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  • Roadbase Soil Stabilizer
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Liquid Limit Test Via Cone Penetration Method


The liquid limit is the empirically established moisture content at which a soil passes from the liquid state to the plastic state. It provides a means of classifying a soil, especially when the plastic limit is also known.

Two main types of test are specified. The first is the cone penetrometer method, which is fundamentally more satisfactory than the alternative (Casagrande method) because it is essentially a static test depending on soil shear strength. It is also easier to perform and gives more reproducible results.


  1. A flat, glass plate of which a convenient size is 10 mm thick and about 500 mm square.
  2. Two palette knives or spatulas.
  3. A penetrometer as used in bituminous material testing complying with BS 2000-49.
  4. A cone of stainless steel or duralumin approximately 35 mm long, with a smooth, polished surface and an angle of 30 ± 1°. To ensure that the point remains sufficiently sharp for the purposes of the test, the cone shall be replaced if, after continued use, the point can no longer be felt when brushed lightly with the tip of the finger when the tip of the cone is pushed through a hole 1.5 ± 0.02 mm in diameter, bored through a metal plate 1.75 ± 0.1 mm thick. The mass of the cone together with its sliding shaft shall be 80.00 ± 0.1 g (see Figure 1).

    NOTE The effect of surface roughness is more significant than small variations in cone angle or bluntness of the tip.

  5. One or more metal cups (55 ± 2) mm in diameter and (40 ± 2) mm deep with the rim parallel to the flat base.
  6. An evaporating dish, of about 150 mm diameter.
  7. Apparatus for moisture content determination of fine grained soils as specified in BS 1377:1990-1 clause 3.2.2.
  8. A wash bottle or beaker, containing distilled water.
  9. A corrosion-resistant airtight container.
  10. A metal straightedge about 100 mm long or a straight-bladed spatula.
  11. A stopwatch readable to 1 s.

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  1. Take a sample of about 300 g from the soil paste prepared and place it on the glass plate.
  2. Mix the paste for at least 10 min using the two palette knives. If necessary add more distilled water so that the first cone penetration reading is about 15 mm.
  3. Push a portion of the mixed soil into the cup with a palette knife taking care not to trap air. Strike off excess soil with the straightedge to give a smooth level surface.
  4. With the penetration cone locked in the raised position lower the supporting assembly so that the tip of the cone just touches the surface of the soil. When the cone is in the correct position a slight movement of the cup will just mark the soil surface. Lower the stem of the dial gauge to contact the cone shaft and record the reading of the dial gauge to the nearest 0.1 mm.
  5. Release the cone for a period of 5 ± 1 s. If the apparatus is not fitted with an automatic release and locking device take care not to jerk the apparatus during this operation. After locking the cone in position lower the stem of the dial gauge to contact the cone shaft and record the reading of the dial gauge to the nearest 0.1 mm. Record the difference between the beginning and end of the drop as the cone penetration.
  6. Lift out the cone and clean it carefully to avoid scratching.
  7. Add a little more wet soil to the cup, taking care not to trap air, make the surface smooth as in Step No. 3 and repeat Step No. 4 to Step No. 6.
  8. If the difference between the first and second penetration readings is not more than 0.5 mm record the average of the two penetrations and proceed to Step No. 9. If the second penetration is more than 0.5 mm and less than 1 mm different from the first, carry out a third test. If the overall range is then not more than 1 mm record the average of the three penetrations and proceed to Step No. 9. If the overall range is more than 1 mm remove the soil from the cup, remix and repeat Step No. 3 to Step No. 8 until consistent results are obtained and then proceed to Step No. 9.
  9. Take a moisture content sample of about 10 g from the area penetrated by the cone and determine the moisture content.
  10. Repeat Step No. 3 to Step No. 9 at least three more times using the same sample of soil to which further increments of distilled water have been added. Proceed from the drier to the wetter condition of the soil. The amount of water added shall be such that a range of penetration values of approximately 15 mm to 25 mm is covered by the four or more test runs and is evenly distributed. Each time soil is removed from the cup for the addition of water, wash and dry the cup.
  11. If at any time during the above procedure the soil has to be left for a while on the glass plate cover the soil with the evaporating dish or a damp cloth to prevent the soil drying out.