better outdoor living at home spring


Garden Retaining Wall


Even low garden-type retaining walls need to be built using proper construction methods to remain a long lasting feature for your yard.

Our Mom has a low retaining wall in the backyard that kept falling over in a number of spots along the length of the wall. It had been installed a number of years ago as a DIY project to create a raised plant bed.

Work had begun before we could take photos of the overall damage to the wall. The extent of the problem is hard to see in the 'before' photos, but the forward tilt of the top course is visible and some of the upper block units that were pushed out of alignment is also visible. Not seen in these photos, is that the distant end of the wall in the photos had begun to topple over.

Top course is sloping forward. Also, visible in the center of photo is a wall block pushed out of place (in the second course from the top)

It can be frustrating for a homeowner when problems with a DIY project arise, and they don’t know why the problem is occurring. Before getting to the source of the problem, here is some info about the wall and the material used to build the wall.

The ground around the wall is slightly sloped (being higher at the side yard and sloping down around the corner of the house to the backyard). The wall is about 6 inches in height at one end and 18 inches at the other end.

This relatively simple retaining wall was constructed of modular concrete block units that are designed to fit together to produce a wall with a batter (stepped back). This batter assists in resisting the forces from behind the wall produced by the soil, and water.

These concrete block units are specifically designed by the manufacturer for retaining a small amount of soil, and for creating a wall of no more than 18 inches in height. They can typically be found at home improvement centers. This type of wall is not meant to be used as a seat wall.

While this type of wall product is designed for small DIY homeowner projects, it still requires the very basic technical construction requirements that a much larger and structural retaining wall would require, in that it needs a solid base and good drainage (a way for water to drain from behind the wall).

Types of Walls

Freestanding walls and retaining walls are designed and constructed differently from one another. A couple of these differences in design results from retaining walls having to address the weight of the soil it is retaining and the issue of water drainage from behind the wall. Water can build up behind the wall creating hydrostatic pressure. This pressure is a powerful force that will cause even a low retaining to fail or topple over, if the pressure is not released.

To release the pressure, retaining walls need to have coarse drainage aggregate directly behind the wall creating voids where the water can quickly collect and then a way for the collected water to be released to reduce the hydrostatic pressure. Larger, structural retaining walls will use drainage pipe, including weep holes, in addition to the aggregate to quickly move the collected water away from behind the wall. A low garden-type retaining wall, such as this, needs drainage aggregate, too. The particular design, and manner of installation, for this type of modular dry-laid concrete wall product will create gaps between the individual blocks where water can exit from the drainage aggregate where it has collected. The gaps between the blocks act as weep holes.

While searching for the source of why the wall was falling over, the first area we looked at was directly behind the wall. We dug the soil out from behind the wall and discovered that the wall was installed without using any drainage aggregate. Any water that found its way behind the wall created hydrostatic pressure from the soil expanding as it absorbed water, and from runoff water not being able to quickly exit from behind the wall. These events caused too much pressure on the back of the wall and resulted in the wall falling over in various spots.

Solution

Since the wall units had been pushed out of place by the pressure that built up behind them, all the block unit courses, except for the base course, along the entire length of the wall were disassembled and re-laid to position each block correctly. We had a width of 6 inches of soil removed directly behind the wall, and to the depth of the top of the base course of the wall. Before installing the drainage aggregate, the excavated area was lined with filter fabric on three sides (bottom, back, and top) to keep soil from entering the aggregate. The top of the aggregate will be a bit lower than the top of the wall. Remember to leave enough filter fabric to fold over the top of the aggregate to prevent the soil above from washing into the aggregate.

Filter fabric is available at builder suppliers and home improvement centers. It typically comes in 4 foot wide rolls. This material keeps soil from filling the voids around the drainage aggregate, thereby giving the water a place to collect. We highly recommended using the filter fabric.

There has been a number of significant rainfalls since this work was completed, and there has not been any problems with the wall.

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