Soil Selection Field Tests

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Soil is an ever abundant resource that has been used for building since the beginning of time. Soil is the Earth’s concrete. It contains gravel, sand, silt, and clay. Silt and clay are the binding materials that stick the sand and gravel together. While silt and clay lose their cohesiveness when wet they can be stabilized using cement, lime, fly ash, and cementitious materials. Almost every soil can be used in situ or after improvements. The best soils for earthen construction is more sandy than silty or clayey. By understanding the characteristics of the soil in your region you can better understand how to create a great building material and a supply chain that maximizes the indigenous materials that are found locally. These initial field tests are a simple free way to learn about the soils around you and how they will perform as a building material. This is also a good excuse to get your hands dirty and play with some dirt.

Grab a few small buckets or containers and a shovel. If you want to test multiple places be sure to document and label where each sample was taken from.

It is important that the soil is taken from 1.5ft( 457mm) below the surface. When making compressed earth blocks YOU DO NOT WANT ORGANIC MATERIAL (roots, sticks, straw, grass, leaves, or decomposing plant matter) in your soil. The problem with organics is that they have a memory of their shape and structure. If organic material is compressed into a block, as soon as the pressure inside the block machines chamber is released the organic material will expand slightly and try to return to it’s previous un-squished shape. Also, we do not want to use topsoil because it has nutrients in it making it valuable for agricultural purposes. When any excavation happens in construction the topsoil should always be scraped off and placed into a separate pile.

All soil samples taken must be dried and sieved to 1/4” (6.35mm) or smaller.

Best Soil Composition

Gravel Sand Silt Clay15% 50% 15% 20%

Wash Test

Take a handful of soil and Smell for Organics

-Soil should not smell musty or rotten

-If the soil smells like potting soil or soil used in agriculture it has organics and should be avoided.

Place your hand with the soil inside of your palm under a slow trickle of water.

Results: Composition wash test a) Soil does not stick to your hands and many large grained pebbles and rocks can be seen - Gravely Soil b) Soil is very granular and easy to wash off - Sandy Soil c) Soil sticks a lot and does not wash away easily from hand - Silty Soil d) Soil makes a thin film and if massaged into palm sticks a lot into the lines of hand - Clayey Soil

Record Results On form Take picture Send to Dwell Earth

Learning PointsRule Out Organics Get Initial Estimations of Soil

Pen TestTake a handful of soil and slowly massage in water until the soil reaches a bread dough like consistency.

Break off a chunk 1/2 the size of your thumb.

Roll it out into a worm the size of a pen on a slightly moistened nonstick surface or in the palm of your hand.

When pen diameter is reached or surpassed pick up the worm and observe the flexibility and cohesiveness by letting it hang off your hand as you move your hand slowly.

Soil Composition Pen Test a) Can not get close to pen diameter - Gravely b) Crumbles just before pen diameter - Sandy c) Rolls out smaller than pen - Silty or Clayey

Soil Plasticity and Cohesiveness a) Could not make soil worm b) Soil worm has little to no flexibility and breaks quickly c) Soil worm is flexible and sticks together even while being moved around and swung slowly

Learning PointsSoil Plasticity and Cohesiveness Soil Composition by Texture

Results:

Record Results

Stick TestTake the remaining soil that has been moistened into bread dough constancy

Make a ball with a 2in (50mm) diameter and stab with a knife

c) Lots of material sticks to knife Clayey

b) Small amount of material sticks to knife Sandy/Clay

a) Little to no material sticks to knife Gravely/Sandy

Results:Soil Composition Stick test:

Learning PointsSoil Composition

Record Results

Shine TestRemake the ball.

Cut ball in half.

Look at flat surface of cross section and move the ball back and forth in the light observing the shining in the light.

Silt vs Clay a) Surface is dull and does not shine in the light - Silt b) Parts of the surface are shiny - Some Clay c) The entire surface is very shiny - Very Clayey

Results:

Learning PointsExamining if there is more silt than

Record Results

Fruit Jar TestFind a flat bottomed clear jar. Use a permanent marker of piece of tape to mark a fill line 2/3 of the way from the bottom of the jar.

Add dry soil to fill line.

Fill the Rest of the Jar with Water leaving 1in (25mm) of space at the top.

Shake vigorously until soil is completely suspended in the water.

Wait 5 minutes and shake again.

Let Settle Overnight.

Do not disturb the jar

Measure how much the settled soil has risen above the fill line

Results: Soil Expansiveness a) Soil did not expand over the line - Non Expansive b) Soil expanded 1/8in over the line - Slightly Expansive c) Soil expanded 1/4in over the line - Expansive d) Soil expanded 1/2in or more over the line - Very Expansive

Fruit Jar Test

Learning PointsExpansiveness of Soil

Record Results

Take a close up picture of the jar.

Make sure multiple samples are labeled correctly.

Estimate the approximate percentage of gravel, sand, silt, and clay

Results: Soil Expansiveness a) Percentage Gravel ________ b) Percentage Sand ________ c) Percentage Silt ________ d) Percentage Clay ________

Fruit Jar Test

Learning PointsSoil CompositionRecord Results

Please take a picture and record all test results send to bobd@dwellearth.com

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