Outbuilding Foundations - raft?

[texutree]

I am looking to build a blockwork outbuilding/workshop (~6.5x4m, 2.5m total height) down at the bottom of my garden, where there are currently several leylandii trees. My intention is to remove these trees and as much of their roots as possible using a stump grinder. 

 

I am concerned that after removing the roots, the ground may not be suitable for a footing of normal depth to support the structure (soil is clay) so I was looking for feedback on the possibility of using some variant of raft foundation to spread the load over a wider area (entire building footprint). Is this feasible? My thoughts were something along the lines of 6"deep concrete slab reinforced with steel mesh... (sitting on 4" of sub-base).

 

Any thoughts or feedback would be much appreciated.

Edited January 5, 2020 by texutree
more accurate

[PeterW]

Welcome

 

I would go for a slightly wider raft and thicken the edges to 300mm x 450mm wide and tie this to a 150mm overall slab  reinforced with A142 mesh - the quickest way to do this is just clear the whole area to 400mm, lay a full 100mm of MOT1 and compact well and then use 100mm EPS to create the thinner section in the centre. It will also keep your slab slightly warmer... 

[Russell griffiths]

I would excavate all the top soil down to a solid base then dig a shallow trench around the perimeter then add 150mm of type 1 on the flat bit,whacker plate solid. 

This way when you pour your slab you will have a thicker section around the outside. 

Use a heavy grade mesh not the thin stuff

150mm concrete through the middle 300 around the outside. 

 

Peter beat me to it. 

 

Edited January 5, 2020 by Russell griffiths

[texutree]

Many thanks indeed for the replies and advice!

[MortarThePoint]

You have a double whammy as the geology is likely high volume change potential (VCP) and the trees removed are high water demand so that will maximise any resulting heave. How high were the Leylandi? If over 9m or 10m (I think) then they would be classed as mature and their full height (>=18m) would be used for any foundation calculations. What are the different strata? Don't forget to consider other nearby trees. NHBC have a foundations mobile app but I don't know if it covers raft. Chapter 4.2 of their guidelines has useful information (NHBC 2020 Chapter 4.2).

 

I have some experience in this area as we are to build a large(ish) garage with high VCP geology (alluvium over gault clay). We can't bear on the alluvium so need to do bulk soil replacement down to 1.25m with MOT Type 1. Thankfully we should be yielding some concrete from demolition. There are some sizable trees and ex-trees near by so a raft was the best bet. Thankfully this isn't the case for the house, but the trench foundations need to be pretty deep there.

 

I'd expect the dimensions of your workshop are consistent with a less fussy approach, but you will certainly need reinforcement and +1 for the thickening round the edges. You could (some might say should) spend a lot on geotec and structural engineering, but that's your call and I don't think I would for the likely cost of what you are proposing. Be careful of building regulations (Outbuildings vs Building Regs) but the size you are proposing (26m2) will be OK if you are >1m from the boundary or substantially non-combustible (i.e. no timber cladding).

 

I'm no expert on any of these areas so don't take the above as gospel.

Edited January 5, 2020 by MortarThePoint
links

[Mr Punter]

I would get the merchant size (3.6 x 2.0) heavier duty A252 mesh as it will fit your slab dimensions and is easier to handle than the full sheets.  You could also cut a sheet up to have a second layer near the bottom around the outside, but probably overkill.

[texutree]

Many thanks all. The trees are approx. 3.5m tall and I am unsure as to the different strata underlying the clay. As it is just an outbuilding, I don't think I will go down the route of paying lots for geotech - in terms of building regs, it is closer than 1m to boundary but I will ensure it is built from substantially non-combustible materials.

 

 

[MortarThePoint]

2 hours ago, texutree said:

The trees are approx. 3.5m tall and I am unsure as to the different strata underlying the clay.

Pretty small trees. NHBC considers the zone of influence for high water demand trees to be 1.25 times mature height so you can be surprised how far away tress can have an affect.

 

 

Quote

I don't think I will go down the route of paying lots for geotech

Makes sense, I wouldn't either. The NHBC guidelines will be cautious and you're not building multiple storey habitable space.

[Temp]

On my outbuilding (timber frame, tiled roof, oak clad) on clay I...

 

Removed about 12" of soil and compacted the ground with a wacker plate.

Laid 6" of hardcore (compacted in two layers) then a thin layer of sand (compacted) about 1ft bigger than finished building all round.

Staked 6" shuttering using rebar stakes (dimensions of slab slightly smaller than finished building so water dripping off cladding would fall outside the slab not on the top of it).

Just about remembered to add a length of mdpe pipe for electrics to come up through slab later. 

Laid membrane into shuttering (prevents water draining out of the concrete).

Filled with 6" readymix concrete and levelled to top of shuttering using a long length of 4*2 (the levelling part was physically hard work).

 

I didn't have trees to worry about but if I had then would have added mesh raised up 3" before the concrete pour. 

 

It would have been easy to add a trench around the edge to make it deeper for block walls. Perhaps make trench wide enough to get the Wacker plate into it?

Edited January 5, 2020 by Temp

[texutree]

1 hour ago, Temp said:

Just about remembered to add a length of mdpe pipe for electrics to come up through slab later. 

This is something I will have to make a mental note of beforehand!