We live on the side of a hill so we dug into the hill mean making the ground level usable. This means the back and both sides of the house have a retaining wall with the highest point being 3.2m and gradually steps down following the contour of the land. it's a block wall and for a wall of this height it needed huge foundations, bottom 5 rows of blocks are 300 series blocks and after that are 200 series blocks. Can't remember the size but the high sections have extra thick steel in every block hole and anything below 2m just had the normal steel dropped down every hole.
Then a pump and 3 trucks of concrete to core fill the wall. After that we put 2 coats of waterproofing membrane on the back, just normal agg pipe of the bottom and while the engineer said I could back fill the 1st metre with recycled concrete and then use some sort of coal ash for the balance to save money, I opted for 60T of recycled concrete and backfilled to half a metre from the top all around. For the extra $400 it saves spending tens of thousands fixing it is 20 or 30 years time.
Seeing this wall is off the house by a metre it doesn't have to be 100% waterproof, so I drilled a couple of holes about a 800mm from the bottom as an extra pressure release measure. While I've only seen these holes in action twice, it has meant there was a lot of water sitting behind the wall judging by how it was squirting out.
After a lot of rain like Brisbane is currently receiving, my wall will weep water down to the drain for about a week.
