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Building a Dry-Stack Wall
by by Krista Kugler-Quinn - March 2012


When the concrete block retaining wall behind the author's house collapsed during a record-breaking
rain event, it left a pile of debris and mud right up to the house’s foundation.

When the concrete block retaining wall behind our house fell during a record-breaking rain event, my husband, Jeff, and I were devastated. We had questioned the integrity of the wall but never expected the whole 110 x 4 foot structure to collapse at one time leaving us with a huge pile of mud and block right up to our house foundation. When we recovered from the shock, we decided to make the most of the disaster by using it as an opportunity to turn an unsightly service area into a focal point of our landscape.

After much research, we decided to build a native stone dry-stack retaining wall to replace the block wall. Often there is a misconception that dry-stack walls are less stable than mortared walls, but, if constructed properly, dry-stack walls are actually very sturdy. In fact, many centuries-old dry-stack walls still stand today. The key to their stability is drainage. Dry-stack walls are different from most other types of retaining walls because they are designed to let water run through them. Thus, water pressure does not build up behind the wall.


The diagram below gives a cross-section view of a properly constructed dry-stack wall.


Selection And Construction
Jeff and I thought a native stone wall would enhance our landscape and look more appropriate in our rural setting. Additionally, building a dry-stack wall requires only simple construction skills — a definite plus, since my husband intended to do most of the work himself. Finally, the materials for dry-stack walls cost substantially less than other types of retaining walls. We also figured that we could skimp on some materials by using the natural abundance of rocks from our own property.

Our first step was to construct an open trench that drains toward either side at the top of the slope behind our house. This was needed to divert water away from the wall. We then had to finalize the design of the wall. In some areas, there are building codes for retaining walls, but we were allowed to come up with our own design. Large walls (over 3 or 4 feet tall) may need to be designed by an engineer. We simply drew detailed pictures and diagrams of how the wall would be built, how the site would be prepared and what the wall would look like from various vantage points. Based on these plans, we selected rock from a local rock yard and ordered about 1 ton of stone for every 30 to 35 feet of wall face. We also had two dump truck loads of washed 3/4 inch diameter gravel delivered.


A trench that drains to either side was dug at the top of the slope to divert water away from the wall.


The bank was excavated to allow plenty of room for the wall thickness and gravel backfill. Angling the bank from bottom to top made it more stable.

We then began preparing the site. First, we had our underground utility lines marked. Next, using a small rented backhoe, my husband removed the pile of debris from the collapse of the block wall and began shaping the bank. He excavated so that the bank angled back from bottom to top leaving plenty of space for the wall thickness and about a foot of gravel backfill. We laid heavy-duty, porous landscape fabric along the bank with about 3 extra feet at the top edge. The fabric helps stabilize the bank and prevents soil from saturating the gravel backfill, which is needed for drainage.

Usually at this point, a trench would be dug for the wall footing, but we were able to use the existing concrete footing from the block wall. Generally, a 2- to 4-inch footing made of compacted pea gravel, rock dust or road base is sufficient for a wall less than 4 feet tall in the Southern regions of the U.S. Ideally, footings should extend a few inches in front of and behind the wall. Footings should have a 5-degree pitch back toward the bank.

The next step was to place the foundation rocks. Jeff used our largest, heaviest rocks for the foundation. Because the width of the foundation should be about one-half of the wall's eventual height and we planned to build a 4-foot tall wall, he made our foundation 2 feet wide. In some cases, he used two stones to reach all the way back to the bank. The top surfaces of these stones were pitched back toward the bank. The stones were set tightly abutting one another. Stone chips and shards were packed into the gaps between stones to make a solid surface.
 


Landscape fabric was laid over the bank to help stabilize it and prevent soil from eroding into the gravel layer. A perforated drainpipe was installed near the bottom of the gravel layer to aid drainage during heavy rain events.

Vertical joints were offset in the wall for stability. Small rocks were used to help level and secure larger rocks. After each course of stones was set into the wall, washed gravel was used to backfill between the wall and the bank.


No Turning Back
Once the foundation was constructed, Jeff began the major construction phase of the project. He sorted our rocks, reserving large, flat stones for capstones, long stones for deadmen, blocky stones for the wall ends and small scraps for shims and wedges. He then began setting layers of stones the whole length of the wall. After each course was set, he backfilled between the wall and the bank with gravel. He maintained about 1 foot of gravel between the wall and the bank. Each course of stone was set back about 1/2 inch so that the wall leans back into the bank at a 5- to 10-degree angle for stability.


Long stones that reached all the way back to the bank were placed throughout the wall to serve as deadmen. Each course of stone was set back about 1/2 inch so that the wall leans back into the bank at a 5- to 10-degree angle for stability.


Completing the last 6 inches of the wall was the most time consuming and tedious part of construction. Just the right mix of stones was needed to make the top of the wall level from side to side with a slight pitch back toward the bank.

As the rocks were set, Jeff was careful to offset the vertical joints by placing two stones over one. Protrusions were trimmed with a chisel to make stones fit better. Where stones did not make a tight fit, the gaps were plugged with small stone chips. Walking on top of the wall helped determine when a stone was unstable. Small wedge stones were used to adjust the angle of the rocks so that the top surface was pitched back toward the bank slightly.

When backfilling the area between the wall and the bank, a piece of rebar or a tool handle was used to force gravel into all of the cracks and crevices on the back side of the wall. Walking on top of the gravel layer and spraying it with water also helped to settle it. A hand tamper was used to compact the gravel layer after each course of the wall was laid.

We installed a 4-inch perforated drainpipe near the bottom of the wall in the gravel layer. Jeff made sure the pipe had at least a 1 percent slope, so that it drained to an exit point at the low end of the wall. Long stones that reached all the way back to the bank were placed throughout the wall to serve as deadmen, which help to anchor the wall.


Wrapping It Up
Finishing the last 6 inches of the wall was probably the most time consuming and tedious part of the construction process. Just the right size stones were needed to fit underneath the large, flat capstones so that the top of the wall would be level from side to side with a slight pitch back toward the bank. Some trial and error was required to find the right mix of stones. Before laying the capstones, the landscape fabric was folded forward to cover the gravel layer, thus sealing the gravel off from any eroding soil. When the capstones were placed, they secured the fabric above the gravel.

Jeff constructed a stone bench into the wall by setting the top half of the wall back about 18 inches where the bench would be. He also capped the bottom section of the wall to serve as a seat. We finished the landscaping project by leveling a small planting terrace at the top of the wall and then riprapping the rest of the slope behind the terrace. At our back door, Jeff constructed a flagstone patio and laid pea gravel in the walkway between the house and the wall.

The project was incredibly successful. Our new back patio is secluded and pleasant, giving us a great place to relax, entertain friends and view wildlife. Additionally, the wall is very attractive and has already stood up to several large rain events that flooded other parts of the town near where we live.


The new retaining wall is both sturdy and attractive. Jeff constructed a stone bench into the wall to
accommodate visitors by setting the top half of the wall back about 18 inches for the backrest.


(From State-by-State Gardening February 2005. Photos by Krista Kugler-Quinn.)

 


Krista Kugler-Quinn is a freelance home and garden writer and Arkansas Master Gardener. She lives and gardens in Conway, AR.

 

       

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