Building a wood retaining wall is an excellent way to improve and enhance the value of your property. It can be built in a couple of weekends by anyone with a little motivation and a good plan.
Want to know why retaining walls fail? See article on Wood Retaining Walls Failures.
Before starting your construction project, plan out your ideas and even make a sketch of the wall as it will be built on your property.
Also, check with your local building department to determine if a permit is required. Some building departments require a permit if the wall has a height of more than 3 feet.
This article will provide guidance on the construction of the wall to insure a strong, stable and lasting wall. Also, see timber retaining walls.
Material can be bought at your local Home Depot or Lowe's.
The most common type of timbers is treated 4x6s or 6x6s. The most common timbers are the 6x6s. They have the most structural strength. Choose timbers that have a 0.40 treatment rating (suitable for ground contact) and seal any cut ends with preservative to prevent rotting. Railroad tie walls are also used where railroad ties are available.
Other materials you will need are:
The drawing above shows the cross section of a timber retaining wall. Each of the horizontal timbers is setback 1 inch for added strength.
Every third timber is anchored with a deadman which extends horizontally behind the wall. The wall gets most of its strength from the 6x6s set perpendicular to the face of the wall. The length of deadman anchors should be approximately the height of the wall.
Also, note that the bottom timber is set below the finished grade. This gives support weight of the soil pressing against the base of the wall. The finished grade should be sloped away from the wall for positive drainage. Water should not be allowed to pond at the foundation of the wall.
Immediately behind the horizontal timbers, backfill with a gravel blanket and install a 4-inch perforated pipe at the footing of the wall. Any water that gets behind the wall will be directed by the gravel to the perforated pipe and drained away from the wall. The 4-inch perforated pipe should daylight for positive drainage.
Finally, a valley or swale should be constructed at the top of the wall to carry water away from the wall. This will prevent water from getting behind the wall and creating hydrostatic pressure. Hydrostatic pressure is a strong force and can damage the wall.
Setting the timbers on a gravel base and providing good drainage behind the wall will result in a wood retaining wall with a long life.
The front view of the wood retaining wall shows the face of the wall. The spacing of deadman anchors are shown at a maximum of 8 feet. The spacing on low height walls can be less than taller walls.
Consult a Professional Engineer if your wall is more than 4 feet tall. The engineer will evaluate the foundation soil and will specify the spacing of the deadmen.
The notes are typically shown on an engineering drawing of a retaining wall taller than 4 feet.
The soil behind the wall should be properly compacted. Two major reasons retaining walls fail is (1) trapping water behind the wall and (2) poor backfill compaction.
The notes are typically shown on an engineering drawing of a wood retaining wall taller than 4 feet.
The soil behind the wall should be properly compacted. Two major reason retaining walls fail are (1) trapped water behind the wall and (2) the backfill material is poorly compaction.
The reason for constructing a wood retaining wall is to improve your property and to increase the value of the property. A well-constructed wall will do both.
A poorly constructed wall will harm the looks and the value of your property.
Poorly constructed walls will bulge from the weight of the soil behind it. They will tilt from hydrostatic pressure created from a poor drainage system behind the wall. And the timbers will rot quickly if water stands around and behind the wall.
Call a Professional Engineer if you have questions about any aspects of the your wall project.