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Posted: Sat Aug 08, 2009 1:18 pm
by mark555
Dear Tony / Experts,
I joined the site last week. I can log on, but it will not let me post a new topic.
I have very much enjoyed reading your web-site - and have learnt a lot.
I have a 'boggy' garden. Having stripped the moss and grass away, I am left with a 'brownfield' site. I am therefore thinking it is a good opportunity to put some drainage in.
My site is approximately 125 m square.
My soil seems to be clay-ish to a depth of around 70cm, then sandy.
Originally I was going to run plastic drainage pipe and attach this to the rainwater drains. Unfortunately on investigation the rainwater drains are set too high up to allow me to get a fall into them from the garden.
I therefore stated to look at Soakaways. Your site is very informative.
In my situation the ground water table is high, at about 70 - 80 cms under the garden.
I have carried out a percolation test, with a 100mm of water dropping in just around a 1000 seconds - giving me a value of 10.
I am therefore thinking that a shallow, but wide soakaway may work.
Your soakaway suggests a minimum depth of 1.2 meters - but can I use a depth of say 70cm, but have it wider?
If this is acceptable would laying the pipe around 30cm's deep seem acceptable?
As an alternative, I considered digging a number (say 10 or so) smaller soakaway pits around the site (without connected pipe) - would this be an alternative?
I hope you are able to give me the benefit of your experience.
Many many thanks, Regards Mark
Posted: Sun Aug 09, 2009 10:59 am
by Tony McC
Apparently, you *can* post a new topic, or we wouldn't be here!
I can't follow this tale: you say the ground is clay to 700mm depth, whereupon it becomes sandy, but then the water table is also at 700mm depth: in sand??? Then you undertake a percolation test, and this works, but how can it with such a shallow water table?
So: to start logically, what are you defining as the depth of the soakaway: the bottom of the storm crates/perforated pipes?
If you are proposing 700mm depth to base of storm crates, this would make the top somewhere around 200-300mm below ground level, which is a bit shallow, but if the water table is at 700mm down, then you can't really go any deeper. Bit, if you are laying perforated pipes, then 700m depth would be fine.
The logical solution, as I see it, would be to install a herringbone dispersal drain, which would allow the water-retaining upper clay layer access to the free-draining sandy lower layer. Allow around 3-4m between each arm of the dispersal drain as too much drainage can be as bad as too little.
Posted: Sun Aug 09, 2009 1:08 pm
by Bob_A
Sorry if this is a daft question but if you have a long garden could you use a couple of long continous pipe rather than using a herringbone configuration. Or is this limited to how deep the pipes will have to be laid.
Reason for asking that it may be simpler to dig and use less pipe connectors.
Also is there a way of estimating how much pipe you need in relation to how big an area you're trying to drain.
Cheers
Bob
Posted: Mon Aug 10, 2009 9:58 am
by mark555
Sorry if my post was a little confusing.
To clarify.
I have a clayish soil to a depth of 70-80cm approx.
At 80 cm I hit both the sand layer AND the water table (it is a costal location so perhaps the tidal level affects this??).
My thoughts (taking the replies into account) are to dig a soakaway hole to 80cm deep (this is as deep as I can go due to the water table). Then run two straight pipes along the garden approx 30cms deep.
As the garden is 7m wide that would allow the pipes to be say 2.5 meters apart, with no part of the garden further than 2.5 meters from a drainage pipe.
Please could you clarify:
1. Would perforated drainage pipes at 300mm be too shallow (i.e. cause the ground to dry out too much in dryer spells?)
2. Are two parallel pipes at 2.5 meters apart OK?
3. Does a series of crates sunk at 700mm (say on a 100mm bed of gravel) and then connected to the perforated pipes sound like a sensible solution.
Thanks and sorry for any repetition, but this topic is new to me.
Thank you, Mark.
(ps. My initial registration would not work, I waited for several days - it would allow me to log in, but not to post...... but on reregistering a new account it became active straight away - anyway account is working A1 now)
Posted: Tue Aug 11, 2009 9:00 am
by Tony McC
On a plot 7m wide, I'd run a ring of land drainage 2-2.5m in from the boundary. This needs to extend down to the permeable sand layer otherwise there is nowhere for the collected water to go.
There's a good argument to be made that a simple filter drain with no pipe would be adequate in such a scenario, but as the pipe is dirt-cheap anyway, it might be churlish to omit it.
Posted: Mon Aug 17, 2009 3:13 pm
by mark555
Thanks Tony - Before I start digging can I just confirm the advice is as follows:
1. I dig a trench say about 200mm wide by 800mm deep (the level of the water table)
2. I put some geo textile fabric in the bottom of the trench
3. I fill the bottom of the trench with say 200mm of gravel.
4. I put a flexible plastic drainage pipe in
5. I fill around the pipe with gravel
6. I fill say 200mm above the pipe with gravel and fold the geo fabric over the gravel
7. I top off to garden height with soil.
In this instance I end up with no dedicated soak way, just a long looped drainage trench.
If the above is correct - the gravel bed will start be 400mm under the soil - is this too deep or too shallow? ( I am aware of your comments that too much drainage is just as bad).
Does the pipe have to be laid level in this instance - or as it is not running to a soak away does it really matter if it varies levels around the trench.
Thank you very much for you continued help.
Posted: Thu Aug 20, 2009 2:45 pm
by Tony McC
In clarification....
1 - Dig trench spade width by depth to water table
2 - Line entire trench with construction grade geo-textile. If gravel section is to be 400mm deep and 200mm wide , you need 400 + 200 + 400 + overlap (say 500) = 1500mm width of geotextile
3 - 100mm gravel in base of trench
This is all shown in great detail here
4 - place perforated pipe is required
5 - backfill with gravel to required depth, say 400mm
6 - overlap geotextile and trim as necessary
7 - complete backfill with soil