PVC vs HDPE- buried under slab 2

[GeneralAssociate]

GeneralAssociate (Mechanical) 22 Aug 05 21:50
Value Engineering was requested by CM/Owner. Commercial Project includes methane mitigation using air intake/exaust/blower system, slab on grade, 3/4" washed stone bedding, new construction.

One item sticks out - 8500LF 4" Perforated SDR 17 HDPE pipe. Using Perforated PVC under slab would lower project costs, seems logical. Engineer prefers HDPE?. But only on perforated? Make sense?

Cost savings would be substantial enough to include in proposed VE. Any thoughts on industry/design standards.

Thanks All, Steven

 

[JasonG]

I'd look into the strength of the 2 materials. I'm pretty sure that HDPE is stronger than PVC. The engineer may be concerned that the strength of perforated PVC may not be adequate beneath the slab and it would get crushed.

 

[Dirtguy4587]

HDPE will take more deflection than will PVC. I typically use HDPE in any area where the pipe will be placed in a fill zone or any areas where the pipe may be subjected to ground movements.

 

[gtrainor]

HDPE is stronger, however I cannot see using it in this situation. I have used lots of 4" PVC for vapor extraction under slabs and driveways where tanker trucks park. Specify sand for bedding for good haunch support. Top that off with pea gravel at least a couple of inches over the pipe with a layer of geo fabric over that. Backfill with native and you should be good to go. If your engineer is worried about deflection enough to break the pvc, then you have much bigger problems than just a pipe breaking.

 

[TerraSouth]

Good topic!

I also have a project where I have to divert runoff from very large roof through down spouts and under a 6" concrete slab to a catch basin. 185' x 245' roof area. 8 down spouts spaced along the 185'. 185 x 180 concrete slab for a staging area.

Would you specify sch. 40 PVC or sch. 80 PVC?
What would be your minimum cover over the pipe?
1.5 feet, or 2 feet perhaps?

This slab is used as a staging area for loading tractor trailors, large fork lifts will be rolling back and forth.
(2500 psf allowable bearing capacity)

Would it be best to tie all the down spout into a single juntion box and then run concrete pipe to the catch basin? (this would shorten the required PVC)

OR just run PVC from each down spout to the junction box (more PVC to run)

How about a trench drain that all the down spouts tie into at the drip line, then run concrete pipe over to the catch basin?

 

[gtrainor]

TerraSouth, not sure if you are asking me, but I'll comment anyway. Now that you talking about roof drainage, that's quite different than methane mitigation. Even if the pipes used for methane transfer defelct, crack, even be crushed, there is a good chance the system will work to some degree especially if it is a grid system. However, for storm runoff for a roof, let's not cut corners. PVC Storm Pipe can be used if a good bedding design is adheared to. But I would use DIP or Concrete at least under the heavy load areas. I would also advise not to use a junction box. If for some reason the one discharge pipe from the j-box backed up, then all your pipes will back up to the roof. Then you got major problems. I recall a friend telling me that on one building the drain pipes were undersized by the plumbing engineer. This caused ponding on the roof and eventually led to roof damage. The civils were cited in the lawsuit because they were responsible for storm water management, whether if it was on a roof or a parking lot. From then on, the civils had to approve of the plumbing engineer design in that design firm. Do a Google search for "roof drains" to see some other suggestions.

 

[TerraSouth]

Thanks for the comments!

Very good points, and I need to do my homework on this one.

Thanks

 

[mitchell54]

GA,

I am in agreement with TerraSouth. PVC is the way to go, keep in mind that you are designing a pathway for methane migration, with appropriate cover 4" PVC will do just fine. I have seen and used this method for air sparging units and piliot studies in which I have placed PVC under active rail lines to conduct soil vapor studies. As long as you make sure that you have proper depth and appropriate cover and compaction of that cover you will be fine.