Soil Classification using AASHTO

Summary

This video tutorial teaches viewers how to classify soils using the AASHTO system, focusing on particle size and plasticity. It explains the significance of sieve sizes 200, 40, and 10 for distinguishing soil types and the plasticity index's role in classifying clay and silt. The video also covers the AASHTO classification table, soil classification chart, group index calculations, and provides exam tips and examples to solidify understanding.

Takeaways

• 📚 AASHTO and USCS are the two primary soil classification systems. AASHTO is primarily used in highway design, while USCS is used for building and highway projects.
• 🔍 AASHTO defines particles by grain size and plasticity, with key boundaries: sieve #200 (0.075 mm) between fine and coarse grains, sieve #10 (2.0 mm) between sand and gravel, and 75 mm (3 inches) between gravel and cobbles.
• 🧮 The plasticity index (PI) of 10 is the boundary between clay and silt soils, while some references also divide based on grain size at 0.002 mm.
• 📊 The AASHTO classification table divides soils into groups: A1s, A3, and A2s for granular soils, and A4, A5, A6, A7s for cohesive soils (silty or clay).
• 🔗 AASHTO uses a percentage passing sieve #200 to distinguish between soil types: more than 35% passing means silty/clay soils, while less than 35% indicates granular soils.
• ⚖️ Soil plasticity is another key factor in classification. Soils with a PI greater than 10 are clay, while those with a PI under 10 are silty or granular.
• 🧪 Group index (GI) is used to evaluate soil quality for highway subgrade. The lower the GI, the better the soil quality, with no upper limit for GI.
• 📏 The soil classification chart helps visualize soil properties, with plasticity index on the vertical axis (PI of 10) and liquid limit on the horizontal axis (LL of 40).
• 🔠 Soil groups like A2-4, A2-5, and A2-6 are positioned in specific areas of the classification chart, aiding in visualizing where soil classes fit based on plasticity and liquid limits.
• 🧠 The process of classifying a soil sample involves determining particle size, liquid limit, and plasticity index, then using process of elimination within the AASHTO classification table.

Q & A

What is the AASHTO classification system used for?

The AASHTO classification system is used for classifying soils, primarily in highway design projects, based on their particle size and plasticity.

What are the two primary soil classification systems mentioned?

The two primary soil classification systems mentioned are the Unified Soil Classification System (USCS) and the AASHTO classification system.

What is the difference between USCS and AASHTO usage?

USCS is mainly used for projects involving highways and buildings, while AASHTO is primarily used in highway design projects.

What are the boundaries for fine-grained and coarse-grained particles according to AASHTO?

According to AASHTO, the boundary between fine-grained particles (silts and clay) and coarse-grained particles (sand and gravel) is sieve #200, which has an opening size of 0.075 mm.

How are clay and silty soils divided according to AASHTO?

AASHTO divides clay and silty soils based on their plasticity index (PI). The boundary is a PI of 10, with PI less than 10 indicating silty soils and PI greater than 10 indicating clay soils.

What are the AASHTO group classifications for soils?

The AASHTO group classifications are A1, A2, A3 for granular soils, and A4, A5, A6, A7 for silty and clay soils. A8 is used for peat and highly organic soils.

What determines whether a soil is classified as granular or cohesive in the AASHTO system?

A soil is classified as granular if less than or equal to 35% of the material passes through sieve #200. If more than 35% passes through, it is classified as cohesive.

How does AASHTO classify gravelly and sandy soils?

AASHTO classifies soils as gravelly if less than 50% of the material passes through sieve #10. If more than 50% passes, it is classified as sandy soil.

What is the purpose of the AASHTO soil classification chart mentioned in the video?

The AASHTO soil classification chart helps visualize the plasticity portion of the classification table, with axes representing the plasticity index and liquid limit.

What is the group index (GI) used for in the AASHTO classification system?

The group index (GI) is used to evaluate the suitability of a soil to be used as a highway subgrade. A lower GI indicates better quality material, while a higher GI indicates poorer quality.