Landscape Retaining Walls

Page Outline:

I.    General Considerations
II.  Concrete Retaining Wall Blocks
III. Rock Walls
IV. Treated Timber Walls
V.   Poured Concrete Walls
VI.  Brick Walls
VII. Planting on a Slope

I. General Considerations

Block wall with capstone

Most permitting agencies (i.e. County, City) require a permit to build a retaining wall over four feet high. Often these larger walls will need to be designed or approved by an engineer. This is a good idea, as there are considerable forces exerting pressure over time on higher walls.

Western Washington has an abundance of hills and slopes that, when well designed, allow for a much more interesting and attractive final landscape than a flat property (Imagine a flat Butchart Gardens – near Victoria, B.C.). Rodda and Sons Landscapes specializes in the installation of these rock, timber, poured concrete, brick or concrete retaining walls in Normandy Park, Burien, Des Moines, Bellevue, SeaTac and other Seattle areas.

II. Concrete Retaining Wall Blocks

Low Stonewall block retaining wall with capstone

In recent years there has been a large increase in the number of manufactured concrete blocks that are engineered for retaining walls. Generally these walls are built on a base of compacted crushed gravel, often with drainage behind higher walls. The blocks come in many sizes, shapes and colors and from many sources (Mutual Materials, Home Depot, Lowe’s, etc.). The most common errors by homeowners are not using a compactor on the crushed gravel base, not using a level on the blocks, and not using engineering that is needed on higher walls.

Broken concrete pieces, such as slabs from removing a sidewalk, can also be used to build a low wall. If this is done well, it can be very attractive.

III. Rock Walls

Dry stacked flagstone wall and steps

Long lasting wall. Higher walls need larger rock. Our guidelines are: One-man rock (up to 300 pounds) can be used for walls under 2 feet high. Two-man (300-1000 pounds) can go to 4 feet high. Higher walls often need even larger rock. Drainage behind the rock wall needs to be considered. Experience in rocksetting is highly beneficial.

Interesting rock walls can also be constructed using a flat, dry-stacked flagstone or slate. Or the flagstone can be mortared together.

IV. Treated Timber Walls

A well-built wall using good quality new treated timbers should easily last 20 years. Timbers can be connected with spikes or screws and the designer needs a knowledge of structures to design a plan that will hold the wall up over time. ‘Deadmen’ are often buried into the hillside and connected to the wall for support. These materials are easier to work with than rock where tractor access is limited; and they take up less space, when space is tight.

In addition to a vertical timber wall, we have developed an open grid timber wall that can stabilize a slope at up to 45 degrees, or more, and has space for planting in the grid. The grid uses less materials and can lower costs.

Low walls can be made using vertical timbers or vertical round peeler poles.

Used RR ties have a considerable range of age and quality and are difficult to cut because of ground in dirt. Also the creosote can come off on your shoes. So we tend to avoid used ties.

V. Poured Concrete Walls

Excellent long lasting material if well designed – if not, these walls do break or fall over in time. More expensive than the above walls due to labor to build forms, etc.

Broken concrete pieces dry stacked for a wall, concrete steps and landscaping

VI. Brick Walls

Broken concrete pieces to retain a slope, concrete stairs, and landscaping

Beautiful, long lasting material also needs to be well designed. More expensive than the above walls due to materials and labor. Very low walls (under one foot) can be just dry or mortared brick. Medium height walls can be mortared double layers of brick. Higher walls can be done using brick as a facing on another structural wall, usually concrete.

VII. Planting on a Slope

Sometimes a retaining wall is not necessary. The soil may be stable on a slope and can be planted and mulched (eg bark) to stop erosion. Much erosion damage on a slope is caused by raindrops impacting against bare soil, loosening soil particles and allowing them to wash downhill. Bark mulch absorbs the impact of the raindrop and protects the soil structure. Mulch protection allows the soil surface to remain more porous, and allows water to soak into the soil much easier. Picture the loose soil under leaves in a forest versus the hard bare soil in a city lot. As plants grow, their foliage and roots help protect and stabilize the slope.

The next erosion problem on a slope is running water. It’s best to intercept the surface water and control its flow down the slope, such as in a pipe.

On larger slopes, hydroseeding (spraying mulch and grass seed in a water slurry) is often effective. The mulch (ground wood fiber) gives immediate protection and the grass grows fairly quickly.

Planting trees, shrubs and groundcovers can save a lot of expense over building walls and can be very attractive.

Large rocks can be set into a slope with plantings for interest, but these rock are added for their beauty and do not add much retaining value.

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- Janet C.