Rocker Pipes - Are they needed?

[BRIS]

In UK it has always been standard practise to install rocker pipes on drains and sewers at the entrance and exit from manhole chambers. (Rocker pipes are short lengths of pipe which will permit differential settlement between the pipe and chamber). In recent years this practise has spread to DI (ductile iron) water pipes and may consultants are now detailing rocker pipes on ductile iron pressure pipes.

My personal view is that this is an unnecessary expense and the introduction of additional joints is a potential source of future leaks. I would argue that for the majority of soil conditions the DI water pipe has sufficient strength and ductility to compress the pipe bedding to accommodate any differential settlement between the pip and valve chamber.

I would be intested in th views of other members of te forum. Does anyone have any experience of failures of DI pipe at the junction with a chamber either due to settlement or experience of failures of rocker pipes due to joint movement?

Brian

 

[hydrae]

Brian

I usually work on the clean side of things, but thinking about it, the rocker pipes are installed to protect the pipe from small movements over time from the manholes. Now switching the pipe from weaker (PVC, Clay tile, Concrete) materials to ductile iron does not remove the movement characteristic, it just strengthens the weakest component, which may lead to the manhole becoming the weaker material or the joint between the manhole and the pipe. The movement will still happen, but what will break?
I would continue to have some sort of joint near the connection point.
Hydrae

 

[steveonline]

Well, if the previous mailer is on the clean side, I am going to comment on the dirty side!

As a UK consultant I have seen a number of instances where in extreme conditions where pre-cast concrete manhole rings have been used to raise landfill leachate collection wells, the chamber has settled by at least half a pipe diameter, and the 225 dia pipe has been damaged. The plastic pipes have either sheared between chamber wall and fill if uPVC, or simply pulled back out of the concrete if flexible plastic. The result is the same - pipe surround and waste fines washed into the chamber and a developing void behind the damaged pipe openings.

Nobody has yet re-entered these chambers which are 5 to 10 metres deep into gassing waste. Nothing less than full PP with breathing apparatus will be needed to gain entry, and until environmental damage starts to occur these chambers they will not be repaired.

Unfortunately, I don't think rocker pipes would have helped much. But we don't actually know whether they were used. I suspect not.

Surely, my example, by stating the extreme just illustrates what good engineering means - taking care to THINK about possible failure and damage mechanisms, and designing responsibly to avoid them. I put rocker pipes, even using DI, where I think the ground may allow larger than normal settlement (particularly say in made or poor ground where peat and soft clay and alluvium is present), but I would not do so for DI where the chamber is on really good virgin founding and the pipe equally stably supported. After all each joint added is an additional risk of leakage as stated previously.

AS far as DI strength is concerned it can be remarkable. Stanton & Staveley once used to distribute a photograph with their sales literature showing an entire coast road washed away, except that at least 10 to 20 metres of DI pipe spanned the void and remained entirely unsupported, in free air, and yet apparently still in use!

 

[BRIS]

I am reveiwing drawings from a leading UK consultant with a very long history of water supply. Their standard drawings require rocker pipes, 2 diameters long, upstream and downstream of all chambers on clean water ductile iron pipelines, regardless of soil conditions. I am wondering whether the provision of rocker pipes regardlees of soil type and potetial settlement has become standard practice in the industry ?.

 

[JedClampett]

If it isn't the standard, it should be. It's very easy for the pipe to accomodate the movement by rotating through the joints. If these "rockers" (not the term we use in the states) are not there, where does the flexibility come from?
The only time we don't use them is when we have very stable soil with very minimal settlement or expansion. Even then we try to provide at least flexible joint just in case.