Plastic Limit Test Abstract The purpose of this lab is to determine the plastic limit of a specific sample of soil, to use the plastic limit and liquid limit from the previous lab in order to determine the plasticity index, and to use the plasticity index in order to classify the soil sample. This is accomplished by adding water to the sample, then rolling the sample into 3mm threads repeatedly until the soil sample begins to crack. Once this point is reached, the soil is at its plastic limit. Three identical tests are done with the same sample. At this point, the soil is placed in the oven to determine the moisture content. This moisture content is the plastic limit. After calculating the average plastic limit from the three trials, the plastic limit of the sample was 23. Since the liquid limit of this soil from the previous lab was 27, we can subtract the plastic limit from this to get 4, which is the plasticity index. This data is compared to a chart in order to classify the soil. The sample from this test lined up with the CL-MI section, which is silty clay. This classification has multiple applications to construction. One of these is the lateral soil load. According to ASCE 7-16, the lateral soil load for CL-MI is 85psf per foot of depth. Another application is found in the bearing capacity. According to the Code of Federal Regulations, the bearing capacity of this soil is 1500psf. Introduction The plasticity index of soil is the point at which a thread of it with a 3.2mm diameter begins to crumble. At this point, it should have a moisture content that is lower than the liquid limit, which is the point where soil begins to flow like a liquid. The properties of soil are well known given the liquid and plastic limit, so once these are found, the soil can be classified. This classification is exceedingly important for construction because it determines the properties of the soil. These properties determine how suitable the soil is for construction purposes. The soil sample used in this lab was collected from Morehead State University’s campus. The purpose of the lab is to determine the plasticity index of a sample of soil, and to use that data in order to classify the soil. Using the classification from this lab, it can later be determined how suitable the soil is for construction. For the classification, it is important to note that the sample was noticeably fine. Materials 1.Porcelain dish 2.Glass plate for rolling the sample 3. Air tight containers to determine the moisture content of the sample 4.Balance of capacity 200gm and sensitive to 0.01gm 5.Oven thermostatically controlled with interior of non-corroding material to maintain the temperature around 105 C and 110 C. Methods 1. Take about 20gm of thoroughly mixed portion of the material passing through #40 sieve obtained 2. Mix thoroughly with distilled water in the porcelain dish until the soil mass becomes plastic enough to be easily molded using fingers. 3. Allow time for the water to permeate throughout the soil 4. Take about 10g of this plastic soil mass and roll it between fingers and the glass plate with just enough pressure to roll the mass into a thread with uniform diameter throughout its length. The rate of rolling should be between 60 and 90 rolls per minute. 5. Continue rolling until a thread of 3 mm diameter is reached. 6. Kneed the soil back together to form a uniform mass and then re-roll. 7. Continue the process until the thread crumbles when the diameter is 3 mm. 8. Collect the pieces of the crumbled thread in the air tight container in order to determine the moisture content. These should be weighed before placing in the oven. 9. Repeat the test to at least 3 times and take the average of the results calculated to the nearest whole number. 10. Place the containers with the soil in the oven for at least 24 hours. 11. Determine the weight after the samples have been placed in the oven. These results are then used to find the plastic limit. The average should be calculated to the nearest whole number. Observation and Reporting Compare the diameter of thread at intervals with the rod. When the diameter reduces to 3 mm, note the surface of the thread for cracks. Results Test 1 Test 2 Test 3 1 Weight of container with lid 20 W1 (g) 20 20 2 Weight of container with lid 26 + wet soil W2 (g) 25 25 3 Weight of container with lid 25 + dry soil W3 (g) 24 24 4 Weight of water W2- W3 (g) 1 1 1 5 Weight of dry soil W3- W1 (g) 5 4 4 6 Water content w (%) W=[( W2- W3)/ (W3W1)]x100 20 25 25 Average Plastic Limit= (20+25+25)/3= 23 The Plasticity Index is calculated using the liquid limit from the previous lab. (27) Plasticity Index(PI) = (LL - PL)= 27-23= 4 This chart is used to classify the soil sample. The Liquid Limit of 27 and the Plasticity Index of 4 puts the soil sample in the area of CL-MI on the chart, which means that it is likely silty clay. Discussion The data from this lab showed that the plastic limit was 23, the plasticity index was 4, and the soil was classified as silty clay. This classification makes sense because silty clay is a fine classification of soil, and the sample used for the test was noticeably fine. In fact, silty clay is one of the more fine kinds of soil there is, according to the Unified Soil Classification System’s flow chart at www.outagamie.org. In addition to this, the lateral pressure can be found for this kind of soil in ASCE 7-16; it is 85psf per foot of depth. The bearing capacity can be found in the Code of Federal Regulations; it is 1500psf. This study could best be improved by implementing a more sensitive scale. Since the change in moisture content after placing the samples in the oven was only by 1 gram for each of the samples, a more precise scale could allow for a more precise plastic limit calculation. This could potentially change the classification of the soil upon recalculation. Literature Cited: “Code of Federal Regulations.” GovInfo, 9 Sept. 2022, https://www.govinfo.gov/help/cfr. “FLOW CHART: UNIFIED SOIL CLASSIFICATION SYSTEM FIELD IDENTIFICATION OF COARSE and FINE GRAINED SOILS.” Outagamie, https://www.outagamie.org/home/showpublisheddocument/47132/636282748007270000. Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-16). American Society of Civil Engineers, 2017.
