Forum: Site Works
Topic: Elevated patio rebuild
started by: JRoberts

Posted by JRoberts on 28 Dec. 2018,22:30
All,

Apologies  for the long post.I am after some some suggestions on the best way to proceed with sorting out our patio which is rapidly falling apart. Our house is slightly unusual in that it is surrounded by an elevated patio. Unfortunately when it was built (About 12 years ago), they didn't do a great job and the retaining wall has moved in multiple places, almost all the tiles are loose and the patio has sunk in a few locations. Anyhow the time has come to sort it out properly. It is about 150 square meters of tiles and the retaining wall is about 70m long so it isn't going to be a small job. To complicate matters I am out in the fens so ground conditions are very poor.

Firstly a few general photos to give some idea of what needs sorting out:





Some pictures showing some of the movement:




The patio itself appears to be sitting on a mix of hardcore and clay with the slabs sitting on a few blobs of cement around the edge:



The concrete foundations are ~30cm wide and where the wall has moved the cracks go right through the foundations. As far as I can tell there is no provision for drainage through the wall. To give some idea of ground conditions here are two borehole scans. The first is <100m from the house, the second is in the same fen but about 1 mile away:

 

Somewhere I have the paperwork for the piles the house sits on which I think are ~7m deep. This tallys with other local borehole scans which list bedrock (clay) at ~6m. For example:




Now to get to my questions :).
# Any thoughts on the reasons for it falling apart? I wondered if the grout failing and letting water get where it shouldn't started everything failing.
# What would be the correct way to engineer the base for the patio to prevent/minimise further movement. I was wondering if geogrid or geotextiles could help?
# Any thoughts on the correct way to engineer the retaining wall to ensure it doesn't move again? This time it needs to be done properly. Given ground movement seems likely would rebar in the footings help?
# Finally if anybody has any bright ideas of how to renovate this space I am all ears :).

Many thanks for any input.

Jonathan

Posted by Tony McC on 29 Dec. 2018,12:17
1 - The peat is the most likely cause of the ground movement, but poor backfilling beneath the patio is also likely to be implicated

2 - A floating slab, dowel-tied to the house foundation, with appropriately positioned movement joints

3 - I'd be sorely tempted to eliminate the retaining wall by lowering the level of the patio. Rely on steps/terraces/ramps to transition from house to patio, but lose all that unnecessary height.

Posted by seanandruby on 29 Dec. 2018,18:44
I think you could at least half the height and extend outwards to use up the hardcore. Maybe come around in sections using geo-grid, hardcore and as tony suggests tie a concrete slab into the wall, floating a slab to spread the load. I think it would be easier to sell topsoil rather than hardcore. Interesting project to get your teeth into.
Posted by lutonlagerlout on 31 Dec. 2018,03:04
As soon as I saw the pictures I thought like the gaffer Reduce the Height
have some expansive steps
your current steps look mean and small
that elevated path around the house all looks a bit pointless?

maybe even consider having a new patio on a floating slab a few metres away and some bigger grander steps and some paths at ground level?
also maybe a faux storm porch over the front door and some railings  :)

as said you may be able to recycle the hardcore if crushed

cheers LLL

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