Atterberg Limit Tests (LL and PL)

Summary

In this video, Dave Whinger from Missouri S&T's geotechnical labs demonstrates the laboratory procedures for conducting the Atterberg limit tests, which include the liquid limit and plastic limit tests. These tests are crucial for classifying the fine-grained portion of soil and have practical applications in geotechnical engineering. The video showcases how to perform these tests on a silt sample from the Mississippi River Valley, which has been modified with bentonite clay to enhance plasticity. The liquid limit is determined by the moisture content at which soil transitions from a viscous fluid to a plastic putty-like consistency, measured by the number of blows required to close a groove in the soil sample. The plastic limit is identified when the soil begins to crumble when rolled into a 3mm thread. The video guides viewers through the necessary equipment, calibration, and step-by-step testing process.

Takeaways

• 🧪 The video discusses laboratory procedures for performing Atterberg limit tests, specifically the liquid limit and plastic limit tests, which are crucial for classifying the fine grain portion of soil.
• 🏗️ Atterberg limits are index tests used in geotechnical engineering for soil description, classification, and correlating to engineering properties like shear strength.
• 📏 The liquid limit is determined by the moisture content at which soil transitions from behaving like a viscous fluid to a plastic putty, defined by the number of blows required to close a groove in the soil pat.
• 🛠️ For the liquid limit test, essential lab equipment includes the CAG Grande apparatus, grooving tool, moisture content cans, water bottle, mixing bowl, spatula, mortar, and pestle.
• 🔍 The plastic limit is identified as the moisture content where soil changes from a plastic putty to a brittle solid, determined by rolling soil into a thread until it crumbles at 3mm diameter.
• 📈 To produce a flow curve, four liquid limit tests are required at specific blow count ranges, and the results are plotted to determine the liquid limit through linear regression.
• 🌡️ Moisture content determination is a critical step, involving weighing soil samples before and after drying to ascertain the water content affecting soil behavior.
• 🔄 The process involves iterative testing and adjustment of soil moisture to achieve the desired blow counts for accurate liquid limit determination.
• ⚖️ Accurate calibration of the CAG Grande apparatus is necessary to ensure consistent and reliable test results.
• 📑 The lab documents, including the handout, ASM standards, and data sheets, are available on Blackboard for reference, guiding the execution and recording of the tests.

Q & A

What are the primary purposes of the Atterberg limit tests?

The Atterberg limit tests, including the liquid limit and plastic limit tests, are used to classify the fine-grained portion of soil and provide insights into how the soil will behave under certain conditions.

What is the significance of the liquid limit in geotechnical engineering?

The liquid limit is significant in geotechnical engineering as it represents the moisture content at which a soil transitions from behaving like a viscous fluid to a plastic substance, typically measured by the number of blows required to close a groove in the soil sample.

What is the plastic limit in the context of soil classification?

The plastic limit is the moisture content at which a soil starts to crumble when rolled into a thread of 3mm diameter, indicating the transition from plastic to brittle behavior.

What is the practical application of Atterberg limits in geotechnical engineering?

The Atterberg limits are used for soil description, quantitative soil classification, and correlating with engineering properties such as shear strength.

What type of soil is used in the described laboratory procedure?

The soil used in the laboratory procedure is a silt from the Mississippi River Valley, which has been mechanically pulverized and passed through a No. 40 sieve to remove large particles, with bentonite clay added to increase plasticity.

What equipment is necessary to perform a liquid limit test according to the script?

The equipment needed for a liquid limit test includes the Casagrande apparatus, grooving tool, moisture content cans, water bottle, mixing bowl, spatula, mortar and pestle, and a scale.

How should the soil be prepared before conducting the liquid limit test?

The soil should be mixed with water until it appears uniform and has a creamy surface, which may take up to 20 minutes of proper blending.

What is the procedure for calibrating the Casagrande apparatus?

The Casagrande apparatus is calibrated by placing the grooving tool underneath the cup and adjusting the thumb screw to ensure the cam just contacts the cup without lifting it, using the 10mm block on the grooving tool.

How are the blow counts determined during the liquid limit test?

The blow counts are determined by rotating the handle of the Casagrande apparatus at 120 revolutions per minute and counting the number of blows required for the groove to close to 13mm.

What is the process for conducting the plastic limit test as described in the script?

The plastic limit test involves rolling a soil sample into a thread until it reaches a 3mm diameter, at which point the soil should crumble. Samples are then collected for moisture content determination to find the average moisture content at the point of crumbling.

How many liquid limit tests must be conducted to produce a flow curve, and what are the required blow count ranges for these tests?

Four liquid limit tests must be conducted, with one test each between blow counts of 15-20, 20-25, 25-30, and 30-35.