Land drainage in clay - Do overground land drains work?

[Alex11]

We have a sewage treatment plant for 7 houses. We are on clay soil everywhere. First we tried a 10cu m soakaway but it doesnt drain fast enough. Then we installed a 200m long underground land drain - single length, in shingle with a membrane on top discharging into a ditch. After 7 years the ditch is pooling (can't cope) and we think the drain shingle is clogged up, so too much effluent stays in the pipe. So now we need to do something. Another limited life underground land drain, or this approach proposed to us for an overground land drain through our woods? Effluent is pumped up to a perforated pipe laid on a shingle bank 1/2 m high and 1m wide at base, covered in grass. Does anyone have experience of these and if they work, what should the spec be? Any other options? EA say no to borehole, we have no mains drainage and no streams. Thanks for this excellent site - looked through all posts in this category - don't think anyone has asked this one.

[Tony McC]

The engineers who put together the proposal should have the calculations (my beloved BRE65 and the almost equally enjoyable Part H2 of the Building Regs) to show that a mound type construction will work on your site.

Drainage mounds *do* work, but many BCOs are very sceptical and an experienced installer is highly recommended.

[Alex11]

Tony
Thanks very much for your prompt reply. I've looked on the Planning Portal , downloaded document H and it's very informative. Please could you clarify what is BRE 65 and where I can find it - I've looked all over the Planning Portal.

With reference to your reply regarding mound drainage. Looking at document H, this refers to mound construction as part of the process to treat the effluent from a septic tank. In our case, the effluent is from a treatment plant and already conforms to EA consent. The proposed mound drain uses shingle not sand and is purely for dispersal of the effluent over a clay soil, considered preferable because when the shingle finally clogs up, it's easier to dig out and replace than if it were underground. I wondered if you had experience of a mound used for this purpose. Thanks again

[Tony McC]

BRE Digest 65 covers the design and construction of soakaways and has been giving me severe migraines for the past 6 weeks as I try to make sense of the calculations used to define the size of a soakaway - to cut a long story short, you have to guess the size that you think will work, run the dimensions through the equations, and then see if the result is acceptable. I can't fathom out why it can't just say a soakaway of xm³ is required, and my familiarity with quadratic equations is not what it was 30-odd years ago when they bored me sh...err...witless in Fr. Bailey's maths classes.

I know you can get it from the BRE Bookshop but they charge 17+ quid for it, whereas it has been published as a free PDF by many local authorities and the old Office of the Deputy Prime Minister for many years.

However, H3 is the main one to use. BRE65 is useful if you need to build a mound over a soakaway, which is *sometimes* used in bad clay, but, to be brutally honest, as it says in the FAQ above, soakaways/leach fields and clays are not good companions.

[lutonlagerlout]

IME most soakaways in clay are a waste of time
all that happens is the water fills up to topsoil level then percolates away through the top soil,irrigating grass plants trees etc
lates job in hemel they want 4 crates or 1.5m3 so we went for the 4 crates and terram,got the crates down to £27 ea.
LLL