CHAPTER: 5 SOIL CLASSIFICATION CONTENTS:

Soil

classification,

USCS

system,

AASHTO

system,

Textural

classification.

5.1 SOIL CLASSIFICATION Soils are all different, depending on their origins, compositions, locations, geological histories, and many other factors. Two soils may be quite different, even though they were obtained from nearby boring holes on the same construction site. However, it is more convenient for engineers when soils are categorized into several groups with similar engineering behaviors. Engineers can understand approximate engineering characteristics of those grouped soils without actual laboratory or field tests. This process is called soil classification, and it helps engineers in the preliminary design stage of geotechnical engineering problems. There are many soil classification standards in many countries. Those standards uses

Atterberg limits and soil gradation information for the classification of soils. Mainly three classification systems are widely used in geotechnical engineering group. These are  USCS system  AASHTO system  Textural classification

5.2 UNIFIED SOIL CLASSIFICATION SYSTEM (USCS) USCS system was firstly developed by Arthur Casagrand for wartime airfields construction in 1942 and the system was modified and adopted for regular use by Army Corps of Engineers and then by the Bureau of Reclamation in 1952 as the Unified Soil Classification System. Currently ASTM (D-2487) adopts it. The system uses six major symbols and four modifiers as in the following:

Soil Classification

Major symbols

Modifiers

G S M C O Pt

W Well graded (for gravel and sand) P Poorly graded (for gravel and sand) H High plasticity (for silt, clay, and organic soils) L Low plasticity (for silt, clay, and organic soils)

Gravel Sand Silt Clay Organic Peat

The system uses soil’s gradation information and plasticity characteristics (LL, PL, PI) for the classification. Soil classification procedure is summarized in Figure 5.1 and 5.2.

Figure 5.1: USCS flowchart

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Soil Classification

Figure 5.2: Plasticity chart for USCS Since USCS uses simple symbols with their meanings, it is easy to understand the nature of soils from classified group names. This system is widely used all over the world for geotechnical engineering purpose.

5.3 AASHTO CLASSIFICATION SYSTEM The U.S Bureau of Public Roads (now the Federal Highway Administration) developed AASHTO soil classification in the late 1920s for road construction. The current version, which was revised in 1945, is used for extended applications in road bases. AASHTO method uses Atterberg limits (LL and PL) and information on grain size distribution curve. The procedure uses an elimination process of column in Table 5.1, from the upper left corner toward downward and right. If the condition on the row is not satisfied, the entire column is eliminated and it is never referred back. After the last row check for PI, one or possibly more than one column may survive this elimination process. If more than one column survived, the first column from left is selected as a group /subgroup. Page | 38

Soil Classification In addition, group index (GI) is calculated as follows 𝑮𝑰 = 𝑭𝟐𝟎𝟎 − 𝟑𝟓 𝟎. 𝟐 + 𝟎. 𝟎𝟎𝟓 𝑳𝑳 − 𝟒𝟎 + 𝟎. 𝟎𝟏 𝑭𝟐𝟎𝟎 − 𝟏𝟓 𝑷𝑰 − 𝟏𝟎 For A-2-6 and A-2-7 subgroups, the following equation is used 𝑮𝑰 = 𝟎. 𝟎𝟏 𝑭𝟐𝟎𝟎 − 𝟏𝟓 𝑷𝑰 − 𝟏𝟎 Note: GI=0 when calculated GI is negative

GI is reported in rounded integer number (say, GI=4.4≈4.0 and GI=4.5≈5.0)

Table 5.1: AASHTO classification system

5.4 TEXTURAL CLASSIFICATION Texture means visual appearance of the surface of a material. The visual appearance of a soil is called its texture. The texture depends upon the particle size, shape of particle and gradation of particles. The triangular classification system suggested b U.S. Bureau of Public Roads in commonly known as the textural classification system. According to the textural classification system, the percentage of sand (size 0.05 to 2. Mm), silt (size 0.005 to 0.05 mm) and clay (size less than 0.005 mm) are plotted along the three sides of an equilateral triangle. The equilateral triangle is divided into zones,

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Soil Classification each zone indicates a type of soil. Textural classification system is outlined in Figure 5.3.

Figure 5.3: Textural classification system

[TRY TO SOLVE] [1] The following information are obtained from soil gradation curve and consistency test: % passing No.4 (4.75mm)=92% % passing No.10 (2.0mm)=87% % passing No. 40 (0.425mm)=63% % passing No. 200 (0.075mm)=28% D(10)= 0.01mm, D(30)=0.090mm, D(60)=0.39mm LL=46%, PL=35% Page | 40

Soil Classification Classify the soil according to  USCS system  AASHTO system

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