Segmental Retaining Walls

A retaining wall holds back soil that wants to move. Wet soil pushes harder than dry soil, and it never stops pushing. A segmental retaining wall (SRW) is built from dry-stacked concrete blocks with no mortar. It works because of its weight, its backward lean, and the drainage behind it. The blocks are the easy part. Most failed walls are failures of what you cannot see: a weak base, no drain pipe, or the wrong backfill. Follow the block maker's installation guide on every job, because it overrides general rules.

What You Will Be Able to Do

• Decide when a wall needs an engineer or a permit
• Figure out the trench, base, and how much block to bury
• Set a level base course
• Build up with proper drainage and backfill
• Explain why geogrid is used on taller walls
• Spot the warning signs of a failing wall

When a Wall Needs an Engineer

Most block systems stand as simple gravity walls only up to a limit, usually about 3 to 4 feet. Past that, walls typically need geogrid and an engineered design. Many towns also require a permit below that height.

Call for design help before building if there is:

• A wall over about 3 to 4 feet, or the limit in the block maker's chart
• A driveway, building, or other load near the top
• A slope above or below the wall
• Tiered walls
• Poor soil or water problems

Check the local building department on every job. A tall wall that fails can kill someone or wreck a home.

How the Wall Stays Up

• Weight: Each block adds mass that resists sliding and tipping.
• Setback (batter): Each course sits a little behind the one below it, often about 3/4 to 1-1/4 inches. That backward lean resists the earth pressure.
• Drainage: Clean stone and a drain pipe let water out so that it cannot build up pressure.
• Geogrid: Layers of reinforcing mesh tie taller walls into the soil behind them.

Water is the main enemy. A missing drain pipe behind the wall is the most common single cause of failure. Soil soaked with water pushes far harder, and in cold climates, it freezes and pushes harder still.

Planning the Base

Call Before You Dig

Call 811, and wait for the marks. Walls often need a trench right along a property edge, so confirm where the property line is too. Walls that cut into a slope can undermine a neighbor's land.

How Much Block to Bury

The bottom course is buried so that the wall cannot slide out at the toe. A common rule is 1 inch buried for each 8 inches of wall height. Many block makers also set a minimum of 6 inches of buried block, no matter how short the wall is. Taller walls, slopes below the wall, and soft soil all call for more. Use whichever number is greater, and follow the maker's guide.

Worked example. A wall is 32 inches tall. By the 1-in-8 rule, 32 divided by 8 gives 4 inches of buried block. If the block maker asks for 6 inches at minimum, bury 6 inches.

Sizing the Trench

The trench must hold the base and the buried block.

Worked example. With a 6-inch base and a 6-inch-deep buried course, dig 12 inches below the finished grade at the front of the wall. A trench about 24 inches wide is common, and a tall wall needs more.

Step by Step

Build the Base

1. Dig the trench. Remove topsoil and soft material.
2. Compact the trench bottom.
3. Place geotextile if the design calls for it. Some walls wrap fabric up behind the drainage stone, so that fines cannot clog it.
4. Add crushed base aggregate. Compact it in thin lifts of about 2 to 3 inches. Many makers call for at least 6 inches of base, though some allow 4 on shorter walls.
5. Level the base from end to end and front to back. Some systems allow a thin layer of sand on top for fine leveling.

If the wall steps up a slope, the base steps too, in increments of block height. Do not slope the base.

Set the First Course

The first course is the most important one. Any error here shows up in every course above.

1. Start at the lowest point, or at a fixed end.
2. Set each block with a level, front to back and side to side, and check it against its neighbors.
3. Use a string line along the back or front edge.
4. Tap blocks down with a rubber mallet. Pull one back up and adjust the base if needed.

Take your time here. It is common to spend as long on the first course as on the rest of the wall.

Build Up

1. Sweep the top of each course clean. A pebble can keep a block from seating, and the error adds up.
2. Stagger the joints, so that no vertical seam lines up with the one below.
3. Engage the lip, pin, or connector that sets the setback.
4. Fill the block cores if the system has them, and fill behind the course with stone as you go.
5. Check for level every course or two.

Drainage

1. Place clean, angular drainage stone at least 12 inches deep behind the wall, unless the plan says otherwise.
2. Lay a perforated drain pipe at the back of the base, at the bottom of the stone.
3. Run the pipe to daylight, or to a proper storm drain. A drain that has nowhere to go does nothing. Give it a steady fall, and add outlets along long walls.
4. Put geotextile between the stone and the soil behind it, as the design requires. It stops soil from clogging the stone.

Do not use clay as backfill. It holds water and swells. Use the material the design calls for.

Backfill and Compaction

Backfill in thin lifts as you build, and compact each lift. Do not wait until the wall is finished. Use a light compactor within about 3 feet of the back of the wall. A heavy machine there can push the wall out.

Geogrid

On a wall that needs it, the plan tells you where and how long.

1. Cut each layer to the length on the plan. Grid often extends back into the soil about 60 to 100 percent of the wall's height.
2. Lay it level on top of the block course and on the compacted soil, running in the strong direction shown in the plan. Check which way it runs. Grid is stronger one way.
3. Pull it tight, and hold it with stakes.
4. Add backfill over it, and do not drive tracked machines directly on it.

Caps and Finish

Glue cap blocks on with adhesive made for concrete. Let the wall settle into place first if the maker says to. Slope the soil on top so that water flows away from the wall. A clay layer or swale at the top can help keep surface water out of the drainage stone.

Staying Safe

Blocks commonly weigh 40 to 80 pounds, and big units take a machine. Use your job's rules for lifting. Cutting block produces silica dust, so use a wet saw or a saw with dust control, and wear the respirator the job requires. A wall trench next to a steep bank can cave in, even if it is shallow. Keep out until it is sloped or protected. Do not undercut existing walls or foundations.

Signs of Trouble

A small crack may not be serious, but a wall that is moving is a safety issue. Keep people clear, and bring in an engineer.

Common Mistakes

Check Your Understanding

1. A wall will be 40 inches high. How much block do you bury by the 1-in-8 rule, and what could change that?

40 divided by 8 gives 5 inches. A maker's minimum of 6 inches would mean 6. A slope below the wall or soft soil would call for more.

2. A customer wants a 5-foot block wall holding up a driveway. What do you do?

Stop and get an engineered design and a permit. It is too tall to be a simple gravity wall, and it has a load on top.

3. A wall built last year is bulging. What should you check first?

Drainage. Look for a drain pipe that has no outlet or is clogged, and look at what was used as backfill. Keep people clear if it is moving.

4. Why do blocks step back as they go up?

The setback gives the wall a backward lean, which helps it resist the soil pushing from behind.

Key Takeaways

• Check height limits and permits before you start.
• Bury the first course. Follow the maker's rule, and use the larger number.
• Spend your time on a compacted, level base and first course.
• Use clean stone and a drain pipe that leads to daylight.
• Compact in thin lifts, with light equipment near the wall.
• Taller walls use geogrid, and only to an engineered plan.