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Rookie2 (Civil/Environmental) (OP)
5 Jun 07 19:56
Hi,
We recently recieved a review comment requesting that we install watertight joints on a RCP storm drain line due to the fact that the HGL showed that the pipe was surcharged.
This particular line is the most upstream segment in the system and the surcharge is due to inlet control.

My question is does this make sense? Is an inlet controlled pipe under pressure? I wouldn't think so due to the fact that inlet controlled pipes do not flow full.

Also we had this same request on some of the downstream lines where the pipe was surcharged by somewhere in the range of 0.1'-0.3'. I would think the pressure on these lines would be minimal. Does it make sense to install watertight joints? If not is there a reference that would help me make that argument?

Thanks
civilperson (Structural)
5 Jun 07 20:21
All joints on correctly installed RCP are water tight. The pressure capacity is not high but you do not have a large head.  Specify a leak test at the completion of the installation, (less than so many gallons per hour per inch of diameter and per hundred feet of length).
cvg (Civil/Environmental)
5 Jun 07 20:49
inlet control does not mean the pipe is surcharged.  If the energy grade line is significantly higher than top of pipe, it could at some time seal and flow under pressure.  I agree with civil person to a point - when installed, even tongue and groove joints are relatively leak resistant under low heads of a few feet of water.  However, after backfilling and in the ground for a few years, you may not be able to expect total leak resistance.  But for storm drains, it generally is not a big deal.  Depending on local practice, many jurisdictions don't even require leak testing on storm drains.  I would recommend that they be soil tight to prevent piping failure.  Also, if you expect high groundwater, they should be gasketed.  
Rookie2 (Civil/Environmental) (OP)
5 Jun 07 22:44
Thanks for the responses

CVG, I'm just curious, why should they be gasketed if there is high ground water? Infiltration?

RWF7437 (Civil/Environmental)
6 Jun 07 1:42
methinks cvg accidentally misspoke when he said "inlet control does not mean the pipe is surcharged."  Hr probably meant that inlet control does not mean that the pipe is NOT surcharged.

However that may be;

remember that you have designed this pipe for only one, or a few,  of many possible storms and that the probable error in your calculations is on the order of plus or minus 50%, if you're very careful.

remember that the cost difference between rubber gasketed and tongue and groove joints is probably minimal compared to the total cost of your project

remember that infiltration or exfiltration  and possible piping or hydraulic failure are real concerns to the agency who has to maintain this storm drain for the rest of its useful life.

a few dollars spent to guard against these things seems a small price to pay, to some of us .

good luck
cvg (Civil/Environmental)
6 Jun 07 11:29
Typically inlet control means the pipe is not surcharged.  However, as I attempted to say that pipes which flow under inlet control are probably on a steep slope with high energy.  If the pipe seals, then it will probably surcharge and may do so with significant hydraulic force which can cause leakage.

soil tight joints will prevent most piping failure.  However, this can also be done by wrapping with filter fabric.  

gaskets will prevent infiltration which can draw down the groundwater.  Dept of Water Resources will not like the idea of your storm drain siphoning off groundwater to the nearest stream.  Local agencies where I currently practice routinely require rubber gasketed concrete pipe.  The pipe costs more, however installation may be quicker than tongue and groove which require mortar to seal the joints.  Leak resistance is also superior for gaskets.

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