PVC vs. RCP? 2

[cse53226]

I'm designing a private parking lot for both cars and school buses. Trying to decide whether to use PVC or RCP. I have between 1.5' and 3' bewteen my top of pipe to the asphalt. PVC is less expensive and would save my client money but I want to make sure it will hold up to the heavier load from buses with the little cover it has. Any thoughts? Anybody ever have problems with PVC? Any problems with cracking or anything due to freezing?

THanks

 

[ccor]

The required minimum cover for flexible pipe is generally set to ensure that the assumptions that Spangler made in the development of the Iowa formula are met, which is that the surface point load is actually distributed evenly over the width of the pipe.

ASTM D2321 requires one pipe diameter (or 2-feet minimum) cover over thermoplastic pipe. However, it is conjectured that this applies to soil-only cover during construction. Also the way cover is measured can vary. Typically, cover is measured from the top-of-pipe to the top-of-pavement for rigid pavements (concrete), and from the top-of-pipe to bottom-of-pavement-section for flexible pavements (asphalt).

Rigid RCP and NRCP in low cover applications may also tend to crack a flexible pavement over the centerline of the pipe. Here in the mountains of Colorado (many freeze-thaw cycles and expansive and aggressive soils) we have used both PVC and HDPE with success in low cover applications. We use 1-foot minimum from the bottom of flexible pavement section, or to the top of a rigid pavement as detailed above.

 

[saruman26]

I would recommend HDPE N-12(I typically spec ADS

http://www.ads-pipe.com)

instaed of PVC. It has a smooth interior just like PVC, but adds a corrugated exterior for greater strength. It also splits the difference in cost between PVC and concrete and is more easily installed.

As for cover, I agree with ccor. The HDPE pipe needs at least two feet of cover to be safe. Where cover is an issue, you could potentially specify RCP Class IV or V for shallow depth, provided that you have 6" clear between the bell of the pipe and the underside of the pavement section (this is PA DOT spec).

Another option is to pour a 6" concrete slab over the pipe to better distribute heavy loading and protect the pipe from breaking - this goes for both HDPE and concrete.

 

[LCruiser]

I've never seen a problem with 1 foot between HDPE and subgrade - Plus, HDPE lasts at least as long as RCP.

 

[dicksewerrat]

If you use a plastic pipe, HDPE or PVC. inspect the installation and backfill compaction.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Sharrick]

I agree with dicksewerrat. Installation and backfill compaction are critical for plastic. A proper base needs to be created, the pipe placed, then sand packed in around the haunches of the pipe. This sand at the haunches should only be placed to about 3/4 the diameter of the pipe until good compaction is obtained. This is the hardest part to get people to take the time to do, but most pipe failure is from not have good compaction under the haunches. The next layer of backfill should only be to about 1' over the pipe, then compacted again. After that the final lifts of fill can be placed up to 3' at a time.

Plastic pipe needs good, solid, non-yielding material around it for it to have any strength.

In our county, our Road Commission no longer allows plastic for driveway culverts, not because the product is bad, but because few people take the time to install it properly.

 

[xantics]

I'd go with RCP Class IV, wall B or HDPE with an equivalent thickness if the city is comfortable. But as dicksewerrat and Sharrick mentioned, plastic pipes derive their strength from how well the soil around it is compacted.