Percolation test. Draining too quickly

[flanagaj]

Admittedly it has been very dry, but I dug to holes 1m down and then a 300 * 300mm square.  One of the pits was into chalk bedrock and the other was soil and very heavy flint.   I saturated both holes last night and then returned this afternoon to carry out a first test as 3pm and then another at 6pm.  The results are below

Chalk hole :

   Reading 1 (342 sec) Vp = 2,3
   Reading 2 (462 sec) Vp = 3.08

 

Flint / Soil 
   Reading 1 (780 sec) Vp = 5.2
   Reading 2 (1140 sec) Vp = 7.6

 

So these readings are bad news, but I am struggling to understand how neighbours on both sides have treatment plants and leech fields.

Anyone have any advice here.   I obviously need to raise the height of the field so that the invert is not near the chalk bedrock.

 

It states that it cannot be < 15 for it to be suitable.

[Temp]

Never heard of it being too permeable. Asked ChatGPT which said...

 

Quote

 

Yes, the ground can be too permeable for a traditional soakaway design — though it's less common than poor permeability.

Why Too Much Permeability Can Be a Problem:

 

1. Risk of Ground Instability
If the soil is very sandy or gravelly, water may drain too quickly, potentially undermining foundations, causing erosion, or leading to subsidence.

2. Pollution Risk
In highly permeable soils, contaminants in surface water (like oils or silt) can rapidly infiltrate down to groundwater without adequate natural filtration.

3. Inadequate Soakaway Function
If water drains away too fast, the soakaway may not provide enough attenuation (temporary storage), which is a key requirement in sustainable drainage systems (SuDS).

 

How to Address It:

Percolation Tests still apply — even in very permeable soils — and will help assess suitability.

You may need to:

• Use lined attenuation crates to slow water release.
• Add filtration layers to prevent pollution.
• Include flow control features to reduce flood risk downstream.
• Avoid soakaways near foundations if soil is loose or unstable.

Regulations Note:

Building Regulations Part H and local SuDS guidelines may restrict soakaway use in very permeable or contaminated areas — especially near aquifers.

 

 

Can't really tell you what I would probably do.

[nod]

You need to bring a Structural Engineer in It seems ridiculous that you can be draining to fast I’ve only heard of the opposite being an issue 

[flanagaj]

Here is the method I used.  Was all done correctly.

 

 

[flanagaj]

24 minutes ago, nod said:

You need to bring a Structural Engineer in It seems ridiculous that you can be draining to fast I’ve only heard of the opposite being an issue 

I was going to speak with a local company that installs the treatment plants, but I suspect they won't be interested as they are not getting the commission.

[nod]

1 hour ago, flanagaj said:

I was going to speak with a local company that installs the treatment plants, but I suspect they won't be interested as they are not getting the commission.

I’ve a friend who has been installing septic and treatment plants for over forty years and all he gets from planners is What’s your qualifications 

Give it to an SE who will do a drawing and they will have there little bit of paper 

[JohnMo]

We're in sand and our permeability test drained way to quickly. You just follow the rules for the permeability being out of limits. 

[flanagaj]

10 hours ago, JohnMo said:

We're in sand and our permeability test drained way to quickly. You just follow the rules for the permeability being out of limits. 

Can I ask what the solution was?  One option I have read is to slow the permeability by used a layer of compacted builders sand.  Keen to understand, as without a DF, our only other option is a septic tank and that is not going to fly.

[JohnMo]

We have a treatment plant and a bigger than usual drainage field. Ours was done to Scottish rules.

 

But read page 31 of this. 

https://assets.publishing.service.gov.uk/media/5a80cf9ded915d74e33fc8ae/BR_PDF_AD_H_2015.pdf

 

Make sure you did the perc test correctly - you did not - you need to repeat 3x. So fill leave overnight, then do a drain away test. Repeat 3x, so the tests should take at least 3 days.

 

Then if the answer isn't what you are needing read 1.38

"Where Vp is outside these limits effective treatment is unlikely to take place in a drainage field. However, provided that an alternative form of secondary treatment is provided to treat the effluent from the septic tanks, it may still be possible to discharge the treated effluent to a soakaway."

 

Your treatment plant is a secondary treatment I believe.

[flanagaj]

5 minutes ago, JohnMo said:

We have a treatment plant and a bigger than usual drainage field. Ours was done to Scottish rules.

 

But read page 31 of this. 

https://assets.publishing.service.gov.uk/media/5a80cf9ded915d74e33fc8ae/BR_PDF_AD_H_2015.pdf

 

Make sure you did the perc test correctly - you did not - you need to repeat 3x. So fill leave overnight, then do a drain away test. Repeat 3x, so the tests should take at least 3 days.

 

Then if the answer isn't what you are needing read 1.38

"Where Vp is outside these limits effective treatment is unlikely to take place in a drainage field. However, provided that an alternative form of secondary treatment is provided to treat the effluent from the septic tanks, it may still be possible to discharge the treated effluent to a soakaway."

 

Your treatment plant is a secondary treatment I believe.

I did fill the holes with water and let them drain away.  I am going back this morning to repeat the tests.   The very dry spell has not helped matters though.

We are using a Graf tank which is the primary treatment not secondary.   I thought the drainage field was the secondary treatment?

 

[JohnMo]

As I said read the document,

 

"alternative form of secondary treatment is provided to treat the effluent from the septic tank"

 

A treatment plant IS NOT a septic tank, so read page 30 for an explanation.

 

It's all laid out for you to follow, but it requires you read the rules.

[JohnMo]

12 minutes ago, flanagaj said:

Graf tank

Your first post said treatment plant, if that's what you are actually referring to and not a their septic tank or cesspool. The soak away is exactly that, an area to soak away cleaned water that exits from a treatment plant, it's not for secondary treatment as it would be from a septic tank.

 

If you are proposing a septic tank, you need to put in a treatment plant if your perc test fails.

 

Big difference in Scottish rules is treatment is the only solution allowed, septic is a no.

[flanagaj]

Sorry for my confusion on terminology.  We are installing a Graf package treatment plant.  

13 minutes ago, JohnMo said:

As I said read the document,

 

"alternative form of secondary treatment is provided to treat the effluent from the septic tank"

 

A treatment plant IS NOT a septic tank, so read page 30 for an explanation.

 

It's all laid out for you to follow, but it requires you read the rules.

I have read that document so am fully aware of the rules.  My question simply to do with a fast percolation reading.

[JohnMo]

5 minutes ago, flanagaj said:

My question simply to do with a fast percolation reading.

The rules are written around a base case assumption of a septic tank - a treatment plant provides primary and secondary treatment (so built in bulk separation, and treatment of waste water prior to leaving the plant).

 

A treatment plant isn't a septic tank (page 30), if you fail a perc test you are not allowed to install a septic tank tank (base case) you have to install a treatment plant which is the primary and secondary treatment system.

 

So if installing a treatment plant, it really doesn't matter if you fail a perc test, but planning I assume would want to see test results, so you need to test correctly? You are already doing the prescribed next step of installation of secondary treatment i.e. a treatment plant for a failed test.

[Bancroft]

13 hours ago, Temp said:

Never heard of it being too permeable.

Water can drain too quickly for the percolation test.

 

We have a clay cap about 3m deep above a bed of chalk.  This excludes a normal drainage field so we're looking at a concrete ring soak away.

 

In the clay, water sits there for days. But, once we did down to the chalk, it drains too quickly.

 

The solution being proposed is to dig out the clay down to the chalk and then put a layer of sand at the bottom.

 

This will allow percolation into the chalk but slow it down sufficiently to be within the correct range.

[torre]

15 hours ago, flanagaj said:

saturated both holes last night

How many times did you fill and refill the holes when saturating?

 

Rule out the chalk hole, then the big slowdown between tests for the second hole suggests that ground may not have been saturated enough to give a reliable reading. I'd fill and drain that a couple more times and then repeat the tests over a day or two

[flanagaj]

5 hours ago, torre said:

How many times did you fill and refill the holes when saturating?

 

Rule out the chalk hole, then the big slowdown between tests for the second hole suggests that ground may not have been saturated enough to give a reliable reading. I'd fill and drain that a couple more times and then repeat the tests over a day or two

Ruling out the chalk hole as that was still draining far too quickly even today after 5 fills, the soil / flint hole readings are as follows.  These are as follows.

2nd May 1800hrs - Filled hole and let drain overnight
3rd May 2 tests

     1530 hrs : 13 minutes
     1800 hrs : 19 minutes
4th May 2 tests
     1200 hrs : 23m 35s
     1529 hrs :  30m 48s

As you can see.  The readings differ wildly and it's left me somewhat confused.  I am going to go up tomorrow and do another 3 readings of the hole.  The hole now appears to have a layer of silt at the bottom.  This is going to skew the readings as by it's very nature a silted hole will not be as porous.  Should the silt layer be removed before each test?

 

 

[JohnMo]

Test is fill with water, leave overnight, do perc test one next day. Fill leave overnight, do perc test two next day. Fill leave overnight, do perc test three next day. It takes three days. Random filling here there everywhere is rubbish.

 

You aren't doing the test correctly. Crap in, crap out, using an intended pun.

[G and J]

36 minutes ago, JohnMo said:

Crap in, crap out, using an intended pun.

We haven’t got a site loo at the mo, but I didn’t think that was what percolation tests meant….  😕 

[flanagaj]

1 hour ago, JohnMo said:

Test is fill with water, leave overnight, do perc test one next day. Fill leave overnight, do perc test two next day. Fill leave overnight, do perc test three next day. It takes three days. Random filling here there everywhere is rubbish.

That's the part that was missing from a lot of the 'How to ..." articles.  I assume you have to perform the fill and test at the same time each day, otherwise, you will get different readings.  So how long do you leave between the overnight fill and the test (in hours) as that too will influence the results.  It all seems rather hit and miss.   A core sample sent away for analysis would be a far better option.

 

[JohnMo]

17 minutes ago, flanagaj said:

That's the part that was missing from a lot of the 'How to ..." articles.  I assume you have to perform the fill and test at the same time each day, otherwise, you will get different readings.  So how long do you leave between the overnight fill and the test (in hours) as that too will influence the results.  It all seems rather hit and miss.   A core sample sent away for analysis would be a far better option.

 

You just read the rules and blindly do what it says. Hit and miss maybe, but a standard hit and miss for everyone. But as I said, result isn't that important you are already going to install a treatment plant so secondary treatment part gets a tick in the box anyway.

 

Core sample is just bonkers, you pay for someone to come along to do core a sample, then a lab to analyse, then kiss a couple of grand goodbye.

[flanagaj]

28 minutes ago, JohnMo said:

 result isn't that important you are already going to install a treatment plant so secondary treatment part gets a tick in the box anyway.

That's not quite true, as it dictates what m2 your leach field needs to be.  Having wildly varying values does make it tricky calculating it.

[BadgerBodger]

Three consecutive days? I know the first “fill” should be on day one and the other tests follow but I actually thought you were meant to test consecutively on the same day so you get a cumulative impact (making the test take longer each time) which results in a larger drainage field. This is more important with poor draining land but not to be ignored. 
 

Either way. I did my own test as above, and got pretty low values (as in fast draining)  but they were accepted (using PSTP). Admittedly I didn’t follow the guidance entirely as it was the middle of storm darragh… but I think it helped my cause.  I also did three trial holes and tests picking the worst two for my final result. 
 

I used this website for guidance…

 

https://www.copeland.gov.uk/sites/default/files/attachments/percolation_surface.pdf

 

 

Edited May 4 by BadgerBodger
Clarity

[JohnMo]

30 minutes ago, BadgerBodger said:

used this website for guidance…

 

https://www.copeland.gov.uk/sites/default/files/attachments/percolation_surface.pdf

That's for a rain water soakaway site?

 

So back to document I linked and you say have read, you don't record the time for 300mm of water to disappear for example like you have. You are not reading the building regs documents, it's all laid out and easy to follow. Will bow out now

 

 

[BadgerBodger]

Apologies, you’re right. That one is for the surface water. I did not need that test in the end as we are using an existing pond which has been used for the same purpose for the last 30 years. 
 

I used a generic test from one of the tank suppliers.

 

https://www.jdpipes.co.uk/knowledge/sewage-treatment/how-to-perform-a-percolation-test.html#:~:text=The procedure measures how long,will come from the drainage

 

And you are right. I did it wrong. I should have measured the time

from 225mm down to 75mm but with the weather and me being in a general rush to get done (doing 3 tests at once) I forgot that and explained it to the BCO at the time.
 

People make mistakes. 
 

In reality, my point was faster speeds can be accepted and that you don’t have to do it on three consecutive days. Which so far as I can still tell, you don’t.