Retaining wall foundation and patio drainage

[onionsa]

I’ve dug out roughly the area for our new patio and want to get started on the retaining wall. The wall will only be about 2’ high - so what sort of foundations would I be looking at for that? I understand that I would need to dig or about 1’ further than the wall to allow for backfilling?

I also question about the patio drainage, as this would normally fall away from the house, if there is a wall there surely the water will just gather? I’ve had someone mention putting a channel type drain against the house and plumb this in to the existing drain that is in one corner of the existing area - does this sound like the best option?

Thanks

 

[Halitosis]

Looks like it wont be holding back too much of a load, so a minimal foundation of maybe 5 inches should be ample (though it should extend front and back beyond the thickness of the wall). How's drainage in your garden (sandy or clay soil and does it easily become waterlogged)? Also what sort of patio will you have (sand jointed can drain down through)? I built a higher retaining wall in my garden and this pavingexpert website was a brilliant source of information - see 3/4 down the page.
I learned that drainage is key, as water retention is the primary cause of retaining wall failures as pressure builds unless there is an outlet. So ensure you have weep holes through the wall or a field drain behind it if the soil doesn't drain easily by itself.
Good luck

 

[onionsa]

Looks like it wont be holding back too much of a load, so a minimal foundation of maybe 5 inches should be ample (though it should extend front and back beyond the thickness of the wall). How's drainage in your garden (sandy or clay soil and does it easily become waterlogged)? Also what sort of patio will you have (sand jointed can drain down through)? I built a higher retaining wall in my garden and this pavingexpert website was a brilliant source of information - see 3/4 down the page.
I learned that drainage is key, as water retention is the primary cause of retaining wall failures as pressure builds unless there is an outlet. So ensure you have weep holes through the wall or a field drain behind it if the soil doesn't drain easily by itself.
Good luck

Thanks for the reply. The ground seems to be a mix of general mud for the top layer, then about 3 inches of sandy material in the middle and clay for the rest. I’ve been here 10 years and can’t say it gets water logged, just the general wet ground when we’ve had a lot of heavy rain. Also, the wall that we’ve taken down didn’t have any drainage at all and was still as solid as it’s ever been so I don’t think there is a drainage issue.

I’ve been all over pavingexpert the last couple of days and learnt a lot. I’m going to go with a linear drain against the house wall as it will plumb in the easiest ton our existing storm drain and will likely have mortar joined patio rather than sand.

The other thing I’m struggling to find an answer on is how level the sub grade (formation layer) really needs to be before you compact the sub base. I’ve got it roughly at 150 but some parts are 120 others 200, when you catch a big rock or root etc it makes it difficult to get it spot on. So I was wondering if the sub base being 70 in parts and 100 in others when it’s compacted would be ok?

I’ve added a photo of the different material that is making up our ground.

 

[onionsa]

Pic added

 

[Halitosis]

Your subgrade looks neater than any I've seen (not that I'm in the trade or any kind of expert!) so no problem there. Try to get your compacted base nice and level but even then your paving mortar can make up any variances (within reason) to ensure your slabs are level.
Are you also lifting that existing patio and wall, or working around it? If lifting it, might be best to do that sooner to ensure you build your base evenly across the piece.

 

[onionsa]

Yes the remaining part of the old patio will be coming up tomorrow and the ground taken down as level as possible to the target of 150mm deep. The wall is already down - that was last nights job. And tonight I’ve levelled that area so it’s lookong better. There are some obvious high points so I’ll work on them tomorrow too.