the standard of care and will be responsible for negligent errors and/or omissions in the utility data for the certified utility quality level .
Additional Information about SUE
- ASCE Standard: Standard Guidelines for the Collection and Depiction of Existing Subsurface Utility Data
- Presentation: "Applying Subsurface Utility Engineering to Highway and Road Projects"
- Purdue University Study: Cost Savings on Highway Projects Utilizing Subsurface Utility Engineering
- Purdue University Study Companion Presentation
- SUE Case Studies
- Sample SUE & Utility Coordination Scope of Work for Consultant Services
- Utility Topics Bibliography