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)