Wednesday - February 22, 2012

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• Retaining Walls
  • Construction Process

Construction Process

Alatini Saulala can discuss in person the importance of 'not cutting corners' in the building of a retaining wall that easily last for your lifetime!

Preparing the Foundation

Probably the most important part of installing interlocking concrete blocks is to prepare the foundation for the wall properly.

You want to make sure that as you dig out the area, you don't disturb the soil that you're going to leave in place. This is called "skimming."

As you dig out, check to make sure that the base is level.

You want the base low enough so that the first row will sink below ground level on the front. As a rule of thumb, you should bury the bottom row about 1" for every 8" of wall height above the ground.

For larger projects dig down about 6 inches below where you want the wall to start, and then put in some class 5 fill.

Compact the fill with a 4 x 4 or hand tamper. Again, make sure it's level all along the wall.

For really large retaining walls on a large hillside, water drainage is usually a problem because it'll flow down the hill and start eating away at the fill behind the retaining walls.

To eliminate this problem you need to lay a moisture membrane behind the wall to catch the water, along with some drain tile to collect the water and drain it away.

Installing First Row

When you set your first row of block in, you're establishing the shape of the whole project, so you want to get it right.

It's real important to check the blocks for level as you set them down. If you need to make minor adjustments, you can use a little sand to raise the blocks. You don't want to use loose dirt because dirt will settle a little over time.

Make sure that the outside edges touch one another.

As you finish each row of a wall, you should backfill in behind, and in between the blocks. For a small wall you can just use dirt for the backfill.

For larger projects use crushed stone or river rock for the backfill.

Clean the tops of each row before you move on to the next row.

Installing Remaining Rows

All of the rows will need to somehow lock to the row underneath it. Some interlocking blocks have a flange on the bottom in back to lock on to the lower row. Some have holes in the top and bottom for pins to lock the rows in place.

If you've got a curved wall and are using the flange-type of block, you'll probably need to knock if off with a hammer. Otherwise, the flange will get in the way.

Most people set up a running bond pattern. This means that the joints between the blocks in the second row fall over the middle of the blocks in the first row.

After the second row of blocks is set, you follow the same procedure: fill around the blocks, compact the fill, clean off the tops of the blocks, and put on the next course. And this repeats all the way up the wall.

For large walls, some professionals use what's called a geogrid. This is a flexible plastic mesh that attaches to the back of the wall and anchors it to the hillside.

If you're using the pin-type of block, you'll need to use a special "cap" block for the top row. You don't want any holes for pins in the tops of these blocks.

We like to put some construction adhesive under this row to hold down the top blocks. This is just a little insurance.

Curved Tree Ring

A tree ring like this is probably one of the simpliest types of retaining walls. This one only took a couple of hours.

The lawn had a pretty good slope to it. We figure it's about 15 inches higher at the back of the tree ring than at the front. So for the first row we just went down to a point where the blocks would be completely buried.

This is a nice area to plant a tree and a few flowers.

Terraced Walls

One of the most typical retaining wall projects is a terraced wall on a house with a walkout lot.

There's always a good-sized hill along the side of the house and it's too steep for any plantings, and it's tough to mow the grass.

Terraced walls create some level areas for flowers and shrubs, and in this case it helped to hide the air conditioning compressor.

And like the tree ring, you can see how the curve dies into the hill.

Large Retaining Walls

The back of this house had a large hill running up from it. We've used a series of retaining walls to dress it up a little.

Along with the walls, we put a couple of level planting areas for some trees and shrubs, or maybe some vegetables and flowers.