A Bending Test for Determining the Atterberg Plastic Limit in Soils

Abstract

The thread rolling test is the most commonly used method to determine the plastic limit (PL) in soils. It has been widely criticized, because a considerable subjective judgment from the operator that carries out the test is involved during its performance, which may affect the final result significantly. Different alternative methods have been put forward, but they cannot compete with the standard rolling test in speed, simplicity and cost. In an earlier study by the authors, a simple method with a simple device to determine the PL was presented (the "thread bending test" or simply "bending test"); this method allowed the PL to be obtained with minimal operator interference. In the present paper a version of the original bending test is shown. The experimental basis is the same as the original bending test: soil threads which are 3 mm in diameter and 52 mm long are bent until they start to crack, so that both the bending produced and its related moisture content are determined. However, this new version enables the calculation of PL from an equation, so it is not necessary to plot any curve or straight line to obtain this parameter and, in fact, the PL can be achieved with only one experimental point (but two experimental points are recommended). The PL results obtained with this new version are very similar to those obtained through the original bending test and the standard rolling test by a highly experienced operator. Only in particular cases of high plasticity cohesive soils, there is a greater difference in the result. Despite this, the bending test works very well for all types of soil, both cohesive and very low plasticity soils, where the latter are the most difficult to test via the standard thread rolling method.

Review of Recent Developments and Understanding of Atterberg Limits Determinations

“…In view of perceived weaknesses and poor repeatability/reproducibility of the handrolling PL, mechanical thread-rolling [43][44][45][46] and alternative methods also based on the onset of brittleness [47][48][49][50] have been proposed for improved PL determination.…”

“…Moreno-Maroto and Alonso-Azcárate [48] developed a fast and simple test method based on the bending of 3 mm diameter soil cylinders, which allowed determination of PL as well as a number of new consistency parameters. Their bending method was refined in subsequent work [49,50], becoming a potential alternative to the standardized hand-rolling PL method. However, apart from the Bobrowski and Griekspoor approach [44], which has been included in ASTM D4318-17e1 [8] and AASHTO T90-00 [56] albeit that the handrolling method remains the preferred approach for PL determination, none of the other described methods have, to date, been adopted more widely.…”

“…Considering that toughness (plasticity) is an exclusive property of clays and clayey soils, and with the PL HR determined using the thread bending test [49,50], a value of I P /LL PC ≈ 0.33 (i.e., setting T max = 0 in Equation ( 7)) defines the upper limit for silts and other non-cohesive soils; plotting as the so-called 'M-line' in their I P − LL chart (Figure 2a). Similarly, considering that clays have high toughness [11], and adopting a T max value of 20 kJ/m 3 (based on Barnes' toughness classification [45,46]) as a boundary between moderately or slightly clayey soils (e.g., clayey silts) and actual clays, results in I P /LL PC ≈ 0.50, defining the so-called 'C-line' in Figure 2a.…”

“…Finally, the two recently developed plasticity-based fine-grained soil classification systems [19,79] presented do not rely on PL HR results (which are known to have high operator variability of up to 10-15%), but instead require PL determination using a thread-bending test (Moreno-Maroto and Alonso-Azcárate [48,49]), or rely solely on the flow index (and LL FC(BSI) ) determined from 80 g/30 • FC LL testing [19].…”

Reappraisal of the ASTM/AASHTO Standard Rolling Device Method for Plastic Limit Determination of Fine-Grained Soils

“…Apart from this standardized method, none of the other proposed device-rolling techniques have been adopted more widely. Note that, in addition to device-rolling techniques, other methods developed based on the "onset of brittleness" concept for improved PL determination include the likes of the indentation test [59] and the thread-bending technique [60][61][62].…”