The water content of soil can significantly change its behavior. To get a reading on how soil will change depending on its level of moisture, and how stable it will be as it changes, engineers can testsoil samples to find their Atterberg Limits. The tests are easy to perform in a lab, but don’t let their simplicity undermine their importance in understanding the safety of a foundation or earthworks site.
Why is Testing Atterberg Limits Important?
Atterberg Limits provide definitions for a soil’s limits: its liquid limit, plastic limit and shrinkage limit. Each of these limits depends on how the soil behaves as its moisture level changes. Knowing Atterberg Limits is especially important for clays and silts, which react the most to moisture changes and have four different states of consistency: solid, semi-solid, plastic and liquid. These physical changes can greatly affect the final design of a structure, which needs to remain stable even as the consistency of the soil below it changes.
Defining Atterberg Limits
Atterberg Limits tests help to accurately distinguish the boundaries between each of a soil’s states, from solid to liquid. So, for example, when a soil sample no longer decreases in volume due to moisture loss, it has reached its shrinkage limit. Atterberg Limits can also be used to classify different types of soil at a site. The limits are:
- Plastic Limit: when a soil sample changes from a plastic to a semisolid state due to decreased moisture content. When this change occurs, the soil will crumble when you attempt to roll it into a thread.
- Liquid Limit: when a soil sample changes from plastic to liquid. When this change occurs, a groove cut into the sample will close easily if its container is jarred from the outside.
- Shrinkage Limit: as mentioned above, this limit describes a soil sample that no longer loses volume even as it continues to lose moisture.
- Plasticity Index (PI): used to classify soil types, this value is determined by subtracting the plastic limit from the liquid limit.
Atterberg Limits Tests
The most commonly used Atterberg Limits tests are for plastic and liquid limits, which are then used to determine a sample’s plasticity index. The shrinkage limit is determined with the ASTM International D4943 method.
To determine a plastic limit, simply roll out a thread of the sample on a flat, non-porous surface. As you roll it out, it should retain its shape until it gets very thin, at which point you can remold it into a ball and repeat. Moisture will begin to evaporate as you roll the sample, causing it to break at gradually larger diameters. When the sample breaks apart at 1/8 inch diameter (3.2 mm), it has reached its plastic limit. Keep in mind that this test is only relevant for soils with a plastic state, like clay.
Liquid limit can be determined using two different methods. The first called the Casagrande Method, involves placing a pat of clay in the bottom of a metal bowl and using a Casagrande grooving tool with a standardized width of 2 mm to cut and separate it into two halves. Next, a motorized liquid limit machine strikes the bottom of the bowl until the grooves touch one another again. The moisture content at which it only takes 25 blows to the bowl to close the groove is the sample’s liquid limit.
The Reliable Fall Cone Test
The second is the fall cone test, also known as the cone penetrometer test. This method was developed to obtain more reliable results since the Casagrande Method requires the judgment of the operator working the liquid limit machine. The samples are penetrated using soil penetrometers, specifically a stainless steel cone with an apex angle of 30° and a mass of 80 g (including its shaft). The liquid limit in this test is defined as the water content of the soil at which the cone can penetrate exactly 20 mm during the 5-second cone release period.
The Atterberg Limits, developed by Swedish scientist Albert Atterberg, is a basic measure of the critical water contents of fine-grained soil. These limits are used extensively in geotechnical engineering and soil science to classify fine-grained soils, particularly clay, and silt, which significantly change their state with varying moisture content. The Atterberg Limits consist of three main parameters: the Liquid Limit, the Plastic Limit, and the Shrinkage Limit.
1. Liquid Limit (LL):
The Liquid Limit is the moisture content at which soil transitions from a plastic state to a liquid state. It is determined through a test where a soil paste is placed in a standard cup and cut by a groove. The cup is dropped repeatedly from a set height, and the number of drops required to close the groove over a specified distance is recorded. The moisture content at this point is the Liquid Limit. It represents the upper limit of soil’s plasticity.
2. Plastic Limit (PL):
The Plastic Limit is the moisture content at which soil begins to behave as a plastic material. To determine the Plastic Limit, a soil sample is rolled into threads until they crumble at a diameter of 3.2 mm. The moisture content at which the soil crumbles is recorded as the Plastic Limit. It denotes the lower limit of soil’s plasticity.
3. Shrinkage Limit (SL):
The Shrinkage Limit is the moisture content at which further loss of moisture will not result in a decrease in the volume of the soil. It is less commonly determined but is crucial in understanding the volume changes in soils due to moisture variations.
Applications and Importance:
The Atterberg Limits are fundamental in soil classification systems like the Unified Soil Classification System (USCS) and the American Association of State Highway and Transportation Officials (AASHTO) system. These limits provide essential information about the behavior of fine-grained soils under varying moisture conditions, influencing their suitability for construction projects. For example, soils with high Liquid and Plastic Limits may be too unstable for foundation support, whereas soils with low limits may be more stable.
In conclusion, understanding the Atterberg Limits is crucial for predicting the behavior of soils in construction, agriculture, and earth science applications. They provide valuable insights into the workability, compressibility, and other mechanical properties of soils, guiding the design and construction of foundations, embankments, and other soil-structure interactions.
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