Building Boulder Walls
Description/Purpose:
Boulders (field stone) are natural materials that can be used to create retaining walls, edging or to complement landscapes.
Amount, Specifications & Supplies:
Boulders are sized in supply yards by approximate overall length. Common sizes are 6-12", 12-18", and 18-24" for small to medium walls. Larger boulders are available for more sizable walls.
Boulders are sold by the ton.
Tools and Equipment:
- Spade for digging a base trench and backfilling
- Construction backhoe for large walls
- String with stakes for setting straight lines
- Pry bar for moving and rotating smaller boulders
- Skid loader (optional) for moving large boulders and backfilling
- Tape measure
- Tamping bar or power plate tamper for compacting soil
Site Considerations:
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 size of the wall will determine the size of the boulders. Shorter walls can be built with smaller boulders (less than 18"). Taller walls holding back more soil should be built with larger boulders because of their weight and holding ability. A boulder wall's strength also depends on good contact between the boulders and about 25% setback into the area being retained.
Step-By-Step Process:
1. Measure and lay out the area where the boulder wall is to be built. Use stakes and string to create a straight level line. Paint or flags are used on curves.
2. Dig a trench deep enough to accommodate 1/3 to ½ of the depth of the first row of boulders. The first row of boulders must be partially buried in the soil to provide stability and strength for the wall.
3. Set the first course of boulders into the trench. The largest boulders should be used in the first row to anchor the wall securely in place. They should be approximately equal in size so the second course is relatively level. Try to position the boulders with the smoothest side up so the next course has a good contact surface to lay on. Additionally, when fitting the base course of boulders, try to make ends fit as tightly as possible to each other.
4. Backfill soil against the back of the first course and tamp thoroughly. Pack soil up to the height of the base course to provide a level surface for the second course of boulders.
5. The second and subsequent courses should be set back into the hill to provide strength for the wall. The setback is generally 25% (3 per every 1 foot of elevation). Place boulders on top of the base course so that seams in the second course do not match seams of the first course. If seams match, it will create a weak spot in the wall. Select each boulder to fit as tightly as possible with the adjacent boulders. Rotate boulders until a good fit is achieved. The wall will be stronger with more surface area contact between boulders due to the friction that is created. Mixing smaller boulders with larger ones usually weakens a wall.
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6. Continue adding each course, backfilling and tamping soil behind the wall after each course is in place.
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.