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Questions and Answers
What do soil classification systems provide?
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?
The soil classification systems are mainly based on which of the following?
- Soil color
- Organic content
- Grain-size distribution (correct)
- Plasticity of soils (correct)
According to the AASHTO classification system, soil is classified into _____ major groups.
According to the AASHTO classification system, soil is classified into _____ major groups.
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What percentage of granular materials passes through the 75 μm sieve according to the AASHTO soil classification system?
What percentage of granular materials passes through the 75 μm sieve according to the AASHTO soil classification system?
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?
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?
The term silty is applied when the fine fractions of soil have a plasticity index of _____ according to the AASHTO soil classification.
The term silty is applied when the fine fractions of soil have a plasticity index of _____ according to the AASHTO soil classification.
Group index (GI) is defined in terms of which parameters?
Group index (GI) is defined in terms of which parameters?
If the calculated value of GI is negative, it is taken as _____?
If the calculated value of GI is negative, it is taken as _____?
If the calculated value of GI is 3.5, it is taken as _____?
If the calculated value of GI is 3.5, it is taken as _____?
The group index of soils belonging to group A-3 is always _____?
The group index of soils belonging to group A-3 is always _____?
The quality of performance of a soil as a subgrade material is inversely proportional to the group index.
The quality of performance of a soil as a subgrade material is inversely proportional to the group index.
Who proposed the original form of the Unified Soil Classification System?
Who proposed the original form of the Unified Soil Classification System?
According to the Unified Soil Classification System (USCS), what defines fine-grained soils?
According to the Unified Soil Classification System (USCS), what defines fine-grained soils?
What is the symbol for silt?
What is the symbol for silt?
The symbol 'O' is used for which type of soils?
The symbol 'O' is used for which type of soils?
Dual symbols are required for which of the following?
Dual symbols are required for which of the following?
What is the group symbol for silty sand?
What is the group symbol for silty sand?
What does the group symbol SC stand for?
What does the group symbol SC stand for?
For soils of group MH, the liquid limit is greater or equal to _____?
For soils of group MH, the liquid limit is greater or equal to _____?
For soils of group GW, what are the conditions regarding the coefficients?
For soils of group GW, what are the conditions regarding the coefficients?
What defines USCS name rules?
What defines USCS name rules?
Provide the formula for Group Index Rules.
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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