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.
Interlocking Concrete Block
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.