soil classification for construction, Lecture notes of Soil Mechanics and Foundations

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CLASSIFICATIONS ANDCLASSIFICATIONS AND

FIELD IDENTIFICATION FIELD IDENTIFICATION

OF SOILS OF SOILS

• I. CLASSIFICATIONS OF SOILS

• (^) A soil classification system is the arrangement of different soils having similar properties into groups and sub-groups based on their application.
• (^) It provides a common language to express briefly the general characteristics of soils.
• (^) There are several methods of classifying soils.
• (^) The most widely used classification systems by engineers are described here.

Gravel Sand Silt Clay Coarse Medium Fine Coarse Medium Fine (b) M.I.T ( Massachusetts Institute of Technology) Classification 2 0.6 0.2 0.06 0.02 0.006 0. Grain size in mm (c) Textural Classification This classification is based on a triangular chart shown below. A soil with known percentage of sand, silt and clay sizes is represented by a given point on a triangular chart of this type. Example 35% sand -, 25% silt-, and 40% clay-size particles, This soil falls into the zone of Sandy clay.

CLAY 50 % by weight of silt 100 0 10 20 30 40 60 80 90 SAND 70 SI LTY SAND SANDY CLAY 30 40 50 20 10 0 0 (^60 7080 ) 40 CLAYEY SI LT SANDY SI LT SI LTY CLAY 20 10 SI LT 30 70 60 50 80 90 SI LT 100 CLAYEY SAND

2.1 AASHTO Classification System

• (^) The AASHTO classification system, also called Public Roads Administration (PRA) classification, is based on the following three soil properties: - (^) Grain-size distribution - (^) Liquid limit - (^) Plasticity index
• (^) This system is generally used by highway engineers, for classification of sub-grade soils for the highway pavements.
• (^) According to this system, soil is classified into seven major groups, A-1 through A-7.

• (^) Soils classified under groups A-1, A-2, and A- are granular materials with 35% or less passing through a No. 200 sieve.
• (^) -Soils with more than 35% passing a No. 200 sieve are classified under groups A-4, A-5, A- and A-7. These soils are mostly silt and clay- type materials.
• (^) To classify a soil using AASHTO soil classification Table, one must proceed from left to right with required test data available. By the process of elimination, the first group from the left into which the test data will fit gives the correct classification.

• (^) a= that portion of percentage of particles passing No.200 sieve greater than 35% and not exceeding 75% expressed as a whole number(0 to 40)
• (^) b= that portion of percentage of particles passing No.200 sieve greater than15% and not exceeding 55% expressed as a whole number(0 to 40)
• (^) c= that portion of the liquid limit greater than 40 and not exceeding 60, expressed as a Positive whole number(0 to 20)
• (^) d= that portion of the Plasticity index greater than 10 and not exceeding 30, expressed as a Positive whole number(0 to 20)

General Classification Granular Materials (35 per cent or less passing No. 200) Silt-clay Materials (More than 35 percent passing No. 200) Group Classification A- A- A- A-4 A-5 A- A- A-7- A-7- A-1-a A-1-b A-2-4 A-2-5 A-2-6 A-2- Sieve analysis per cent passing No. 10 No. 40 No. 200 50 max 30 max 15 max 50 max 10 max 51 min 10 max 35 max 35 max 35 max 35 max 36 min 36 min 36 min 36 min Characteristics of fraction passing No. 40 sieve Liquid limit Plasticity Index 6 (max) N.P 40max 10max 41 min 10max 40max 11 min 41 min 11min 40 max 10 max 41 min 10 max 40 max 11 min 41 min 11 min Group index 0 0 0 4 (max) 8 (max) 12 (max) 16(max) 20 (max) Usual types of significant constituent materials Stone fragments gravel and sand Fine sand Silty or clayey gravel and sand Silty soils Clayey soils General rating as sub- grade Excellent to good Fair to poor The A-7 group is subdivided into A-7-5 or A-7-6 depending on the plastic limit. For P.L.<30, the classification is A-7-6; for P.L  30, it is A-7-5.

• (^) The name and symbols used to distinguish between the typical and boundary soil groups are given below.
• (^) a) Coarse-grained soils G = Gravel , S = Sand
• (^) The gravel and sand are further divided into subgroups as;
• (^) W = Well graded P = Poorly graded
• (^) b) Fine-grained soils » (^) M = Inorganic silt » (^) C = Inorganic clay » (^) O = Organic silts and clays » (^) Pt = Peat

• (^) The above groups of fine-grained soils (excluding peat) are further subdivided according to their liquid limits into » (^) L = Low plasticity (ℓ < 50%) » (^) H = High plasticity (ℓ > 50%)
• (^) Plasticity Chart: - The plasticity chart is a plot of plasticity index versus liquid limit. Fine-grained soils are subdivided into soils of low, medium and high plasticity following the criteria cut-lined below
• (^) Low plasticity (ℓ < 30%)
• (^) Medium plasticity (ℓ between 30% and 50%)
• (^) High plasticity (ℓ> 50%)
• (^) The diagonal line drawn in the plasticity chart is called the “A” line and is given by the equation P = 0.73 (ℓ - 20). Clays fall above the A-line and silts below it.

• (^) 1. Coarse-grained Soil
• (^) -Coarse-grained soils are mineral fragments that may be identified primarily on the basis of grain size. The different constituents of coarse-grained soils are sand and gravel.
• (^) The size of sand varies from 0.075 mm to 2 mm
• (^) and that of gravel from 2 mm to 60mm.
• Sand can further be classified as coarse, medium and fine.
• (^) The description of sand and gravel should include an estimate of the quantity of material in the different size ranges as well as a statement of the shape and mineralogical composition of the grains.
• (^) The mineral grains can be rounded, sub- rounded, sub-angular, or angular

• (^) 2. Fine Grained Soils
• (^) A. Inorganic Soils: - The constituent parts of fine-grained materials are silt and clay fractions.
• (^) The classification tests used in the field for preliminary identification are; a. Dry strength: The strength of a soil in a dry state is an indication of its cohesion and hence of its nature.
• (^) It can be estimated by crushing a 3 mm size dried fragment between thumb and forefinger.
• (^) A clay fragment can be broken only with great effort, whereas a silt fragment crushes easily.