Stone retaining walls can bring a great natural look to your yard. Retaining walls keep the soil in place so that you can add a flat patio, pool, or planting area on a slope. Medium or large boulders make great retaining walls that can be attractive, natural-looking, and long-lasting.
You will spend between $100-$600+ per ton on boulder rock for a retaining wall. The price varies both by the rock type you want and your region. Landscapers will charge $45-$75 per hour to install the wall. The average cost for a 25 ft. long wall at $300 per ton of boulder is $5,000
Whether you are building the wall yourself or working with landscapers, doing the details right will give you a long-lasting wall. Keep reading to learn about building a boulder rock retaining wall, why some walls fail, and other questions you may have about these retaining walls.
How to Estimate the Cost of Your Boulder Retaining Wall
An effective retaining wall is built on a gravel base sunk 1-2 feet below the height of the new wall. The wall is built on the base and extends to the soil height. This allows water runoff to flow over the wall and not up to the wall and through it.
To determine the cost of your rock, figure out the length of the wall you want. Based on how much dirt will be supported by the rock, decide how wide the wall will be. Will it be one boulder width or 2-3 boulders wide? Then decide how high the wall will be.
A 2 ft. visible wall should be 1 ft. more below the soil line. In this example, a 25 ft. long wall that is 2 ft. high above the ground and 1 ft. underground and 2 ft. wide needs 11.25 tons of 150 lb. boulder rock.
- 25 ft. long wall x 3 ft. high (2 ft. above ground, 1 ft. below ground) x 2 ft. wide = 150 cubic feet
- 150 cu. ft. x 150 lbs. boulder weight = 22,500 lbs
- 22,500 lbs divided by 2,000 lbs (1 ton) = 11.25 tons
The Labor Cost
Labor cost for landscapers varies by region averaging between $45-75 per hour. It may be even higher in major metropolitan regions. You may also need a structural engineer to confirm the wall design. These run $125-$250 an hour.
Removal and Excavation Costs
The construction of a retaining wall has three main parts: removals, excavations, and construction. You may need to have a tree or fence removed. Then you must excavate for the wall. A 2-ton mini excavator costs $250-$400 per day to rent.
Gravel costs, on average, $30 a ton plus delivery. You may need to use a geogrid for placing the rock and filter fabric for sandy soils. Also, budget for any permits you may need or visits from the power company to mark pipes and wires.
How Do you Build a Boulder Retaining Wall?
The big question is whether you want to do this project yourself or hire a landscape professional. Retaining walls requires moving a lot of dirt, so deciding how you will do the excavating will help you decide whether this is a DIY project or better left to the experts.
Another important consideration is what is being retained. If this boulder retaining wall is decorative, this is more of a DIY project. If the new wall protects a deck from sliding or helps to shield part of your home from shifting dirt, then a professional landscaper may be needed.
What is a Retaining Wall?
A retaining wall protects a flat section of a yard from a 35 degree or larger slope face. Long-lasting retaining walls usually have the soil of the slope run right up to or over the top of the wall. The wall is often at least a foot into the ground, resting on a gravel base.
What is a Boulder Retaining Wall?
A boulder retaining wall is a rock barrier constructed with natural boulders placed with their biggest side into the soil. The larger the stone side into the soil, the stronger the wall. The boulder rocks can be anywhere from a foot long to 6 feet long. Medium boulders are said to be less than 2 feet long.
What are Man-Sized Boulders?
The term man-sized boulder is quite confusing. When working with your supplier, make sure you speak the same language. Some states refer to a man-sized boulder as big enough for a person to lay across it. Other states use a rock description that says:
- Half-Man rock - 6"-12"size of a basketball - 25 - 50 lbs
- One-man rock - 12"-18" - 50 - 100 lbs
- Two-man rock - 18"-28" - 200-700 lbs
- Four-man rock - 36"-45" - 2,000 - 4,000 lbs
Soil Considerations for a Boulder Retaining Wall
The type of soil you have affects whether or not you use a geogrid or filter fabric along the back of the boulder retaining wall. The geogrid helps hold the soil in place, so the rocks fit more firmly. The filter fabric is used with sandy soil to keep the soil from moving through the wall.
Don't use a filter fabric if the soil has a clay content, as the water absorbed by the clay will fill up the fabric filter, which may cause it to bulge out, pushing the boulders forward.
How to Build a Boulder Retaining Wall
Here are the steps for building a boulder retaining wall:
- Remove any trees, fences, old walls, or old concrete patios from the space
- Excavate down and back to create a space for the gravel footing and for the wall itself
- Lay 5" deep gravel at least six inches wider than the boulders
- For sandy soils, lay a filter fabric along the bottom and back of the wall
- If using a geogrid, lay it on the soil or filter fabric
- Use the excavator to move the boulders into place
- Fit the boulders with the large flat sides against the soil wall
- With the excavator, shake the boulders to make sure they are solidly in place
What Type of Rock to Use
Your region will give you the most affordable options. Rock that is not common to your location is possible to use but can be considerably more expensive. Fieldstone will often have regional names as well. Common rock types for boulder retaining walls are:
- Lava rock
How Long Do Boulder Retaining Walls Last?
A well-constructed boulder retaining wall should last 30+ years as natural rock will last for years and years in the elements. The rain and freezing cycles will affect the shape of the boulder retaining wall. Done right, the wall can last well beyond 30 years.
Why do Boulder Retaining Walls Fail?
Boulder retaining walls fail most often because they have been incorrectly constructed. People who cut corners on properly excavating and preparing the footing will have problems. Walls made with the largest rock faces on the outside may be susceptible to moving since the inside of the wall has more cracks and spaces for the soil to move against the rock.
Are Boulder Retaining Walls Good?
Yes, boulder retaining walls are a great option. The natural rock created in the earth will not suffer erosion. The rocks will look great with overhanging greenery or with flowering bushes above them. Water will flow over the top, so the wall maintains its integrity for years.
How High Can you Build a Boulder Retaining Wall?
You can build a boulder retaining wall up to four feet. Over four feet, and you may need a building permit. If you want a wall bigger than five feet, you will want an internal structure keeping everything secure.
If the height is more than three to four feet, consider doing two or three tiers of boulder retaining walls. These tiers will make space for more planting, making a beautiful site. Tiers are more attractive than a large wall.
What is the Most Cost-effective Retaining Wall?
The most cost-effective retaining wall material is poured concrete and interlocking concrete block. Pine logs are cost-effective as well but not as long-lasting. The boulder retaining wall with local fieldstone can be cost-effective as well.
Building a boulder retaining wall will give you a beautiful landscape piece that will last for years. Whether you build it yourself or have the landscape pros do the work, you can have a great retaining wall that is attractive, functional, and can be affordable.
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