What does a bad percolation test mean?

[broadex]

I’m applying to build a two dwellings in my back garden. There is already a main sewage on the main house.

 

The Planning office insisted on having percolation tests (I guess this is to manage rain water).

 

The tests came back really bad as the soil is too clay, and may not be suitable for soakaways but great soil for building on.

 

The engineers have identified a nearby ditch that runs along its length approx. 5m to the rear of garden and it drains to a pond and then to a formal watercourse. They have asked me to approach the land/farm owner whom but I’m dreading this option as the farm ownership is complex. I had been trying to contact the owners over the years to resolve another matter without any luck.

 

Are there any other options one can take if the percolation tests are bad?

[Marvin]

Depends if there is layered clay. How deep did they dig?. I went down for my soakaway and at the bottom broke through to a sandy stony layer and the water just disappears! Pity I didn't find this before the rest was dug out!

[Moonshine]

10 minutes ago, broadex said:

 

Are there any other options one can take if the percolation tests are bad?

 

what is the sewage from the main house? does it connect to a combined sewer?

 

Our ground conditions don't allow for a soakaway, and luckily there is a combined sewer in the road in front of our house.

 

We wrote to the local water authority who granted us to discharge surface water to the combined sewer.

[Phaedrus]

47 minutes ago, broadex said:

There is already a main sewage on the main house.

So the presumption should be that you connect your wastewater from the new builds to the sewer.  Unless there are insurmountable obstacles, you're not looking at an off-mains wastewater treatment plant and drainage field.  Percolation tests in accordance with BS6297 are required for the design of a drainage field. 

 

How is the rainwater from the main house dealt with?  If it is to soakaway then that suggests this option could work for the new builds too.  Infiltration rate tests in accordance with BRE 365 are used for soakaway design.  If these are what you had carried out and gave bad results then I'd be looking at further tests to assess the variability of the soils across the site and with depth before ruling out this option. 

 

Alternatively, interception of the rainwater and attenuation before discharge to the ditch could be a reasonable solution.  Discharge of combined wastewater and rainwater to a sewer is normally not allowed.

 

I'd have expected your engineers to have put all this in a Foul & Surface Drainage Strategy Report.

 

 

[Ferdinand]

The usual presumption for rainwater would be a Sustainable Drainage System these days, I think.

[Temp]

On clay soakaways may need to be very large or not work at all.

 

Most water companies don't like you putting rainwater into the foul sewer and may or may not allow it. If they allow it its normally called a combined system.Some allow existing properties to continue with a combined system but dont allow but not new ones to do so.

 

Some options..

 

You could dig deeper and see if a deeper soakaway works better.

You could try applying for planning permission on the basis that a combined system is proposed. The water co will be consulted and may or may not allow it.

You could propose storm water attenuation system with outflow to the foul sewer. The water co will be consulted and may or may not allow it.

You could propose a rainwater recycling tank with overflow to the foul sewer. The water co will be consulted and may or may not allow it.

You could approach the land owner of the ditch for permission to use that. They may allow it but may also want a significant payment if you have no other options so careful what you say when you approach them. If your land has riparian rights you might have a right to use the ditch. 

 

I suggest you need to look at the costs of different options. We opted to propose a rainwater recycling tank with overflow to a roadside ditch. It was allowed. Your mileage my vary.

[Faz]

21 minutes ago, Ferdinand said:

The usual presumption for rainwater would be a Sustainable Drainage System these days, I think.

+1 - this will need a SUDS solution which will involve and unrealistic amount of soakaway crates. 

 

I am in the same position and, in my case, the whole this is completely unnecessary - the ground make up is 600mm peat over clay so the water goes straight through the peat and washes over the clay until it drains away. They are going to make me put rainwater storage in for a 1 in a 100 year event + 40% addition for 'climate change'. 40 sq. m of crates overall.....  All to avoid a puddle in the garden once a century - which would never happen because of the peat!

 

Madness.

 

 

[Ferdinand]

1 minute ago, Faz said:

+1 - this will need a SUDS solution which will involve and unrealistic amount of soakaway crates. 

 

I am in the same position and, in my case, the whole this is completely unnecessary - the ground make up is 600mm peat over clay so the water goes straight through the peat and washes over the clay until it drains away. They are going to make me put rainwater storage in for a 1 in a 100 year event + 40% addition for 'climate change'. 40 sq. m of crates overall.....  All to avoid a puddle in the garden once a century - which would never happen because of the peat!

 

Madness.

 

 

 

If you have a big garden perhaps take advantage of the demand to use some of it as a swale / bog garden?

 

The volume of that should count as buffer.

 

If you have the bottom of an excavated pool as pond, then can you count the 2ft to the top as storage volume? 

 

Just musing.

[Faz]

Or agree to what they want and take a more practical solution and quickly backfill ?

[broadex]

Thank you all. This site has been extremely helpful. I am eternally grateful. 

[saveasteading]

The requirements, and solution, may depend on the local rules, especially in regard to flooding. Picking up several points, not only yours.

 

The rain currently lands on a garden and presumably soaks, very slowly, into the ground or evaporates.

When you then build houses with hard roofs, the rain is quickly concentrated into wherever you put it. Hence outfall to a sewer or ditch could add to flooding downstream.

The crate solution is expensive, and still requires a constrained outflow and outlet. This is often the easiest option for a designer and planner, but at your cost.

 

I agree totally with Ferdinand to try for a swale or pond first if you have space.  It may be quite large but is very cheap, and can be good for wildlife. It then works by having the necessary storage, a large surface over the ground for soaking into ground and a large evaporating surface, especially in heat or wind. A longish swale often finds a 'leak' in the ground from previous trees etc.

In my experience they have worked better than calculations indicate, probably due to plant respiration, and faults in the ground. It will need maintenance or it will gradually fill in.

 

If the volume doesn't quite suffice then you could overflow to drain or soakaway.

 

Flooding is very serious for those downstream, hence the imposition of SUDS practises, and every litre adds up.

 

One test though, for any proposal, is where will the water go in extreme circumstances and overflow? 

 

[Modernista]

This is probably a silly additional question but it seems to fit in the context of the discussion. The existing house we are about to refurbish has no gutters. So rainwater runs straight off onto the ground as a 'design feature'. The eaves projection is around 600mm (see pic) so the waterfall that ensues is discharged away from the walls and there is an additional projecting shelter above the external doorways to avoid getting soaked as you come and go in the rain. I've been wondering seeing various posts here if we can anticipate the LA (Scotland) not being happy with that and if as soon as we submit plans we'll have to manage rainwater according to current regs. That would be costly and challenging, but not impossible and I'd rather avoid it if I can. In heavy rain the water discharged doesn't seem to pond anywhere - it runs away into the surrounding garden, which is heavily wooded. So I guess my question is if you don't collect rainwater from roofs at all, might we be required to and then deal with it via a new soakaway etc?

[saveasteading]

The building regulations do not tell you to build drains and soakaways, or that gutters are necessary . They do tell you to protect the building from water, and that any water must be disposed of appropriately. 

On top of that, planning may impose limits to outflow.

 

If you can prove that your current, and proposed, building does not require any change then that may suffice. You may need to establish what happens to the water now.

I am guessing that there are french drains to the perimeter, beneath the drip points, then that soaks away or is connected to a drain.

 

However, with climate change rainfall quantities, it may be difficult to predict what will happen in heavy rain. 

 

Perhaps there is a compromise, to allow partial additional storage.

1. retain existing situation

2. for climate change, add swale or soakaway for that extra amount.

 

Good luck in convincing a building inspector......some will welcome the innovation/common sense, some will resist.

[Modernista]

Thanks, that's really helpful. I'll need to investigate further in terms of the possibility of french drains and any existing soakaway. In some areas the water discharges onto what looks like a 400mm wide strip covered with large pebbles - but they appear to be set in mortar of some sort rather than loose / granular but perhaps the mortar is permeable (or at some point has been a french drain now covered over) but that's got me thinking - and as you say I'm sure the level of pragmatism varies greatly in terms of building inspectors and this type of thing. Thanks again.

[saveasteading]

Perhaps worth investigating the large pebbles. positioned where they are. It is likely to be as a drain. and perhaps someone thought it was decorative and grouted it.

The last similar I looked at was round pebbles at the top, then coarser below, but was filled with decades of muck. It would be a chore to rake out, clean and return, but would improve the capacity, and might make your options easier. 

Let us all know findings and solution please.

[Modernista]

Will do, thanks very much. Will post a picture next time I'm at the site

[Modernista]

Just by way of a postscript here is a pic of the possible french drain. I can't dig it out to inspect the layers underneath until I own the place next month but it does look like the stones aren't set in any mortar or grout - so it looks as if you are right about the decades of muck - thank you! The alignment isn't directly under the roof run off (see the wet line in the pic) but presumably that reduces the chance of any gravel infill etc being eroded by falling water. Looks like a maintenance job required - but I'll also need to find out where it goes as the foul drains are choked with tree roots and any drain runs  from these french drains to any soakaway will be no different probably..... These are only present where there is paving. Elsewhere the run off is direct to ground (although again it may be that any original french drains are just covered with layers of general leaf and soil debris.)

[Temp]

Deleted.

Edited May 6, 2021 by Temp

[saveasteading]

useful pic. may or may not go deeper, and may not even link to a useful soakaway. when damp and dirt were everyday things, a slight improvement in the outdoor puddles situation might have sufficed.  looks like someone has painted the bricks in bitumen to keep the worst of the splashing off.

 

I am tending towards putting as much modern drainage away from the house as you can. If this, possibly, French drain works then is  a bonus.

[Modernista]

Thanks. I'll post again on this when I find out more. 

[broadex]

Has anyone used rainwater harvesting as an alternative? The planning request for percolation tests were for surface water management and i just thought before i go back to planning, i can suggest using some sort of rainwater harvesting. 

[joe90]

However if you don’t use all the rainwater the excess has to go somewhere so a soakaway is still required.

[Ferdinand]

20 minutes ago, broadex said:

Has anyone used rainwater harvesting as an alternative? The planning request for percolation tests were for surface water management and i just thought before i go back to planning, i can suggest using some sort of rainwater harvesting. 

 

All of use it to some degree. I have a few big butts (water butts).

 

But nearly all go for the "for the garden" sort rather than a second plumbing system to flush loos.

 

The second sort is very complicated for the benefit, and you have to have more holes in your house. Shower heat recovery is an easier halo, if you want something.

[joe90]

I discounted rain water harvesting on cost grounds despite being on solid clay, when the BC told me I needed a soakaway I told him “you mean a swimming pool?”,  “what” he said”, I replied that any hole I dug in our clay would fill with water and stay there, I pointed to an existing ditch that was taking any surface water away and proposed I piped it there and he accepted that.

 

since the build I dug a pond in the field next to the garden and that’s filled with water and apart from evaporation it stays there. Clay can be good sometimes ?

[saveasteading]

Anything that reduces water flow will help, but perhaps not much in extreme or long duration storms.

Harvesters are easy enough with newbuild but messy with retrofit/rebuild. They reduce all flow to drain because you have the same rain but reduced bought in water.

As Ferdinand says, rainwater butts are easy. They are also a recognised solution in new developments where there not be another solution, or simply as standard.

If already full, then they don't help much.

But you could do what I do, to take the load of my old drains and soakaway in the garden. In summer the several butts are used for the garden, but in winter I turn on the tap slightly and let it dribble to empty over a couple of days, thus doing what a complex storm system does, and slowing the flow to downstream problem areas.

Anything can be presented formally as a solution, and if well presented with logic, has a chance.

You might need a lot of butts though.