In a previous post, we discussed how to build a 2-ft tall timber retaining wall. In this post, we will discuss how to build a timber retaining wall greater than 2-ft by using deadman anchors. The weight of the timbers can only resist a minimal amount of soil by themselves, so the deadman anchors provide additional support to allow for taller walls.
As we learned in this post, all wood in a timber retaining wall must be pressure treated for ground contact to UC4A or UC4B standards (UC4B preferred). The timbers shall be 6″x6″ or 8″x8″.
Note, this post applies to walls that are less than 4-ft in height, without toe slopes or surcharges, and built on good soils (i.e. not fat clays or soft soils). If any of these situations apply to your project, please reach out to an engineer.
Tools
- Hand Tamper*
- Plate Compactor*
- Shovel*
- Gloves*
- Wheel Barrow
- 4-ft Level*
- Torpedo Level*
- Sledge hammer* or Deadblow*
- Drill (if using screws)
- Circular Saw or Chainsaw
Materials
- Pressure Treated Timbers
- Galvanized or Stainless Nails/Screws
- Clean Crushed Rock
- Filter Fabric*
- Rebar*
- Paintable copper based wood preservative*
- Construction Adhesive*
- Perforated Drain Pipe*
- Pipe outlet or Wall Drain Pro*
Foundation
Dig a trench that is at least 12-inches wide. For walls greater than 2-ft tall, dig an 10-inch minimum depth. Compact the bottom of the trench using a hand tamper. Check out more information on preparing the foundation here.
After compacting the foundation, place the base material. The base material must consist of at least 4 to 6-inches of clean crushed rock such as No. 57 stone. The clean crushed rock will remove water from under the wall helping to prevent water damage to the timbers. For more information on retaining wall backfill materials, check out this post.
Compact the base material using the plate compactor.
Setting First Row of Timbers
Place the first row of timbers directly on the crushed rock base. Start at a corner or structure that you are abutting against. Leave no gap between the timbers. Then, ensure the timbers are completely level along the length of the timber by using a 4-ft level*. Use a torpedo level* to level the timber from front to back. Use a deadblow* or small sledge hammer* to adjust the timbers.
Drill two holes at 4-ft apart on each timber on the bottom row. The holes should be the same diameter as the rebar*. The length of the rebar should be at least 3-ft. Drive the rebar through the holes with a sledge hammer.
Deadman Anchors
The deadman anchor consists of a timber tie installed perpendicular to the wall face and attached to both the wall face and a timber cross plate. The length of the deadman anchor should equal the height of your wall. So a 4-ft tall wall would require 4-ft long deadman anchors. Deadman anchors are spaced 8-ft apart on each row of timber, except the top and bottom coarse. Pro tip: buy timbers in 8-ft sections to limit the amount of cutting.
The length of each timber cross plate should be 30″ wide. Attach each deadman to the cross plate and the below facing timber using two nails or screws (shown as 60d spikes below). Stagger the nails/screws to prevent the deadman from rotating. Space roughly 2-inches from the wood edge to prevent splitting.
The nails or screws should be 3/8″x12″ galvanized nails*, 12″ stainless steel screws*, or 12″ ACQ approved coated screws*. Because the copper in the pressure treatment is corrosive, use hot-dipped galvanized or stainless steel nails/screws. Do not use electroplated (or zinc plated), uncoated, painted, or other type of coating unless it is rated for ACQ pressure treatment. These alternative coatings are fine for an above ground deck, but they do not provide the necessary corrosion protection needed for pressure treated wood that is in contact with the ground.
Subsequent Rows of Timbers
Stagger the joints of the next row of timbers from the ones below. This helps the stability of the retaining wall. Aim to place at least a quarter of the length of the timber past a joint (for 8-ft timbers, this is at least 2-ft). Also, set each row of timbers with a 1/2-inch setback from the one below. This dramatically helps with the stability of the wall. Install a nail/screw every 2-ft along the length of the timber (5 minimum per an 8-ft long timber) and stagger them between the front and back face.
Subsequent Rows of Deadman’s
Stagger the deadman anchors horizontally from the ones above and below them by 4-ft. There should be a vertical column of deadman anchors every 4-ft with deadman anchors in every other row. See below.
Installing Drainage
Water cannot easily flow through wood like it can through porous segmental blocks. This means you need special accommodations to ensure water does not build up behind the wall. Place a minimum of 12-inches of drainage stone (a clean crushed rock such as No. 57 stone) behind the timbers. This, combined with the clean crushed stone used for the base, will help the water drain underneath the wall.
Run a 3-in or 4-in perforated pipe* behind the wall and outlet every 30 ft. Plan an outlet for the pipe the lowest points on the wall. The lowest point may either be at the end of the wall or somewhere in the middle. An outlet in the middle will require a hole in the timbers. Or avoid the hole in the timber by using a Wall Drain Pro*.
Compact the drainage stone with a hand tamper or plate compactor. Continue placing the drainage stone to within 4-inches of the of the wall. When you get to the top of the wall, place filter fabric* above the drainage stone. Extend the fabric up the back of the timbers and attach with construction adhesive*. Extend the filter fabric at least 6-inches past the drainage stone.
Cutting Wood
Timbers are too thick to cut with a standard circular saw in one cut. To use a circular saw, you will need to cut one edge, rotate the timber, cut again, and continue this until you have complete sawn through the timber. Alternatively, use a chainsaw. Wear a mask to avoid breathing in any dust from cutting pressure treated wood.
Pressure treatment only extends an inch or so into the wood. So, when you cut through the wood or drill a hole, the cut edge will be untreated. Treat any cut edge with a paintable copper based wood preservative*. Use a foam paint brush* to apply the preservative.
Final Thoughts
When built correctly, wood retaining walls can last for many years. However, they will deteriorate over time and require replacement. With today’s more environmentally friendly pressure treatments, you can expect a wood retaining wall to require replacement in 5-20 years depending on your climate. If you intend to build a retaining wall that will stand the test of time, consider a segmental block retaining wall.
