EXPERIMENT 3 : Plastic Limit Test Introduction The plastic limit, PL of a soil is the lowest water content at which the soil remain plastic, and when plotted against the liquid limit on the plasticity chart enables the classification of cohesive soils. The plasticity index of a soil is the numerical difference between the liquid limit and the plastic limit. It is the moisture content at which the soil is in a plastic state. Usually, the determination of the plastic limit is made in conjunction with the determination of the liquid limit. It is recognized that the results of the test are subjected to the judgement of the operator, and that some variability in results will occur. It is important in civil engineering awareness of soil science for seismic mitigation of structure placed on soil foundation consists of soft soil. Objective -
To determine the samples of soil which the limit between plastic state and
-
semi-plastic state. The plasticity index of a soil is the numerical difference between the liquid limit and the plastic limit. It is the moisture content at which the soil is in a plastic state.
Apparatus and Equipments 1. Soil-Mixing Equipment (Spatula or Palette Knives, Flat Glass Plate, Distilled Water)
2. Sample of soil ( the left of sample after taking out from the Oven – 105-110◦C) 3. Balance (accurate to 0.01 g) 4. 4 standard containers 5. Oven
Procedure 1. About 20 g of soil sample paste took from the liquid limit test, a little preprepared dried soil added to it and shaped it into a ball. 2. If soil in its natural state is used, allow it to dry partially on a glass plate before shaping it into a ball. 3. The ball sample is molded between the fingers and rolls it between the palms until the sample is dried sufficiently for slight cracks to appear in its surface. The sample is then rolled on the glass plate with the palm. This sample is divided into two sub samples of about 10g each and carries out separate determination on each portion. Divide each sub sample into four more or less equal parts and treat it. 4. The soil molded in the fingers to equalize the distribution of moisture. From the soil into thick threads by rolling the soil between the palms and then the threads is rolled between the fingers (from finger-tip to second joint) of one hand and the surface of the glass rolling plate. Alternate between mould and rolling to effect the gradual drying of the soil. 5. The sample is rolled into threads between the fingers, from the finger-tip to the second joint of one hand and the palm of the other hand until the thread shears both longitudinally and the palm of the other hand until the thread shears both longitudinally and transversely when it has been rolled to about 5 mm in diameter size. Do not gather the pieces of sample together after they have crumbled, in order to reform a thread and to continue rolling, the first crumbling point is the plastic limit. 6. 2 metal containers are taken and recorded their identification on the Table 4.1. Each container is weighed separately and record weights are recorded.
7. The crumbled thread is placed into one container and process of step 4-5 is repeated on the three other remaining portions, placing all in the same container. The container containing the crumbled threads is weighed and the weight recorded. 8. The step 4-6 is repeated on the duplicate sample formed in step 3 and the crumbled threads are placed into another container. The container is weigh and contents, and the weight recorded.. 9. The two containers are placed in the oven at a temperature of 105 – 110 overnight (16 hours to 24 hours). After drying, the containers are removed from the oven and allow them to cool in a desiccator. .Tthe dry weight is recorded and the moisture content is determined as specified in the Moisture Content Test. 10. The two moisture content values of the soil sample are recorded in table. Precaution 1. We cannot rolled the soil paste on the surface of the table but just can rolled on our palm to avoid further loss of moisture. 2. Gradual drying of the soil is affected by alternatively rolling and molding, not by continual rolling, either as a ball or as threads, which produces a dried crust. With soils that are marginally plastic it is often difficult to obtain the correct crumbling condition. Plasticity Chart
Results and Calculations By the same process as the liquid limit measurements, we get the value of the plastic limit:
Plastic limit
1
a) Container No
(781)
b) Mass of wet soil + container
(g)
13.73
a) Mass of dry soil + container
(g)
12.32
b) Mass of container c) Mass of dry soil
(g) (g)
5.76
d) Mass of moisture
(g)
e) Moisture content
(%)
f)
Plastic limit, PL
6.56
(%)
1.41 24.48 24.48
M1 = Mass of container
= 5.76 g
M2 = Mass of container + wet soil
= 13.73 g
M3 = Mass of container + dry soil,
= 12.32 g
M4 = Mass of dry soil
(M3 – M1)
= 5.76 g
M5 = Mass of moisture (M2 – M3)
= 1.41 g
M6 = Moisture content
= 24.48 %
(M5 x 100) M4
Therefore, Plastic Limit, WP = 24.48%
Discussion The plastic limit is the empirically moisture content at which a soil becomes too dry to be plastic. It used together with the liquid to determine the plasticity index and It is recognized that the results are subject to the judgment of the operator, and that some variability in result will occur. It is convenient to carry out the test on a portion of the prepared material prepared for one of the liquid limit test procedures. Through this experiment, we managed to achieve the objective of this lab that is to measure the plastic limit to the given soil sample. During the progress of the experiment, we found out that there are a lot of factors that could influence our experiment outcome. Below are the list of precautions that should be followed in order to achieve the objectives and also better results; 1.
We must make sure that our palms are dry when we rolled the sample into ellipsoidal size soil paste which is of 5 mm width.
2.
The weight of the soil sample should be increased in order to obtain the moisture content which will achieve better and more accurate result.
3.
Ensure that there is existence of slightly crack on the entire thread of the soil sample that had been rolled.
4.
When weighing the containers or samples, ensure that there is no excess force exerting on the balance due to its sensitivity.
5.
In all times, the container and any other apparatus should be clean before using to avoid errors and weight of excess particles.
Conclusion From the result we obtained, the value of moisture content as well as the plastic limit is 18% indicate the characteristic of the soil whereas the water contents of the soil should not less than 18% due to the crack limit of the soil. As we get the result of plastic limit, it will use to obtain the Plastic index by the formula of Liquid Limit deduct Plastic Limit. This calculation will be shown below: Plastic Limit, PL = 24.48% Liquid Limit, LL = 39.62% Plastic Index = LL – PL = 39.62 – 24.48 = 15.14% As a conclusion, 15.14% is plastic index indicates the range between the liquid limit and plastic limit whereas the percentage of water content should fall between these intervals.
Appendix
Reference 1. BS 5930 (Sample Preparation & Soils Classification system) 2. Laboratory Manual & Guideline, Soil Laboratory, Department of Civil Engineering, School of Engineering Infrastructure 3. Principle of Geotechnical Engineering, Braja M. Das, 6 th Edition, Thomson Inc
