Drainage Design Query

[bigreadie]

Hi All,

 

We are almost over the start line to commence our build in Faringdon but the planners have added a pre-commencement condition relating to surface and foul drainage.  

 

We are connecting to the sewer in a neighbours property so that should be straight forward but the soak away not so.  I am trying to keep professional fees to a minimum but our structural engineer is saying we need a drainage design completed which is costing in the region of £2500. 

 

Does anyone else have any experience of soak aways and did you hire a 'drainage consultant' to fill a couple of holes with water and then produce a plan?

 

Comments much appreciated!!

 

Wayne

[joe90]

Ha, our BC officer said I needed to dig a soakaway pit fir the rain water, I said you mean a “swimming pool!, I explained we were on solid yellow clay and any hole I dug would fill with ground water and stay there, he then said “where are you putting the rainwater then”, I said “in that ditch over there where all the other rain water ends up” ! he shrugged his shoulders and walked away. Just had my build signed off and no questions asked about soakaways ?

[Temp]

 

Quote

 I am trying to keep professional fees to a minimum but our structural engineer is saying we need a drainage design completed which is costing in the region of £2500. 

 

What does that include?

 

Perhaps ask the BCO what he needs for the drainage. If he knows the ground is free draining he may not need you to justify the size of the soakaway. Search online for guides like this..

 

https://www.drainagepipe.co.uk/soakaways-and-membranes/how-do-i-know-what-size-my-soakaway-should-be/

 

http://www.pavingexpert.com/drain08.html

 

If you have clay or the BCO wants to see it all justified with data you may need a percolation test. If you also need a ground condition report to justify the foundation design do both at same time. If you can provide a JCB and driver it will be cheaper (ask your builder/ground worker?). I think if they have to provide  JCB you could be looking at £1500 with the rest being for the drainage design. I might be out of date on costs.

 

The BRE publish a guide to soakaway design (BRE365) which needs data from the percolation test to calculate the size. As I recall the design method in BRE365 is complicated but you might find a simpler method on line that is conservative (eg gives a bigger than necessary size). That's not necessarily a bad thing.

 

In addition to sizing the soakaways you need to specify the location on the site plan and the layout of the pipes (soil stacks, man hole etc). A lot of this is in the relevant approved documents but it takes some reading. If you posted your house and site plans here someone might be able to suggest a layout but you couldn't sue us if it all went wrong.

Edited July 29, 2019 by Temp

[Temp]

A version of BRE365 digest is here..

 

https://www.eden.gov.uk/media/1072/bre_digest_soakawaysdesign.pdf

 

 

[joe90]

1 hour ago, bigreadie said:

we need a drainage design completed which is costing in the region of £2500. 

 

Tosh, ask the BCO what he wants, if he will not tell you I am sure if you post some drawings people on here will be able to come up with a design fir you?

[ToughButterCup]

8 hours ago, Temp said:

[...]

If you have clay or the BCO wants to see it all justified with data you may need a percolation test.

[...]

 

@Temp is such a nice bloke. Straight as an arrow.

Here's what actually happens in real life in West Lancashire.

 

Adapted from my blog

Let me take you with me on my first steps on the road to cynicism in the building sector.

For some reason (sewage smells?) many people appear to delay attention to the soft and smelly until it’s either too late or until they’ve backed themselves into the smallest room in the house; and then, trousers round their ankles, they allow someone to lock the door from the outside.  Evidence? Use the search terms refusal and percolation on our LPA website. ‘Refused pending percolation test results’ is all too common.

 

Imagine then my incredulity when, on the recommendation of a colleague, a company turns up to do a ‘PERK test (mate)’ for a drainage field on our land.   Just a bit of context…. we live in sight of what was a clay quarry, within cricket ball throwing distance in fact So, there might just be a bit of clay around 

 

“Yeah, that looks fine (mate) you’ll get a drainage field on here no bother” he says without so much as lifting a shovel.

“Tickety boo ” I say. “Gonna do the percolation test then?”

“Aye… I’ll get ‘t shovel from ‘t van”

“Where’s your machine then?” I ask. “

No need for one (mate)”

“I’ll get the tea on then” . Tea duly made…. yer man’s gone

 

A square foot of the turf has been gently disturbed in one spot, and carefully replaced.  An alarm bell sounds in the dim, dark recesses of my brain. And instantly switches off. Time for the pub.  Friday is International Party Night in our place.

 

Monday – Here are verbatim copy and past unedited (anonymised) passages from the written report;

 

…I can confrm [sic!] that we have carried out a porosity test to determine the suitability of the sub soil. The percolation results indicate that an excavation area of 23m2 for the sub surface irrigation system is required….

…Condition of soil: Loam soil to the base of the excavation….

…Number of excavations: No.3 trail [sic!] pits to a depth of 550mm….

…Percolation values: Pit 1 – 16 secs/mm. 

Pit 2 – 20 secs/mm.

Pit 3 – 19 secs/mm.

Average percolation value: 19 secs/mm….

(Condition of soil: Loam soil to the base of the excavation.)

 

The briefest look at H2 Drainage Fields and drainage mounds page 31 to 33 shows the requirement for hole to be dug to 300mm below the intended invert (para 1.33 page 32). In our case that would be a two meter deep hole at least. ( See @Temp's comment above)

So, trying to be fair, I suggest to the company that I pay for a properly constituted percolation test.

Here’s part of the emailed response from the company

 

. ..However [our report] would be based on the procedure we carried out [reported in the quotation above] to confirm the first report which we have done 100’s of times and never been questioned by Building Control/Planners once.

 

Talking to a different company rep about the matter and he grins disarmingly.  And tells the story of a completely built house in Newcastle which had neither access to  off mains drainage or a sewer.     Or in the rep's unforgettable phrase -

 

Get this wrong and get your own back. 

 

[MikeSharp01]

10 hours ago, joe90 said:

Ha, our BC officer said I needed to dig a soakaway pit fir the rain water, I said you mean a “swimming pool!, I explained we were on solid yellow clay and any hole I dug would fill with ground water and stay there, he then said “where are you putting the rainwater then”, I said “in that ditch over there where all the other rain water ends up”

Same story here - clay and ending up in ditch. Its just about making the case.

 

 

[Jeremy Harris]

Same here, we're also on clay.  We had to (as does everyone, AFAIK) comply with the SuDS regulations for surface and roof water drainage.  The solution was to bury 20 drainage crates in a hole under the drive.  By good fortune there was a small patch of porous soil in one corner of the hole, through which rainwater percolates away and ends up in the adjacent stream.  We weren't permitted to drain the water into the stream, but apparently hiding the drainage path so that it isn't on the surface is allowable.  The crates we used were the 196 litre heavy duty Aquacells, which I managed to buy on eBay for a fraction of the normal price.  They are a recognised compliant solution, so just a matter of working out the volume you need from the chart in Part H.  About a day's work to dig the hole, run all the pipes, install the crates (wrapped in terram) and backfill.  Seems to work OK.

[Gone West]

The water table around here can be up to road level so the BCO and Highways agreed that we could discharge roof drainage straight onto the lane so we bypassed SuDS. Saved a lot of money.

[nod]

11 hours ago, bigreadie said:

Hi All,

 

We are almost over the start line to commence our build in Faringdon but the planners have added a pre-commencement condition relating to surface and foul drainage.  

 

We are connecting to the sewer in a neighbours property so that should be straight forward but the soak away not so.  I am trying to keep professional fees to a minimum but our structural engineer is saying we need a drainage design completed which is costing in the region of £2500. 

 

Does anyone else have any experience of soak aways and did you hire a 'drainage consultant' to fill a couple of holes with water and then produce a plan?

 

Comments much appreciated!!

 

Wayne

This is another example of so called professionals milking you for every penny they can 

 

Joes experience pretty much mirrors ours

Our  Architect has done a drain layout showing Treatment plant 

and top water running to a soak away We also where Clay So no go there 

I did pretty much the same as Joe 

BC was ok with it But asked if it could be on a proper drawing 

The Architects assistant  spent 20 mins putting it on a drawing and emailed to me 

 o charge 

 

The reality is your Groundworks company may well find a better solution and tweak  things anyway

Or find you may have to add an extra manhole or two and sort this out with BC while the drains and foundations are going in

 

Everyone  has to earn a living  

but theses fees are unreasonably 

[bigreadie]

Thanks all.  Really helpful as usual.

 

Breakdown of quote is a follows:

 

£350 to visit site and recording infiltration test

£1150 - report, calcs and drawings

£650 - foul drainage design

and then £500 if he submits to Thames water for sewage connection

 

They have also mentioned that any warranty provider might insist on a the test as well as well as a soil sample...do they?

 

Speaking the the GW (who lives down the road), he built the extension to our sellers house and says that the ground is good, no clay and free draining due to its sandy makeup.  BC (who lives around the corner) has said the same....

 

Site layout attached for visuals...

Site_Layout.pdf

[Oldsteel]

On 31/07/2019 at 20:00, bigreadie said:

Thanks all.  Really helpful as usual.

 

Breakdown of quote is a follows:

 

£350 to visit site and recording infiltration test

£1150 - report, calcs and drawings

£650 - foul drainage design

and then £500 if he submits to Thames water for sewage connection

 

They have also mentioned that any warranty provider might insist on a the test as well as well as a soil sample...do they?

 

Speaking the the GW (who lives down the road), he built the extension to our sellers house and says that the ground is good, no clay and free draining due to its sandy makeup.  BC (who lives around the corner) has said the same....

 

Site layout attached for visuals...

Site_Layout.pdf 2.18 MB · 7 downloads

 

I am at a similar stage in my project close to you in Gloucestershire so this thread is very useful to me too! I have a planning condition that states:

 

Prior to the commencement of development, a full surface water drainage
scheme shall be submitted to and approved in writing by the Local Planning
Authority. The scheme shall include details of the size, position and construction of
the drainage scheme and results of soakage tests carried out at the site to
demonstrate the infiltration rate. Three tests should be carried out for each soakage
pit as per BRE 365 with the lowest infiltration rate (expressed in m/s) used for design.
The development shall be carried out in accordance with the approved details prior to
the first occupation of the development hereby approved.

 

Is your requirement a planning condition? I cant see any way around getting a contractor in to do this. My building regs guy could do the drawing (at extra cost no doubt) , I could get the GW guy to dig the pits but all this would require oversight by someone with BRE365 knowledge, and then there is the final report.  We are on a sloping site, no sign of clay and a stream runs across the bottom of the site. So as my BR guy pointed out 'it's a piece of cake'. But I still have to discharge the planning condition ..... hence my question!

[PeterW]

32 minutes ago, Oldsteel said:

... get the GW guy to dig the pits but all this would require oversight by someone with BRE365 knowledge, and then there is the final report...

 

BRE365 is self explanatory and if you read it, you could probably do it yourself. 

 

There is a very helpful copy online here and you can see the process needed. 

 

But it be very careful getting any equipment on site if you have a CIL Liability as it may be seen as some (money grabbing) councils as a quick way to get your CIL money.  

[Oldsteel]

On 13/08/2019 at 10:31, PeterW said:

 

BRE365 is self explanatory and if you read it, you could probably do it yourself. 

 

There is a very helpful copy online here and you can see the process needed. 

 

But it be very careful getting any equipment on site if you have a CIL Liability as it may be seen as some (money grabbing) councils as a quick way to get your CIL money.  

 

Yes, I will call the planners re the CIL commencement, when I can get them the answer the phone ?. What is the definition of 'commencement', especially when the conditions state 'prior to commencement .... SuDS .. BRE365 .. etc'

[Ben100]

My BC officer said just dig a hole 'roof area x 0.3'

[Ferdinand]

On 14/08/2019 at 11:28, Oldsteel said:

 

Yes, I will call the planners re the CIL commencement, when I can get them the answer the phone ?. What is the definition of 'commencement', especially when the conditions state 'prior to commencement .... SuDS .. BRE365 .. etc'

 

You need that either stated or confirmed in writing.

 

 

Edited September 4, 2019 by Ferdinand

[redtop]

i have looked into this loads and think you are being massively overcharged.  Lots of online guides on how to do a perc test  yourself (dig away or get digger / driver for a day for a few hundred quid) then pay a few quid and use a tool (I paid for one from 'yourspreasheets.co.' others are available :-) )  This takes the data and calculates the size required.  Submit the output to thames water for their approval.  The other option, dep on roof size, is to use more than one soakaway; if the roof size being drained is less than 100m sq then perc test isnt usually required and the calculation is very simple.  Have to say for ours the architect just put 1m cubed soakaway and BC / water authority didnt bother questioning it.  And having done the calcs we need one closer to 3m cubed...

[Pete]

My B/Insp said I was ok with soakaway crates. We worked out the roof area and just put in the appropriate number of crates. I will add we are on excellent draining land and the plot is reasonably large so had a lot of room to spread the soakaways apart. I also do not have any trees near me as the B/Insp was concerned about them silting up in the long term and the roof is metal so we should be ok I hope.?