When I first started work, it was in the building trade. The bricklayer told me to go and ask what height the airbrick was meant to go in the retaining wall

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It didn't sound unreasonable because retaining walls often have "weep holes" in them anyway where a bit of pipe is embedded in the wall so that water doesn't build up behind it. I just assumed that the air bricks were to be used as weep holes.
For those who may not know, air bricks actually exist. They are placed in the brickwork of buildings that have wooden floors. They are built in between ground and floor level to keep an air flow under the boards which helps avoid rot. Obviously such a brick would not normally be used in retaining walls but I still think they would be a far tidier way of doing things than embedding pipes in the wall.