I would like to install an allan block retaining wall (3' at its highest). I'm going to build a shop 25' downhill from this retaining wall, and want to have a poured slab for parking vehicles on. In researching foundations, it is recommended not to use a concrete foundation, so how to I ensure that there is not excessive water traveling under to the slab? the drain will prevent the water table from exceeding the drains height locally, but the soils below could still be fully saturated. Or is this OK?

As a 2nd question, how should I interface the gravel footer of the retaining wall and the driveway apron's slab? (Red question mark area in figure)

Lastly, why does allan block recommend a 4" x 18" wide footer? it seems the excess width provides no value, unless it is to ensure a plate compactor has the space get good compaction in the gravel.

Located in Zone 4C (Seattle area)

Drainage Sketch

  • Did you ever get this resolved? If so, please give a check-mark to the answer or write up your own answer explaining what you did to get it fixed and give yourself a check mark. That will help others with this kind of problem know that this has a resolution and is a good place to look for their answer.
    – FreeMan
    Aug 7, 2020 at 18:27
  • The final build varied somewhat: I utilized a rockery instead, it had a draintile behind it per the figure above. I did not build the shop. everything has worked as expected since it was built.
    – jlipstate
    Sep 15, 2020 at 18:50

1 Answer 1


I know a little about 2 subjects. This is one. I'm a landscape architect and two easy examples of how to fix your problem came to mind. BTW...nice drawing.

I HIGHLY RECOMMEND YOU DO THIS...simply get some diamond tip masonry drill bits and drill some holes into your retaining wall about an inch above the ground. Also do the same from the other side of your wall at different levels in your wall, but that's below the ground surface that your retaining.

Doing this is not optional if you want your wall to last through the next rainfall in your area. This is to allow water from any sudden downpours as well as water trapped below the surface to escape, while leaving your yard in place.

The other option that's easily done is called a French drain. This is the most aesthetically pleasing way to move water because the entire drain is buried and cannot be seen. Easily done with the right equipment (varies widely from a mini excavator to a pick and shovel) or neighborhood kid to dig the trench for your pipe to lay in.

The opening to the drain would be where the question mark is on your drawing. And you would have the pipe traveling down the hill and UNDER where your retaining wall will be. Just 3" below the surface of the ground where your wall will be is sufficient to keep it from being pinched or crushed.

Make sure you level out the ground where the first layer of bricks that make up the base of your wall, and bury your pipe 3 inches below that. Cover with dirt. The only things you HAVE TO BE SURE OF is that your pipe must be laid in a straight line, and ALWAYS keep the fall. Meaning don't allow a hump of dirt make your pipe lift more than 1/8 inch or water and sediment will gather there, eventually clogging the drain. It only needs to be as long as about 6 inches at the least past your wall. But you can run it as far as you want. No one will even know its there. You can purchase everything you need at Walmart or your local home improvement store for cheap.

Oh yeah, avoid drilling any holes in the actual mortar/cement you use to hold the bricks in place to prevent weakening the structure. The amount of holes you need depend on the amount of water that comes your direction from other places around you. A 1" hole every 2 to 5 feet above the ground. With smaller holes placed closer together below the surface on other side of wall.

  • Thanks for your answer. If you would edit that massive wall of text into a couple of paragraphs to make it a bit easier to read, it would be very helpful
    – FreeMan
    Aug 7, 2020 at 18:28

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