Elsevier

Engineering Geology

Volume 302, 5 June 2022, 106623
Engineering Geology

Discussion
Discussion of “Mohajerani method: Tool for determining the liquid limit of soils using fall cone test results with strong correlation with the Casagrande test” by E. Hrubesova, B. Lunackova and M. Mohyla [Engineering Geology 278 (2020) 105852]

https://doi.org/10.1016/j.enggeo.2022.106623Get rights and content

Highlights

  • In present global context, percussion-cup and fall-cone generally considered equally valid for liquid limit (LL) determination

  • Fall cone (FC) approach generally has superior repeatability and reproducibility

  • Recommended to consistently redefine LL uniquely as water content at which universal FC penetrates specified depth into remoulded specimen

  • Plastic limit (plastic/brittle boundary) condition uniquely established using standard thread-rolling method

  • Any agreement between water contents of plastic limit (PL) and PL100 for a given fine-grained soil essentially coincidental

Abstract

The Authors have presented an interesting paper (Hrubesova et al., 2020), which aims (in part) to validate the Moharjerani (1999) calibration approach for establishing the 80 g/30° fall-cone penetration depth equating to the liquid limit by the Casagrande percussion-cup approach, determined according to the British Standard. In this paper, the Discussers present some clarifications on, and state various observations regarding, the approaches adopted in the Hrubesova et al. (2020) investigation, as well as the Moharjerani (1999) calibration approach employed therein. The Discussers also present a description of some relevant literature not covered in the Authors' paper (Hrubesova et al. 2020) aimed at making further clarifications on this important area of geotechnical practice.

Introduction

The Authors' paper (Hrubesova et al., 2020) and this Discussion paper are concerned with determinations of consistency limits, particularly for LL, of fine-grained soils (i.e., for the saturated, remoulded soil fraction passing the 425-μm sieve size). Since Atterberg, 1911a, Atterberg, 1911b described the consistency limits, there have been many modifications to the classification framework for fine-grained soils. These include the mechanisation of the percussion cup (PC) liquid limit (LL) test by Casagrande (1932) (i.e., LLCas,ASTM), and subsequently the refinement of the Casagrande plasticity chart (Casagrande, 1947; Howard, 1984), as well as the development of other plasticity-based systems for fine-grained soil classification (see the reviews of O'Kelly (2021b) and Moreno-Maroto et al. (2021)). Although the PC LL approach remains in favour and in widespread use for many regions, the fall cone (FC) approach for LL determination (i.e., LLcone) has been adopted as the preferred method in numerous parts of the world, appearing in the British Standard in the 1970s.

Given the apparent limitations of using the thread-rolling test for determination of the plastic limit (PL), some research work has focused on means of soil classification not reliant on its measurement, including a recent proposal by Vardanega et al. (2021) employing the FC flow index and a redrawn plasticity chart, thereby allowing fine-grained soil classification to be achieved solely from FC data.

Apart from fine-grained soils, research on the variation of LLcone with clay content for sand–low plasticity clay and fine gravel–kaolin mixtures is reported in the papers by Cabalar and Mustafa (2015) and Kumar and Muir Wood (1999), respectively, along with the link between saturated, remoulded, undrained shear strength (su) and water content (w) for different clay contents in the mixtures. Likos and Jaafar (2014) investigated the FC penetration depth (d) as a function of saturation level for four sandy soils, studied using an effective stress approach linking the soil–water retention and suction–stress characteristic curves. Mahajan and Budhu (2009) discerned the viscous drag as the FC penetrates fine-grained soils with w > LL, and they showed that clay shear viscosities at LI (liquidity index) of <1.5 may be approximated from penetration time – d data.

Section snippets

For the liquid limit

The mechanically different PC and FC approaches, which imply arbitrarily criteria-chosen low undrained shear strengths at the LL, are generally considered in the present global context equally valid means for LL determination. As such, the Discussers do not agree with the Authors' contentions, citing Mohajerani (1999), that (i) determining the LLcone based on the same condition (i.e., using the 80 g/30° FC with d = 20 mm universally) for all fine-grained soils was “incorrect”, and (ii) the

Summary

In the present global context, the PC and FC approaches are generally considered equally valid for LL determination. Systematic differences in the experimental values from both methods can occur for a given fine-grained soil since these approaches are mechanically different. Given that the FC approach has arguably superior repeatability and reproducibility, the Discussers contend that the LL should be consistently redefined in terms of the water content at which a universal FC (agreed cone

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Notation

d
Cone penetration depth (mm)
IP100
Alternative "plasticity" index (= LLconePL100)
LLCas,ASTM
Liquid limit determined by Casagrande percussion-cup according to American Standard (hard base) (%)
LLCas,BS
Liquid limit determined by Casagrande percussion-cup according to British Standard (soft base) (%)
LLcone
Liquid limit determined by fall cone test (%)
LLMoh
Liquid limit determined by Mohajerani method (%)
PL100
Plastic strength limit (assuming R* = 100) (%)
R*
Ratio between su,PL and su,LL-FC
su
Saturated,

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