The National Academies

NCHRP 20-07/Task 248 [Completed]

Guidelines for Utility Encasement Policy for Highway Crossings
[ NCHRP 20-07 (Research for AASHTO Standing Committee on Highways) ]

  Project Data
Funds: $50,000
Research Agency: Gas Technology Institute
Principal Investigator: Dr. Khalid A. Farrag
Effective Date: 3/9/2009
Completion Date: 12/8/2009
Comments: Completed final report submitted to AASHTO

NCHRP 20-7 (248)

Research Problem Statement

Guidelines for Utility Encasement Policy for Highway Crossings





It has been a long-standing policy of many States to require that when a utility crosses a highway that those facilities be encased.  Some of the perceived benefits of the encasement policy are as follows:

  • Increased structural integrity for the highway
  • Vent pipes in case of leaking to allow the gas to escape to the atmosphere
  • Encasement pipe, if hit by excavation equipment, would notify the operator of the existence of the pipe before impacting the carrier pipe itself.
  • If the encasement remains undamaged, the carrier pipe could be replaced by extracting the existing pipe and threading in new pipe without a new bore under the roadway.


Utilities are putting pressure on DOT’s to relax encasement requirements, claiming the following:

  • Cathodic protection is stated to be more difficult to maintain with encased pipe – this is said to lead to accelerated corrosion of the pipe.
  • Improved welding and inspection techniques (for natural gas lines) greatly reduce the possibility of leaking pipe.
  • Thicker walled pipe can provide the same structural strength as encased pipe.
  • Uncased pipe crossings are easier to install.


While some States have the ability to grant a variance to the encasement requirement, research is needed on whether or not encasement policies need to be revised.  There seems to be a growing movement in the natural gas industry to consider allowing uncased crossings for many low pressure crossings.  A study needs to be done to determine whether current DOT encasement policies are appropriate.



This research will determine whether current DOT encasement policies are appropriate.


If it is determined that uncased utility highway crossings may be a safe alternative to encased crossings, a guideline should be developed for uncased crossings that includes the following:

  • Type of road crossings allowed.  Should uncased crossings be allowed on freeway facilities?  Should they be limited to two-lane highways?  etc.
  • Maximum pipe pressure
  • Maximum pipe diameter
  • Minimum depth of pipe
  • Minimum pipe wall thickness



Suggested tasks to achieve the objective include the following:

  • Identify the original basis of need for encased utility crossings
    • Research the history of past natural gas accidents/explosions and other high pressure facility failures at highway (railroad crossings) and determine the causes
    • Compare the life of cathodically protected uncased versus cased pipe and the safety of each
    • Determine, if possible, the probability of a failure and/or explosion from a leaking uncased pipe under a highway
  • Determine if safety requirements can be met using thicker, uncased pipe
    • Examine the safety improvements in the industry that could negate the reasons stated above for originally setting the present policy, such as welding techniques and inspection methods
  • Identify any potential alternatives to encased pipe, including alternative materials or methodologies
    • Investigate the policies of other countries and their safety records
    • Conduct a cost comparison between encased versus thicker-walled pipe crossings
  • Develop a report detailing the findings and supply to the Subcommittee on Right-of-Way and Utilities for use in updating appropriate AASHTO guidance




TIME AVAILABLE: Nine months, including three months to produce a report.



STAFF RESPONSIBILITY: David Reynaud, 202/334.1695 (dreynaud@nas.edu)

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