What is plastic limit of soil?
This article will discuss the ASTM D4148 test methods used to determine the plastic limit of soil. The plastic limit is the point at which soil starts behaving like a plastic material. At this moisture content, the soil will break into pieces when rolled into 3.2mm (1/8in) diameter threads.
Apparatus
In order to carry out certain tests related to the plastic limit of a substance, several tools and equipment are required. One such item is a ground glass plate that should be at least 30 cm (12 in.) square by 1 cm (3?8 in.) thick. This is necessary for rolling plastic limit threads. Another tool that is optional, but may be helpful, is a plastic limit-rolling device. A spatula or pill knife is also needed, with a blade that is about 2 cm wide and between 10 to 13 cm long.
A drying oven is required in order to dry out the substance being tested, and a metallic rod that is 3.2mm in diameter and 100mm long is also necessary. Water content containers will be needed to hold the samples being tested, and a balance that conforms to Specification D 4753, Class GP1 is required. The balance must have a readability of 0.01 g. These tools and equipment are essential for conducting accurate tests related to the plastic limit of a substance.
Fig.1: Set of apparatus used for plastic limit test
Soil sample preparation
To prepare for the liquid limit test, it is necessary to select a portion of soil that weighs 20 grams or more. The soil should be carefully chosen to ensure it represents the entire sample being tested.
Once the soil has been selected, it is essential to reduce its water content to a level where it can be rolled without sticking to the hands. This can be achieved by spreading or mixing the soil continuously on a glass plate or in a mixing/storage dish.
To speed up the drying process, an electric fan can be used to expose the soil to a steady stream of air. This will help to remove excess moisture from the soil, making it easier to handle and manipulate. It is important to ensure that the fan is not too powerful, as this could cause the soil to become airborne and contaminate the surrounding area.
Fig.2: Plastic limit sample
Plastic limit test procedure
To form an ellipsoidal mass, first, select a portion from the plastic-limit specimen that weighs between 1.5 to 2.0 grams. This selection is crucial to ensure that the resulting ellipsoidal mass is of the desired size and shape. Once the appropriate amount is chosen, shape it into an ellipsoidal form. The ellipsoidal shape can be achieved by manipulating the specimen with the fingers or by using a shaping tool. This process is important for conducting various tests on the specimen, as an ellipsoidal shape provides a standardized shape that can be used for comparison between specimens. Overall, selecting and shaping the plastic-limit specimen into an ellipsoidal mass is a crucial step in conducting accurate tests and obtaining reliable results.
Fig. 3: Ellipsoidal plastic limit sample
The given context describes the method of rolling a soil mass either by hand or using a rolling device. When using the hand method, the soil mass should be rolled between the palm or fingers and a ground-glass plate with just enough pressure to form a thread of uniform diameter throughout its length. The diameter of the thread should be further reduced on each stroke until it reaches 3.2 mm, which should take no longer than 2 minutes. It is recommended to perform 80-90 strokes per minute, with each stroke defined as a complete forward and backward motion of the hand to the starting position. However, for very fragile soils, the rate of rolling should be reduced.
Fig. 4: Rolling plastic limit sample
Fig.5:performing plastic limit test on soil sample
The rolling device method is a technique used to determine the plastic limit of a soil sample. This method involves using a plastic limit-rolling device, which consists of two plates and slide rails. To begin the process, attach smooth unglazed paper to both plates to prevent the soil from sticking to them. Then, place the soil mass at the midpoint between the slide rails on the bottom plate, and place the top plate in contact with the soil mass.
Next, apply a slight downward force and back and forth motion to the top plate to make it come into contact with the side rails within two minutes. During the rolling process, it’s important to ensure that the end of the soil thread doesn’t touch the side rail. When the diameter of the thread reaches 3.2 mm, break it into several pieces, squeeze them together, knead them between the thumb and first finger of each hand, reform them into an ellipsoidal mass, and re-roll.
This rolling and kneading process is repeated until the thread crumbles under the pressure required for rolling, and the soil can no longer be rolled into a 3.2-mm diameter thread. By determining the number of times the soil can be rolled and kneaded before it reaches this point, the plastic limit of the soil can be calculated.
Fig.6: Lean clay soil at plastic limit
To determine the water content of soil, the following steps should be followed in accordance with Test Method D 2216. Firstly, a plastic limit specimen of soil should be taken and thoroughly mixed to ensure homogeneity. Then, a 1.5 to 2.0-g portion of soil should be selected and crumbled into small pieces. These pieces should be placed in a container of known mass, and the container should be immediately covered.
This process should be repeated until the container has at least 6 g of soil. Another container should also be prepared in the same way to ensure sufficient soil for accurate testing. Once both containers have been filled, the water content of the soil can be determined.
The determination of water content is an important factor in soil analysis and can provide valuable information about the properties of the soil. By following these steps, the water content of soil can be accurately determined using Test Method D 2216.
Calculation
To determine the plastic limit (PL), the two water contents obtained from conducting the trial plastic limit test must be averaged. The resulting average value should be rounded to the nearest whole number. However, it is important to ensure that the difference between the two trial plastic limits is within an acceptable range of 1.4 percentage points. If the difference exceeds this range, the test must be repeated to obtain more accurate results.
Table 1 Data Sheet for Plastic Limit Test
| Sl. No. | Observations and Calculations | Determination No. | ||
| 1 | 2 | 3 | ||
| Observation | ||||
| 1 | Moisture content container number | |||
| 2 | Mass of empty container (M1) | |||
| 3 | Mass of container + wet soil (M2) | |||
| 4 | Mass of container + dry soil (M3) | |||
| Calculations | ||||
| 5 | Mass of water = M2 – M3 | |||
| 6 | Mass of dry soil = M3 – M1 | |||
| 7 | Water content = w = [(5) / (6)] x 100 | |||
Result
Plastic limit of the soil = _____%