Unstable ground - Can i build on soft-ish ground?

[Bodger]

I recently removed a tree from the corner of my garden leaving a ~4' diameter hole, ~2' deep in the middle. I hastily filled it in with soil and now it's a bit soft. I've used a pummel to compact it, but it's certain to sink somewhat over time. I have build a quadrant shape arbour in the same corner, 6' radius and now wish to cover the ground underneath with something to put a small table & chairs on.

To me, my choices appear to be:
1) put down a weed fabric and cover with pebbles or similar
- pro's: quick & easy
can top-up the pebbles as the ground sinks
- con's: I don't like it (chairs sink & pebbles / chippings walk)

2) compact the best I can,
lay a concrete slab with a brush finish
- pro's: fairly cheap & easy
- con's: harder work

3) dig out all the soft stuff,
fill the hole with scalpings,
lay a concrete slab with a brush finish
- pro's: ground is stable
- con's: even more hard graft...
need to find a home for the soft stuff

At the moment I'm tending towards (2) as I think this is a good trade-off between cost & effort / aesthetics. So,
a) will a concrete slab be strong enough to remain flat (i.e. in one piece)? and
b) if so, how thick, & will reinforcing be required / desired?

NB: Some of my friends are heavy.

[lutonlagerlout]

if you make a 150mm thick concrete slab with re bar in it and c35 concrete it wont crack,it may sink or tip but it wont crack
its absolutely fatal filling holes in with mud/soil if you intend building anything there
cheers LLL

[Bodger]

Thanks for your reply lutonlagerlout.
6" thick seems like a lot of concrete.
1) What ballast:cement ratio is C35? (is that about 4:1? I don't really understand the "mixing concrete" page...)
2) Would it be possible to use less concrete (say 4") of a stronger mix?
3) What if I go with option 1 for a year or so, would it then have settled sufficiently to use less concrete?

[Tony McC]

Whatever surface you go for, the prep needs to be right, and the only way to get the ground to a fit state is to excavate all the backfill you piled in, and then re-do it all by placing layers of crushed aggregate (50mm crusher run/Type2/804/DTp1, etc) not more than 150mm deep and thoroughly compacting each layer before placing the next, gradually building up the level as you proceed.

Concrete slab: as long as there's no vehicular traffic, you could get away with 100mm of C30-C35 streangth concrete, laid as shown on the main website.

To make sense of the Prescribed Mixes table, you first identify the required strength on the left hand column. We'll go with 30 newtons (C30).

Next, select the size of coarse aggregate you'll be using, which is most likely to be 20mm for a DIY mix. Look across the top row (in red) and you'll find the 20mm column is split into two sub-columns for "workability", which is a way of saying how wet the mix will be. To keep things simple, use the Medium workability which has a slump of 25-75mm (stiffish to porridge-like, but not overly sloppy)

Follow this column down to the C30 rows (purple) and, reading off one at a time, you can see that for each cubic metre of concrete you will need....

400kg cement (16 bags)
1700kg aggregate

...and the aggregate need to contain 35% of zone 2 fine aggregate (concreting sand).

35% of 1700kg = 600(ish) kg

...so that's 600kg of concreting sand and (1700-600=) 1100kg of 20mm aggregate.


It may look complicated but it's actually quite simple once you get used to it. Sdaly, much of it has been superceded by the new concrete standards which are incredibly baffling and, at the end of the day, the concrete doesn't know the standards have been Brussels-ified, so if you dont tell it, neither will I!

[Bodger]

Thanks for that Tony. I think maybe I was in denial. The table makes sense to me now also.
Due to time constraints I will probably have to go for option (1), for now at least. I'd like to know though,
1) Is the ground ever likely to settle long term? (Tree)
2) Rather than go for 100mm of C30-C35, could I use say 75mm and use reinforcing mesh (so I can mix by hand)?
3) would there be any benefit in mixing any stronger?

Good web site by the way. I wish I had found it before laying my patio. Could have done a much better job.
Patio 1 Patio 2 Patio 3 Oh well...

[Tony McC]

1 - Yes: all ground settles over time, so all we can do is try to minimise the inevitable.

2 - No: 75mm will not give the requisite minimum cover of 50mm top and bottom for the mesh. You could use fibres, but you would then need a damn good mixer to ensure the fibres are evenly distributed. 100mm is a better option.

3 - not really. C30 is plenty. We would mnormally only go above that for heavy structural work. Some of the hardstandings that we constructed in freight yards had to be capable of taking a 20T container carried by a 42T FLT, and for those we used (IIRC) C37.5 albeit 200mm thick with B-series mesh top and bottom.

[Bodger]

I chose option 1 and it's been like that since (about 2 1/3 years). It's now a pebble / leaf mix and I still don't like it. Do you think enough time has passed to go with option 2 or shall I leave it another year or two?

[DNgroundworks]

Call me stupid if you like, but why do you need different types/grades of crushed aggregate in layers quote - 50mm crusher run/Type2/804/DTp1 Just for for some garden chairs?

And just out of interest what is the difference between these different aggregates ive only ever used type1?

[Tony McC]

I didn't mean layers of each of those aggs, but layers built up using one or other of them.

All build-up has to be done in layers when it exceeds 200-225mm in depth, as compaction kit can't effectively compact much more than that.

50mm crusher run is a coarser r=grade with no precise grading envelope, whereas Type 1 and Type 2 are quite strictly defined in terms of particle sizes and ratios. 804 (and 803) are the neames used for Type 2 and Type 1, respectively, in Ireland. DTp1 is shorthand for Type 1

[GB_Groundworks]

just as a side to this i just tried something similar capping a big hole we'd dug with a serious lump of concrete, 400mm thick, 3000x3000 c40 ultra reinforced slab used to be a bridge support in a river, i tried to use it to cap off a wet hole and use as man hole base at the same time. even with that massive concrete raft probably 3 tons in all it still sank in one corner. had to struggle to pull the thing out and dig down to good ground come up from there.