Clay on top of 100mm waste pipe OK?

[Molynoox]

I was reading Part H last night and one part got me thinking, see screenshot.

Most of the soil that I extracted from the ground was clay - does this mean that I cannot backfill it again with clay based on what the regs say about 100mm maximum clay lump size? It seems it's OK to do that from 300mm above the pipe, but for the first 300mm looks like I need to buy some new soil and get rid of the old stuff?

I also show a picture of my trench so far, seems the clay is drying out, should I cover it to stop it doing that? I feel like it won't go back in the hole again very well if it's all rock hard.

thanks
Martin

 

[dickm]

That does remind me of the nightmare, shrinking Oxford clay near Milton Keynes; ended up having one corner of the house underpinned because of shrinking.....

 

[RichardG]

Yes, I’m on Norfolk clay and had to use a different infill, I had loads of sharp sand left over from another job and some pea shingle which I mixed to use. The building inspector was happy anyway.

Nice trench, there’s something very satisfying about a trench, especially when you can sit and admire it with a beer and reminisce about the hard graft.

 

[Woody2Shoes]

5mm pea shingle under and beside the pipe, and clay with stones removed (or more pea shingle) on top for 100mm would be fine. Your clay will go back to mush when wet surely? (it's been very dry for the last week or three). I wouldn't be too concerned about the lumpiness of the clay as long as the pipe has no stony bits near it. I'm on heavy weald clay which looks just like yours!

 

[Spectric]

With waste and drainage pipes you use pea shingle, it gives good support and fully surrounds the pipes.

 

[Molynoox]

cool, thanks all. The regs seem a bit over the top to me, I can understand the 100mm below around and on top, but I don't understand the 300mm on top.
I am going to have excess soil if I put in 200-300mm of shingle. Not sure how I'm going to get rid of that....

 

[Woody2Shoes]

cool, thanks all. The regs seem a bit over the top to me, I can understand the 100mm below around and on top, but I don't understand the 300mm on top.
I am going to have excess soil if I put in 200-300mm of shingle. Not sure how I'm going to get rid of that....

"Selected fill" just means you've sieved/visually checked the material you backfill with. Clay, in particular, behaves like water over the long term - stuff moves about in it!

 

[Molynoox]

That makes sense.

 

[MARK.B.]

Pea shingle all round the pipe this will allow for slight movement of the pipe and protect it, If you put large lumps of clay back in you will fill the trench but there will be voids in the clay which over time will settle to fill the voids and your surface layer of Grass will sink below the level of the rest of your lawn.

 

[Woody2Shoes]

Pea shingle all round the pipe this will allow for slight movement of the pipe and protect it, If you put large lumps of clay back in you will fill the trench but there will be voids in the clay which over time will settle to fill the voids and your surface layer of Grass will sink below the level of the rest of your lawn.

I think it's fair to say that nearly all soils will do the same kind of thing - as fines get washed into the shingle etc. I haven't found it a particular problem on heavy clay here, but I did go to some lengths to compact the clay as it went back in. An additional consideration is that clay shrinks/swells with moisture (but then so does the adjoining subsoil!). The whole process works best when the clay is damp and "squidgy" (technical term ).

 

[MARK.B.]

Yep Squidgy is it is then true there will nearly always be some settlement so compacting to remove the voids will help a lot and thats where the pea shingle comes into its own as it will give a little as you beat the clay into submission thus avoiding any potential damage to the pipe

 

[sammy.se]

I've dug trenches and a large hole before, It's hard work.
Now, whenever I see someone in a movie dig a hole (usually human sized!) in the space of a couple of hours, I always scream BULL-CARP at the TV

Well done on the hard work!

 

[Molynoox]

thanks for all the comments, mega helpful!

update is that I will be using 4 tons of pea shingle now which will be enough for 100mm below above and around
regarding the clay, I think I will keep it wet by spraying with hose pipe and maybe keep under a tarp too. Compacting it as I fill it is also excellent advice.

Sammy, yes its hard work digging by hand, it took me 2 days to dig the hole in the picture (7m of 750mm depth) and another day to do the final run (4 metres down to 600mm). I slept very well those nights :-D