Photo by Morgan Caraway
An astonishing variety of things can be built from bags filled with earth, also known as “earthbags.” One of the most versatile forms of natural building, earthbags can be used in many applications that would be too wet for most conventional and natural building methods. Earthbag pools are a perfect demonstration of this.
Before I describe two pool projects I’ve been involved in, let’s discuss some of the advantages of earthbag building.
From the Ground Up
Earthbag structures are strong, which is why they’ve traditionally been used for military fortifications and flood control projects. The “superadobe” technique produces structures from an unbroken spiraling polypropylene tube with barbed wire between the layers. Other builders prefer “hyperadobe,” or long polyester tubes made of open netting, which allows the fill to blend between the layers, thereby holding them together. To build our earthbag pool, we filled and stacked 14-by-26-inch polypropylene bags stabilized with rebar. These rectangular bags produce uniform walls 10 to 11 inches thick — about 1-foot-thick after they’ve been coated with protective plaster.
The building crew fills bags with inorganic material, and carefully places the pool liner after ensuring no sharp rocks threaten its integrity.
Almost any kind of inorganic fill dirt can fill the bags we use. The polypropylene bags offer mechanical support, so the mix inside doesn’t need to be a specific ratio of sand and clay, unlike with superadobe and hyperadobe. Traditional cob and straw bale construction covers a lot of straw in wet mud. If the mix isn’t allowed plenty of time to dry, or if it absorbs too much humidity from the air, the straw could rot and mold. This poses a threat to the air quality and health of those living within the structure. With a standard earthbag, no such danger exists, because there’s no organic material. Walls can be built as fast as a team can construct them; a large group can put up an entire earthbag structure in one day.
My wife and I built our first earthbag house in 2009. We’ve worked on many earthen structures since then, including our own earthship-inspired house and an earthbag tiny house, and we now operate the Sustainable Life School to teach natural building methods to others.
Photo by Morgan Caraway
For years, I had an ambition to build a pool below the natural spring on our property. I knew that filling the pool would be easy, because gravity would move the water. (I try to take advantage of natural forces in everything I build. My tiny house also has gravity-fed water, as well as geothermal refrigeration.) My dream finally came true in 2018. Here are the steps my wife, Mary Jane, and I took to build our spring-fed pool.
The author’s earthbag pool became a community project.
Planning. I was thinking we’d build something small, but Mary Jane wanted bigger — and she was right! After agreeing on the size of our pool, research revealed other decisions we needed to make. I learned that gradual curves can become slipping points when algae grows on the pond liner. Terraces are more practical and allow for clearly demarcated depths for swimmers of different skill levels. We bought a long piece of 4-inch PVC for a drainage pipe, and, knowing we’d need to drain the pool occasionally, we also bought a tee and a clean-out plug. We planned to place the drainage pipe at the lowest point of the excavation to make the pool easy to drain when it needed to be cleaned. Mary Jane learned that a bog with aquatic plants might cut down on the algae, so we decided to build one out of rocks and cement-stabilized earth.
Excavation. We needed a liner for our pool, as our soil is porous. Most of our pool footprint could be created by excavation, and we hired a local fellow with a backhoe attachment on his tractor to dig it. Although I applaud purists who argue that modern tools and materials shouldn’t be used in natural building, I believe the best use for fossil-fuel energy and synthetic materials is for the cause of sustainability. Digging a pool-sized hole by hand would take a long time and a lot of sweat.
Bagging. We held a combination workshop and workday for our students and friends, and divided them into two groups. One group filled the bags, and the other placed them. Although the excavation had created most of the pool shape, we still had to build up height with three rows of earthbags in some areas. For extra support against the weight of the water within, we hammered short lengths of 1⁄2-inch rebar through the earthbag wall, making sure not to leave any metal above the tops of the bags. Around the big curve, we added 2-foot lengths of rebar every 2 to 3 feet. Note that backfilling against a wall will also make it resist pressure from the water side.
Lining. After making sure there were no sharp rocks sticking up inside the excavated area, we placed the liner. We chose an HDRPE (high-density reinforced polyethylene) liner because it’s less expensive and lasts longer than EPDM (ethylene propylene diene monomer), and resists rips and tears. Such liners are generally guaranteed for 40 years. Order extra feet all around to make sure the liner is big enough for your planned pond. We pushed down the liner as far as it would go in our excavation, and then we
delicately and precisely cut the hole for it to slip over the drainage pipe. Then, we assembled the rest of the drain and the overflow pipe. The height of the overflow pipe will establish the depth of the water in your pond, because the water will flow out of the pipe when it reaches the lip (see illustration, below).
Diagram by Morgan Caraway
Our wonderful community mamas covered the outer edge of the membrane with rocks for extra protection and beautification.
The pool has performed swimmingly, and has become a community center during our scorching hot and humid summers. I usually have to clean the pool weekly at the peak of summer to prevent excessive algae growth. Luckily, the HDRPE liner we chose is durable, and scrubbing doesn’t endanger it.
The most expensive part of our natural pool was the $550 liner. We also paid about $300 for the excavation, and a small amount for inexpensive PVC pipe. Our total cost was about $900.
My friends and students Chris and Lisa Huskey built their own earthbag pool in steamy South Carolina. Because their pool is aboveground, the design incorporates earthbag buttresses to counteract the weight of water pushing against the wall.
Photo by Morgan Caraway
Chris and Lisa measured their pool radius with a rope tied to a center post — 9 feet, because they wanted an 18-foot-diameter pool — and marked it with a permanent marker on the plastic they’d dropped as a moisture barrier. This easy technique works great for marking the footprint of any rounded earthbag walls. Just measure to the inner edge, or whichever side you’ll be placing bags from, so you can easily see the line you need to follow.
Photo by Morgan Caraway
Next, they built the wall, including the buttresses. When building the wall, they knew exactly where to place the pump inlet and outlet. To remember such details, I write notes on the first couple of courses with a permanent marker to remind me what needs to go above. That way, the crew won’t forget anything important.
After they finished bagging, they placed sheet insulation against the inside wall, and covered the grass in the bottom of the pool area with dirt to protect the liner and give it plenty of support. (Thousands of gallons of water are extremely heavy.) They then placed the liner, which has a rope tensioned around the top to keep it in place, and hooked up the pump. Pool party!
The walls of this above ground pool will need to be plastered to protect he earth bags from the sun’s damaging ultraviolet rays.
Chris and Lisa’s earthbag pool cost about $1,300, including delivery of fill dirt and $150 for a liner. Damage caused by the sun’s ultraviolet rays is one of the biggest dangers to an earthbag wall, so be sure to plaster the walls to protect the bags from UV degradation. Simple earth plaster isn’t sufficient, because it will be washed off by rainfall, so the plaster you use will need to be cement-stabilized or lime-based.
As you can see, earthbags can be a cost-effective way to build a pool, and they’re much stronger than the average aboveground pool. If you properly protect the earthbags from sun, they can last indefinitely in almost any environment.
We urge you to take safety precautions during and after any earthbag pool build. Fence the project area as soon as possible to keep out children. I know parents who’ve lost unsupervised children to bodies of water. As a builder, it’s your responsibility to follow all codes and requirements, and neither I nor the Sustainable Life School take any responsibility for the use or misuse of this information.
Morgan Caraway and his wife, Mary Jane, live off-grid and conduct natural building classes and sustainable living workshops both in person and online, including “Building an Earthbag Tiny House,” at Sustainable Life School.