Type of foundation backfill for this job

[Radian]

Hi, first post on these forums. Our building project has got to the stage where foundation trenches have been poured and block & beams are supported on blockwork. Before we go on, the ground has proved very difficult - heavy clay on a sloping site and water frequently pools on top of the foundation and hence under the floor.

 

Because of this we are going to lay 100mm perforated pipe all around the perimeter and discharge it into the rainwater drainage exit pipework (below foundation level). The question is what material to use for backfilling over the pipe. I know it will require something like gravel with a Geotex wrap to prevent sediment getting into the pipe but I don't like the idea of bringing any of the clay back in as it will shrink and settle like crazy. There are to be paths and patios to most walls so whatever backfill is used it has to be well compacted. Would 10mm gravel (around the pipe) topped with Type 1 be sensible or any better ideas? The depth required varies but there's a considerable volume to fill so cost is highly significant.
 

[Mr Punter]

Crushed concrete is cheap and works fine. You may want to top it off with a bit of proper type 1 to get a smoother finish for the final paving or tarmac.

[joe90]

We have similar ground and my builder backfilled with 50mm drainage stone, no membrane (as he reckons it clogs with clay silt) and this was linked to the rainwater drainage to get all surface water away from the build. We did the same with the garage and it has worked very well. I also talked to a land drainage company about our field which is like a bog in places and he said the same about membrane or pipes blocking and stone in a French drain worked best. 

[epsilonGreedy]

My foundations are open at the moment and in the wet weather running up to Christmas the trenches partially filled. I currently have 600mm x 600mm of poured concrete and two courses of blocks which brings the blockwork up to ground level on one side and 1/2 block below ground level on the other side of the house due to a gradient. The trenches filled to about 1.25 blocks deep at worse but following some pumping I realized this was caused by surface run off external to the oversite and also some run off from the contained oversite.

 

The good news is that following a less wet period I have discovered that the winter water table is much lower than the flooding indicated after digging some drainage sumps 400mm below the poured concrete top. I have not used the sump pump for 3 weeks and have found the sub soil (silt plus some clay) has a natural self draining capacity after moderate periods of rain.

 

During the most sustained wet periods the trench pooling did not overtop the oversite so your situation might be worse.

 

I am less concerned about the over all situation because once the roof is on that will eliminate one source of foundation flooding. Like you I might consider a drain where the foundations face the uphill plot gradient just to encourage surface water to take a faster route around the property footprint and down to a soak away on the lower side of the plot.

Edited January 25, 2019 by epsilonGreedy

[Declan52]

Pea gravel around the pipes.  But it doesn't compact so will be no good for paths. 

As far paths any kind of crushed stone will do. Type 1 or 3 will do.  Get the cheaper one. 

[Radian]

I'm very impressed with the replies here and the depth of experience and knowledge. Thank you all.

The silting of the permeable membrane mentioned by joe90 is something I've wondered about before. In our previous property the land drains became disfunctional after a few years but the increasing levels of flooding came so gradually I'm ashamed to admit we kind of ignored it. Thinking logically, any silt stopped by the membrane will accumulate there - and with clay it can be surprisingly impermeable. I've been pumping the newly poured trenches and when getting down to the last few mm, the inevitable fall on the concrete can easily be dammed by the last bit of silt which has to be brushed along to get the last drop to flow down to the pump sump.

So it might make sense to let the silt freely pass into the perforated channel so long as a reasonable amount of water goes through as well. On that score I have very few doubts.

 

6 hours ago, Mr Punter said:

Crushed concrete is cheap and works fine. You may want to top it off with a bit of proper type 1 to get a smoother finish for the final paving or tarmac.

 

I like the sound of that although Type 1 has a downside...

 

4 hours ago, Declan52 said:

Pea gravel around the pipes.  But it doesn't compact so will be no good for paths. 

As far paths any kind of crushed stone will do. Type 1 or 3 will do.  Get the cheaper one. 

 

Type 1 has higher content of 'fines' and seems to develop a 'paste' when wet so I'm a bit leary of topping the pipe with that. Type 3 has fewer 'fines' and consequently costs a fair bit more. Given the extremity of the surface water issue it could prove to be worth it.

[Declan52]

You will cover the pipe fully with the pea gravel. At least 100mm of gravel all around the pipe.  10mm of any type of stone won't do anything.  Dig the trench with a 300mm bucket and the pipe goes in the middle and will leave you the 100 each side.   Then you can cover it over.  If the clay is dry you can use this to  put over the  gravel and build it up to what ever height suits your path.  Then the crushed stone whatever one you go for. Type 1 is used for paths and roads as it has the  fines so compacts really well and holds together. 

[Oz07]

Providing the pipe is low enough below sub base don't worry about pea gravel not compacting. You will compact your sub base above the pea gravel and will spread the weight. As been said could backfill with clay but you want surface water to be able to migrate down into pipe? What about 6f2 for the main fill with a no fines material for last 100mm below path. Type 3 or whatever that is. 

[jfb]

+1 for not putting a membrane around the French drain if it is clay. If it were sandy better to have a membrane.