Good earthbag fill is a sandy clay of about 70% sand and gravel to 30% clay, and you check it with three field tests: a jar test that shows the layers, a ball test that should hold its shape but crumble when poked, and a ribbon test that should reach 3-5 cm before breaking. If the ribbon breaks at once the soil is too sandy and needs clay or cement, and if it exceeds 8 cm the soil is too clayey and needs sand or gravel.
Part of Earthbag Building
Fill selection is done with simple hand tests on a soil sample, repeated whenever the soil source changes.
Three Hand Tests Replace A Laboratory
The fill decides whether a tamped bag becomes a firm, set block or a sack of loose sand. A mix of about 70% sand and gravel to 30% clay compacts firmly and sets hard. You do not need equipment to find out how close your soil is: three tests take a day at most and cost nothing. For a deeper treatment of the jar test itself, see Soil Testing, and for clay-specific checks see Clay Testing.
What You Need
- A clear jar with a lid, water, and a sample of the soil from below the topsoil layer (organic topsoil is not fill).
- A handful of wet soil for the ball and ribbon tests.
- A ruler or a mark on your hand for measuring ribbons.
Method: Run The Three Tests
Step 1: Jar test. Shake soil and water in a jar, then let it settle for 24 hours. Sand drops first, then silt, then clay. Measure each layer’s thickness and compare it with the total.
Step 2: Ball test. Wet a handful and squeeze it into a ball. If it holds its shape but crumbles when poked, you are close to the right mix. A ball that slumps is too wet or too clayey; one that will not form at all is too sandy.
Step 3: Ribbon test. Push wet soil between thumb and finger to make a ribbon. A ribbon 3-5 cm long before breaking indicates good clay content for earthbag.
Step 4: Correct the mix. A soil that is too sandy, with a ribbon that breaks immediately, takes clay or cement at 5-10% by volume. A soil that is too clayey, with a ribbon over 8 cm, takes sand or gravel.
Step 5: Use gravel only for the first courses. The first two to three courses are filled with gravel alone, with no clay or soil, so moisture cannot wick up into the upper courses. This forms the stem wall and moisture break; see Gravel-Filled Stem Courses.
Worked Example
Suppose a jar of 10 cm total settled depth shows 7 cm of sand, 1 cm of silt and 2 cm of clay. These numbers are illustrative.
- Operation: layer height divided by total height, times 100.
- Sand: 7 / 10 x 100 = 70%. Silt: 1 / 10 x 100 = 10%. Clay: 2 / 10 x 100 = 20%.
The sample has 70% sand but 20% clay, below the target 30%, and has silt that the target ratio does not mention. The jar only gives a rough guide, so settle the question with the ribbon test and adjust by trial.
Common Mistakes
- Trusting the jar alone. Layers are hard to read, and the target is a hand-feel check, not a figure to the percent.
- Sampling topsoil. Organic topsoil is a poor fill; dig below it.
- Skipping the gravel stem courses. Soil fill at ground level wicks moisture upward.
Safety
Digging a test pit
Do not sample from a pit deeper than about 1.2 m, or from an undercut bank. Soil bank collapse buries people. Take samples from an open, sloped cut and never alone.
What’s Next
- Learn next: Fill Ratios and Consistency — blending and moisture in depth
- Combine with: Stabilizers — when cement or lime stiffens a weak mix