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A Bending Test for Determining the Atterberg Plastic Limit in Soils

Published: June 28th, 2016



1Department of Physical Chemistry, Faculty of Environmental Sciences and Biochemistry, University of Castilla-La Mancha

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 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.

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 "thread rolling test" is the most popular and standardized method for its determination2,5. This test is based on rolling soil into....

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1. Collect, Dry and Sieve the Test Sample

  1. 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.
  2. Reduce the sample by quartering in the laboratory if this is too voluminous (use a soil sp.......

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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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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.......

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


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Name Company Catalog Number Comments
Shovel Any NA It is preferable a round point metal shovel so that it can penetrate easily in the soil.
Trowel Any NA 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.
Polyethylene bags Any NA 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.
Soil splitter  PROETISA S0012 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.
Oven SELECTA 2001254 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.
Lab trays Any NA Metal trays are preferred over plastic because the first ones tolerate the oven temperatures better than the second ones.
Mortar and pestle MECACISA V112-02 A ceramic mortar is valid.  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.
0.40 mm sieve (or 0.425 mm sieve) FILTRA 0,400 (or 0,425) Make sure that the sieve mesh is in perfect conditions of use (it should not be neither broken or worn).
Brush Any NA It is useful for passing the soil during the sieving.
Wash-bottle Any NA It should have an approximate capacity of one litre and it should be easy to control the amount of water that it releases.
Distilled water Any NA Distilled water can be purchased or obtained by filtering from tap water (in this last case, a filtering system is necessary).
Nonabsorbent smooth glass plate  Any NA The plate should have a minimum area of approximately 30 × 30 cm.
Metal spatula Any NA The metal blade of the spatula must be flexible. Dry it with a paper after water-cleaning to prevent rusting.
Latex gloves Any NA Latex, vinyl, nitrile or other impermeable materials are valid. They should be thin enough to sense the soil with the hands.
Cling film Any NA Normal cling film is valid.
Airtight bags Any NA Remove the air before closing them.
Thread molder Any NA It is a tool designed in this experiment (drawings with dimmensions are included in this paper).
Steel pushers Any NA It is a tool designed in this experiment (drawings with dimmensions are included in this paper).
Damp cloth Any NA A normal damph cloth is valid.
Roll of paper Any NA Normall rolls of paper used to dry hands are valid.
Caliper Any NA It must have an accuracy of at least 0.1 mm.
Paper and pen Any NA Paper and pen are used to write the results.
Containers with covers Any NA 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.
Precision or analytical balance BOECO BPS 52 PLUS It must have an accuracy of at least 0.01 g.
Protective gloves Any NA Protective gloves are used to catch the metal trays from the oven.
Tongs Any NA Tongs are used to catch the hot containers from the oven.
Desiccator MECACISA A036-01 A normal glass desiccator with silica gel is valid to prevent the dry soil absorbs moisture from the air after oven drying.

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