Relief Plate and Casing Pipe

What is a relief plate in the context of buried pipelines? Functionally what is the difference between a casing pipe and relief plate.

A relief plate is a plate on a roadway that protects the pipeline from as shown below.

http://www.corrosolutions.com.mx/Documentos/INS2009%20Installation%20guide%20for%20GRE%20pipe%20systems%20%201112.pdf

Casing pipe is put around the pipeline to afford mechanical protection.

In Department of Transportation World, casing pipes do more than just protect the carrier pipe:

• If the carrier pipe were to break for some reason, the roadway above the carrier pipe is protected from the failure. For instance, a water main break inside a casing pipe forces the water to the ends of the casing pipe and the roadway or railroad remains intact.
• The casing pipe allows for replacing the carrier pipe without excavating / disturbing the roadway above the casing pipe.
• Installation of the casing pipe allows for later install of a carrier pipe. So it could be a situation of timing. For instance, installing a casing pipe (conduit) while roadways are being constructed allows for the later install of underground electric or gas after the construction of the roadway is completed.
• Casing pipe that is installed by boring methods or horizonal direction drilling significantly reduces the possibility of differential settlement. And if some settlement does occur, the casing pipe protects the carrier pipe.
• Casing pipes protect carrier pipes that may be carrying sensitive material that absolutely cannot be compromised (i.e. water main) from possible sources of contamination.
• There are some hazards. Casing pipe can concentrate gases. If the carrier pipe is a gas line, or maybe even a sewer line, it is wise to consider venting the casing pipe.
• If a casing pipe is carrrying a pressurized line, if there is a leak, even a small one, the casing pipe may become pressurized itself. So be careful when first exposing it.
• Relief plates have none of these pros or cons. They are mostly used to bridge pipes that cannot be exposed to live loads or point loads.