Boulder Retaining Wall: Things You Should Know
A boulder retaining wall can bring structure and definition to your landscape design. Unlike a concrete block retaining wall, a boulder retaining wall retains a natural look that blends rather than competes with other outdoor features.
Boulder and other rock retaining walls that are well-built should last upwards of 100 years or more. These walls are some of the most long-lasting landscape structures, lasting long after houses and barns have been destroyed. Many of these historic large rock retaining walls are still in use today. One of the earliest surviving dry stacked rock retaining walls is Hadrian’s Wall built in 122 C.E.
This article will guide you through everything you need to know about boulder or large rock retaining walls. With this information, you will be able to decide if this rugged wall form is a good option for your home landscaping.
What is a Boulder Retaining Wall?
A boulder retaining wall is one that you build using large rocks or boulders. Different from other stones, boulders are a distinct variety of rocks. Boulders are large rocks or rock fragments which experts define as larger than 10 inches (25.6 cm) in diameter. Boulders make one of the best building materials because they are solid, resistant to erosion from the weather, and they can withstand an immense amount of pressure from both the soil behind the wall and small movements in the earth.
Pros and Cons of a Boulder Retaining Wall
Boulders or large rocks work well as wall-building materials. But there are factors which mean these are not a good option for some home landscapers.
- Visual appeal – Boulders have an innate beauty and natural style which creates stunning retaining walls. These walls have an organic style that blends into the outdoor environment.
- Durability – The size of boulders makes them impervious to erosion caused by the wind and the rain.
- Versatility – No two boulders are alike, so you can create unique and distinctive retaining walls that are unlike anything you see in other yards.
- Maintenance – A rock retaining wall will require little maintenance in the future if it is well-built in the first place.
- Cost – Depending on the area you live, a large rock retaining wall can be one of the most cost-effective options among different boulder rock wall styles. The cost of boulders will vary by location. It will cost less in areas where natural boulders are easy to obtain.
- Space – Retaining wall boulders are large and can create a massive sized retaining wall. In small yards, they may not be an appropriate option because of the available space. Many boulder retaining wall construction uses heavy duty equipment which will require wide entrances into your yard.
- Height – The type of construction of boulder retaining walls limits the prospective height of these walls. A concrete retaining wall is a better option if you need a tall retaining wall.
- Construction – Depending on the size of the boulders, these large rocks are not always a good DIY option. Lifting them and building the wall may require heavy duty equipment and expertise.
- Cost – Living in a part of the country where boulders are not prevalent will increase your cost for these rocks. Also, if you use large boulders you will need professional expertise and equipment. This will increase the cost of your retaining wall.
Construction Method for Boulder Retaining Walls
There are two main types of construction for boulder retaining walls: dry-stacking, also called the gravity method, or with mortar known as the wet-laid method. You can also choose an option that combines both methods. Some contractors may use one method and be unfamiliar with any other. Talk with them first to make sure you are on the same page regarding which building method they prefer and why.
Dry Stack (Gravity) Method
Using gravity as a way to keep the boulders in place is one of the easiest and most popular ways to build a rock retaining wall. This method relies on the weight of the rocks to retain the shape of the wall. Dry stacking requires rocks stacked in an interlocking pattern. This interlocking is achieved by using specific rock contours, rock weight, and frictional interaction.
This method is a good choice if you are using heavy boulders and have moisture in the ground. The cracks between the rocks allow water to seep through and relieve the pressure that would build up behind the wall. This innate drainage system will maintain the integrity of the wall structure and increase its lifespan. Make sure that you or your contractor packs small rocks rather than packed soil behind the wall for increased drainage. This is most important if you have wet soil or heavy rainfall in your area.
This method can be more difficult than using mortar because it requires time, patience, and expertise to find rocks that fit together in a way that is seamless and solid.
Wet-Laid (Mortar) Method
Some contractors may opt for using mortar or cement if they are constructing a retaining wall with boulders. The mortar provides extra strength to the wall. This is a good method if you live in a dry environment and do not worry about moisture buildup in the soil.
You can still build a rock wall with mortar if you live in a moist environment, but you must add drainage. All retaining walls need drainage stone behind them to keep heavy moist soil from pressing into the wall. Depending on how much water is in your environment, you may also need to add drainage pipes or holes to allow water to come through the wall and release the pressure that is building up behind it.
The type of mortar you use is important in this method. Use the type “M” or “S” mortar for your project. Type “M” is the strongest mortar type as it has the highest concrete content. This mortar works well for stone as it mimics the strength of the stone itself. Type “S” is also appropriate for smaller boulder walls and is able to withstand excessive moisture better than type “M”.
You can also consider using a combination of both methods to achieve a natural look with increased strength. This method utilizes mortar along the rear of the wall. This creates the illusion of a dry-stacked wall in the front. You can also use a mortar that is pigmented to match the stones to decrease the visibility of the mortar lines throughout the wall.
Rocks for Boulder Retaining Walls
Retaining wall boulders come in many varieties including granite, sandstone, quartz, limestone, basalt, and bluestone. There are three main types of boulders that landscapers use to create retaining walls: fieldstone boulders, quarry boulders, and water-washed boulders. Both fieldstone and quarry boulders have sharp edges, flat planes and non-symmetrical divots.
- Fieldstone Boulders – Fieldstone boulders are found on the earth’s surface. Character and age are intrinsic to these boulders. They often feature moss and dirt as they have been subject to outdoor weather conditions.
- Quarry Boulders – Quarry boulders are dug up from the ground and have less age and character than fieldstone boulders. These rocks are easier to work with because they have fewer angles. These stones will also create a more stable wall if you are an inexperienced DIYer.
- River Rock – River rocks are rocks that have been washed in the river over time. The constant running water smooths the edges of the rock to create the rounded style that we associate with river rocks. These rocks come in various sizes including those that experts define as boulders. River boulders are the best option if you want to create a wall with rounded stones.
Other Tips When Considering a Boulder Retaining Wall
Consider these tips to ensure that you know all the facts before you invest in a retaining wall with boulders.
- Wall Color – Natural rock retaining wall colors vary to a wide degree. You can use boulders that are off-white, pink, orange, gray, black, or reddish. The color of your wall will have a huge impact on the look of the finished wall.
- Wall Pattern – There are two main types of patterns in large rock retaining wall configurations. One is a modular pattern which involves using boulders that have a similar size and shape. The other is a random pattern configuration that employs boulders of many shapes and sizes. This type of wall takes more time, planning, and skill to implement.
- Permits – Most retaining walls over 4 feet need to be permitted.
- Professional Help – Large boulder retaining walls are difficult for the average DIYer because of their weight and size. You also may need the help of a licensed contractor to help with the construction.
Boulder Retaining Wall Cost
As with everything, the cost for a boulder retaining wall varies depending on many factors. According to HomeGuide, the range for the price of a rock wall is between $2,000-$10,000. The average rock retaining wall cost is around $6,000. Boulders are priced around $50-$200 per linear foot (including labor and materials) or $100-$350 per ton for just the boulders. The cost for a large boulder retaining wall will set you back more at $600 per ton.
Factors That Impact the Cost of a Boulder Retaining Wall
Keep in mind these extra factors that will determine the cost of the completed retaining wall.
- Size and Design – The length and height will impact the cost of the wall as more materials and labor are needed for the wall. The complexity of the wall design will impact the design. A straight wall will cost less than a wall with curves, columns, and stairs.
- Foundation and Drainage – Higher walls require deeper foundations. Most mortared walls will require drainage of some kind and increase the cost.
- Labor Costs – Plan on paying at least $40-$80 an hour for labor or $10-$30 per square foot.
- Engineered Plans – Plan on hiring a structural engineer to draw up plans for your wall if it is over 4 feet high.
- Heavy Construction – Depending on your site, you may need ground cleared, old walls removed, or the site leveled. This will require heavy equipment and operators.
- Geogrid – Geogrid is a flexible mesh that builders put behind rock walls. This helps to stabilize the soil behind the wall. This is helpful if the soil is sandy and fine, but it is not always necessary to use Geogrid. It can increase labor costs up to 50%.
Boulder Retaining Wall Ideas
Now that you know the basics about boulder retaining walls, here are some designs that will excite you with the possibilities.
Variegated Boulder Retaining Wall
This landscape designer used boulders in variegated hues of red, pink, off-white, and a soft bluish-green. The boulder wall climbs up the hill in a way that blends with the surroundings. These designers softened the look of the wall with additions of plants and flowers throughout.
Dry Stacked Flat Boulder Retaining Wall
This contractor utilized the dry-stack method for this retaining wall. It also features a random pattern where the diverse stone sizes fit together like a puzzle. The boulder variety is an example of the deep orange hues common in fieldstone.
Mortared Stone Retaining Wall
This wall features mortared construction and smaller round boulders. Small stones mix with the boulders to create a stone wall with varied shapes and sizes.
Boulder Retaining Wall for Seating
Add extra seating to your space by adding a curved boulder retaining wall around a firepit. Use boulders that are around the same size for a uniform appearance and flat on the top for comfortable seating.
Mixed Material Retaining Wall
Boulders have a distinct natural form, but they work well with many other materials. In this design, the landscaper combined flat, dry-stacked boulders with custom weathered steel.