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Boulders are rocks big enough to create retaining walls and are used to complement landscapes. The boulders are sold by the cubic yard or sold individually. One cubic yard equals approximately 1 ½ tons or 3,000 lbs. These boulders are double sorted on cement and individually pushed into separate bins. This makes for a cleaner and more consistently sized product than what is typically available from boulder suppliers. For landscape walls this will aid in wall construction by having more right sized rocks to choose from while constructing the wall.

Handling the Boulders

Boulders are very heavy. For example a 12" rock weighs about 100 lbs a 18" rock weighs about 300 lbs and a 24" rock weighs about 700 lbs. An 18" rock may be able to be moved with a pry bar and hand cart. It would be extremely difficult to move a 24" rock with those types of hand tools. We do recommend using a skid steer (Bobcat) loader when handling the 12" to 24" or 24" to 36" boulders. If you do not have experience moving big rocks and do not have a skid steer loader (bobcat), using the 8" to 12" rock is recommended because the next larger size category can include rocks up to 700 lbs.

Choosing Your Size

Consider the height of the structure; and also the size of the rocks you can handle. Most professional landscapers use fairly large rocks to construct walls such as a combination of 12" to 24" and 24" to 36" boulders. Larger well-placed boulders can look natural, simple and attractive. Walls up to about 2½ high can be constructed using the 8" to 12". Higher walls should use larger rocks because of the need for additional structural support or consider a design using 2 levels or terracing with the smaller rocks.

Boulders will be delivered in a large heavy-duty dump truck. Keep in mind that the truck may not be able to get to the location of the wall. Have a couple possible dump locations. Dumping boulders on a cement or asphalt driveway is not recommended since it will damage the driveway. A 20' x 20' area is needed to dump a 10 cubic yard load because the boulders can spread out when they come out of the truck.

The 8" to 12" rock is typically a little large for garden border stones.

A 4" to 8" stone is available on a limited basis. These stones are a good size to use as border stones around beds. They can be hand picked and priced by the rock or when available can be delivered by the cubic yard.

Estimating Your Needs

To estimate how much you need, calculate the surface area as you are looking at the wall by multiplying the height by width to get square feet of surface area. Divide your estimated surface area by the rough estimate of coverage per cubic yard for each boulder size. When using a combination of 12" to 24" and 24" to 36" rocks our estimate for coverage is 15 square feet of surface area per cubic yard. Figure more coverage for smaller rocks (18 to 20 square feet) and less coverage if using bigger rocks (12 to 15 square feet).

Basic Installation Instructions

One approach is to place rocks so the flattest part of each rock faces outward. Piece the rocks together like a puzzle minimizing the gaps while maximizing flat outward facing surface while maintaining good rock to rock contact for structural strength. Use the biggest rocks on the bottom. Angle the wall back a minimum of 3" for every 1' high, fill in behind rocks as you go to stabilize. Use the roundest rocks on top. Other approaches to constructing walls are placing larger boulders in a more haphazard way to create a natural look, or egg shaped rocks can be stacked in a more symmetrical pattern using similar sized rocks throughout.

Suggested Tools and Equipment

• Spade/round shovel for digging a base trench and backfilling
• String with stakes and marking paint for setting up location
• Pry bar for moving and rotating smaller boulders
• Skid loader or backhoe for moving large boulders and backfilling
• Tamping bar or power plate tamper for compacting soil
• Tape Measure

Detailed Instructions

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 1/2 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.
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 inches 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.
6. Continue adding each course, backfilling and tamping soil behind the wall after each course is in place.

Wall Style

In Lower Michigan we have mostly round rocks. Rocks from gravel pits are generally more round than rocks sourced from farm fields. Round or egg shaped rocks can create a unique and attractive wall when done right. Our priority in sourcing boulders is to get rocks which have some irregular shape, so the rocks can be constructed with a fitted look and rocks with attractive color. The boulders are carefully size sorted on cement so they come with very minimal extra dirt or debris.