Timber Wall Installation
Landscape timbers are frequently used to create retaining walls, steps or planting boxes.
Amount, Specifications & Supplies:
Pressure treated timbers are recommended and usually come in 5" x 6" x 8' or 6" x 6" x 8' sections. Untreated wood will not hold up when exposed to constant moisture and weather and should not be used for retaining walls.
The specific construction detail is dependent on the size of the wall and existing site conditions. Timber walls should be professionally designed because of the complexity of their construction. Approximately 25% of the timbers needed will be hidden behind the wall to tie back or hold the wall in place.
3/8" galvanized spikes 10 - 12" long - a minimum of 4 spikes per 8' timber is recommended, with 2 spikes at the tieback connection points.
Tools and Equipment:
- Flat back shovel or backhoe for digging trench
- Spade for backfilling
- 1 to 3 lb hammer or sledgehammer for driving spikes
- Level for setting timbers
- Chain saw or large circular saw for cutting timbers
- 3/4" drill and 3/8" extended bit for piloting holes
- String with stakes for setting straight lines
- Tape measure
- Tamping bar for compacting soil
The size of the wall will depend on the site. It is usually better to build two shorter, terraced walls than one tall wall. The tiebacks are composed of timbers called headers and deadmen. The number of headers and deadmen needed for the wall are determined by the size of the wall, soil conditions, drainage, topography and anticipated loads (surcharge) above the wall. If soil is not well drained, a drainage aggregate, pea gravel, sand or an angular clear stone should be placed directly behind the wall to reduce water pressure on the wall. A 4" perforated drain tile is used to move water away from the base of the wall.
1. Measure and lay out the area where the timber wall is to be built. Use stakes and string to create a perfectly straight line and to determine the elevation for the base course of timbers. Timbers do not work well for curved walls but can be used to develop angular or modular walls.
2. Dig a trench approximately 8" wide (14" if a drain tile is used) and 1-2" deeper than the height of the timber, usually 7" deep. Apply 1-2" of sand to the base of the trench. The sand makes leveling of the base timbers much easier.
3. Tiebacks, headers and deadmen are portions of timbers placed into the soil behind the wall to provide support for the wall. Excavate for headers and deadmen into the hill.
The size, length and location of headers and deadmen will be design specific. Headers should extend past the wall face 1½ - 2" to add strength and prevent splitting.
Several header configurations are shown.
4. Place and level the first course of timbers using the sand in the trench. Be sure to have this first course perfectly level. This first course will be completely buried in the ground. When timber ends fit together poorly, use a chain saw or large circular saw and run down the seam between the timbers. This will make the timbers match up better and will avoid gaps. However, this process does remove some of the treated lumber.
5. Lay the second course of timbers over the base course, staggering the seams. This staggering should continue throughout the wall, so that the seams of two adjoining courses are never in the same place. Each course should also be set back approximately 1/2" from the course below. This is called battering. Leaning the wall into the slope in this way adds strength to the wall.
6. As each course is added, drive 3/8" galvanized spikes through to the course below. Spikes should be placed 6-12" from each end with 1-2 additional spikes in the center of the timber. Predrilling makes this job much easier.
7. Soil should be backfilled behind each course and tamped thoroughly. If the soil is not well drained, or the wall is over 5' tall, place a drainage aggregate 8-12" wide against the wall to collect surface water and reduce water pressure on the timbers. A 4" perforated tile should be placed at the bottom of the wall to collect the water as it moves through the drainage aggregate.
8. The third and subsequent courses of timbers will require headers and deadmen. For an average small wall under 5', headers and deadmen should be placed after each 8' timber in every third course of the wall face. If enough headers and deadmen are not used, the wall will fail, a very common occurrence for walls not professionally designed.
Headers should be secured to timbers with two spikes in the face of the wall and to deadmen in the soil behind the wall. The length of the header depends on its placement in the wall, the height of the wall, the soil type, soil and surface water movement, and any surcharge at the top of the wall. Surcharges can be roads, homes, parking lots, and other walls. Think of the soil being retained as the cone formed when an elevator is used to make a pile.
The headers need to be long enough to anchor the wall into that cone shaped pile. The area inside the cone is stable soil. The area outside the cone is active or unstable soil. Headers at the bottom of the wall will be shorter than those near the top of the wall.
Deadmen are placed to cross the header making a T. While deadmen length is design specific, they are usually not less than 60% of the header length. The top two courses of timbers can be set without tiebacks to avoid interference with planting above the wall.
9. Backfill soil after each course is secured and tamp thoroughly.
This implementation report is based on landscape projects completed by University of Minnesota students enrolled in landscape design and implementation courses, Department of Horticultural Science.