Encasing water main in concrete

[LCid]

What would be the main reason(s) for encasing in concrete a 48-inch steel main crossing perpendicularly under another 48-inch steel main? The main is to be encased even if the specified minimum (12") clearance between the mains is achieved.

 

[francesca]

Is one of them clean and the other sanitary?

 

[LCid]

Both mains are clean water.

 

[JedClampett]

I couldn't open your link.
However, it's possible that you encase a pipe in concrete when you're worried about compaction over the bottom pipe. Poor compaction might result in loss of support for the top pipe or even settlement of the soil above.

 

[bimr]

Are the pipes located in a street? Please provide some further details on the installation.

 

[LCid]

bimr,
The pipes are located in a street, with heavy traffic; cross each other at an intersection; both are steel, 48-inch in diameter; clearance between the mains will be 12" or greater. About 30 feet of the bottom main are to be encase in concrete (wire mesh + 8" concrete all around)

JedClampett, excellent comment.

 

[bimr]

Steel pipe is very flexible and is subject to buckling. Other than that, it is difficult to know without a geotechnical report.

You might want to get a copy of : Buried Pipe Design By A. P. Moser, Steven L. Folkman. Design issues are addressed there.

JedClampett has made some valid comments.

 

[cvg]

differential settlement is generally a concern where pipes cross. Support for the upper pipe is on undisturbed soil on each side and on compacted fill at the crossing point. You should bridge the lower pipe by not placing a joint over the existing pipeline. Span it with as long of a pipe spool as possible. The concrete encasement on the bottom pipe replaces the trench backfill with a stronger material which is less susceptible to settlement.