Compaction over drains

[Oz07]

Got some drains to put in below a ground bearing slab. One long run ends up only 50mm ish below the top of the sub base (to top of pipe), other two should easily be 100mm minimum.

Owing to the depth and the fact that these drains will start out in the sub soil but rise up into the sub base whats the best method?

 

1. Install drains before spreading sub base. Keeping drains surrounded in pea gravel as per norm then compacting in one go (roller)
2. Get sub base spread and compacted with roller, then re dig out channels for drains.

 

Option 1 would be my preferred. I'd just be wary about loosing any of the fall with compaction. Is this a valid concern or will it only be the subbase compacting around the pipes? Would it help if I played planks over the pipe runs and missed these with the roller and went over with a smaller plate?

Option 2 would be a bit messier and kind of doing things twice, but i'd be 100% sure of the fall on my drains then.

[Carrerahill]

11 minutes ago, Oz07 said:

Got some drains to put in below a ground bearing slab. One long run ends up only 50mm ish below the top of the sub base (to top of pipe), other two should easily be 100mm minimum.

Owing to the depth and the fact that these drains will start out in the sub soil but rise up into the sub base whats the best method?

 

1. Install drains before spreading sub base. Keeping drains surrounded in pea gravel as per norm then compacting in one go (roller)
2. Get sub base spread and compacted with roller, then re dig out channels for drains.

 

Option 1 would be my preferred. I'd just be wary about loosing any of the fall with compaction. Is this a valid concern or will it only be the subbase compacting around the pipes? Would it help if I played planks over the pipe runs and missed these with the roller and went over with a smaller plate?

Option 2 would be a bit messier and kind of doing things twice, but i'd be 100% sure of the fall on my drains then.

I would lay the drains first - only logical way.

 

You can lay the drains and use some weak mortar mix to give support at various points if needed.

 

 

[MikeSharp01]

The drains should be laid in Pea single, We had exactly the same situation and I used the sub base to create channels along the pipe runs then filled these with pea shingle and once I had 50mm pea above the pipe I could cover again with type 1. So there were places across the slab where the pipe was buried below 100mm Type 1 and 50mm pea single and one the other side the pipe was above the sub base level with visible pea shingle and, in our passive slab case, I had to shape the EPS 50mm. The SE said the concrete of the slab and its reinforcement would bridge there bare pipe portion and type 1 actually compacts very little below 50mm down provided it is compacted in 50mm layers.

[Oz07]

Interesting thanks @MikeSharp01 I was going to go for full 150mm depth compaction with roller in one go. 

obviously was going to bed in 10mm and cover in 10mm too

[MikeSharp01]

No @Oz07you must build it up in thin layers unless you have industrial equipment the ride in compactors- the knobbly wheeled things you see building roads, and similar sized rollers. On a self build 50mm layers is about a you can be sure of with a self propelled vibrating roller. 

[Oz07]

Had access to twin drum but like you say probably sensible to spread it out and go with something smaller

[Carrerahill]

Always compact in layers and dampen the hardcore as you go, water is a fantastic compaction aid. 100mm is the sort of maximum you should do in one go, but for small projects like houses I would always say drop that and go 50-75mm in one go - the rest of your build sits on this so get it right and take your time now. It also helps with levelling as some areas will compact more than other depending on how the hardcore was spread about so you can end up with an undulating surface if you do too much too soon - do it in layers and you can get the first fill compacted and levelled out - top up bits with a barrow or dumper load here and there. Then you can spread more out and be fairly certain it will work out level. 

 

When we did our garage we needed about 38 tons of hardcore, the first 5-6 tons went in with cement dusted through the hardcore, dampened and compacted, it made a really really solid fill (basically a weak no-fines concrete of sorts) that was going no where and helped to keep things right until we could get the levels worked out. It also meant that the retaining wall for the back of the garage was under much less load as the hardcore it was retaining was self supporting and didn't want to go anywhere anyway.