(Civil/Environmental)
(OP)
28 Jul 05 10:40Any suggestions on a problem with corregated plastic storm drain pipe floating? I have 12" To 18" of cover(per manuf. specs.) and the ground water is approx. 3' below the IE of the pipe, its been in the ground since 2001. Looking for any solutions or similar experiences and corrections.
(Civil/Environmental)
28 Jul 05 11:21Why is it floating? Does water pond on top of the pipe during storm events and cause it to float? See how much concrete or flowable fill it would take to offset the bouancy of the pipe. If you have enough volume in the 12 to 18 inches of cover backfill with concrete/flowable fill. If not excavate below the pipe and pour the required concrete volume as a ballast below the pipe and tie the pipe to the concrete ballast with rebar or metal straps.
(Civil/Environmental)
28 Jul 05 15:44Try the "PPI HANDBOOK OF POLYETHYLENE PIPING MARINE - INSTALLATIONS. It should answer your questions.
(Civil/Environmental)
(OP)
29 Jul 05 07:34Thanks BMIR the handbook looks like a good suggestion, but the pipe was in the ground for almost 4 years then popped out of the ground, was looking for a more feasible cost solution. If I were to require the referenced concrete ballast to be installed, then why not just use concrete pipe. The reason the plastic pipe was used in the first place was the cost savings.
(Civil/Environmental)
29 Jul 05 10:35Concrete pipe that is 24 inches in diameter or larger will float if plugged on both ends. What is the purpose of your pipe? Is it dry when water is ponding on it? Put an area drain in that location and allow some inflow into the pipe. If it was full of water and had some cover it wouldn't be floating. You have to throw some resistance to the bouancy by tying the pipe down or by filling the pipe up when the water is high. I don't believe there is any other solution.
(Civil/Environmental)
(OP)
29 Jul 05 11:42The town eng. required that my client pipe an existing ditch that was offsite, he refused to issue a permit until the ditched was piped, due to pressure from the adjacent subdivision. The ditch had nothing to do with my site. Therefore, I opted to use the plastic storm drain pipe. The pipe was installed, per manufacturers specs. The pipe is open for the existing ditch flow up stream and tied to a catch basin on the other end (anchored). The pipe popped out of the ground mid way between the open inlet and the catch basin.
(Civil/Environmental)
29 Jul 05 12:22Typically the manufacturer's specs only concern is that there is sufficient cover for his pipe to carry a certain load without damage and maybe backfill material/compaction issues. Bouancy issues are typically the engineers problem but I'll admit we usually only do those calcs for detention pond outlet structure risers (but we don't have a high water table). You might have experienced the high end of a variable water table to have the pipe expose itself like that. Or another possible cause would be it could be really steep and the velocity of the water could have caused this. Similar reasoning as to why utilities require anchors be placed on steep sanitary sewer lines. Did someone inspect the installation? Was the backfill material compacted according to the specs? I'm not sure you have provided enough information as to the cause of the problem. Let your client sue the city for requiring it in the first place.
(Civil/Environmental)
29 Jul 05 22:03I don't know the setting; but, possibly, the pipe was under inlet control and could not carry all of the water from the ditch. Under inlet control, the pipe was not flowing full so it had air in it. Not handling all of the ditch flow, the remaining water flowed above the pipe causing bouyancy and causing it to float as the ground above was saturated and weak.
You will have to reinstall the pipe to be sure it is to proper grade. Then you will probably need to construct concrete anchors as covered above. These can be constructed on both sides of the pipe even after it is installed and then straps (SS cables, etc.) run across the pipe and fastened to the anchors on either side.
(Civil/Environmental)
30 Jul 05 15:04you might also think about concrete encasement which may be easier than the anchors
(Civil/Environmental)
31 Jul 05 00:30Some concrete pipe manufacturers make 'horse collars'. These can be place over the pipe to help weigh it down.
(Civil/Environmental)
2 Sep 05 12:10While I guess it might be somehow possible to burrow this lightweight piping back down into the earth, at least if one were to assume neither it nor its joints have been damaged in the flotation incident (e.g. with some sort of discontinuous concrete weighting, or even full concrete encasement, assuming the pipe could be suitable tied down to accomplish this safely?), it may also be necessary to assess what stress/off-set effects and or abrasion etc. there might be due to the concrete(edges) to the soft pipe and/or its joints in service during/after this is done.
While it is true that all pipes can float (the first incident I ever heard of was to a (I think 66") diameter concrete waterline now a quarter century or more ago, that I think made quite a mess of joints etc.!), I think it is also true that per Archimedes you are more likely to get a flotation problem if the "bulk density" of what you're dealing with is very markedly less than the bulk density of a fluid surround plus any effective (submerged etc.) earthload bearing down on same . It would perhaps also help if the pipe has at least a little out(side) surface texture, that one would think would help minimize any tendency toward vertical movement relative at least to the soil surround?
Unfortunately, I think the specific gravity or bulk density of the particular pipe you are dealing with may well be, even if assumed to be full of water, near or less than that of even pure water, let alone a greater density of soil that might become liquified, or fluid concrete if this is chosen as a weighting medium?
Unfortunately also, it appears a perception promulgated by some that "lightweight" and "cheap" are good can sometimes collide with Engineering reality (or "Acts of God" not necessarily foreseen by Archimedes a couple thousand years ago?) just a few years after original installation??
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