Foundations... on Clay!

[rfdesigner]

Hi

I've got pretty much everything else planned for the obsy.. but the foundations are giving me hell.

I'm on clay, and there were trees until 9 months ago. I've found data on heave following removal of trees and it looks like the top meter will heave substantially (well over 2 inches, maybe 4) over the 8 years... also I want my obsy to stay standing for a good long time.

So.. I'm going to have to dig down, at least a meter, I've accepted the need for a mini-digger (£200 for a week.. no probs) and the garden is big enough to soak up the spoil (1/2 acre)

The problem I'm having is what to use as a foundation. Ideally I'd like a concrete poured trench, as that will stabilise everything nicely. However I can't manhandle the quantity of concrete required (~3cu mtrs.) and I got a quote for doing the job but it was really too small a job so got a silly number (£4k.. errm, no thanks). I can't get a concrete lorry anywhere near the site, so it would all have to be mixed on site.

I could just throw rubble in the bottom and use blocks laid on top, but that will mean any differential in the heave will start to destabilise the whole thing. If it were just a shed I really wouldn't worry, but I dont want to find in 8 years that my roll off roof is on an incline, if it is there's a strong chance it won't shift.

Anyone else been here?

Derek

[nephilim]

Couldnt u hire a dumper truck for the day & take the concrete straight off the wagon into that? I did 4 metres by hand (wheelbarrow) off a truck a while back which wasnt pleasant & by the time we got to the last metre it was starting to go off which meant remixing that was also nice.

Steve

[rfdesigner]

Thanks.. and good to know 4 cu meters is just about doable.. ps how big are you?.. I'm only 10 stone.. relatively strong for my weight, but still there are jobs I just can't do because I'm just too light.

The dumper would need to get through a ~3 foot gap... we can only just get a micro-digger on site.

Derek

[nephilim]

Ah, shame about the gap, i'm also 10st & 5' 8", I was 37yr old at the time so not really suited to hard labour, I did it at the height of summer which was why we (there were 2 of us) were having to remix it at the end, it took us about 4hrs. Depending on how far the road is from your garden you can also get it piped from the wagon to site although I cant remember how far this will go (they have extension pipes available) i'd ring around & see what your options are.

Steve

[rfdesigner]

Thanks

I'm 40, and not adverse to 'hard work' but not keen on 'hard labour' if you catch my drift, at least I know what I'm in for.

it's a fair distance from the nearest access maybe 100 feet. I'll ring around. thanks.

Derek

[nameunknown]

You should be able to get it piped or barrowed - plenty of firms do that. Consider slumping it down with one of those petrol powered vibe things to knock the air out.

After the trenches are done, consider building the dwarf walls, lining with Visqueen, filling with crush-and-run, sand-blinding and putting the floor slab in with steel mesh (you won't need much concrete for the slab, so just hire a portable mixer) - that way you will have a damp proof floor in case you sell the house and the new occupant wants a summerhouse instead of an obs.

If you do go for rubble under concrete then put something in to stop the water draining out of the concrete before it sets.

P

[George]

Boulder clay is quite stable.....at least here

I went 2 foot down for my pier footings in 2007 and the pier has not budged a mm.

[rfdesigner]

All sounding better.

I've now got a quote for the concrete and a pump.

£450~550 for 3~6cu meters of concrete, delivered, inc VAT, I'm inclined to take the 6cu-meters and fill more or less to the surface.

£336 for a pump for 24 hours hire.

Those were both the first quotes I got, and will give me super-stable foundations, so I'm happy... clearly I'd prefer a price of £0, but this is managable.

Derek

[nephilim]

Excellent, much better than the 4k quote . Good luck with the build.

Steve

[Gina]

Don't forget to separate your pier and foundation from the main floor to avoid possible transfering vibration to your pier.

[Tinker1947]

I was 50'ish when i dug the footing for my workshop 25' by 16' , the premix i collected in a trailer about 1 tonne a trip 9 trips, the cement i collected in my van, hired a 2½ barrow mixer, placed the mixer over the footing on scaffold boards then mixed the cement, straight from the pile of premix, shoveled into the mixer then tripped into the hole, pushed round either way untill at the pre-mix runout, took about 9 hours, a obby won't be so big, if you can get all the cement ect to the site next to the hole its saves hours of labour....

[rfdesigner]

Don't forget to separate your pier and foundation from the main floor to avoid possible transfering vibration to your pier.

That's in the plan.. partly why I'm going for trench foundations as opposed to a raft foundation.. always worthing bringing that detail up though, it's a problem you don't want to find out about after pouring.

I was 50'ish when i dug the footing for my workshop 25' by 16' , the premix i collected in a trailer about 1 tonne a trip 9 trips, the cement i collected in my van, hired a 2½ barrow mixer, placed the mixer over the footing on scaffold boards then mixed the cement, straight from the pile of premix, shoveled into the mixer then tripped into the hole, pushed round either way untill at the pre-mix runout, took about 9 hours, a obby won't be so big, if you can get all the cement ect to the site next to the hole its saves hours of labour....

That's precisely the problem.. I can't

I've got 100 feet.. uphill, with a ~3foot pinch point to overcome. I'd love to get a trailer next to the site.

The problem isn't so much the effort, it's the timescale. I need to make sure all the concrete's wet when the last load goes in. Thinking this through.... I could shift all the materials on one day, then mix and fill on the next.. which brings me to the next question... can I get a mixer through the gap?... quick check and.. ohh.. yes.. cost... £50 a week, so.. just need a new wheel barrow!

My local builders merchants do free delivery for me (quite close).... so... maybe I can do it for

£200: digger

£50: mixer

£40: tamper

£300~600ish: concrete (3 to 6 cu meters)

£150ish: rebar

£10: tea.. going to need a lot!

£750 ~ £1050 all in..

truck:

£200 digger

£450~£550 concrete lorry (3 ~ 6 cu meters)

£330 concrete pump

£40: tamper

£150ish: rebar

£1270 ~ £1370 all in.

neither is free, but £500 for a couple of days work (3cu m).. effectively post tax.. and I'm up for it, it only tips over into not making sense to me if it goes beyond 4 days just for the concreteing

for £300 (6cu m) for what looks nearer 4 days work.. a concrete lorry looks preferable.

Derek

[pete_l]

The problem isn't so much the effort, it's the timescale. I need to make sure all the concrete's wet when the last load goes in.

Could you not use a "divide and conquer" approach?

Instead of digging a single trench and then having to fill it all before the goop goes off, could you not hire a motorised auger of (say) 12 inch diameter? That way you could excavate a nice deep footing that would require less concrete and could be filled in a day. Then the next day dig another footing nearby and fill that one, quickly. Repeat until you have poured enough "piles" and then put a concrete pad over the footings. You'd probably need to work out where to put the rebar, to hold it all together. But at least you wouldn't have to pour the whole thing in 1 day.

[Tinker1947]

Just make it a bit wetter than normal, i done mine around July time, and could still tamp it to get it level after the first mix 9 hours earlier, have a look at additives, there's all sorts you can bung in........

[Gina]

I used 6 square concrete pads for mine for the shed itself. Each 18" square and 15" deep which reached the dense clay "bedrock". Then I used 3 8"x3" second hand barn roof beams with joists and floorboards on top. Railway sleepers are available still which though wood are virtually indestructible and would provide a very solid alternative.

[tony]

Only a tenner for tea?

[malc-c]

Hiring a mini digger is essential IMO if you have heavy ground. I was fortunate in that the chap next door uses these most days in his line of work, and shared the cost of a weekends hire between a couple of neighbors

Not sure of the size of your planned build, but mine is 4.8m x 2.2m. The footings were hand mixed over an 8 hour day using an electric mixer hired for the weekend. I used two tonne of ballast and 14 bags of cement, all mixed and then poured either directly from the mixer or into a barrow and then into the footings which were between 500mm and 750mm deep as the ground slopes.

I left the footings to cure for a week and then commenced building the dwarf wall

The resulting base ready for preservative and covering in 18mm ply.

If access to your site is restrictive, and you have no other means than pumping readymix then that may be the best option. But mixing and laying over 2 tonne of concrete in one day in one go is doable.

[rfdesigner]

thanks for everyones help.

After 10 straight days of hard work, the foundations are now in, as is 20 meters of new drain.. so please be my guest and enjoy the wonderful weather (new drain = dry weather & new concrete = hard frosts so clear nights)

I can't quite believe the weather I did get.. one hour of rain on day 1 and that was it. The cold was dealable with good clothing, someone was looking after me!

I used a mini-digger to dig the foundations.. and get a MASSIVE tree stump out (1.5 days hard digging)

The concrete took 2 days solid mixing and barrowing but saved £300 compared to readymix + pump, I know concrete should really all go in in one go, but short of digging all through the night it wasn't going to happen, I do have 4 lengths of 12mm rebar forming a box reinforcing each section all bent and wired together to keep everything tight and the main 8m lengths are all poured in one piece. I also included 8 steel straps to hold the wooden posts down onto the bricks when I get that far, if it tries to fly away it's going to have to take the foundations with it. The trenches went down to really hard clay where the digger simply refused to go any deeper, which means it should be reasonably stable. The semi-perforated drain protects the high side of the foundations from excessive water and will take the rainwater from the roof away to a ditch on the other side of the garden.

Total cost for 20m drain + 8 x 2.7m rebar-ed concrete trench foundations came to around £1000 all in.. I'll need to get my calculator out for a more precise figure.

At some point I will get the pictures up, probably in a new obsy build thread.

Derek