How Deep Should a Retaining Wall Base Be?

Two parts of the depth

A block retaining wall trench has to hold two things:

1. The base pad: a layer of compacted crushed stone, commonly about 6 in (150 mm) deep, that spreads the wall's weight and keeps it level.
2. The buried course or courses: part of the wall sits below the finished ground in front of it. This embedment stops the bottom of the wall from sliding out.

trench depth = base pad depth + buried block depth

How much of the wall to bury

A widely used rule of thumb for segmental block walls is to bury about 1 in for every 8 in of wall height, and never less than about 6 in. In practice that usually means one full course underground. Taller walls, walls on a slope and walls on soft soil need more. Always follow the block manufacturer's installation guide, which gives figures for its own system.

Round the buried depth up to a whole course of your block. With 6 in blocks, 7.5 in means two buried courses.

How wide to dig

The base should extend in front of and behind the block. A common rule of thumb is a trench twice as wide as the block is deep: about 24 in (600 mm) for a 12 in deep block, giving roughly 6 in of base in front and the rest behind. Behind that, leave room for a drainage zone of crushed stone, often about 12 in wide, for the full height of the wall.

Worked example

A planter wall 20 ft long with 2 ft showing, built from 8 × 16 in blocks. Three courses stand above ground and one is buried, for a total height of 32 in. The rule of thumb gives 32 ÷ 8 = 4 in of embedment, so the 6 in minimum applies. With 8 in blocks you cannot bury part of a course, so one full 8 in course goes below grade. The trench is 6 in of base plus 8 in of buried block: 14 in deep and about 24 in wide.

The retaining wall calculator counts the materials for this wall: 4 courses of 15 blocks, 63 blocks with 5 percent extra, 16 caps, and 2.72 cubic yards of gravel for the base pad and the drainage zone, roughly 3.8 tons.

Building the base

• Dig down to firm, undisturbed soil. Remove topsoil, roots and soft spots and replace them with compacted stone.
• Use crushed angular stone with fines for the pad (often sold as road base or crusher run). Some systems call for clean crushed stone instead; follow the manufacturer.
• Compact the pad in layers of a few inches with a plate compactor.
• Level the pad carefully along its length and front to back. The first course is the one that matters most.
• Step the base up in whole-course increments on sloping ground, rather than tilting it.
• Lay a perforated drain pipe at the back of the base, run to daylight or a drain.

Common mistakes

• Setting the first course on loose soil or on a thin, uncompacted base, so the wall settles unevenly.
• Skipping the buried course to save a row of blocks. The bottom of the wall then has little to stop it sliding.
• Backfilling with the excavated clay right against the blocks instead of free-draining stone.
• Leaving out the drain pipe, or running it uphill so water has nowhere to go.

When you need an engineer or permit

Many codes require a permit, and often an engineered design, for retaining walls over about 4 ft (1.2 m), usually measured from the bottom of the footing or base, not just the visible face. Engineering is also usually needed for any wall that holds back a slope, a driveway, parking, a pool or a building, regardless of height. Many block systems limit gravity walls without soil reinforcement to about 3 ft. Taller walls use layers of geogrid reinforcement in the backfill, designed for the soil and loads. Rules differ by city and county, so check local code before you dig. A wall that fails can cause serious injury and damage.

Metric quick reference

Frequently asked questions