An accurate, quick and simple method to determine the plastic limit and consistency changes in all types of clay and soil: The thread bending test

Repeatability and agreement between methods for determining the Atterberg limits of fine‐grained soils

“…Due to the large variation in methodology, some studies have evaluated the limitations of the methods for different datasets. The rolling device by Bobrowski and Griekspoor (), the Barnes apparatus and Barnes test by Barnes () and the thread bending test by Moreno‐Maroto and Alonso‐Azcarate () were developed to replace the thread rolling approach due to methodological challenges such as different applied pressure on the soil sample, different contact area between hand and sample, friction between hand and sample, variety of rolling speed, the risk of contaminating the sample and operator dependency (Sivakumar et al., ). Additionally, the drop cone method is preferred by some due to some limitations of the Casagrande method such as susceptibility to operator error, different dimensions and weight of the cup, and difficulty in using it for less plastic soils (Spagnoli, ).…”

A Bending Test for Determining the Atterberg Plastic Limit in Soils

“…When the two bending tests are compared (Figure 5A) only in soil M8 is a greater PL difference observed, whereas the soils M8, M9 and S4 are the three ones that exhibit greater PL variations when the new bending test is compared with the traditional thread rolling test ( Figure 5B, Table 2). In these samples the new bending test overestimates the results, especially in M8 and S4 that are two soils with particular characteristics: on the one hand, M8 was reported in the previous authors' study as an unusual soil because despite the fact that it has high LL and PI, it exhibits a poor resistance to bending that could be caused by its composition (it has a great deal of calcite combined with smectite clay) 25 , and on the other hand, S4 is a sepiolite which is a very rare clay in which very high values of PL and PI are normal 26 . Soils M8, M9 and S4 have in common high PL values (greater than 30).…”

“…The PL equation shown in the step 6.1 of the protocol was achieved through a statistical study of the 24 soils tested in a previous study of the authors 25 ( Table 1). The objective was to know the most probable bending slope (the term m in the bending curve equation, which appears in Figure 1A) and the average value of B on the bending curve at which PL was obtained according to the original bending test (the original test was conducted with more than 3 experimental points and graphs were needed to obtain the PL, as shown in Figure 1).…”

“…In a previous study by the authors, a new PL approach was proposed 25 : the original thread bending test (or simply bending test) allowed the PL to be obtained from a graph in which it was represented the relationship between water content and bending deformations. The authors obtained and plotted several experimental points for each soil (the protocol followed to get these points was the same as that indicated in the present paper), so that the correlation of the points could be defined in two ways without compromising in any way the correct definition of the point path: as a parabolic curve, named the bending curve ( Figure 1A), and as two intersecting straight lines with different slope, named the stiff-plastic line and the soft-plastic line.…”

“…The PL results obtained with the new bending test (PL nb ) proposed in this paper, and those corresponding to the original bending test (PL ob ) and the standard rolling test by a highly experienced operator (PL st ) are shown in Table 2. Apart from the 24 soils studied in the previous research (soils M1 to M24) 25 other 6 different soils (soils S1 to S6) were tested in order to check the feasibility of the method with independent soils, i.e., these other soils were not used in the initial statistical study to get the PL equation and, in addition, only two experimental points were obtained in these soils, therefore PL was calculated as shown in step 6.3. The low standard deviation and coefficient of variation (CV) values shown in Table 2 are indicative of a good repeatability of the method, i.e., the PL results obtained from each experimental point are very similar to each other with the new bending method; in fact, all soils except M8 have a CV value that is less than 10 so the dispersion of the results can be considered as low.…”