Basement Backfilling - stone, gravel, soil?!

[Conor]

How have you all backfilled your basements? 

 

I'm seeing various methods - from the American style of a perimeter drain and the rest earth, to various European standards of stone combinations.

 

I'm installing the 100mm French drain with clean stone along the slab esge. The architect has suggested filling the entire excavation with 20-40mm dia clean stone from the drain to ground level. Issue with this is it will cost several thousand pounds.

 

An alternative I've seen is after the drain is installed, stone is placed in layers against the face to a thickness of about 300mm, the rest of the trench can then be local soil.

 

Some details show that you need a geo membrane between the clean stone layer and the soil.

 

Any suggestions? I'm keen to sue up some of the massive mounds of risings on site...

 

https://www.sciencedirect.com/science/article/pii/S036013231930784X

 

 

 

[Bitpipe]

We used clean stone to backfill - fist sized lumps of various recycled stone (brick etc) but no fines. Acts as a giant French drain (perforated land drain under that which you don't want to get clogged). On one very wide section we had a 1m width of stone and some spoil vertically separated by geotextile.

 

The most important thing about stone is it is free draining and does not compact, which is very important if you're filling a 3.5m deep x 1m wide trench around your whole house. Top layer can be spoil as you may be laying rainwater pipes etc in this but it must be well compacted.

 

Our contractors filled in most of the basement excavation area to spec but while I was away they obviously cut a few corners at the end and used site spoil to the rear. While it all looked flat and lovely when complete, a few months later, when the scaff was up, it started to settle and dropped almost a foot. It continued to settle for the next year and then the landscaper put a lot of compacted crush on top to make up the levels and act as a solid base for the patio.

 

Unfortunately there has been a bit more settlement and quite a few of the flags are sagging - it had a lovely even fall when laid but now getting puddling when it rains. I've lifted and relaid a few, using CT1 to bond them back down but if it gets worse I'll need to lift the whole lot and re-lay on a clean bed. Where it was backfilled to spec it is rock solid.

 

The SE did suggest that site spoil could be used but it had to be mechanically compacted in 150mm layers - this is very time consuming but necessary otherwise it will eventually settle.

 

So either go for stone or pay for the extra labour to compact well in 150mm layers.

 

[Conor]

Thanks @Bitpipe.

 

How did you lay the geomembrane between the stone and spoil? I can see it being rally akward. This is what I'm thinking for the gable end as it'll one ever be garden and quite a wide excavation due to soft ground.

[tonyshouse]

+1  on settlement, mine took 7 years to stop I think

 

I have a French drain at bottom edge of slab with 250mm peashingle surround to a borehole soakaway, 

 

I put EPS against the walls, thought about drainage layer but didn’t do it, backfilled with natural sun soils compacted in layers. 

[tonyshouse]

Sorry, it should of course say “sub solis”

[Thorfun]

our SE plans show stone backfill...

 

 

looks expensive but we really don't want it to settle so will bite the bullet and pay what it takes.

[Bitpipe]

5 hours ago, Conor said:

Thanks @Bitpipe.

 

How did you lay the geomembrane between the stone and spoil? I can see it being rally akward. This is what I'm thinking for the gable end as it'll one ever be garden and quite a wide excavation due to soft ground.

 

The contractor did it - basically they unrolled about 15m of it (it's 4m wide) and weighed it down with a few rocks and flapped it away from the rock infill side. They put a layer of rock down and then flapped it the other way and put a layer of subsoil, repeat until you reach the top.

[Bitpipe]

8 minutes ago, Thorfun said:

our SE plans show stone backfill...

 

 

looks expensive but we really don't want it to settle so will bite the bullet and pay what it takes.

 

Pretty identical to ours. Few notes...

 

I'd see if you can get away with EPS200 or 250 under the slab, just see what the loading is from the SE and then get them to check this table https://www.specifiedby.com/styrene-packaging-insulation-ltd/stylite-eps-geofill-void-formers/geofill-compressive-strength-factsheet_4eeaa0ea.pdf

 

We were able to get away with EPS200 which saved more than a few quid and were easier to move around - I used 2400 x 1200 x 300mm blocks which were about 30kg each. Didn't attempt to cut them but just laid them flat on the blinding. As we were using traditional shuttering, I didn't need to use the EPS to do that and I let the EPS extend the slab by about half a metre all round. They did not need securing, once flat and butted up to each other they were going nowhere.

 

I laid the radon membrane over the slab EPS and the crew just built up the steel frame and shuttering on that, had a 150mm kicker to start the walls.

 

We used GRP light wells (MEA Bausysteme) which come with a 200mm XPS insulated collar which tied in nicely with the EPS - I put those on first. The crew had formed rectangular window holes to the dimension of the light well so the XPS went neatly round these. Was fun keeping it in place until the special adhesive went off - lots of horizontal and vertical props.

 

I glued on the vertical 200mm sheets of EPS70 with LE foam (must have used about ten cans and a gun) - spray the concrete with water first to get a good adherence. I then covered the EPS with 2mm core to give it a bit of protection during backfill. Just used spare nails on site (there were 100s) to pin it in place. 200mm EPS was cut with handsaws and an electric chainsaw - makes a lot of mess so do it inside the basement box and create your own giant snow globe

 

Now our land drain was at the base of the slab to a soakaway - is your groundwater level too high to do that? Meant that and percolated rain water had a route away from the slab base. I'm not sure why you'd have one so high up tbh as the water will run past it and down through the stone.

 

Backfill was pea shingle to landdrain then stone. When we got to light well level, we laid a bed of pea shingle and secured the lightwells to their collar (bolts and supplied sealant) and filled around them. 

 

 

[Thorfun]

52 minutes ago, Bitpipe said:

 

Pretty identical to ours. Few notes...

 

I'd see if you can get away with EPS200 or 250 under the slab, just see what the loading is from the SE and then get them to check this table https://www.specifiedby.com/styrene-packaging-insulation-ltd/stylite-eps-geofill-void-formers/geofill-compressive-strength-factsheet_4eeaa0ea.pdf

 

We were able to get away with EPS200 which saved more than a few quid and were easier to move around - I used 2400 x 1200 x 300mm blocks which were about 30kg each. Didn't attempt to cut them but just laid them flat on the blinding. As we were using traditional shuttering, I didn't need to use the EPS to do that and I let the EPS extend the slab by about half a metre all round. They did not need securing, once flat and butted up to each other they were going nowhere.

 

I laid the radon membrane over the slab EPS and the crew just built up the steel frame and shuttering on that, had a 150mm kicker to start the walls.

 

We used GRP light wells (MEA Bausysteme) which come with a 200mm XPS insulated collar which tied in nicely with the EPS - I put those on first. The crew had formed rectangular window holes to the dimension of the light well so the XPS went neatly round these. Was fun keeping it in place until the special adhesive went off - lots of horizontal and vertical props.

 

I glued on the vertical 200mm sheets of EPS70 with LE foam (must have used about ten cans and a gun) - spray the concrete with water first to get a good adherence. I then covered the EPS with 2mm core to give it a bit of protection during backfill. Just used spare nails on site (there were 100s) to pin it in place. 200mm EPS was cut with handsaws and an electric chainsaw - makes a lot of mess so do it inside the basement box and create your own giant snow globe

 

Now our land drain was at the base of the slab to a soakaway - is your groundwater level too high to do that? Meant that and percolated rain water had a route away from the slab base. I'm not sure why you'd have one so high up tbh as the water will run past it and down through the stone.

 

Backfill was pea shingle to landdrain then stone. When we got to light well level, we laid a bed of pea shingle and secured the lightwells to their collar (bolts and supplied sealant) and filled around them. 

 

 

thanks for the great advice as usual @Bitpipe. our SE is TSD and they've designed the insulated slab using the Kore system. I'm not sure what the loads are but at some places they're so high that we've got to add Foamglas instead of the EPS to deal with those loads. As such I trust that they've done their calculations correct when specifying the EPS300. 

 

I'm not sure why the land drain is specified so high I just assumed it should be but you make a valid case and I will question this one and ask why it's not at the base of the slab level. I wonder if it's due to the basement having to tie in with the 'arms' of our building and so the land drain is dealing with the water from those insulated slabs as well? I will ask the question though thanks.

[Bitpipe]

3 hours ago, Thorfun said:

thanks for the great advice as usual @Bitpipe. our SE is TSD and they've designed the insulated slab using the Kore system. I'm not sure what the loads are but at some places they're so high that we've got to add Foamglas instead of the EPS to deal with those loads. As such I trust that they've done their calculations correct when specifying the EPS300. 

 

I'm not sure why the land drain is specified so high I just assumed it should be but you make a valid case and I will question this one and ask why it's not at the base of the slab level. I wonder if it's due to the basement having to tie in with the 'arms' of our building and so the land drain is dealing with the water from those insulated slabs as well? I will ask the question though thanks.

 

I did wonder if it was Kore given the detailing. Are you putting any UFH in the basement slab? We decided against and haven't regretted it as that part of the house is always at a comfortable 20^o year round.

[Thorfun]

29 minutes ago, Bitpipe said:

 

I did wonder if it was Kore given the detailing. Are you putting any UFH in the basement slab? We decided against and haven't regretted it as that part of the house is always at a comfortable 20^o year round.

as yet undecided. thinking about just running some pipes just in case as the cost should be minimal if I can do it myself. then if we find we need heating it can be plumbed in. the same with the first floor. I'd rather lay it and not need it than not lay it and need it!

 

but the basement will be the last thing we fit out and that may not occur for a few years until we can get the funds to do so. during that time we can see how we get on with the temperature down there. our plans are very fluid at the moment!

[Bitpipe]

23 minutes ago, Thorfun said:

as yet undecided. thinking about just running some pipes just in case as the cost should be minimal if I can do it myself. then if we find we need heating it can be plumbed in. the same with the first floor. I'd rather lay it and not need it than not lay it and need it!

 

but the basement will be the last thing we fit out and that may not occur for a few years until we can get the funds to do so. during that time we can see how we get on with the temperature down there. our plans are very fluid at the moment!

 

I was of the same mind but when you have trades on site, first fix, plastering and second fix plus some flooring etc in that space were not massive additions to the budget.

 

Always worth getting the quotes with and without basement for each stage and see where you get.

 

Given you have insulation on the outside wall, you can always do the industrial look and get surface mount electrics on bare concrete walls  

Edited November 8, 2020 by Bitpipe

[Thorfun]

1 minute ago, Bitpipe said:

 

I was of the same mind but when you have trades on site, first fix, plastering and second fix plus some flooring etc in that space were not massive additions to the budget.

 

Always worth getting the quotes with and without basement for each stage and see where you get.

 

Given you have insulation on the outside wall, you can always do the industrial look and get surface mount electrics on bare concrete walls  

 

exactly, it keeps things flexible if we have those quotes upfront and we can decide how far to go with it. tempted to leave it at first fix stage though but we will see. maybe I'll win the lottery.

[Adam2]

On 07/11/2020 at 08:18, Conor said:

How have you all backfilled your basements?

 

Slightly different to the above:

 

100mm perforated drain around perimeter of slab toe sitting in pea shingle about 100mm below and 100-200 above. Membrane placed around the shingle. The drain then flows off into our soakaway. Above this we have 2 types of back-fill - where the back-fill will be built on we can't use site material as the SE says is not granular enough (mostly sand/sandstone) so have to use 6F5 compacted in 150mm levels to a certain number of passes depending on type/size of compactor. Where we're not building  but will landscape, we'll mix in some 6F5 with the sand so we buy less 6F5 and pay less to remove our lovely sand!

[BotusBuild]

Really interested to see no mention of waterproofing on the outside of the EPS wall. Are people not doing this? Is this not being specified by your architects or required by building control?

FYI - my building regs drawings (just approved) show backfill over RIW protective membrane over RIW double drain (or similar) over self adhesive waterproof membrane, with a land drain below the concrete slab

[Adam2]

I figured this was about reusing onsite materials for backfill so didn't mention that.

 

Me - BASF membrane, double drain, EPS sheet 25mm then backfill. Also under slab membrane and water bar at junctions of pours (ICF on concrete raft).

[Conor]

So we've come with 14mm chippings for the base and around the drain. Keeping at least 300mm of that against the face and we fill up. 

 

Ordered 100mm clean stone for the rest... Fat from clean but we're on a tight schedule and have to make do! 

 

Going to lap the geomembrane over the top of the clean stone to the face once we are about 300mm from FGL and compact come type 3 to make the surface.