How to Determine Soil Types With the Atterberg Limits Test

Knowing how fine soils behave – and whether they are clay or silt –helps engineers predict how soil will respond under various circumstances. Atterberg “limits of consistency” are seven international indeces devised by Swedish scientist Albert Atterberg. They are performed on fine-grained silt and clay soils to determine their relative activity in relationship to moisture content. The liquid limit is the one test most often used today to classify fine soils, though the plastic limit is also still used and, rarely, the shrinkage limit. The liquid limit identifies the level of moisture at which a particular soil changes from a plastic or flexible state to viscous fluid and begins to flow.

Prepared soil sample
Porcelain (evaporating) dish
Wash bottle filled with distilled water
Small spatula
Cellophane wrap
Balance scale
Four moisture cans
Liquid limit device
Flat grooving tool with gauge
Drying oven

Instructions

- 1
  Begin with a soil sample that has been air-dried, pulverized and passed through a Number 40 sieve. Place the sample in the porcelain dish. Add water and stir until the sample becomes a creamy paste. Cover the dish with cellophane to conserve moisture.
- 2
  Weigh four of the metal moisture containers with their lids. Record the can numbers and their respective weights.
- 3
  Adjust the liquid limit device by checking the cup’s drop height. The part of the cup that touches the base should rise to 10 millimeters. The grooving tool’s block end is also 10 millimeters high, and can be used as a gauge.
- 4
  Practice using the cup to determine the rate to rotate the crank so the cup drops about twice per second -- the speed you’ll need to conduct the limit test.
- 5
  Place the wet soil sample into the liquid limit device. Spread the sample evenly around the cup -- just as water would fill it -- so the sample is horizontal and about 10 mm thick at its deepest point, where the cup touches the base.
- 6
  Cut a clean straight groove down the center of the cup, with the grooving tool perpendicular to the surface. Make sure no soil slides.
- 7
  Turn the crank to achieve two drops per second. Count the number of drops, or N, until the two halves of soil connect -- flow together -- at the bottom of the groove for about a half-inch, or 13 millimeters. Record the number of drops. If there are more than 50, record no data and go to Step 9.
- 8
  Use the spatula to remove an even soil sample stripe, from where the groove flowed together. Sample from edge to edge. Place the soil into a metal can and put on the lid. Weigh the can immediately. Remove the lid and put the can into the oven to dry for at least 16 hours.
- 9
  Place the remaining soil back in the porcelain dish. Wash and dry the cup and the grooving tool. Remix the soil, increasing the water so it will take fewer drops to close the groove. Repeat the entire trial again, at least two more times, producing successively lower numbers and weighing the samples to determine water content.

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