A Bending Test for Determining the Atterberg Plastic Limit in Soils

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.