Passive slab peripheral drainage.

[MikeSharp01]

I am just setting out to finalise some ground works and have realised, I guess I could have worked this out, that bottom level of the crushed stone base for the passive slab will be slightly below our existing drainage outlet which is a ditch we have running across the front of our property that takes the ground water away, slopes down about 200mm across the width of the property (1 in 60). Now comes the hard part, for me, as I am not sure exactly how these peripheral drains function - other than the obvious, because they are perforated so water goes in and out at will they are only any good for rapid transport of water when the localised water table gets above the drain invert. If I don't need to let the water out and its just a mechanism for distributing water build up under the slab then I have no worries. If I don't need a slope on the peripheral drain, IE its just a path for water above a certain level to get out - it will be perforated after all, I can just about get it out into the highest part of the ditch. If I need a slope I can get a very slight one and take it to the other end of the ditch. However, in either case, if the ditch blocks it will back up water under my slab - not ideal. If I must have a good slope then I am going to need a soak away into the clay, more of a pool than a soak away, which the BC guy has already said he would probably allow to overflow into the ditch so I am back where I started. Any thoughts anyone?

[Jeremy Harris]

The main thing is to ensure that the crushed stone under the foundation can eventually freely drain somewhere, it doesn't much matter where as long as water can't collect in the depression where the stone sits and just stay there.  Our soil was pretty rock hard and impermeable gault, so without a drain the 150mm deep area filled with stone would just end up full of water.  We fitted perforated land drains around the edge, at the bottom of the stone layer, and connected them to our big Aquacell surge tank/soakaway, but that's always above the water table. 

 

In heavy rain a fair bit of water will find it's way under our slab, I'm sure, the main thing is to allow it to drain away somewhere slowly, without washing away anything, which isn't very likely anyway, as water is really just slowly percolating into that area and slowly draining back out again.  In your case, I'd be inclined to just make sure there's a porous route for water to get out to the ditch, perhaps a French drain down to the ditch, that would allow water to trickle through but act as a bit of a barrier if the water backs up might be the best solution.

[MikeSharp01]

Thanks Jeremy, the french drain idea is a good one and also means that I don't have to put a vermin barrier in the peripheral pipe at the ditch end so I can probably get away with putting a percolation path to the lower end of the ditch and not worry too much about backing up because that will clear / be cleared fast enough not to be a long term problem. We are on stiff clay and so once the water gets in a hole it stays there unless I pump it out or await a prolonged period of sun and then it evaporates. As the ditch is used by all the houses on our side of the road as drainage it is kept reasonably well clear as nobody wants to flood their property and the person with the blockage gets the first of the overflow! Thanks for you thoughts.

[PeterW]

If its clay then you may want to use clean 20mm stone and also line the french drain with geotex so the clay doesn't get washed into the gaps . A "V" shape is sometimes better on clay soils but its a pig to dig as you can't use a machine !

[joe90]

I am on yellow clay that does not drain at all. The BI said I must provide a soak away and I told him that would be a pond in our clay. We had strip founds and when I backfilled them I used geotechto line the area and filled with 40mm clean stone, then at a point closest to the boundary ( where a ditch exists in the lane and flows most of the year) I laid a 150mm drain pipe also in 40mm stone. The BI signed off the garage without asking about the drainage.  The area has never been so dry in winter, a very good result which I am repeating around the house. I am waiting to see if the BI wants a soak away for the house and hopefully I can repeat what I did with the garage.

[TerryE]

Like @joe90 we are on blue Oadby member clay and there was no way that anything would drain from what was a local low point.  So we dug a sump hole and had a small (pond) pump in it.  We just pumped it out onto the road.  Worked brilliantly.  The water table is still at ~25cm at the moment (my water meter is under 150mm water!) 

[ToughButterCup]

3 hours ago, PeterW said:

[...]  A "V" shape is sometimes better on clay soils but its a pig to dig as you can't use a machine !

 

You may be able to: if I can, anyone can. Get a 'clay spade'. Worth its weight in gold to us on glacial till.  

[MikeSharp01]

Thanks all... Can see my way through it all now.

 

On another matter..

24 minutes ago, recoveringacademic said:

Worth its weight in gold to us on glacial till

I know what you mean I was held up by one in my local supermarket yesterday
 

[Calvinmiddle]

I don't think you really ned to worry about this, our passive slab was engineered designed with 750mm depth of ground dug out beneath our slab and then filled back with stone.   Needed this as we were on what the results said was desiccated clay and there was a danger of heave.  As our ground slopes and plus the thickness of the slab it mean at one point we were about 1.6m below original ground level, there was no way this could drain anywhere.

Luckily we dug it out in summer when it was dry, as in the winter when we were digging out the hole for the treatment plant we can across a very slight transition layer between two type of clay, that had water running along it, had to get a pump to put the treatment plant in the hole.

Joke was with the ground workers was that we have the largest soak away ever, as I know water must be getting into the slab stone, even our soak-away for rain water is well above the level of the bottom of the stone, so even through they aren't connect there is a good chance it is ended up in there - but is there an issue, if there is one I can't see what it would be, there is terram at the bottom of the hole, its been whacked down in a number of layers and has 100mm of concrete sitting on top - house isn't going anywhere!