Soil Classification Chapter 4

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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?

Soil color

Organic content

Grain-size distribution (correct)

Plasticity of soils (correct)

According to the AASHTO classification system, soil is classified into _____ major groups.

What percentage of granular materials passes through the 75 μm sieve according to the AASHTO soil classification system?

35% or less Signup and view all the answers

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?

Fair to poor. Signup and view all the answers

The term silty is applied when the fine fractions of soil have a plasticity index of _____ according to the AASHTO soil classification.

10 or less Signup and view all the answers

Group index (GI) is defined in terms of which parameters?

All of the above Signup and view all the answers

If the calculated value of GI is negative, it is taken as _____?

0 Signup and view all the answers

If the calculated value of GI is 3.5, it is taken as _____?

4.0 Signup and view all the answers

The group index of soils belonging to group A-3 is always _____?

0 Signup and view all the answers

The quality of performance of a soil as a subgrade material is inversely proportional to the group index.

True Signup and view all the answers

Who proposed the original form of the Unified Soil Classification System?

Casagrande in 1948. Signup and view all the answers

According to the Unified Soil Classification System (USCS), what defines fine-grained soils?

50% or more passing through the 75 μm sieve. Signup and view all the answers

What is the symbol for silt?

M Signup and view all the answers

The symbol 'O' is used for which type of soils?

Organic silts and clays. Signup and view all the answers

Dual symbols are required for which of the following?

Both A and B Signup and view all the answers

What is the group symbol for silty sand?

SM Signup and view all the answers

What does the group symbol SC stand for?

Clayey sand. Signup and view all the answers

For soils of group MH, the liquid limit is greater or equal to _____?

50 Signup and view all the answers

For soils of group GW, what are the conditions regarding the coefficients?

Cu ≥ 4 and 1 ≤ Cc ≤ 3 Signup and view all the answers

What defines USCS name rules?

All of the above Signup and view all the answers

Provide the formula for Group Index Rules.

GI = (F - 35)[0.2 + 0.005(LL - 40)] + 0.01(F - 15)(PI - 10) Signup and view all the answers

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.