Discussion of “Determination of liquid limit by the fall cone method”*

The readers appreciate the comparative study that the authors have made on the liquid limit (LL) test results obtained by both the Swedish Standard (SS) fall-cone (LL_FC) and the Brazilian standard percussion (LL_cup) methods presented for soil samples collected from different geological formations in Brazil. The LL obtained by current standard methods are not definitive values but indicators of soil when its conditions reach the limit state (Manafi, 2019). The two LL methods measure different physical quantities (Haigh, 2012). Compared with the percussion (or Casagrande) method, the fall-cone method is less error-prone (Claveau-Mallet et al., 2012).

The most common LL fall-cone devices are the Swedish cone (60^o-60g fall-cone) and the British/French cones (30^o-80g fall-cone) (Leroueil & Le Bihan, 1996). LL measured by fall-cone test is not standardized in Brazil, and the readers would like to include some additional comments on a comparison of LL_cup with LL_FC values obtained by the Standard BS 1377 (BSI, 1990) fall-cone method considering different soils having LL_cup < 100%. The investigated data come from different operators and laboratories, and it may be expected that some uncontrolled factors during the LL measurements have played a role in the observed differences between the LL_cup and LL_FC values.

Figure 1 illustrates a comparison of LL data obtained by the SS (Clemente et al., 2020) and BS (Bicalho et al., 2014) fall-cone methods with those obtained by the Brazilian standard percussion method (hard rubber base cup). The solid square symbols are the data reported by the authors (SS fall-cone). The open square symbols are the data from BS fall-cone reported by the Bicalho et al. (2014). The LL for different natural inorganic low plasticity clays from different locations in Brazil were compilated by Bicalho et al. (2014) and included data by Pinto & Castro (1971) and Silveira (2001) with LL_cup ranging from 14 to 98% and LL_FC from 18 to 98% (BS fall-cone method). The clays are essentially kaolinites and illites. A fitted empirical relationship (LL_FC = LL_cup+ 2.7, R² = 0.98) shows that LL_cup values are generally 2.7% lower than LL_FC for the data, Figure 1. The linear empirical LL_FC - LL_cup correlation proposed by Queiroz de Carvalho (1986) for 27 samples of lateritic soils from Brazil (LL_cup ranging from 13 to 48%) in which kaolinite is the only clay mineral is also presented in Figure 1. The comparison of data from different sources shows variations in the LL results based on Casagrande and fall-cone methods (Figure 1). It can be observed from Figure 1 that most data fall within LL_FC = 0.8LL_cup and LL_FC = 1.2LL_cup lines. The data consistently indicate higher LL being obtained for the fall-cone devices compared to the Casagrande cup for LL_cup < 40%, while the difference in LL_FC and LL_cup is more spread out for LL_cup > 40% for the investigated fine-grained soils. Also, the LL_FC/LL_cup ratio may range to values even greater than 1.2 at low LL_cup values (i.e., LL_cup < 40%). It is therefore worthwhile to examine the differences in the LL_cup and LL_FC (SS and BS fall-cone) of fine-grained soils when applying LL values obtained by different standards in soil classification systems and empirical correlations in geotechnical engineering, even for LL_cup < 100% where the LL values obtained with the fall-cone and Casagrande methods are often considered approximately equal (Wasti & Bezirci, 1986; Spagnoli, 2012).

Figure 1
Comparison of LL test results obtained by the SS and BS fall-cone methods with those obtained by the Brazilian standard percussion method for LL data between 14% and 110%.

List of symbols

LL: Liquid limit

LL_FC: Liquid limit obtained by the fall-cone method

LL_cup: Liquid limit obtained by the standard percussion method

SS: Swedish Standard

BS: British Standard

R²: Coefficient of determination in linear regression

• Discussion open until August 31, 2021.
• *
  Appears in Soils and Rocks, 43(4), 661-667.

References

• Bicalho, K.V., Gramelich, J.C., & Santos, C.L.C. (2014). Comparação entre os valores de limite de liquidez obtidos pelo método de Casagrande e cone para solos argilosos brasileiros. Comunicações Geológicas, 101(3), 1097-1099. In Portuguese.
• British Standards Institution – BSI. (1990). BS 1377: methods of test for soil for civil engineering purposes. Part 2. Classification tests. London.
• Claveau-Mallet, D., Duhaime, F., & Chapuis, R.P. (2012). Practical considerations when using the Swedish fall cone. Geotechnical Testing Journal, 35(4), 618-628. http://dx.doi.org/10.1520/GTJ104178
  » http://dx.doi.org/10.1520/GTJ104178
• Clemente, C.W., Faro, V.P., & Moncada, M.P.H. (2020). Determination of liquid limit by the Fall Cone method. Soils and Rocks, 43(4), 661-667.
• Haigh, S.K. (2012). Mechanics of the Casagrande liquid limit test. Canadian Geotechnical Journal, 49(9), 1015-1023. http://dx.doi.org/10.1139/t2012-066
  » http://dx.doi.org/10.1139/t2012-066
• Leroueil, S., & Le Bihan, J.P. (1996). Liquid limits and fall cones. Canadian Geotechnical Journal, 33(5), 793-798. http://dx.doi.org/10.1139/t96-104-324
  » http://dx.doi.org/10.1139/t96-104-324
• Manafi, M.S.G. (2019). Soil plasticity variability and its effect on soil classification. Geotechnical Testing Journal, 42(2), 457-470. http://dx.doi.org/10.1520/GTJ20170273
  » http://dx.doi.org/10.1520/GTJ20170273
• Pinto, S., & Castro, P.F. (1971). Determinação do limite de liquidez pelo método do cone de penetração Brasília: Departamento Nacional de Estradas de Rodagem. In Portuguese.
• Queiroz de Carvalho, J.B. (1986). The applicability of the cone penetrometer to determine the liquid limit of lateritic soils. Geotechnique, 36(1), 109-111. http://dx.doi.org/10.1680/geot.1986.36.1.109
  » http://dx.doi.org/10.1680/geot.1986.36.1.109
• Silveira, L.C.S. (2001). O cone de penetração como ensaio alternativo na determinação da plasticidade de solos. In Anais do Congresso Brasileiro de Ensino de Engenharia (NTM:16-NTM:19), Porto Alegre. Porto Alegre: ABENGE. In Portuguese.
• Spagnoli, G. (2012). Comparison between Casagrande and drop-cone methods to calculate liquid limit for pure clay. Canadian Journal of Soil Science, 92(6), 859-864. http://dx.doi.org/10.4141/cjss2012-011
  » http://dx.doi.org/10.4141/cjss2012-011
• Wasti, Y., & Bezirci, M.H. (1986). Determination of the consistency limits of soils by the fall cone test. Canadian Geotechnical Journal, 23(2), 241-246. http://dx.doi.org/10.1139/t86-033
  » http://dx.doi.org/10.1139/t86-033

Publication Dates

• Publication in this collection
  23 June 2021
• Date of issue
  2021

History

• Received
  29 Jan 2021
• Accepted
  12 May 2021