GRP Outlet in Concrete Structure Detail

[stanier]

The application is a DN1800 pipe drain of a concrete structure. The design consists of a 90 degree bend directly beneath the structure. The structure is expected to settle 20mm. There is a concern that the bend beneath the structure will be exposed to strain in excess of its design parameters. Pipe is designed to ISO 10639.

A proposal was made to concrete encase the lot but this was not preferred as there could be problems in the horizontal section adjacent to the conrete casement and the cost.

An alternative solution was a vertical slip joint with O rings fitted seal to allow the spigot encased in the concrete to slip in a female fitting to take up the settlement. This is not preferred because of the adequacy of O rings in axial movement, rolling of the O rings rather than sliding, no means of testing the structure for water tightness and debris getting into the O rings. Chevron type seals may be better but some problems still exist.

Has anyone experience of this type of problem and the detail that can be used to overcome the difficulties seen?

The structure carries sea water for an intake into a desalination plant. Hence leakage intot he soil is not preferred.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[civilperson]

Build a compressible layer between the top of pipe and the bottom of the footing,(assuming the footing will bridge across the trench). Alternately, surround the pipe will a structural cover with clearance between the cover and the top of pipe.

 

[Ussuri]

The way I see it you either prevent (or minimise) the settlement or accomodate the movement.

For option one, could you consider mini or micropiles, a larger foundation width/length, remove the soil and replace with something better, improve the ground by ground stabilisation, reduce the weight of the structure. All possible but would seem to me excessive for a 20mm settlement problem as that is not really a lot. In which case you need to accomodate the differential settlement.

The most common method I can think of is rocker pipes ( a short length of pipe adjacent to the structure) which will act as a knuckle. However, this implies spigot and socket joints which I guess may not be acceptable based on your OP.

What about having the GRP bend fabricated with a flange which then connects to a short double flanged length of MDPE. The inherent flexibility of the MDPE would accomodate your settlement.

 

[stanier]

Thnaks for responses.It is difficult without an ability to provde a sketch.

Civilperson, The structure above the bend is a water retaining structure and the bend connects into the floor of that structure. If the floor settles 20mm and the pipe doesnt then it will leak at the joint.

Ussuri, The answer I think is to concrete encase the bend and provide rocker joints in the horizontal plane. PE fittings would take too long to procure for Australia. I am unsure if we have the welding equipment available for DN1800 in any event.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[Ussuri]

I would also check you sufficient room for a rocker of that size. You might be looking at a length of 2.5-3m.

As i recall if the length of the rocker is too short, less than the diameter, the relative displacement at the joints results in a larger rotation than for a longer pipe. I seem to remember this was a problem for the joint integrity.

 

[rconner]

I think in general there are at least a couple sort of different, and perhaps arguably good, reasons I've seen presented for some sort of dependable “rocker” piping arrangement/joints adjacent various structure penetrations with flexible joints (such as are available for use in at least some steel and ductile iron pipe systems). One is expected “settlement” (due to relative ground etc. movement or some inevitable changes in constructed/existing void ratios etc.) that I believe is talked about in many engineering and geotechnical programs of civil engineering and may also be presented in this thread. Another that is perhaps worth mentioning is a generally much more abrupt or pronounced, relative movements of buried pipe and structure such as could occur in whatever direction in seismic events. With regard to more or less normal settlements at least with reasonably engineered design and construction support, there are now perhaps many text book references for this subject (and e.g. I noticed not long ago e.g. an apparent long-term case-study of water tower settlement over some rather stiff though cohesive soil strata at

http://www.waterinfocenter.com/articles/Water_Standpipe_5_01.pdf).

I think there are likewise many references available now on seismic behaviors and piping provisions that could be obtained with a good search engine, and that talk about even more movements in some areas/cases. While maybe a little over-stated by some film media etc. potential for seismic effects are perhaps more widespread than many folks realize, and in this regard I think I heard there was even some USA congressional testimony not long ago “75 million Americans in 39 states are directly vulnerable to serious earthquakes.”
I recently noticed also that rocker pipes were apparently discussed in another thread a couple years ago on these forums at

http://www.eng-tips.com/viewthread.cfm?qid=84121.

I guess it wouldn’t hurt to mention also that relative movements of particularly large structures and pipes can occur due to flotation as well as settlement.

This of course does not mean that there will necessarily be any problem with simply running welded or full-length pipes of any material in and out of whatever buildings wherever (as I know e.g. that at least steel and ductile iron pipes are very strong as beams and are additionally, well, “ductile”, andof course atleast the latter in effect represents a six meter or tewnty feet "rocker"!). However, this is just to let people know that in effect “hinges” are available at least with these piping materials for some applications that can minimize bending/shear loads on same (and having to worry about complicated, and some additive to other design parameters?, wall/strain design effects of same).

 

[stanier]

Thanks for the input.

We decided against a rocker joint because of the length required and the low strain tolerance of GRP.

The final detail involves wrapping the bend in 20mm compressible rubber and embedding in a sinbgle size gravel. Slab poured. A sliding joint in the vertical fitted with a pair of rubber sliding seals. A flange is to slipped over the male pipe end and then fitted to the concrete floor with SAF 2507 chemset anchors and a non shrink epoxy grout beneath. Once completed the gap betweeen pipe and slip on flange to be filled with a flexible grout.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au