Sinking garden / laying down cement

the_galway_fry

Our garden is sinking/has sunk in parts, up to 8 or 9 inches in places, by the two boundary walls and in weird patches around the garden. The house is 20 years old and you'd think the garden would have settled by now.
I want to build an outdoor oven and am wondering how far I should dig down given the way the garden is for putting cement down
Thanks

Doop

Is there any chance of a drain/drainage pipe having collapsed underneath the affected areas?

BarryD2

More information needed as to site I'd think. I'd be wondering was the land previously used for some sort of landfill prior to building? And perhaps some materials that were formerly rigid have corroded & collapsed? Or is it built on flat land, formerly bog? Ask some of the older residents/ houses in the area. Hopefully any subsidence just affects how the gardens were filled.

Gumbo

Have you removed any heavy vegetation or trees recently?

You have to address the sinking before you do anything further in my opinion.

GreeBo

Gumbo wrote: »

Have you removed any heavy vegetation or trees recently?

You have to address the sinking before you do anything further in my opinion.

Hopefully its just this OP.

If not I would worry about what was happening under the house...I know of a whole row of houses that were built on the site of an old dump.

the_galway_fry

Thanks for all the replies. I don't think the site was built on a bog but it's in Knocknacarra /rahoon so can never be 100%

Dont think it would be a drsin either due to where it is but not an expert so...

I've definitely not removed anything really, removed a dozen large stones when I moved in about 10 years ago but 8 filled them bavk in with clay at the time

Not sure what other information I can give you on the garden,

GreeBo

If it was me I would grab a spade and start digging in one or more of the sunken areas until I figured out what was going on.
It may be that the previous owner removed some large trees and the root system is starting to break down, but that would give more defined sunken areas and would be more obvious.

onrail

If the houses are staying pretty sound, you shouldn’t be in too much bother ( unless the houses were piled!)

Other than the spade, you could get a length of rebar or bamboo and push down through the topsoil until you feel a reasonable resistance. The depth where you feel strong resistance is the depth you’d have to dig out and replace with a decent sub base.

If the rebar keeps falling, then you might have to think a bit harder about it!

the_galway_fry

I wouldn't have thought it would be the trees option as its not that big a garden and the sunken parts are all over though most noticeable by the edges as I'd plastered and painted it do the bits now exposed stand out


The rebar idea might work though I suspect all I will hit is rocks given what I pulled out previously and even this month I pulled a few decent Sized ones that somehow ( due to the heat maybe?) had started to come up out of the earth

Thanks again for the advice

TheBoyConor

Usually contractors building houses backfill and landscape the gardens in an very roughshod and imprecise manner, using the opportunity of burying general site rubbish and wastes and most certainly there was no proper compaction of the banckfill. The cheaper and faster the better. Bury it, cover it up, out of sight of of mind until we're well gone off site.

This is a very likely reason for the subsidence.

the_galway_fry

Yeah that certainly would be possible given who built these and how quick they were being thrown up.
Would it still be settling /moving 20 years on from being built?
Would the rockbreakers that were working on the dunnes extension have contributed to it or is that impossible

the_galway_fry

Did the rebar test as recommended
In the area I'm hoping to cement it never went further down than 14 inches, average was about 12
Also got the same result in other sunken patches around the garden.

One other thing I didn't mention which I would hope isn't an issue is the line running from the tank to the boiler is under where I'm planning on doing, will that be OK?

Thanks again

TheBoyConor

Depends on how deep it is.
Turn off the oil valve and be careful digging out the ground not to damage the pipe.
Chances are, it is going to be shallow - again, plumbing subcontractor would have taken the fastest option with the least amount of work.

If you expose the pipe, lay it down on the excavated ground and put a slieve of foam insulation around it so that it is protected against damage from any sharp stones or if the concrete settles a little over time.

The best option really though is to carefully remove the pipe intact, then lay a 50mm duct between the boiler and tank and place your concrete down as usual. Then pass the pipe through the duct.
That way, if the pipe ever did develop a leak in the future you could easily replace it in the duct without breaking up the concrete. It also gives the flexibility to place a cable in it if the need ever arises.

GreeBo

the_galway_fry wrote: »

Did the rebar test as recommended
In the area I'm hoping to cement it never went further down than 14 inches, average was about 12
Also got the same result in other sunken patches around the garden.

One other thing I didn't mention which I would hope isn't an issue is the line running from the tank to the boiler is under where I'm planning on doing, will that be OK?

Thanks again

Damaging your oil line is a significant problem that requires expert help to clean correctly, its a costly business.

Its only a foot, grab a spade and dig out one of the areas and see what you find.
Perhaps its just full of voids from builders crap that is now settling, but perhaps its not.

the_galway_fry

Thanks again for the replies and advice, I will do what you say and let you know how I get on