To reach an earthbag fill of about 30% clay and 70% sand and gravel, blend a clay-rich soil with a sandy one by volume, then add water until a squeezed ball holds its shape but crumbles when poked. Fill that is too wet springs back under the tamper and fill that is too dry will not bind, and both give weak courses that no amount of extra tamping repairs.
Part of Earthbag Building
Fill quality has two separate controls: the proportion of clay to sand, set by blending, and the water content, set by wetting or drying the pile before bagging.
Clay Proportion And Water Content Are Two Different Dials
Clay binds the fill and sand and gravel give it bulk and strength, which is why the target is about 70% sand and gravel to 30% clay. Water is a separate control. Soil compacts best at a particular water content: too wet and the tamper squeezes water around the grains so the bag feels springy, too dry and the grains will not pack together. The hand ball test finds this point without equipment.
Most sites do not offer a perfect soil. Typically one pit is clay-rich and another sandy, and the job is to blend them.
What You Need
- Two or more soil sources of known behaviour, from the tests in Fill Materials and Testing.
- A mixing surface such as a tarp or a flat board, shovels, and water.
- Optional stabiliser: cement at 5-10% by volume for a soil that is too sandy; see Stabilizers.
Method: Blend, Wet, Test, Adjust
Step 1: Estimate each source’s clay content from the jar test.
Step 2: Calculate the blend (see the worked example) and mix dry by volume with shovel counts or equal buckets.
Step 3: Wet gradually while turning the pile.
Step 4: Ball test. Squeeze a handful. It should hold its shape but crumble when poked. If it slumps it is too wet; if it will not hold, too dry or too sandy.
Step 5: Ribbon test as a cross-check: 3-5 cm indicates good clay content.
Step 6: Re-test every time the pit changes. Soil varies with depth and place.
Worked Example
These soil values are illustrative and come from an assumed jar test, not from a measured site.
- Soil A is clay-rich, 50% clay. Soil B is sandy, 5% clay. Target is 30% clay.
- Let x be the fraction of soil A in the blend. The clay content is 0.50 x + 0.05 (1 - x) = 0.30.
- Expand: 0.50 x + 0.05 - 0.05 x = 0.30, so 0.45 x = 0.25, so x = 0.25 / 0.45 = 0.556.
- Result: about 5.6 parts soil A to 4.4 parts soil B by volume, or roughly five buckets of A to four of B.
This works if clay percentages simply average by volume, which is only approximately true. Treat the answer as a starting mix and let the ball and ribbon tests decide.
Diagnosing Bad Fill In The Wall
| Symptom | Likely cause |
|---|---|
| Bag feels springy under the tamper | Fill too wet |
| Bag will not flatten, grains crumble | Fill too dry or too sandy |
| Course sets soft | Too little clay |
| Surface cracks as it dries | Too much clay |
Common Mistakes
- Fixing wet fill by tamping harder. It only squeezes water around.
- Mixing by guess. Count buckets so the blend is repeatable.
- Ignoring weather. A pile left in rain changes water content overnight; cover it.
Safety
Cement and dust
Cement is caustic when wet and its dust irritates eyes and lungs. Wear goggles and a dust mask when mixing it into dry fill, and wash skin contact with plenty of water.
What’s Next
- Learn next: Tamping Technique — compacting the bag you have just filled
- Combine with: Clay Testing — more ways to read a clay