Why Cone Penetrometer Method is more accurate than Casagrande Method when determining Liquid Limit of Soil?

Question

Atterberg Limit Test

Answer 1

The plasticity of soils is determined by using relatively simple remoulded strength tests. The plastic limit is the moisture content of the soil under test when remoulded and rolled between the tips of the fingers and a glass plate such that longitudinal and transverse cracks appear at a rolled diameter of 3mm. At this point the soil has a stiff consistency.The liquid limit of a soil can be determined using the cone penetrometer or the Casagrande apparatus (BS 1377:1990:part 2, clauses 4.3, 4.5).

One of the major changes introduced by the 1975 British Standard (BS 1377) was that the preferred method of liquid limit testing became the cone penetrometer. This preference is reinforced in the revised 1990 British Standard which refers to the cone penetrometer as the ‘definitive method’. The cone penetrometer is considered a more satisfactory method than the alternative because it is essentially a static test which relies on the shear strength of the soil, whereas the alternative Casagrande cup method introduces dynamic effects.

In the penetrometer test, the liquid limit of the soil is the moisture content at which an 80 g, 300 cone sinks exactly 20 mm into a cup of remoulded soil in a 5s period. At this moisture content the soil will bevery soft. When determining the liquid limit with the Casagrande apparatus, the base of the cup is filled with soil and a groove is then made through the soil to the base of the cup. The apparatus is arranged to allow the metal cup to be raised
repeatedly 10mm and dropped freely on to its rubber base at a constant rate of two drops per second. The liquid limit is the moisture content of a soil when 25 blows cause 13mm of closure of the groove at the base of the cup. The liquid limit is generally determined by mixing soils to consistencies just wet and dry of the liquid limit and determining the liquid limit moisture content by interpolation between four points BS 1377:part 2:1990, clause 4.6 provides factors which allow the liquid limit to be determined from one point (Clayton and Jukes 1978).

The plastic limit test relies heavily on the skill of the operator, and is almost entirely subjective despite attempts by the British Standard to define procedure rigidly. The Casagrande cup method of determining the liquid limit is also rather operator dependent, and in addition suffers from apparatus maintenance problems.

Answer 2

Casagrande Liquid Limit Device

Answer 3

Cone penetration test preferred in the determination of liquid limit test because casaglande cup method introduced by dynamic effects or erros shuch as
(1) Tendency of a soil to low plasticity due to slide and liquefy with shock in the cup rather than to flow plastically.
(2) Sensitivity to operator technique and to small differences in apparatus.