HELP, failed perc test and on clay soil

[nowtie]

Wondering if anyone has any last ditch suggestions. We have full planning permission to demolish an existing bungalow (I bought it 2yrs ago, previous owners in there for 40+yrs) and replace as well as build a detached house in the garden. Woop woop I thought.....but there's a condition that we need to discharge around the disposal of surface water. (foul is fine, there's a main foul only sewer to the rear of the property) 6 months ago I thought well that's not a problem, we've proposed a soakaway system under the drive except it all started going wrong when the percolation test failed. SO I went to the water company as it said if all your SUDS fail we could approach them to discharge our surface water into their foul only sewer. They have said no as on the maps they can see there's a highways gully drain outside the property (not a stormwater drain) We then went to highways and they've just said no as there's current flooding on the road and they're saying we'll contribute more to it. We're now trying to get a meeting with them onsite so we can try persuade them otherwise but we're clutching at straws here I think as the road has a history of flooding and the parish council are all over it with highways maintenance teams so they're fully aware there's an issue with flooding on the road.

Just as a sidnote, we have CCTV cameras done to see where the existing bungalow surface water was going and it was going into the ground (so effectively into the highways drain- although Highways are saying that connection should have never been there)

So in short, we're completely STUCK. Any ideas? Half of me is thinking...perhaps we can re-do the perc test in different areas on the site (dug to 1.2 and 2.4m deep) and see if anything works? Borehole soakaway? Although ridiculously expensive and we're on Mercia mudstone so have been told by a few sources it just isn't permeable. Aggggggghhhhh.

[garrymartin]

Similar situation and some good suggestions here 

 

[JohnMo]

1 hour ago, nowtie said:

percolation test failed

How deep did you go? Go deeper? Just means your drainage field will be deeper in the ground. Your hole should 300mm below the invert level. See how low the heavy clay is and do the drainage field below if you can.

 

https://www.jdpipes.co.uk/knowledge/sewage-treatment/how-to-perform-a-percolation-test.html

Edited June 20 by JohnMo

[Alan Ambrose]

Do you have a soil investigation and/or have you looked up BGS records of any bores nearby. It would be interesting to see how deep the clay is and what’s under it.

[Mr Punter]

Put in an attenuation tank and specify permeable paving.  It possibly won't do much good, but it should get signed off.  As there is an existing building you will not be making matters worse.  Worst case you are allowed to discharge to a foul sewer, although this is not to be encouraged.

 

If it is clay, the more you dig out, the bigger the pond you create.

 

Percolation tests / drainage mounds are for foul water systems and you have a main sewer connection.

[Conor]

If there is an existing house, where does the surface water currently go? You have an automatic right (if not increasing runoff) to use it.

[Jilly]

Don't panic all is not lost, you just have to be creative.

 

Fort the record, I did a rough perc test on 5 places our drive to get an idea (clay) and there was a marked difference at the different places. 

I found it difficult to get calculations for a single site as the Wallingford website is for developments, but I recall one of the companies which sells the crates will do the calcs which should satisfy Building Control or the planners if it's a planning condition.

Also draw up a system of water butts, rain gardens and swales to slow the water down, plus permeable driveway.  

[Temp]

On 20/06/2024 at 14:50, nowtie said:

We then went to highways and they've just said no as there's current flooding on the road and they're saying we'll contribute more to it. We're now trying to get a meeting with them onsite so we can try persuade them otherwise but we're clutching at straws here I think as the road has a history of flooding and the parish council are all over it with highways maintenance teams so they're fully aware there's an issue with flooding on the road.

 

We had a near identical situation. Clay soil and flooding 100m down the hill. Highways said "they wouldn't want us to make the flooding worse".

 

We proposed an underground Rainwater Storage tank with overflow to the highway drain. The planners approved it discharging the condition without further consulting highways even though ours doesn't actually provide storm attenuation when full. We use it to wash the car, water the garden in summer.

 

These days people are more aware of the need for genuine storm attenuation so you might need to propose a tank that just does attenuation or both storage and attenuation. For example a 3000L tank might store 1500L and provide 1500L of storm attenuation.

 

They need a fairly deep hole with concrete at the bottom to prevent the tank floating out of the ground when empty. Also a nearby electric supply for a submerged pump.

Edited June 23 by Temp

[Dave Jones]

put in a normal soakawy crate, run an overflow to the foul connection.

 

Should have made your perc test pass (its paperwork) and avoided the whole headache.

[nowtie]

6 hours ago, Dave Jones said:

put in a normal soakawy crate, run an overflow to the foul connection.

 

Should have made your perc test pass (its paperwork) and avoided the whole headache.

Hi Dave,

This was our back up plan but Wessex water has said no based on the fact they can see a Highways gully drain directly in front of the property?

OK thank you regarding the perc test but surely just "making a perc test pass" on clay soil is only going to mean issues for the gardens of both houses in the future during storms no?

 

Many thanks 

[nowtie]

15 hours ago, Temp said:

 

We had a near identical situation. Clay soil and flooding 100m down the hill. Highways said "they wouldn't want us to make the flooding worse".

 

We proposed an underground Rainwater Storage tank with overflow to the highway drain. The planners approved it discharging the condition without further consulting highways even though ours doesn't actually provide storm attenuation when full. We use it to wash the car, water the garden in summer.

 

These days people are more aware of the need for genuine storm attenuation so you might need to propose a tank that just does attenuation or both storage and attenuation. For example a 3000L tank might store 1500L and provide 1500L of storm attenuation.

 

They need a fairly deep hole with concrete at the bottom to prevent the tank floating out of the ground when empty. Also a nearby electric supply for a submerged pump.

Thank you for your reply. Oh wow that's lucky they approved discharging the condition. We have proposed almost the exact same thing but have still have received a no from planning and highways. There's basically a land drain from a farmers field behind our site which pipes through a culvert underneath the site and then straight into a Highways gully in front of the site (which can't cope during storm conditions as it is) I think our issue is that since the road is like a river (as a result of the land drains) they're reluctant to discharge our planning condition. So my last ditch attempts are just trying to meet with them on-site but they won't meet so I'm in stalemate sat on a site which we can't progress! I'm guessing now we've said a perc test has failed we can't do a u-turn and try different holes and try find something that might work? Just out of options

[nowtie]

On 21/06/2024 at 13:39, Jilly said:

Don't panic all is not lost, you just have to be creative.

 

Fort the record, I did a rough perc test on 5 places our drive to get an idea (clay) and there was a marked difference at the different places. 

I found it difficult to get calculations for a single site as the Wallingford website is for developments, but I recall one of the companies which sells the crates will do the calcs which should satisfy Building Control or the planners if it's a planning condition.

Also draw up a system of water butts, rain gardens and swales to slow the water down, plus permeable driveway.  

Thanks Jilly. Prob is that we've already submitted this alternative SUDS attenuation and reduced outflow into Highways drain stating that the perc test failed??! I mean OMG if I had known how AWFUL it is trying to deal with Highways and get them to agree to meet with us then I would have done 20 different holes on the site! Were you 5 places at greater depths? Our two holes were 1.2m and 2.4m so deep enough?  I've been told since we're on Mercia mudstone there's no chance of making the perc tests work?

Should we try again just to double check? Would it just be a no if we went back to planning and say "Oh actually, sorry it DOES work after all" I mean that's just not feasible right?

[nowtie]

On 21/06/2024 at 10:26, Conor said:

If there is an existing house, where does the surface water currently go? You have an automatic right (if not increasing runoff) to use it.

Hi Conor,

Yes there was a bungalow there which myself and husband demolished. When we had it CCTV'd, the surface water from that one was going into the ground and then I guess into Highways drain eventually. I'm less worried about that one (as you say, I think I have a riparian right to discharge how it was before but with improvements in run off rate) but I'm more worried about the other detached dwelling in the garden of the bungalow which would be new connection?

[nowtie]

On 20/06/2024 at 14:56, garrymartin said:

Similar situation and some good suggestions here 

 

Thanks Garry

 

I have read through these and the only suitable one looks like the attenuation tank and utilisation of a rainwater harvesting system but surely they'd have to have a back-up for when this tank was overflowing and rainwater wasn't getting used quickly enough if you see what I mean? Currently we have attenuation tank with slow release into Highways drain (our engineer proposing a "betterment of the current discharge") but they're not buying it at all.

[Alan Ambrose]

>>> I mean that's just not feasible right?

 

I think that's possible. On my site, it's mostly clay with a small amount of 'sand lenses'. Next door, hearsay says they started digging out their 'moat', hit some sand, and all the water ran away.

 

Find similar sand lenses that drain and you're in business. I don't know if such a thing is possible on mudstone though.

[Blooda]

We didn't do one, as our ground investigation showed it would fail, as we were on clay.   

Water board said we had to reduce to 2l/s in order to be able to put into the foul sewer.

So, the SE produced a scheme that involved crates and attenuation via vortex valve.  All very expensive, and not really suitable for single properties anyway.

 

With a bit of investigating we ended up with rainwater harvesting with attenuation.

 

SuDS with RWH Archives - Rainwater Harvesting

 

Probably a bit more expensive, when you consider the plumbing,  but at least we flush the toilets on Rain water.  and save a bit on water bill.  It will never pay for it's self. 

 

 

[nowtie]

On 20/06/2024 at 16:45, JohnMo said:

How deep did you go? Go deeper? Just means your drainage field will be deeper in the ground. Your hole should 300mm below the invert level. See how low the heavy clay is and do the drainage field below if you can.

 

https://www.jdpipes.co.uk/knowledge/sewage-treatment/how-to-perform-a-percolation-test.html

Hi John,

We went 1.5m and then 2.4 in two test holes. Both holes filled up. We're on Mercia mudstone apparently so I've had a few geologists from my enquiries say a soakaway/deep borehole soakaway wouldn't work there

[nowtie]

4 minutes ago, Blooda said:

We didn't do one, as our ground investigation showed it would fail, as we were on clay.   

Water board said we had to reduce to 2l/s in order to be able to put into the foul sewer.

So, the SE produced a scheme that involved crates and attenuation via vortex valve.  All very expensive, and not really suitable for single properties anyway.

 

With a bit of investigating we ended up with rainwater harvesting with attenuation.

 

SuDS with RWH Archives - Rainwater Harvesting

 

Probably a bit more expensive, when you consider the plumbing,  but at least we flush the toilets on Rain water.  and save a bit on water bill.  It will never pay for it's self. 

 

 

Thanks but unfortunately Wessex Water are saying no as there's a Highways gully drain outside of our property and they've said due to this we've got to ask Highways for permission (who have also said no) Who is SE? We already have drainage engineers on board and it sounds v similar to what we were proposing to Highways. With your rainwater harvesting with attenuation, where did the excess surface water go when the tank was full? Surely watering the garden and flushing the loos won't get rid of all the surface water year round?

This is our issue with it, we don't have permeable soil, any watercourse or stormwater drain nearby. Only the foul only sewer (that was the no from Wessex). Can these attenutation tanks be physically pumped out as an alternative I wonder?

[Blooda]

35 minutes ago, nowtie said:

With your rainwater harvesting with attenuation, where did the excess surface water go when the tank was full?

It goes into the foul sewer, but attenuated to 2l/s. 

This is when it reaches 55% full

 

Example of drainage hierarchy. [will be the same or similar for wessex water] 

 

The Drainage Hierarchy | Essex Design Guide

 

Looks like you are at the combined sewer stage.

 

[Blooda]

SE - Structural Engineer

[Jilly]

2 hours ago, nowtie said:

Thanks Jilly. Prob is that we've already submitted this alternative SUDS attenuation and reduced outflow into Highways drain stating that the perc test failed??! I mean OMG if I had known how AWFUL it is trying to deal with Highways and get them to agree to meet with us then I would have done 20 different holes on the site! Were you 5 places at greater depths? Our two holes were 1.2m and 2.4m so deep enough?  I've been told since we're on Mercia mudstone there's no chance of making the perc tests work?

Should we try again just to double check? Would it just be a no if we went back to planning and say "Oh actually, sorry it DOES work after all" I mean that's just not feasible right?

It’s probably not worth doing it then. But you can propose something else which you can prove works. 

[Temp]

10 hours ago, nowtie said:

Thanks Garry

 

I have read through these and the only suitable one looks like the attenuation tank and utilisation of a rainwater harvesting system but surely they'd have to have a back-up for when this tank was overflowing and rainwater wasn't getting used quickly enough if you see what I mean? Currently we have attenuation tank with slow release into Highways drain (our engineer proposing a "betterment of the current discharge") but they're not buying it at all.

 

Attenuation (only) tanks are sized to cope with a storm surge. After the storm passes they empty slowly so in theory they are empty by the time the next storm arrives.  This type doesn't store water for recycling use, it only slows down the rate of discharge to the drains but thats enough to minimise flooding.

 

Combined tanks that do both Attenuation and rainwater recycling are available. After a storm they drain out slowly but only down to about the half full level. So they still have space for the next storm but also provide some storage.

[Dave Jones]

18 hours ago, nowtie said:

Hi Dave,

This was our back up plan but Wessex water has said no based on the fact they can see a Highways gully drain directly in front of the property?

OK thank you regarding the perc test but surely just "making a perc test pass" on clay soil is only going to mean issues for the gardens of both houses in the future during storms no?

 

Many thanks 

 

why did you tell them ?

[nowtie]

2 hours ago, Dave Jones said:

 

why did you tell them ?

Good question! In hindsight we should have persevered with take soakaway/storage crate solution. What are your thoughts on us going back to planning and saying we have now dug different test holes with different results (which work)? Or is that ridiculous idea? 

[nowtie]

9 hours ago, Temp said:

 

Attenuation (only) tanks are sized to cope with a storm surge. After the storm passes they empty slowly so in theory they are empty by the time the next storm arrives.  This type doesn't store water for recycling use, it only slows down the rate of discharge to the drains but thats enough to minimise flooding.

 

Combined tanks that do both Attenuation and rainwater recycling are available. After a storm they drain out slowly but only down to about the half full level. So they still have space for the next storm but also provide some storage.

Hi, thanks. I spoke to a guy from Rainwater Harvesting and he said attenuation and rainwater combined would still need an overflow solution to somewhere when there is excess water so couldn't suggest anything other than try a borehole soakaway (which would be at least £15-20K PLUS I've been told it won't work so I'm trying to avoid going down that route) Could we re-submit perc test figures to planning that do work if we try different test holes across the site?