Building a wall for a car pit

My biggest concern would be hydrostatic pressure. 1600mm is deep. Where's your water table at in winter?

No way in hell I would get into that pit with the 4" block. They will not support the sides at that depth and with cars parked on it.

the pit is outside is it? Yer mad if it is

It will act as a sump without water proofing .But Blocks on flat ,heavy gauge polythene around the outside of the blocks and cori board up against the polythene to stop it getting pierced .Damp under concrete slab would be neeeded but bit late for that now.

On flat be better but you’ll get away with on edge for the first course.Polythene on floor needs to join with polythene for sides .You can use a water resist tape.Think of it as wrapping the complete build in a plastic bag ,any gaps and water will get through it.

Here's a very comprehensive video series on a guy building a pit for farm machinery

It seems from photo that slab is too narrow, if, as it seems, the inside width is 36 inches with a 4 inch block - if you add another block to take block on flat you will end up with a pit 28 inches wide. Unless you're a midget that is much too narrow to operate in comfortably. The cheapest solution is to remove blocks make pit wider and pour a new wider slab over existing with steel reinforcement. Build walls with blocks on flat and finish off top with 9 inch steel reinforced concrete ring beam overhanging on both sides to suit width or car. Leave a 4 inch x 2 inch deep rebate in inside top of ring beam to take 9 inch x 2 inch planks to cover pit when not in use.

Also leave a duct for electric power (socket, lights etc) in pit, and steps to access, egress pit.

But before proceeding any further consider something like a 4 ton 2 post lift for €2,300.00. Just back fill pit and pour a suitable steel reinforced slab.

I was thinking of making a pit but ended up buying a mid height lift.

Handier and safer and more useful - you can lift vehicle off its wheels.

I'd also be worried about ventilation. One reason they aren't used that much anymore is that they are a health and safety nightmare. Run an engine near them and there is a potential for asphyxiating anyone using the pit, also any spilt volatile liquids tend to collect waiting for a source of ignition.

Thats without human stupidity. Friend in UK said his mate could put his Reliant Robin over the pit to do some work. The driver just rocked up wondered why he was being waved and shouted at and drove straight forward into the pit - an extreme example but it has happened.