The dirt is the most fundamental element of earthbag construction. We strive for an optimal, rammed earth-soil ratio of approximately 30 percent clay to 70 percent sand. According to David Easton, in The Rammed Earth House (see Resource Guide), most of the world's oldest surviving rammed earth walls were constructed of this soil mix ratio. We like to use as close a ratio mix to this as possible for our own projects. This assigns the use of the bags as a temporary form until the rammed earth cures, rather than having to rely on the integrity of the bag itself to hold the earth in place over the lifetime of the wall. However, the earthbag system offers a wide range of successful exceptions to the ideal soil ratio, as we shall discover as we go on. First, let's acquaint ourselves with the components of an optimal earth building soil.
Clay plays the leading role in the performance of any traditional earthen wall building mix. Clay (according to Webster's dictionary) is a word derived from the Indo-European base glei-, to stick together. It is defined as, "a firm, fine-grained earth, plastic when wet, composed chiefly of hydrous aluminum silicate minerals. It is produced by the chemical decomposition of rock
2.1: Wild-harvested clay lumps ready for pulverizing and screening.
of a super fine particulate size." Clay is the glue that holds all the other particles of sand and gravel together, forming them into a solid conglomerate matrix. Clay is to a natural earthen wall what Portland cement is to concrete. Clay has an active, dynamic quality. When wet, clay is both sticky and slippery, and when dry, can be mistaken for fractured rock (Fig. 2.1). Sands and gravels, on the other hand, remain stable whether wet or dry.
One of the magical characteristics of clay is that it possesses a magnetic attraction that makes other ingredients want to stick to it. A good quality clay can be considered magnetically supercharged. Think of the times a wet, sticky mud has clung tenaciously to your shoes or the fenders of your car. Another of clay's magical traits can be seen under a microscope. On the microscopic level, clay particles resemble miniscule shingles that, when manipulated (by a tamper in our case), align themselves like fish scales that slip easily in between and around the coarser sand and gravel particles. This helps to tighten the fit within the matrix of the earth building soil, resembling a mini rock masonry wall on a microscopic level.
Not all clays are created alike, however. Clays vary in personality traits, some of which are more suitable for building than others. The best clays for wall building (and earth plasters) are of a relatively stable character. They swell minimally when wet and shrink minimally when dry. Good building clay will expand maybe one-half of its dry volume. Very expansive clays, like bentonite and montmorillonite, can swell 10-20 times their dry volume when wet. Typical clays that are appropriate for wall building are lateritic in nature (containing concentrations of iron oxides and iron hydroxides) and kaolinite. Expansive clay, like bentonite, is reserved for lining ponds and the buried faces of retaining walls or for sealing the first layer on a living roof or a buried dome.
Fortunately, it is not necessary to know the technical names of the various clays in order to build a wall. You can get a good feel for the quality of a clay simply by wetting it and playing with it in your hands. A suitable clay will feel tacky and want to stick to your skin. Highly expansive clay often has a slimy, almost gelatinous feel rather than feeling smooth yet sticky. Suitable clay will also feel plastic, and easily molds into shapes without cracking (Fig 2.2). For the purpose of earthbag wall building, we will be looking for soils with clay content of anywhere from 5 to 30 percent, with the balance made up of fine to coarse sands and gravels. Generally, soils with clay content over 30 percent are likely to be unstable, but only a field test of your proposed building soil will tell you if it is suitable for wall building.
2.2: A plastic, stable quality clay can be molded with minimal cracking.
Silt is defined as pulverized rock dust, although its particle size is larger than that of clay yet smaller than that of fine sand. Silt is often present to a certain degree along with clay. It differs dramatically in behavior from clay as it is structurally inert. It mimics clay's powdery feel when dry, but has none of clay's active responses. It doesn't swell or get super sticky when wet. Too high a percentage of silt can weaken a wallbuilding soil.
Microscopically, silt appears more like little ball bearings than flat platelets like clay. It has a fine roly-poly feel that is designed to travel down rivers to be deposited as fertilizer along riparian corridors. All of nature has a purpose. Silt is just better for growing gardens than it is for building walls. Soils with an excessively high silt content should either be avoided or carefully amended with clay and sand before building with them. Building with soft, silty soil is like trying to build with talcum powder. In some cases, adding cement as a stabilizer aids in increasing binding and compression strength.
Sand is created from the disintegration of various types of rocks into loose gritty particles varying in size from as small as the eye can see to one-quarter-inch (0.6 cm), or so. Sand occurs naturally as a result of eons of erosion along seashores, riverbeds, and deserts where the earth's crust is exposed. Giant grinding machines at gravel yards can also artificially produce sand. Sand (and gravel) provides the bulk that gives an earthen wall compression strength and stability.
Sands have differing qualities, some of which are more desirable for wall building than others. As a rule of thumb,"well graded" (a term used to describe sand or soil that has a wide range of particle sizes in equal amounts), coarse, jagged edged sands provide more stable surfaces for our clay binder to adhere to. Jagged edged sand grains fit together more like a puzzle, helping them to lock into one another. Sand from granitic rock is usually sharp and angular, while sands from disintegrated sandstone are generally round and smooth.
Gravel is made of the same rock as sand only bigger. It is comprised of coarse jagged pieces of rock varying in size from one-quarter-inch pebbles (0.6 cm) up to two- or three-inch (5-7.5 cm) "lumps" or "cobbles." A well-graded soil containing a wide variety of sizes of sand and gravel up to one inch (2.5 cm) contributes to the structural integrity of an earthen wall. A blend of various sized sand and gravel fills all the voids and crannies in between the spaces created by the sand and gravel. Each particle of sand and gravel is coated with clay and glued into place. Sand and gravel are the aggregates in an earthen soil mix much the same as they are for a concrete mix. In a perfect earth-building world the soil right under our feet would be the optimal mix of 25-30 percent stable clay to 70-75 percent well-graded sand and gravel. We can dream, but in the meantime, let's do a jar test to sample the reality of our soil's character.
The jar test is a simple layman method for determining the clay to sand ratio of a potential soil mix. Take a sample of the dirt from a shovel's depth avoiding any humus or organic debris. (Soil suitable for earth building must be free from topsoil containing organic matter and debris such as leaves, twigs and grasses to be able to fully compact. Organic matter will not bond properly with the earth and will lead to cavities later on as the debris continues to decompose.) Fill a Mason jar half full with the dirt and the rest with water. Shake it up; let it sit overnight or until clear. The coarse sands will sink to the bottom, then the smaller sands and finally the silt and clay will settle on top. You want to see distinctive layers. This will show the approximate ratios. To give a rough estimate, a fine top layer of about one-third to one-quarter the thickness of the entire contents can be considered a suitable soil mix. If there is little delineation between the soils, such as all sand/no clay or one murky glob, you may want to amend what you have with imported clay or coarse sand or help stabilize it with a percentage of cement or lime (more on stabilization in Chapter 4).
2.3: The jar Test. Three sample soils and their appropriate uses.
Choose the best soil for the job. In some cases the choice of an earth building soil mix may depend on the climate. After a wall is built and standing for a few seasons some interesting observations can be made. Earthbag walls made with sandy soils are the most stable when they get wet. Cement/lime stucco over earthbags filled with a sandy soil will be less likely to crack over time than bags filled with a clayey soil. The richer a soil is in clay, the more it will shrink and expand in severe weather conditions. When building exposed garden walls in a wet climate, consider filling the bags with a coarse, well-draining soil and a lime/cement base plaster over stucco lath. Dry climates can take advantage of earthen and lime plasters over a broad variety of soil mixes as there is less chance of walls being affected by expansion and contraction.
Soils of varying ratios of clay and sand have unique qualities that can often be capitalized on just by designating them different roles. A soil sample with a high clay content may be reserved for an earthen plaster amended with straw. A sandy/gravelly soil is ideal for stabilizing with a percentage of lime or cement for a stem wall/foundation (Fig. 2.3).
Once we know our soil ratios from the jar test, we can go ahead and make a sample bag to observe the behavior of the soil as it dries and test its strength when cured. Seeing and feeling help us determine if we want to amend the soil with another soil higher in whatever may be lacking in this one, or give us the confidence that this soil is bombproof the way it is. If the soil is hopelessly inadequate for structural purposes, have no fear. Even the flimsiest of soils can still be used as non-load-bearing wall infill between a structural supporting post and beam system (refer to Chapter 5). Later on in this chapter, under "Soil Preparation and Moisture Content," we'll walk through how to make sample test bags.
Gravel Yards: Imported Soil. A convenient and common source for optimum to adequate building soil is often obtained at more developed gravel yards. This material is usually referred to as "reject sand" or "crusher fines." It is a waste by-product from the manufacture of the more expensive gravel and washed sand sold for concrete work. Reject sand is often the largest pile at the gravel yard and is usually priced dirt cheap. Our local reject sand has a ratio of approximately 20 percent clay to 80 percent sand/gravel. The primary expense is in delivery. For us it costs $58.75 to have 15 tons (13.6 metric tonnes) of reject sand delivered ($1.25 a ton for the dirt and $40.00 for the trucking). Another option for good wall building material is often called "road base." Road base usually has a higher ratio of gravel within its matrix, but still can be an excellent source for wall building especially as a candidate for cement stabilization for stem wall/ foundations.
Pay a visit to your local gravel yard before ordering a truckload. Take some buckets to collect soil samples in to bring home for making sample tests. You may find unexpected sources of soil that are suitable for your needs. This has largely been our experience when perusing gravel yards. Since a 600 square foot (58 square meters) structure can easily swallow up 5080 tons (45-73 metric tonnes) of material, it is our preference to pay the extra cost of importing this clean, uniform, easy to dig (FQSS!), suitable clay/sand ratio mix for the sheer labor and time saving advantages. However, the beauty of earthbag building allows us the freedom to expand our soil options by using most types of soil available on site.
Exceptions to the Ultimate Clay/Sand Ratio
Steve Kemble and Carole Escott's Sand Castle on the Island of Rum Cay, in the Bahamas, is a wonderful example of the adaptability of earthbag architecture. All that was available to them was a mixture of coarse, crushed coral and sand so fine it bled the color and consistency of milk when wet. This material was obtained from the commercial dredging of a nearby marina. Because of the coarseness and size variety within the matrix of the fill material, it packed into a very solid block in spite of a clay content of zero percent (Fig 2.4).
A workshop in Wikieup, Arizona, introduced us to a similar situation of site-available coarse granitic sand that in spite of its low clay content (less than six percent) produced a strong compacted block of rammed earth. The sharp coarseness of this decomposed granite fit like a jigsaw puzzle when tamped, locking all the grains together.
Marlene Wulf hand dug into a clay-rich slope of lateritic soil to build a bermed earthbag yurt in Georgia. (Fig. 2.5). The structures at Nader Khalili's school in Hesperia, California, are built of soil with only five percent clay content. Yet this coarse sandy mix has proven to endure shear and load bearing tests that have exceeded Uniform Building Code (UBC) standards by 200 percent.
Smooth surface sands from sandstone are generally considered weak soils for wall building. We've added cement to stabilize this type of earth and made it about as strong as a gingerbread cookie. Occasionally a situation arises where this kind of sand is our only option. Here's where the built-in flexible form allows us the opportunity to greatly expand our options from the ideal soil ratio. This is when, yes, we do rely on the integrity of the bag to a certain extent to stabilize the earth inside. In this case, we may consider building an above ground post and beam infill, or a partially-buried round kiva style structure to support the brunt of the wall system (we would not consider building a dome with this weaker soil).
Water plays a significant role in the preparation of the soil that will become the building blocks of our structure. Although we coined the phrase flexible-form
2.4: Doni harvesting crushed white coral in the Bahamas.
rammed earth technique to describe the method to our madness, we have expanded our soil preparation recipes beyond what has been traditionally considered the ideal moisture content for a rammed earth soil. Before making a sample bag, we need to determine the ideal moisture content for the particular soil we are working with. All soils are unique and behave differently from each other. Each soil also behaves differently when prepared with differing amounts of water.
2.5: Although labor intensive, this carefully excavated site did little to disturb the surrounding vegetation and provided the builder with the soil needed for her construction project.
The water content for rammed earth has traditionally been around ten to twelve percent. This percentage of moisture in an average suitable building soil feels fairly dry. It is damp enough to squeeze into a ball with your hand and hold together without showing any cracks (Fig. 2.6). A simple test is to moisten the soil and let it percolate evenly throughout the soil sample. Squeeze a sample of the earth in your hand. Next, hold the ball out at shoulder height and let it drop to the ground. If it shatters, that approximates what 10 percent moisture content feels and looks like.
2.6: Squeeze a sample of the earth in your hand. There should be enough moisture that the soil compacts into a ball.
This has long been considered the optimum moisture content for achieving thoroughly compacted rammed earth walls and compressed bricks. Ten percent moisture content allows a typical rammed earth soil mix to be pounded into a rock hard matrix and is hence considered the optimum moisture content. We too have followed the optimal moisture content practice in most of our projects.
However, we and fellow earthbag builders have made some discoveries contrary to the "optimum moisture content" as prescribed for rammed earth. We then discovered that our discoveries were previously discovered in laboratory tests conducted by FEB
Building Research Institute, at the University of Kassel, and published in the book, Earth Construction Handbook, by Gernot Minke. We found these test results fascinating for a couple of significant reasons.
Here's what we discovered. We can take a soil sample of an average quality earth mix of 17 percent clay, 15 percent silt, and 68 percent sand and gravel, and add about ten percent more water than the traditional ten percent moisture content prescribed for a rammed earth mix. The result produces a stronger yet less compacted finished block of earth. For those of you who are getting acquainted with building with earth for the first time, this may not seem like a big deal, but in the earth building trade, it flies in the face of a lot of people's preconception of what moisture content produces the strongest block of dirt.
Let's explore this a little further. Rammed earth is produced with low moisture and high compaction. When there is too much moisture in the mix, the earth will "jelly-up" rather than compact. The thinking has been that low moisture, high compaction makes a harder brick/block. Harder equals stronger, etc. What Minke is showing us is that the same soil with almost twice the ideal moisture content placed into a form and jiggled (or in the earthbag fashion, tamped from above with a hand tamper), produces a finished block with a higher compression strength than that of a ten percent moisture content rammed earth equivalent. What Minke is concluding is that the so-called optimum water content does not necessarily lead to the maximum compressive strength. On the contrary, the workability and binding force are the decisive parameters. His theory is that the extra moisture aids in activating the electromagnetic charge in the clay. This, accompanied by the vibrations from tamping, causes the clay platelets to settle into a denser, more structured pattern leading to increased binding power and, ultimately, increased compression strength.
We can take the same soil sample as above with lower moisture content and pound the pudding out of it, or we can increase the moisture content,"jiggle-tamp" it, and still get a strong block. What this means to us is less pounding (FQSS!). Tamping is hard work, and although we still have to tamp a moister mix to send good vibes through the earth, it is far less strenuous to jiggle-tamp a bag than to pound it into submission. Our personal discoveries were made through trial and error and dumb luck. Weeper bag or bladder bag are dirtbag terms we use when the soil is what we used to consider too moist, and excess moisture would weep through the woven strands of fabric when tamped. The extra moisture in the soil would resist compaction. Instead of pounding the bag down hard and flat, the tamper kind of bounced rather than smacked. The weeper bag would dry exceedingly hard, although thicker than its drier rammed earth neighbor, as if it hadn't been compacted as much.
We once left a five-gallon (18.75 liter) bucket of our favorite rammed earth mix out in the rain. It became as saturated as an adobe mix. We mixed it up and let it sit in the bucket until dry, and then dumped it out as a large consolidated block. It sat outside for two years, enduring storms and regular yard watering, and exhibited only the slightest bit of erosion. We have witnessed the same soil in a neglected earthbag made to the optimum 10 percent moisture specification (and pounded mercilessly), dissolve into the driveway in far less time. So now we consider the weeper bag as not such a sad sight to behold after all.
Our conclusion is that adapting the water content to suit the character of each soil mix is a decisive factor for preparing the soil for building. We are looking for a moisture content that will make the soil feel malleable and plastic without being gushy or soggy. The ball test can still apply as before, only now we are looking for a moisture content that will form a ball in our hands when we squeeze it; but when dropped from shoulder height, retains its shape, showing cracking and some deformation, rather than shattering into smithereens (Fig. 2.7).
Personal preference also plays a role in deciding one's ideal mix. A drier mix produces a firmer wall to work on. Each row tamps down as firm as a sidewalk.
Earthbag construction is a seasonal activity. Need we say a frozen pile of dirt would be difficult to work with? Earthbag walls need frost-free weather to cure properly. Otherwise, nature will use her frost/thaw action to "cultivate" hard-packed earth back into fluffy soil. Once cured and protected from moisture invasion, earthbags are unaffected by freezing conditions.
2.7: Three sample balls of soil dropped from shoulder height to the ground. The samples (left to right) show moisture contents varying from 10 to 20 percent.
If you have a big crew capable of constructing several feet of wall height in a day, a drier mix will be desirable. The moister the mix the more squishy the wall will feel until the earth sets up some. With a smaller crew completing two or so rows of bag work a day, a moister mix will make their job of tamping easier. You will have to be the judge of what feels best overall and meets the needs of your particular circumstances.
2.8: Using a sprinkler to pre-moisten a pile of dirt in preparation for wall building.
2.9: In some cases where water is a precious resource or needs to be hauled to the building site, the earth can be flooded and held in check by tending little dams, allowing it to percolate overnight.
Prepping soil (Fig. 2.8). Some soils need time to percolate in order for the water to distribute evenly throughout the pile. High clay soils require repeated watering to soften clumps as well as ample time to absorb and distribute the water evenly (sometimes days). Sandy soils percolate more quickly. They will need to be frequently refreshed with regular sprinklings (Fig. 2.9).
Make some sample test bags. To best understand soil types and moisture content, it's good to observe the results under working conditions, so let's fill and tamp some bags. When making test bags, try varying the percentage of water starting with the famous ten percent standard as a minimum reference point. For some soils ten percent may still be the best choice. For now, lets pre-moisten our test pile of dirt to about ten percent moisture.
Once the proper moisture content has been achieved (plan on a full day to a few days for this), fill some sample bags (refer to Chapter 3 for details on the art of diddling and locking diddles for making the most of your test bag). After filling, fold each bag shut and pin it closed with a nail. Lay the bags on the ground and tamp them thoroughly with a full pounder (see Chapter 3 for description of pounders and other tools). Let them cure for a week or more in warm, dry weather, protected from frost and rain. Thick rammed earth walls can take months to fully cure, but after a week or two in hot, dry weather, our test bags should feel nice and hard when thumped. Vary the moisture content in these test bags to get better acquainted with how they differ in texture while filling, how they differ while being tamped, and what the final dried results are.
After the bags are sufficiently cured, we test each one by kicking it, like a tire. We jump up and down on it and drive three-inch (7.5 cm) nails into the middle of it. If the soil is hard enough to hold nails and resist fracturing, it is usually a pretty good soil. If the soil is soft or shrunken, it will need to be avoided or amended or used as infill for a post and beam structure. We do these tests to determine which moisture ratio is best suited for this particular soil (for more scientific code-sanctioned tests concerning modulus of rupture and compression, we suggest consulting the New Mexico Uniform Building Code) (Fig. 2.10).
Our personal feeling is that earthbag construction should be tested as a dynamic system rather than an individual unit. It is the combination of all the ingredients — bags, tubes, soil, barbed wire, careful installation, and architectural design — that
2.10: (top) This informal test demonstrates the weight of a 3/4-ton truck on top of a fully cured earthbag, resulting in no deformation whatsoever. 2.11a: (top right) The owners of this tall earthbag privacy wall, located on a busy intersection in town, woke up to find that the earthen plaster on one area of their wall had fallen off. The reason is shown in the next picture. 2.11b: (lower right) During the night, an unintentional "test" was conducted by an inebriated driver, which helped answer our questions about the impact resistance of an earthbag wall — the wall passed; the car failed.
determine the overall strength of an Earthbag building (Fig. 2.11a & b).
Earth is a simple yet complex substance that you can work with intuitively as its merits become familiar. Experimentation is a big part of the earthen construction game. Once the test bags have dried, and the right soil mix and the suitable moisture content for the particular job has been chosen, the building crew is ready to go to work. A team of six to eight people can go through about 25 tons (22.5 metric tonnes) of easily accessible material in three days. Kept pre-moistened and protected with a tarp, it's ready for wall building throughout the week. If the building process is simple, the progress is quick.
2.12: Bag ensemble (left to right): way-too-big; 100-lb. misprint; 50-lb. misprint; 50-lb. gusseted misprint; 50-lb. burlap.
Was this article helpful?
How would you like to save a ton of money and increase the value of your home by as much as thirty percent! If your homes landscape is designed properly it will be a source of enjoyment for your entire family, it will enhance your community and add to the resale value of your property. Landscape design involves much more than placing trees, shrubs and other plants on the property. It is an art which deals with conscious arrangement or organization of outdoor space for human satisfaction and enjoyment.