Maximum slope of underground drain

[sharpspark]

I'm installing a new foul water drain fed by one shower/toilet/sink for occasional use. The guide lines I've read for this application either say 1:40 or 'minimum 1:40' slope on the drain. I know that solids can get left behind if too steep but nowhere can I find a figure for the maximum slope. The word 'minimum' that is so often used suggests that it can be steeper. Going steeper would be a benefit as it is in steeply sloping ground. Can anyone advise a suitable maximum slope?

 

[Softus]

Foul water drains
Foul water drainage systems are generally designed to run at a
maximum of three quarters full bore. Pipe gradients should be
established such that the velocity does not fall below 0.70m/s to
ensure adequate self-cleansing. The proportional velocity relating to
pipes running part full must be taken into consideration when
determining compliance with this requirement.

When calculating flow rates for foul water drains it is recommended
that reference is made to BS EN 12056-2: 2000 and BS EN 752-4: 1998
to determine peak discharge rates. The procedure detailed in the
above standards takes into consideration appliance discharge unit
values and frequency of use to derive peak flow rates from which pipe
sizes and gradients can be established.

The table below is taken from BS EN 752-4: 1998 and provides
guidance on minimum gradients for different size drains.

[code:1]Peak Flow (a) PVCu Pipe Size Minimum
litres/second (mm) Gradient
.lt 1 82 1:40
110 1:40
.gt 1 82 1:80
110 1:80 (b)
160 1:150 (c)

(a) Peak flow based on probability flow calculation method
(b) Minimum one WC
(c) Minimum five WCs
[/code:1]
In cases where discharge rates are very low or where continuous flow
containing solid matter will be less than 1.0 l/s a steeper gradient of
not less than 1:40 will be required. In addition, where ground
settlement is likely, it is also recommended that steeper gradients are
used and shorter 3m lengths of pipe are installed.

So there you have it, straight from the

 

[doitall]

Whatever you do has to be approved and checked by the building inspector anyway.

Why not ask the guy to make a site visit for advice.

 

[ChrisR]

I remember reading "between 9 and 90mm per metre".

 

[Nige F]

I'm installing a new foul water drain fed by one shower/toilet/sink for occasional use. in steeply sloping ground. Can anyone advise a suitable maximum slope?

No, I can`t .............use backdrop manholes how steep is your steep anyway .......

 

[sharpspark]

Thanks guys. The slope is about 20 degrees on average and about 20m long. I'm already planning back drop manholes in places but I want to make the most of what pipe slope I can. As the article from Softus shows, for less than 1L/m use minimum 1:40. What I am after is the maximum. The word minimum implies to me that I can go steeper and that would be useful.
Interestingly the building control guy has said he does not need to be involved in this due to the nature of the project (not a permanent residence). We've already considered using saniflow and small bore and ruled it out due to possible future development we might do, so it's going to be in 110mm. All the requirements for access, laying, working to building regs etc I'm OK with, I just want to get a figure for the maximum slope. I will try and talk to building control again but I thought I'd see what I could get from the forum first.

 

[Gasguru]

20 meters long at 20 degree's equates to a pipe depth of 7.3 meters at the end of the run. Apart from the self cleaning velocity probably being exceeded you would need a very wide bucket and trench shoring to lay the pipe safely ie very expensive.

I suggest you keep the run as shallow as possible (with the recomended gradient) so you can use low cost access chambers and then construct a backdrop chamber at the end of the run to drop down. It will need to be large enough to climb down into with step irons incorpoprated. You are then ready for future development.

Building on slopes is always the most expensive way to construct

 

[ChrisR]

WHy not look at the Building Regs??!! Or ask BC?!!

9 to 90 mm per metre run as I remembered, is for branches up to 15m long (Table 2)
Flow rates for sewers are quoted for slopes from 1: 10 and shallower.


http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADH_2002.pdf