Upside: Timber is often used to create retaining walls since procuring timber is often easier and cheaper than ordering block or poured concrete used for constructing a concrete retaining wall. Also, the color and the overall texture of a wooden retaining wall is made from aged timber adds a more natural appeal to the yard or the garden space.
Downside: Timber retaining walls do not offer the resilience that is offered by concrete-enforced retaining walls. Pressure-treated timber has to periodically retreated, which is a time-consuming and tedious process. In terms of the wall height, timber-made retaining walls are limited since the wood-based walls have a restriction in terms of height to ensure greater safety.
Use 8-foot-long, 6x6-inch pressure-treated wood designated "For Ground Contact," and have all materials delivered. Follow all rules for landscape fabric, drainage and backfill. All timber walls require deadmen every 4 feet at midwall height or higher. Pin the first tier of timbers to the ground with #4 rebar.
Upside: Segmented retaining walls use interlocking-block systems from Keystone, Risi, Rockwood, Tensar, Versa-Lok, among other brands. These retaining walls are mortar-free and easy to assemble. Units are small and modular, so walls can taper, turn, wrap, and curve. Concrete Block retaining walls are available in many textures, shapes, and colors. These engineered systems, which can be used for walls up to 20 feet high, rely on several techniques including:
•Keyed and battered design (blocks fit into each other and are stacked so they lean into the hillside)
•Backfill trap (block shapes allow backfill to be shoveled into the block webbing, trapping each block individually)
•Geo-grid webs (geo-grid plastic-net tiebacks attach to the blocks and are buried 5 feet in the hillside at specific heights).
Downside: You can't mix and match manufacturer's systems. Most block systems that use metal pins to tie blocks together can be a challenge to line up exactly.
Arrange before delivery from the masonry yard where materials will be stockpiled in your yard and if the forklift used to off-load the truck will fit through backyard gate, etc. Follow all rules for landscape fabric, drainage, and backfill. Use manufacturer's calculators to determine how many blocks, pins and tiebacks you'll need. When stacking blocks, sweep off each layer; small pebbles can disrupt the pattern. Cap walls with flat units or stone held down with silicone caulk.
Manufactured concrete block retaining walls are becoming popular as a way to create large permanent retaining walls to hold large volumes of soil, on large slopes. Concrete retaining walls are engineered using blocks of many sizes, shapes and colors. Concrete blocks are stacked and interlocked atop of compacted, crushed gravel.
Retaining walls reduce the angle of a slope so it is easier to plant, walk on or use as an accesible landscape. Retaining Walls are typically made by 'cutting and filling'; a practise of excavating a portion of the slope and then using the cut soil to backfill behind the retaining wall creating a relatively flat area. A single retaining wall may be enough to support a short stretch of slope, while a series of parallel retaining walls is often used to support larger slopes.
The taller the retaining walls are and the closer they are together, the flatter the slope is above them. Terraces are created when a retaining wall is tall enough to create a level surface above it. Terraces are necessary for pathways, lawns, patios, vegetable gardens and other areas where people need to move around comfortably. But if the goal is only to plant the landscape with ornamentals, the retaining walls can be shorter -- most groundcovers will thrive on slopes as steep 45 degrees, but getting the slope down to about 20 degrees opens up more possibilities for landscaping.
Retaining walls are built with natural stone, concrete blocks, railroad ties and a variety of other materials. Seattle Rockeries uses material that match the look of your house and fit with the overall style of your landscape. Sticking with the same material for all the walls and stairs is a wise choice because it gives cohesiveness to the overall design.
Retaining walls over three feet tall generally require a building permit and should be built by a professional contractor. Retaining walls of any size need to be built on compacted subsoil to prevent them from settling over time and need a perforated drain pipe and a layer of drainage gravel behind them so water can flow away rather than build up behind the wall. Walls may be constructed with a perfectly vertical profile or angled slightly into the hill which reduces the complexity of building them.