How to build an Allan Block retaining wall

Before you dig

Allan Block walls hold soil back with their own weight and setback. How tall you can go without geogrid reinforcement depends on the block, your soil and what's above the wall. In clay soil with level ground above, an AB Classic wall tops out at about 3½ ft. A slope, driveway or patio above the wall lowers that limit. Taller walls need layers of geogrid and a design. Check with your municipality before building a tall wall, since engineering and permits often apply.

Step by step

1. Lay out and excavateMark the front of the wall with stakes and string, or a garden hose for curves. Strip all grass, roots and organic soil. None of it can go back behind the wall.
2. Dig the base trenchStart at the lowest point. For walls under 4 ft, dig 18" wide and 4" deep, plus about 1" for every foot of wall height so the first course is buried. For walls 4 ft and taller, dig 24" wide and 6" deep, plus the same 1" per foot. Compact the trench bottom with at least two passes of a plate compactor. Replace soft clay or wet soil with compacted granular fill.
3. Add the drain pipe if neededUse a drain pipe for walls over 4 ft, reinforced walls, poorly draining soil, walls next to paving, and walls with a slope above. Lay it low at the back of the trench and vent it to daylight at least every 50 ft.
4. Build the basePlace at least 6" of wall rock (clean crushed rock with no fines) in the trench, compact it with two passes, and check the whole length for level.
5. Set the first coursePlace blocks lip up and forward, centred in the trench. Level each one side to side and front to back, check alignment against a string line, and tap with a dead-blow hammer. You can shim with up to ½" of coarse sand. Take your time here: any error in the first course grows as the wall gets taller.
6. Fill and compact every courseBackfill in front of the base course with soil. Fill the block cores and 12" behind the wall with wall rock, then backfill behind that with soil in layers of 8" or less. Compact with at least two passes running parallel to the wall, working from the block toward the back. From the second course up, run the compactor on the blocks too. Keep heavy equipment 3 ft back from the wall.
7. Stack the next coursesSweep the top of each course clean, then stack the next one with the vertical joints offset by at least ¼ of a block. Where the design calls for geogrid, roll it out from the back of the front lip, pull it tight, and never drive or compact directly on it.
8. Finish the topOn the last course, backfill behind the blocks with topsoil instead of fill. It holds plants and keeps surface water from soaking in behind the wall. Grade the top so water runs away from the wall, then add caps.

Summarized from Concrete Masonry & Hardscapes Association (formerly ICPI) and Allan Block installation guidance. Follow the manufacturer's instructions and any engineered plans for your project.

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