This research has been partially funded by a grant (Beca de Investigaci&#242;n Ambiental) from the Servicio de Medio Ambiente de la Diputaci&#242;n Provincial de Toledo (gran number 133/10) and the research project PEII-2014-025-P of the Junta de Comunidades de Castilla-La Mancha.

1. Collect, Dry and Sieve the Test Sample
<ol>
	<li>Collect a soil sample in the field (use a shovel or a trowel) and store it in a polyethylene bag. 
	Note: The volume of the sample varies depending on the type of soil: in fine soils (clays and silts) between 100 and 1,000 g is generally sufficient, but in sandy soils and those containing gravel and pebbles, large amounts may be required, from a few to several kg.</li>
	<li>Reduce the sample by quartering in the laboratory if this is too voluminous (use a soil sp...

<ol>
	<li>Atterberg, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=&#220;ber+die+physikalische+Bodenuntersuchung+und+&#252;ber+die+Plastizit&#228;t+der+Tone.">&#220;ber die physikalische Bodenuntersuchung und &#252;ber die Plastizit&#228;t der Tone.</a> Internationale Mitteilungen f&#252;r Bodenkunde. 1, 10-43 (1911).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> ASTM Standard ASTM D 4318. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. , (2005).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> UNE 103-103-94. Determinaci&#242;n del l&#236;mite l&#236;quido de un suelo por el m&#233;todo del aparato de Casagrande. , (1994).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> BS 1377-2. Methods of test for soils for civil engineering purposes-Part 2: Classification tests. , (1990).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> UNE 103-104-93. Determinaci&#242;n del l&#236;mite pl&#225;stico de un suelo. , (1993).</li><li>Whyte, I. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Soil+plasticity+and+strength:+a+new+approach+using+extrusion.">Soil plasticity and strength: a new approach using extrusion.</a> Ground Eng. 15 (1), 16-24 (1982).</li><li>Temyingyong, A., Chantawaragul, K., Sudasna-na-Ayudthya, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Statistical+Analysis+of+Influenced+Factors+Affecting+the+Plastic+Limit+of+Soils.">Statistical Analysis of Influenced Factors Affecting the Plastic Limit of Soils.</a> Kasetsart J. (Nat. Sci.). 36, 98-102 (2002).</li><li>Bobrowski, L. J., Griekspoor, D. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+the+Plastic+Limit+of+a+Soil+by+Means+of+a+Rolling+Device.">Determination of the Plastic Limit of a Soil by Means of a Rolling Device.</a> Geotech. Test. J., GTJODJ. 15 (3), 284-287 (1992).</li><li>Rashid, A. S. A., Kassim, K. A., Katimon, A., Noor, N. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+Plastic+Limit+of+soil+using+modified+methods.">Determination of Plastic Limit of soil using modified methods.</a> MJCE. 20 (2), 295-305 (2008).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> ASTM Standard ASTM D 248. Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). , (2000).</li><li>Casagrande, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Research+on+the+Atterberg+limits+of+soils.">Research on the Atterberg limits of soils.</a> Public Roads. 13 (8), 121-136 (1932).</li><li>Casagrande, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Classification+and+Identification+of+Soils.">Classification and Identification of Soils.</a> Transactions, ASCE. 113, 901-991 (1948).</li><li>Sokurov, V. V., Ermolaeva, N., Matroshilina, T. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plastic+limit+of+clayey+soils+and+its+subjetive+determination.">Plastic limit of clayey soils and its subjetive determination.</a> Soil Mech. Found. Eng. 48 (2), 52-57 (2011).</li><li>Andrade, F. A., Al-Qureshi, H. A., Hotza, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measuring+the+plasticity+of+clays:+A+review.">Measuring the plasticity of clays: A review.</a> Appl. Clay Sci. 51, 1-7 (2011).</li><li>Harison, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Using+the+BS+cone+penetrometer+for+the+determination+of+the+plastic+limits+of+soils.">Using the BS cone penetrometer for the determination of the plastic limits of soils.</a> G&#233;otechnique. 38 (3), 433-438 (1988).</li><li>Feng, T. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fall-cone+penetration+and+water+content+relationship+of+clays.">Fall-cone penetration and water content relationship of clays.</a> G&#233;otechnique. 50 (2), 181-187 (2000).</li><li>Feng, T. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Using+a+small+ring+and+a+fall-cone+to+determinate+the+plastic+limit.">Using a small ring and a fall-cone to determinate the plastic limit.</a> ASCE, J. Geotech. Geoenviron. Eng. 130 (6), 630-635 (2004).</li><li>Lee, L. T., Freeman, R. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dual-weight+fall+cone+method+for+simultaneous+liquid+and+plastic+determination.">Dual-weight fall cone method for simultaneous liquid and plastic determination.</a> ASCE, J. Geotech. Geoenviron. Eng. 135 (1), 158-161 (2009).</li><li>Sivakumar, V., Glynn, D., Cairns, P., Black, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+method+of+measuring+plastic+limit+of+fine+materials.">A new method of measuring plastic limit of fine materials.</a> G&#233;otechnique. 59 (10), 813-823 (2009).</li><li>Sivakumar, V., O'Kelly, B. C., Henderson, L., Moorhead, C., Chow, S. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measuring+the+plastic+limit+of+fine+soils:+an+experimental+study.">Measuring the plastic limit of fine soils: an experimental study.</a> P. I. Civil Eng. - Geotec. 168 (GE-1), 53-64 (2015).</li><li>Wroth, C. P., Wood, D. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+correlation+of+index+properties+with+some+basic+engineering+properties+of+soils.">The correlation of index properties with some basic engineering properties of soils.</a> Can. Geotech. J. 15 (2), 137-145 (1978).</li><li>Haigh, S. K., Vardanega, P. J., Bolton, M. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+plastic+limit+of+clays.">The plastic limit of clays.</a> G&#233;otechnique. 63 (6), 435-440 (2013).</li><li>Barnes, G. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+apparatus+for+the+plastic+limit+and+workability+of+soils.">An apparatus for the plastic limit and workability of soils.</a> P. I. Civil Eng. - Geotec. 162 (3), 175-185 (2009).</li><li>Barnes, G. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+apparatus+for+the+determination+of+the+workability+and+plastic+limit+of+clays.">An apparatus for the determination of the workability and plastic limit of clays.</a> Appl. Clay Sci. 80-81, 281-290 (2013).</li><li>Moreno-Maroto, J. M., Alonso-Azc&#225;rate, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=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.">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.</a> Appl. Clay Sci. 114, 497-508 (2015).</li><li>Bain, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+plasticity+chart+as+an+aid+to+the+identification+and+assessment+of+industrial+clays.">A plasticity chart as an aid to the identification and assessment of industrial clays.</a> Clay Miner. 9 (1), 1-17 (1971).</li></ol>

The Atterberg plastic limit1 is a very important parameter in soils, mainly because it is widely used for geotechnical purposes10,11,12. The standard thread rolling test for PL determination has been widely criticized because it is highly dependent on the skill and judgment of the operator who is conducting the test and consequently new approaches to obtain the PL are claimed6,7,9,13,15-20, 23-25. However the simplicity, low cost and quick performance of the standard PL test give it an ad...

Liquid Limit (LL) and Plastic Limit (PL) are the two most important soil consistency limits of those defined by Atterberg in 19111. LL marks the boundary between liquid and plastic states, and PL between plastic and semisolid states. LL is obtained around the world according to several standards through the Casagrande method2,3 or the penetration test4. Both methods are conducted mechanically by devices; thereby, minimal operator interference is involved. In the case of PL, the so called &#34;thread rolling test&#34; is the most popular and standardized method for its determination2,5. This test is based on rolling soil into 3 mm threads by hand until the operator considers the soil to be crumbling. For this reason it has been widely criticized because the skill and judgment of the operator play a critical role in the outcome of the test. Standard rolling test is importantly affected by many uncontrolled factors, such as the pressure applied, the contact geometry, the friction, the speed of rolling, the size of the sample and the type of soil6,7. The American Society for Testing and Materials (ASTM) developed the ASTM D 4318 standard which includes a simple device in order to minimize the operator interference2,8, however significant differences have been reported in some soils when comparing the manual rolling test against the test performed by the ASTM D4318 device9.
PL is a very important parameter for geotechnical purposes, since Plasticity Index (PI) is obtained from it (PI = LL - PL); PI is used to classify the soil in accordance with the Plasticity Chart shown in ASTM D 248710, based on the research of Casagrande11,12. Errors in the PL affect negatively this classification13, and for this reason, a new test for PL determination is required.
Pfefferkorn test, cone penetrometer, capillary rheometer, torque rheometer or stress-strain tests are some examples of alternative methods for measuring soil plasticity14, but these are not adequate to obtain the PL. With the special instance of fall cone tests, a large number of researchers have attempted to define a new methodology for PL determination using different penetrometer designs15-20, but without reaching any real agreement. Furthermore, all of it is based on the assumption that the shear strength at the PL is 100 times that at the LL21, which is not true22.
Barnes23,24 developed an apparatus that emulated the rolling conditions of soil cylinders in an attempt to lay down a clear criterion for PL determination. Nevertheless, some shortcomings are identified with this approach, such as its complexity, test duration and mainly the questionable means of calculating the PL25. The success of the standard rolling test lies in its simplicity, quick performance and low cost, so no alternative method will be able to replace it, unless it meets these three requirements and other ones, such as high accuracy and low operator interference.
In a previous study by the authors, a new PL approach was proposed25: 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 stiff-plastic line is the steepest one, and PL was calculated from it as the moisture percentage corresponding to the cutoff point of this with the y-axis (Figure 1B). In this cutoff point the bending produced is zero, which is in accordance with the concept of plastic limit, i.e., PL is the moisture content at which the soil is not capable of withstanding deformations below this threshold (semisolid state) but it does bear them above it (plastic state). Although in the original study, the PL could not be obtained directly by the bending curve (this does not intersect the y-axis), this line was very useful because considering that the bending curve and the intersecting lines follow very similar paths, the bending curve equation obtained from the experimental data was used to obtain extra points to, firstly, correct any deviation, and, secondly, to carry out the test with just a few points as shown in Figure 1B.
<img alt="Figure 1" src="/files/ftp_upload/54118/54118fig1.jpg" /> 
Figure 1. Graphical representation of the B-W points in a tested soil by the original bending test. (A) The correlation of the points is represented as a parabolic curve, named the bending curve whose equation is included. (B) The correlation of the points is defined by two intersecting lines and other extra points are added (they were calculated from the bending curve equation). B values are obtained as B=52.0-D (where D is the average distance measured between the tips at the time of cracking in mm) and the PL is calculated as the water content corresponding to the cutoff point of the stiff-plastic line with the y-axis. This figure has been modified from Moreno-Maroto &#38; Alonso-Azc&#225;rate25. <a href="https://www.jove.com/files/ftp_upload/54118/54118fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
All the results were in excellent agreement with those achieved through the traditional thread rolling method by a highly experienced operator. However, the original bending test remained slower than the standardized thread rolling test. In an attempt to further economize test time, a one-point version was put forward. It was based on the average bending slope (m) obtained in the 24 tested soils, which was 0.108 (m is the slope of the bending curve when it is represented in double logarithmic scale; m appears on the bending curve equation in Figure 1A). By means of an equation where this factor was included, both the stiff-plastic and soft-plastic lines were graphically drawn, and thus the PL was estimated. These results were also highly correlated with both the multi-point bending test and the standard rolling test. In spite of this one-point version being even faster than the traditional test, the PL calculation was more complex because plotting was necessary. For this reason, on the basis of statistical criteria a new equation for PL calculation has been developed in this study, so that plotting is not required and results can be achieved with only one point, whereas the experimental protocol is the same as the original bending test. This new version meets the necessary requirements to replace the outdated thread rolling method.

<table border="0" cellpadding="0" cellspacing="0" width="978">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:244px;">Shovel</td>
			<td style="width:76px;">Any</td>
			<td style="width:117px;">NA</td>
			<td style="width:541px;">It is preferable a round point metal shovel so that it can penetrate easily in the soil.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Trowel</td>
			<td>Any</td>
			<td>NA</td>
			<td>It should be easy to handle both in field and laboratory, so approximately 500 g of soil should be the maximum of soil that could pick up.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Polyethylene bags</td>
			<td>Any</td>
			<td>NA</td>
			<td>The size of the bags depends on the collected soil volume. If we were interested in preserving the natural moisture, use sealing tape to close the bag.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Soil splitter&#160;</td>
			<td>PROETISA</td>
			<td>S0012</td>
			<td>It is not mandatory, because the quartering can be performed with the shovel, but in case of using it: it must be big enough to split several kg of sample in the cases of soils with large amounts of gravel or pebbles.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Oven</td>
			<td>SELECTA</td>
			<td>2001254</td>
			<td>The oven must be able to maintain constant temperature and should have some sort of slot or outlet opening to facilitate the release of water vapor.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Lab trays</td>
			<td>Any</td>
			<td>NA</td>
			<td>Metal trays are preferred over plastic because the first ones tolerate the oven temperatures better than the second ones.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Mortar and pestle</td>
			<td>MECACISA</td>
			<td>V112-02</td>
			<td>A ceramic mortar is valid.&#160; It is recommended to use a rubber covered pestle because if the pestle was of other different materials (like metal or a ceramic), it could break the sand particles.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">0.40 mm sieve (or 0.425 mm sieve)</td>
			<td>FILTRA</td>
			<td>0,400 (or 0,425)</td>
			<td>Make sure that the sieve mesh is in perfect conditions of use (it should not be neither broken or worn).</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Brush</td>
			<td>Any</td>
			<td>NA</td>
			<td>It is useful for passing the soil during the sieving.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Wash-bottle</td>
			<td>Any</td>
			<td>NA</td>
			<td>It should have an approximate capacity of one litre and it should be easy to control the amount of water that it releases.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Distilled water</td>
			<td>Any</td>
			<td>NA</td>
			<td>Distilled water can be purchased or obtained by filtering from tap water (in this last case, a filtering system is necessary).</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Nonabsorbent smooth glass plate&#160;</td>
			<td>Any</td>
			<td>NA</td>
			<td>The plate should have a minimum area of approximately 30 &#215; 30 cm.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Metal spatula</td>
			<td>Any</td>
			<td>NA</td>
			<td>The metal blade of the spatula must be flexible. Dry it with a paper after water-cleaning to prevent rusting.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Latex gloves</td>
			<td>Any</td>
			<td>NA</td>
			<td>Latex, vinyl, nitrile or other impermeable materials are valid. They should be thin enough to sense the soil with the hands.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Cling film</td>
			<td>Any</td>
			<td>NA</td>
			<td>Normal cling film is valid.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Airtight bags</td>
			<td>Any</td>
			<td>NA</td>
			<td>Remove the air before closing them.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Thread molder</td>
			<td>Any</td>
			<td>NA</td>
			<td>It is a tool designed in this experiment (drawings with dimmensions are included in this paper).</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Steel pushers</td>
			<td>Any</td>
			<td>NA</td>
			<td>It is a tool designed in this experiment (drawings with dimmensions are included in this paper).</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Damp cloth</td>
			<td>Any</td>
			<td>NA</td>
			<td>A normal damph cloth is valid.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Roll of paper</td>
			<td>Any</td>
			<td>NA</td>
			<td>Normall rolls of paper used to dry hands are valid.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Caliper</td>
			<td>Any</td>
			<td>NA</td>
			<td>It must have an accuracy of at least 0.1 mm.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Paper and pen</td>
			<td>Any</td>
			<td>NA</td>
			<td>Paper and pen are used to write the results.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Containers with covers</td>
			<td>Any</td>
			<td>NA</td>
			<td>Small cylindrical glass containers are valid. If they do not have covers, watch glasses can be used as covers. Covers are useful to avoid the loss of water during the test and also to prevent the dry soil absorbs moisture from the air after oven drying.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Precision or analytical balance</td>
			<td>BOECO</td>
			<td>BPS 52 PLUS</td>
			<td>It must have an accuracy of at least 0.01 g.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Protective gloves</td>
			<td>Any</td>
			<td>NA</td>
			<td>Protective gloves are used to catch the metal trays from the oven.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Tongs</td>
			<td>Any</td>
			<td>NA</td>
			<td>Tongs are used to catch the hot containers from the oven.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Desiccator</td>
			<td>MECACISA</td>
			<td>A036-01</td>
			<td>A normal glass desiccator with silica gel is valid to prevent the dry soil absorbs moisture from the air after oven drying.</td>
		</tr>
	</tbody>
</table>

a bending test for determining the atterberg plastic limit in soils

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 &#34;thread bending test&#34; or simply &#34;bending test&#34;); 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.

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 authors25 (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...

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A Bending Test for Determining the Atterberg Plastic Limit in Soils

The traditional standardized test for determining the plastic limit in soils is performed by hand, and the result varies depending on the operator. An alternative method based on bending measurements is presented in this study. This allows the plastic limit to be obtained with a clear and objective criterion.

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 ...

The overall goal of this new procedure is to determine the plastic limit in soils by measuring bending deformations. The traditional thread-rolling method is highly influenced by the skill of the laboratory operators, so an alternative approach is presented. This method allows the plastic limit to be obtained with a clear and objective criterion.Therefore the obtained results are highly reliable to classify and identify soils for botanical purposes. The main advantage of this technique is that it is not only quick, simple, and cheap, but also accurate and free of subjective interpretations. Beginning with a fine soil sample, dry it at 60 degrees Celsius or less, as needed.Next, disaggregate the soil manually using a mortar. Preferably, use a rubber-lined pestle to avoid breaking up the sand particles. Then, pass the sample through a 0.4 millimeter sieve and discard the soil collected in the sieve.Now, take about 20 to 40 grams of soil and wet it down on a non-absorbent glass plate. Then, knead the sample with a metal spatula until a homogeneous soil-water mixture is obtained. Wearing latex gloves, roll the mixture into a ball that is between three and five centimeters in diameter.Now, make a second ball with more or less water than the first, so the balls have different moisture content. Both soil balls must not crumble or stick when they are handled. Then, wrap each soil ball with cling film and place them in an airtight bag.Store the bag for 24 hours under hermetic conditions. After the tempering period, wearing latex gloves, flatten the soil ball on a smooth glass plate into a nearly three-millimeter thick patty. Keep the patty's surface covered with cling film to avoid moisture loss.Then, using a thread moulder, get the thickness down to precisely three millimeters. Now, cut the jagged edges of the flattened soil mass with a spatula, and make strips of soil that are at least 52 millimeters long, and three millimeters wide. Next, roll the soil strip with the thread moulder until the square section just becomes round, so it is three millimeters in diameter.If the strip is too dry and crumbly, or too sticky, repeat the test, using soil with more or less water content. Next, place the soil thread next to the front side of the thread moulder, and use the width of the thread moulder to make the cylinder exactly 52 millimeters long. Now, turn the thread moulder upside-down, and put the cylindrical piece in contact with the central part of the soil thread.Put the steel pushers in contact with the center of the soil thread. Then, carefully move the steel pushers towards the tips of the soil thread in a circular path. Repeat this motion, until the point of cracking.If the first crack appears near one of the thread tips, keep bending the other tip until the second crack appears at the opposite end. Now, remove the thread moulder and use a caliber to measure the distance between the central part of each tip to a precision of 0.1 millimeter. Now, put the soil thread into a container of known weight, and cover it to prevent moisture loss.If the bending deformations are so large that even the thread tips come into contact, remove the pushers and thread moulder, and continue bending the soil thread by hand, until the point of cracking. Then measure the distance between the thread tips, and record a negative value. From the excess soil, shape other additional threads, but do not cut their tips.Store these in the same container, and cover it again. Now, make a second soil thread that is 52 millimeters long, like the first. Repeat the bending process on the second soil thread.If the results are similar, do not bend more threads. If they differed, shape and bend at least one more soil thread, until agreeable measurements are obtained. Once a minimum of five grams of threads are obtained, record the weight of the container with the soil threads to a precision of at least 01 grams.Now, repeat the entire process for the second ball of soil. Begin with placing the two containers of soil threads uncovered in an oven at 105 degrees Celsius to dry the soil for at least 18 hours. The next day, put the containers in a desiccator.Once they have cooled to ambient temperatures, record their weights to a precision of at least 01 grams. Then, place the containers back into the same oven for at least six more hours. Again, cool them in a desiccator, and measure their weight.Continue repeating this process until two consecutive weight measurements are completely identical, which is the weight of the soil when dry. Now, for each soil ball, using the known water content, W, and its bending at cracking, B, calculate the plastic limit of each ball. Then, use the average value to characterize the soil.The number, minus 0.108, in the plastic limit equation, corresponds to the average value of ending a slope obtained in a previous study with a multiple inversion in which 24 soils were tested. The number 2.135 refers to the average bending value corresponding to the plastic limit in the bending curve for those 24 soils, in accordance with our original multi-point test. The plastic limits calculated from 30 soils with the described bending test, were compared to the original multi-point bending test, and the standard rolling test.Each test was executed by a highly experienced operator, and the results were very similar. The plastic limit results obtained through the new bending test were highly correlated with the original bending test, and the standard thread-rolling test. Most results were perfectly correlated, even with very low-plasticity soils.After watching this video, you should have a good understanding of how the plastic limit can be obtained instantly and accurately with a simple test in which the operator does not play a critical role in the final outcome. Once mastered, this technique can be done in five to ten minutes. So it can be even faster than the traditional test.In addition to its quick performance, the simplicity, low cost, and accuracy of this test, make it easily affordable for any laboratory. After its development, this technique paved the way for researchers and companies in the field of geotechnics, agriculture, ceramics, and soil science, to obtain more reliable plastic limit results.

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20.2K Views.  University of Castilla-La Mancha. The overall goal of this new procedure is to determine the plastic limit in soils by measuring bending deformations. The traditional thread-rolling method is highly influenced by the skill of the laboratory operators, so an alternative approach is presented. This method allows the plastic limit to be obtained with a clear and objective criterion.Therefore the obtained results are highly reliable to classify and identify soils for botanical purposes. The main advantage of this technique...