Questions and Answers
What do soil classification systems provide?
A common language to express general characteristics of soils.
The soil classification systems are mainly based on which of the following?
According to the AASHTO classification system, soil is classified into _____ major groups.
7
What percentage of granular materials passes through the 75 μm sieve according to the AASHTO soil classification system?
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What is the general subgrade rating for soils of group A-4, A-5, A-6, and A-7 according to the AASHTO soil classification?
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The term silty is applied when the fine fractions of soil have a plasticity index of _____ according to the AASHTO soil classification.
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Group index (GI) is defined in terms of which parameters?
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If the calculated value of GI is negative, it is taken as _____?
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If the calculated value of GI is 3.5, it is taken as _____?
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The group index of soils belonging to group A-3 is always _____?
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The quality of performance of a soil as a subgrade material is inversely proportional to the group index.
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Who proposed the original form of the Unified Soil Classification System?
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According to the Unified Soil Classification System (USCS), what defines fine-grained soils?
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What is the symbol for silt?
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The symbol 'O' is used for which type of soils?
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Dual symbols are required for which of the following?
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What is the group symbol for silty sand?
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What does the group symbol SC stand for?
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For soils of group MH, the liquid limit is greater or equal to _____?
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For soils of group GW, what are the conditions regarding the coefficients?
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What defines USCS name rules?
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Provide the formula for Group Index Rules.
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Study Notes
Soil Classification Systems
- Soil classification systems establish a standardized language for describing soil characteristics.
- Key factors in classification include grain-size distribution and plasticity.
AASHTO Classification System
- The American Association of State Highway Officials (AASHTO) recognizes seven major soil groups.
- Granular materials are characterized by 35% or less passing through a 75 μm sieve.
- Soils classified as A-4, A-5, A-6, and A-7 typically receive a fair to poor subgrade rating.
- The term "silty" applies to soils with a plasticity index of 10 or less.
Group Index (GI)
- The Group Index (GI) is calculated using the percentage passing 75 μm, liquid limit, and plasticity index.
- If GI is negative, it is assigned a value of 0; for a calculated GI value of 3.5, it is rounded to 4.0.
- Soils in group A-3 inherently have a GI of 0.
- Soil performance quality as subgrade material decreases as the GI increases.
Unified Soil Classification System (USCS)
- The original USCS was proposed by Casagrande in 1948.
- Fine-grained soils contain 50% or more passing a 75 μm sieve.
- Soil symbols include "M" for silt and "O" for organic silts and clays.
Dual Symbols and Group Symbols
- Dual symbols are necessary for sands and gravels containing 5 to 12% fines.
- The group symbols "SM" represent silty sand, while "SC" denotes clayey sand.
- Group MH soils have a liquid limit of 50 or greater; group GW soils have a coefficient of uniformity (Cu) of 4 or more, with Cc between 1 and 3.
USCS Naming Rules
- Coarse-grained soils with less than 50% passing through the No. 200 sieve use "G" for gravelly and "S" for sandy group symbols.
- Fine-grained soils with 50% or more passing through the No. 200 sieve start with "M" (inorganic silt), "C" (inorganic clay), or "O" (organic silts and clays).
- Peat, muck, and highly organic soils are represented by the symbol "Pt."
Group Index Calculation
- The formula for calculating GI is based on fines, liquid limit, and plasticity index.
- If the calculated GI is negative, it is adjusted to 0; rounding rules apply for values close to whole numbers.
- The GI is zero for specific groups: A-1-a, A-1-b, A-2-4, A-2-5, and A-3.
- For groups A-2-6 and A-2-7, a partial group index for PI is used for calculating GI.
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Test your knowledge on soil classification systems with these flashcards! This quiz focuses on definitions and key concepts from Chapter 4 related to soil characteristics, classification based on grain-size and plasticity, and the AASHTO system. Perfect for students studying soil science.