Twin wall sewage drains

[Steve Firth]

Does anyone have any experience of these?

I have to install a sewage drain in a location where the fall is around

10 metres over a 60 metre run. My intention was to build several backfalls keeping the runs between at 1:40. However the local builders' merchant is claiming that twin wall tube can be run following the contour of the land at 1:6 and will not block despite the slope being (IMO) far too steep.

Obviously if it is true, then it is a very attractive option, much less digging to do and safer working. However I can't (so far) find any useful information on twin wall drainage and what little I can find seems to imply that it should be installed using the same criteria as

110/160mm PVC drains.

Niggling at the back of my memory though was that in a construction journal some years ago I read an article about the use of twin wall tube for exactly this purpose and the report was of the cost saving from being able to have steep falls without the use of backfalls and deep concrete inspection chambers.

Oh, and to forestall rants about bs en 12566-3, building regs, building inspectors, etc. This installation is not in the UK and different rules apply.

[Tabby]

How would the wall construction have any effect on drainage bhaviour?

NT

[Tim Watts]

Why does having too steep a fall cause blockages? I'm not disputing it but I can't see why and I'd like to know...

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Tim Watts saying something like:

I suspect the solids get left behind by the liquid.

[Tabby]

Non-compliant drain piping often works perfectly, or fine upto a point, but can block every so many years. BR more best practice on this than essential.

NT

[Hugo Nebula]

...

Well, the law of gravity has been adopted in most jurisdictions. However, Hepworths (clay pipes) say in their design guide, "recent research has shown that steep gradients do not cause increased erosion of pipes or deposition of solids". Plastic pipes would be even less susceptible so steep gradients are no problem.

[Steve Firth]

I think the builders' merchant has taken the view that twin wall tube has (supposedly) smoother internal walls than PVCu or clay. Another consideration is that twin wall is available in longer lengths than PVCu and the joints between sections are done using a different seal designs that does not cause a ledge on which sewage could get stuck.

I can't see anything else that would account for the claim that it's OK to use following the slope of the land which is around 25% at the steepest point and that averages 16-17%. Obviously the big attraction is the reduction in the scale of groundworks.

I did find an American site that claimed that twin wall was perfectly acceptable to use at slopes as steep as 25% and another that stated that any slope greater than 45° is effectively vertical. I can certainly construct a drain that has falls steeper than 1:1 and sections in between of 1:40. It's very tempting to do all of that in twin wall that can be made to follow the trench contour because it's more flexible than PVCu.

[Tim Lamb]

I know little about the subject:-)

It occurs to me to wonder what effect a *slug* of water/brown jobs, rocketing down your pipe will have when it eventually reaches an inspection pit or a stretch of 1:40 gradient. Will man hole covers lift to emit a burp of trapped air? Will the brown bits get splattered up the walls?

More seriously, how do they overcome these problems in volcanic islands where 30deg slopes are common?

regards

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