Swedish scientist Albert Atterberg was the first person to define the limits of soil consistency for the classification of fine-grained soils. Nearly 100 years later, his methods are still being used to determine the Liquid Limit, Plastic Limit and Shrinkage Limit of soils. This blog post will define the main components that encompass Atterberg Limits, explain the significance of these tests, and cover basic equipment used in these procedures.
Why are Atterberg Limits Tests Important?
Soils for engineering use are often classified based on properties relative to foundation support or as they might perform under pavements and in earthworks. Geotechnical classification systems today are designed to make it easy to equate field observations to estimates of engineering properties. As moisture content of a clay-like soil increases, it goes through four distinct states of consistency: solid, semi-solid, plastic, and liquid. Each stage is defined by significant changes in strength, consistency and behavior, and Atterberg Limits tests accurately delineate these boundaries using moisture contents at the specific points where the physical changes occur. Knowledge of these values helps in foundation design of structures and to predict behavior of soils in fills and embankments. The values can contribute to estimates of shear strength and permeability, prediction of settlement, and identification of potentially expansive soils.
What is Liquid Limit and Plastic Limit?
Now that you understand the importance of Atterberg Limits, let’s define the components.
- Liquid Limit is the water content at which soil changes from a plastic to a liquid state, when the soil specimen is just fluid enough for a groove to close when jarred in a specified manner.
- Plastic Limit is the water content at the change from a plastic to a semisolid state. This test attempts to deform a soil specimen below the moisture by rolling it into a thread, resulting in the soil crumbling.
- Shrinkage Limit is the water content where further loss of moisture does not cause a decrease in specimen volume.
- Plasticity Index (PI) is calculated as the Plastic Limit subtracted from the Liquid Limit and is an important value when classifying soil types.
How Does the Procedure Work?
- Liquid Limit is measured by spreading a portion of the soil sample in the brass cup of a Liquid Limit Machine and dividing it using a grooving tool. The moisture at which the groove can be closed for a 1/2in distance using 25 specified drops of the cup is defined as the Liquid Limit.
- Plastic Limit is determined by repeatedly pressing together a small ball of plastic soil with your hands and rolling it out into a 1/8in thread. The Plastic Limit is the moisture content at which the ball will crumble before it can be completely rolled out. The Plastic Limit Roller is a device that can also be used to perform this test.
Liquid and Plastic Limit tests are outlined in further detail in ASTM D 4318, and Shrinkage Limit tests are detailed in ASTM D 4943.
What Equipment Do You Need?
Liquid and Plastic Limits:
- Motor-driven or hand-crank operated Liquid Limit Machine
- ASTM (Casagrande) Grooving Tool or AASHTO Grooving Tool
- Balance with 0.01g readability to meet the spec
- Mortar and Pestle for particle size reduction
- Porcelain Evaporating Dish used to mix specimen to the desired moisture content
- Spatula for mixing, forming and smoothing soil specimen
- Glass Plate used to roll out threads of soil
- Optional Rolling Device for faster and more consistent rolling of threads
- Wash Bottle to easily dispense water when mixing with soil
- Washing Pan for convenient clean-up of bowls and spatulas
- Aluminum Containers for moisture content determinations
You also have the option to use a Soil Grinder to expedite the sample preparation process for Atterberg Limits.
Shrinkage Limit (in addition to equipment listed above):
Regardless of what equipment you decide to use, it’s important to remember that Atterberg Limits play a key role in the early stages of structural design to ensure that soil will carry the structure without excessive settlement and not too much change in volume as it shrinks and expands with different moisture contents.