Passive raft and a sloping site

[oldkettle]

Hi, 

 

Found a somewhat related topic (https://forum.buildhub.org.uk/topic/567-passive-slab-and-sloping-site/) but with a much smaller difference in levels than ours. 

As you can see from the attached pictures, we have quite a fall currently. Close to a meter back to front and >0.6m left to right. The right wall sits well above the ground with 14 courses of bricks below dpc only. So now that we are almost ready to accept the inevitable (demolish and rebuild) I want to understand whether there is a reasonable chance to build a raft here. One builder mentioned the existing walls can be used as retaining/supporting the perimeter, but he didn't see the pictures. The concern is obviously the cost: if we are going to have to start adding piles etc. this may become too expensive. Party Wall act may also come into play.

 

Any clues or do I need to go straight to Hilliard? 

Edited September 26, 2020 by oldkettle

[ProDave]

I like all the advantages of an insulated passive raft, but like you, we have a sloping site, and the only way I could see is to build up a lot of the site level with a LOT of infill, compacted in layers.  The snag being you also have to level an area extending beyond the actual house footprint.

 

So we opted instead for strip foundations and an insulated suspended timber ground floor.

[oldkettle]

Yes, I remember, Dave. In our case the ground has been there for 60+ years so nothing needs to be added... hopefully. 

Edited September 26, 2020 by oldkettle

[Russell griffiths]

I also wanted an insulated slab, but would need to build up over 600mm to get the height needed, this wasn’t really a problem but retaining that 600mm of Stone was. 

It will be the same in your case you would need a retaining wall at the highest point to retain all the stone needed to level the site. 

 

Alternatively you can have a step in the house half way through the floor area. 

 

Before you talk to anybody you would be wise to get a topo survey done so you all know the height difference you are talking about. 

[scottishjohn]

maybe you could use  foam glass to both level site up t and give you insulation and drainage

https://unitylime.co.uk/shop/insulation/foam-glass/geocell-foam-glass-1m3/

[Alexphd1]

You could build a 600mm thick eps raft "super insulated raft".......

 

Joking apart I have seen EPS used on commercial sites to build up the ground levels. 

[Bitpipe]

+1 on Topo, you'll need this for the house design process anyway, planners will want to see it etc.

 

Do you have the budget to consider building a passive basement below the house? Effectively the externally insulated walls become strip foundations to meet the structure above and the insulated raft becomes the basement floor.

 

You then have a suspended timber floor at ground level but don't need to insulate it. UFH can go in basement (but we didn't bother as the ambient temp is constant year round) instead we put it under the ground floor using all spreader plates.

 

Effectively it's a more extreme version of what @ProDaveDave is suggesting.

 

 

[oldkettle]

Thanks everybody, I've almost lost hope I'd get more replies. 

 

@Bitpipe Actually I've been going through your posts on the basement yesterday evening. It did cross my mind that this was an option, we are already 2.1m in the ground from the dpc level and I'd happily lift the FFL another 0.5m. What stopped me eventually was your note that an extra meter of width was required for the excavation. In our case that'd mean moving another 2.3 metres to the left which would certainly upset our neighbours on the left. My wife is also not keen: "why do you need an extra 100m2 in the basement on top of the 50+m2 outbuilding". I feel like we are back to square one which is extremely frustrating.

 

@Russell griffiths

I understand a retaining wall is needed, I am just trying to figure out whether the existing wall can serve this purpose to an extent : somehow it has not moved anywhere nor allowed the slab to move.

 

@Russell griffiths

@Alexphd1

I thought I didn't really need to build the ground level up as it is already there, just remove the slab and about 500mm below and start building back up from there. Am I wrong? 

 

Off to read about topo surveys. 

Edited September 28, 2020 by oldkettle

[oldkettle]

Does anybody know whether basement raft can have a step in it? May be cheaper to add plenty of steel into this than deal with Party Wall.