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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

 

 

PROBLEM STATEMENT:

 

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.

 

OBJECTIVE:

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

 


TASKS

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

 

FUNDS AVAILABLE: $100,000.

 

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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