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I'm looking to build a retaining wall 800mm (2.5ft) high and 225mm wide at the side of my house.

During the original construction groundworks, the ground around my house had to be raised up about 800mm for access purposes,100-200mm hardcore was used.

How deep into the hardcore do I need to go with my foundation and how deep does the concrete in the foundation need to be?

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  • What exactly does your "hardcore" consist of ?
    – Alaska Man
    Apr 4 '20 at 20:02
  • What is hardcore?
    – Ack
    Apr 4 '20 at 20:04
  • Also, what is the distance from the wall to the foundation of your home?
    – Ack
    Apr 4 '20 at 20:05
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    If you drill and pin with rebar I would just get it flat. If the wall is straight the footing needs to be wider depending on the back fill if curved not much of a footing would be needed at all.
    – Ed Beal
    Apr 4 '20 at 20:06
  • @Ack Hardcore is drainage style aggregate, sharp, rough edges. It'll be 2m from the house but will not be bearing the load of the house as the house foundations go down around 1500mm
    – Fony Tinlay
    Apr 4 '20 at 20:10
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You need a good base for the wall to be stable so I would remove the hardcore and place the footing at least down to original compacted soil.

For the depth of the footing, if you mean the depth of the concrete, 225 mm is good. If you mean how far down in the soil, then either the depth of the footing (225 mm) or provide a decent sized width of footing on the soil retained side. There are two main items to address with the wall, overturning and sliding. The forces are minimal for a 800 mm tall wall assuming no surcharges and it's drained well so no hydrostatic loads. The sliding resistance has to come from somewhere. Some of it is soil friction. Some of it is resistance on the front of the footing from the soil. If the front of the footing is in hardcore then it will not provide much resistance. To increase friction you need weight. To get wait, you have the depth of soil on the retained side. Make the footing wider here by say 400 mm and this should be enough. This will also increase the resistance to overturning.

To get better numbers on sizes and what else to do would require doing a design.

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  • Thanks for the reply. If I was (for the sake of argument) to dig about 300mm deep into the hardcore (450mm wide), making sure it was compacted and then embedded a Geogrid mesh between every other course of block and ran it 1m back towards the house between lifts of soil backfill, would this negate the sliding and overturning problems? Illustration Here I'm planning to put a french drain behind the wall and there is no surcharge on the wall other than the backfilled soil.
    – Fony Tinlay
    Apr 4 '20 at 21:00
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    yes, in fact this is a common technique and in fact you would not need much of a footing, or any at all since all you need is to not have more weight than the allowable soil bearing, there is no additional forces from the overturning. Still, it's a good idea to have one. Note that your wall is relatively short for a retaining wall and the forces are low. With the geogrid, you can think of each rise (block height) somewhat like a mini retaining wall, then the blocking is simply there to keep the soil from sloughing in between the layers.
    – Ack
    Apr 4 '20 at 21:52

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