Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
Why identify soil? Before you design a foundation, road or embankment, you need to know what kind of soil is on site. The soil type tells you how strong it is, how much water it holds, and how it will behave under load.
Field tests are quick checks you can do on site with just your hands, some water and a glass jar. No laboratory needed. They tell you within minutes whether a soil is mainly sand, silt or clay.
First, Know the Soil Types
Soils are named by the size of their particles (grains). The picture shows them from biggest to smallest:
All Tests at a Glance
| Test | Sand | Silt | Clay |
|---|---|---|---|
| 1. Feel test | Rough, gritty | Water comes out when squeezed; dries fast, dusts off | Soapy, sticky; dries slowly |
| 2. Thread test | Cannot make a thread | Weak thread, breaks easily | Long, strong thread |
| 3. Shaking test | Very quick reaction (fine sand) | Moderately quick reaction | No reaction |
| 4. Dry strength test | No strength, falls apart | Very slight strength | High strength, hard to break |
| 5. Jar test (settling time) | About 1.5 minutes | 5 minutes or more | 10 minutes or more |
Test 1: Feel Test (Wet and Manipulated Test)
How to do it: Take a small amount of soil in your hand, add a little water if it is dry, and rub it between your fingers.
🟠 Sand
Feels rough and gritty, like sugar.
⚪ Silt
When squeezed, water comes out. It dries quickly and can be dusted off the fingers, leaving only a stain.
🔴 Clay
Feels soapy and smooth. It sticks to the fingers and dries slowly.
What it tells you: whether the soil is mainly sandy, silty or clayey.
Test 2: Thread Test
How to do it:
- Take a piece of moist soil about the size of a 1 cm cube (a sugar cube).
- Roll it between your palms, or on a smooth flat surface, into a thread about 3 mm thick (about the thickness of a matchstick).
- If it does not crumble, fold it, knead it and roll it again. Keep repeating. The soil slowly loses water as you work it.
- Stop when the thread starts to crumble. This point is called the plastic limit. How the thread behaves near this point tells you the soil type.
Test 3: Shaking Test (Dilatancy Test)
How to do it:
- Remove the coarse particles by passing the soil through a 425-micron IS sieve (No. 40).
- Make a small pat of moist soil about 2 cm in size. Add water so that it is soft but not sticky.
- Place the pat in your open palm and shake it sideways, tapping your hand hard against the other hand several times.
- Watch the surface, then squeeze the pat between your fingers.
Very fine clean sand
Quickest and clearest reaction.
Inorganic silt
Moderately quick reaction (e.g. rock flour).
Plastic clay
No reaction. The surface stays dull.
Test 4: Dry Strength Test
How to do it: Pass the soil through a 425-micron IS sieve, mix it with water to the consistency of putty, and shape a small pat. Let it dry completely (in the sun, in air or in an oven). Then try to break and crumble it with your fingers.
| Soil | How it breaks | How the powder feels |
|---|---|---|
| Clay (CH group) | Very hard to break: high dry strength | (stays in hard pieces) |
| Silt | Breaks easily: very slight strength | Smooth, like flour |
| Silty fine sand | Breaks easily: slight strength | Gritty, like fine sugar |
Test 5: Jar Test (Settling Test)
How to do it: Put a spoonful of soil in a tall glass jar (or tube), fill it with water, shake well and place it on a table. Watch how fast the particles settle.
Bonus: Measure the thickness of each layer at the bottom. This gives a rough idea of how much sand, silt and clay the soil has.
Quick Checks: Silt or Clay?
Silt and clay look similar. Use these two quick checks:
- Crush a dry lump with your fingers. A silt lump crushes easily, but a clay lump is much harder.
- Rub test: wet a spot on a lump and rub your finger across it, then look at the surface.
Sand or Gravel?
This one is easy: just look. Grains bigger than 4.75 mm (about the size of a small pea) are gravel. Smaller visible grains are sand.
Practical Site Checks for New Engineers
When you visit a site, you can get a good idea of the soil before taking any sample to a lab. Use these simple checks while walking the site and digging a small pit.
Check 1: Look at the Site
Wide cracks in the ground
Deep, wide cracks in dry weather mean expansive clay (such as black cotton soil). It swells when wet and shrinks when dry, so foundations need special care.
Puddles after rain
Water standing for a long time means clay (water cannot soak in). If the ground dries quickly with no puddles, it is sandy.
Mud on your shoes
Thick, sticky mud that clings to shoes means clay. Loose grains that fall off easily mean sand.
Check 2: Soil Colour
Colour gives a first clue. Use it only as a hint, and always confirm with the hand tests.
Colour is only a clue: the same colour can mean different soils in different areas.
Check 3: Digging Test (Spade Test)
| What you notice while digging | Likely soil |
|---|---|
| Spade goes in easily; sides of the pit fall in | Loose sand |
| Very hard to dig when dry, sticky when wet; pit sides stand straight | Clay |
| Spade hits stones and pebbles | Gravel or murram |
| Soft, dark, with roots and a musty smell | Organic soil (not fit for foundations) |
Check 4: Ball Test and Ribbon Test
Take a handful of moist soil (wet enough to stick together, but not dripping).
- Ball test: squeeze it firmly into a ball in your fist, then open your hand and poke it gently.
- Ribbon test: press the soil between your thumb and first finger, pushing it upward to form a flat ribbon. See how long the ribbon gets before it breaks.
Sand
Ball: falls apart, no ball
Ribbon: no ribbon
Silt
Ball: forms a ball but cracks when poked
Ribbon: short, under 2.5 cm
Clay
Ball: firm ball, keeps its shape
Ribbon: long, over 5 cm
The longer the ribbon, the more clay in the soil.
Check 5: Water Soak Test (Simple Percolation Check)
Dig a small hole about 30 cm deep, fill it with water, and see how long the water takes to soak away:
Check 6: Smell Test
Smell a fresh, moist sample. A strong musty or rotten smell means organic soil (with decayed plants). Organic soil is weak and compresses a lot, so it is not suitable for foundations and should be removed.
- Walk the site: look for cracks, puddles and the soil colour.
- Dig a small pit (about 30–50 cm) and note how easy it is to dig.
- Take a handful of soil: do the feel, ball and ribbon tests.
- Do the thread test and the shaking test on a small sample.
- Pour water in the pit and note how fast it drains.
- Smell the sample to rule out organic soil.
- Note your results, and send samples to the lab for important structures.
Key Takeaways
- Soil type is decided by particle size: gravel, sand, silt, clay (biggest to smallest).
- Feel: sand is gritty, silt dusts off, clay is soapy and sticky.
- Thread: clay makes a strong 3 mm thread, silt breaks, sand cannot form one.
- Shaking: fine sand reacts fastest, silt moderately, clay not at all.
- Dry strength: clay is hard to break, silt and sand break easily.
- Jar: sand settles in about 1.5 min, silt 5+ min, clay 10+ min.
- On site: look for cracks and puddles, check colour, digging, ball and ribbon, water soak and smell.
- These are quick field checks. For design work, confirm with laboratory tests.
References: IS 1498: Classification and identification of soils for general engineering purposes.