suitable bedding for RCP storm drain

[pevans]

We are in the process of monitoring the installation of approximately one mile of 18" Reinforced Concrete Pipe for storm drain purposes along a major road improvement project. The pipe will have 5 feet to 10 feet of cover. The contractor originally intended to use a one-half sack or better cement/ABC slurry from the trench bottom to one foot above the top of pipe. The contractor has asked us to consider allowing the use of 'chips' from top of bedding to one foot above the top of pipe. Density tests would be taken at springline. The source of the chips is a concrete aggregate supplier's 'reject' pile, and as such consists of clean, durable aggregate with approximately 90 % fractured faces. Several gradations are available, ranging from # 8 grading (ASTM C-33) to finer than # 8 grading. The finer material is preferred by the contractor, and it would be 100% passing 1/4" sieve, 40% passing the #8 sieve, 5% passing the #30 sieve, and virtually no -#200 material (1 or 2%). The American Concrete Pipe Association has been completely unresponsive, and of no help. Anyone with knowledge of, or experience with, this type of situation, I would truly appreciate some advice

 

[BillHolt]

If I were in your place, I'd certainly consider it - but you'll need to dig out your copy of the Concrete Pipe Design Manual to assess the adequacy of the backfill for the anticipated loading.

As you describe the material, it sounds ideal, but you'll need some additional thickness above the pipe to give the same degree of protection to the pipe. Of course, the conc encasement may very well be an overspecification to begin with ...

Also, you want to be especially alert to the need for the material to be compacted around and above the pipe, although this should be easy to accomplish with the gradation you describe.

Of course, your client should see a substantial discount if this is allowed.

Bill Holt

 

[echosix]

RCP with 5 to 10 feet of cover is a pretty routine installation. Why are you embedding it in concrete? Is there a special load condition which you are encountering?

We normally use compacted, granular bedding to the springline for routine pipe installation. This is generally consistant with the concrete pipe manual. Without bothering to check the load tables, this sounds like Class II pipe.

You should be able to get the Canadian Concrete Pipe manual on-line. It appears to be pretty consistent with the USAian one.

Some jurisdictions, like Chicago, prefer sand bedding and sand backfill because they are easy to compact and inspect.

I second Bill's comment about getting some money back from the Contractor if you go with granular bedding.

-Dennis Novak, P.E.

The foregoing constitutes general discussion, not professional advice. The reader is encouraged to retain a competent Professional Engineer or other appropriate professional prior to basing any action or decision on this discussion.