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Chapter 3 classification

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Chapter 3 classification

  1. 1. Soil Classification 3. SOIL CLASSIFICATION 3.1 Field Identification of Soils The methods of field identification of soils can conveniently be discussed under the headings of coarse-grained and fine-grained soil materials. Coarse-Grained Soil Materials The coarse-grained soil materials are mineral fragments that may be identified primarily on the basis of grain size. The different constituents of coarse-grained materials are sand and gravel. As described in the earlier sections, the size of sand varies from 0.075 mm to 4.75 mm and that of gravel from 4.75 mm to 80 mm. Sand can further be classified as coarse, medium and fine. The engineer should have an idea of the relative sizes of the grains in order to identify the various fractions. A small magnifying glass can be used to identify the small fragments of shale or mica. The properties of a coarse grained material mass depend also on the uniformity of the sizes of the grains. A well-graded sand is more stable for a foundation base as compared to a uniform or poorly graded material. Fine-Grained Soil Materials Inorganic Soils: The constituent parts of fine-grained materials are the silt and clay fractions. Since both these materials are microscopic in size, physical properties other than grain size must be used as criteria for field identification. The classification tests used in the field for preliminary identification are 1. Dry strength test 2. Shaking test 3. Plasticity test 4. Dispersion test 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. Shaking test: The shaking test is also called as dilatancy test. It helps to distinguish silt from clay since silt is more permeable than clay. In this test a part of soil mixed with water to a very soft consistency is placed in the palm of the hand. The surface of the soil is smoothed out with a knife and the soil pat is shaken by tapping the back of the hand. If the soil is silt, water will rise quickly to the surface and give it a shiny glistening appearance. If the pat is deformed either by squeezing or by stretching, the water will flow back into the soil and leave the surface with a dull appearance. Since clay soils contain much smaller voids than silts and are much less permeable, the appearance of the surface of the pat does not change during the shaking test. An estimate of the relative proportions of silt and clay in an unknown soil mixture can be made by noting whether the reaction is rapid, slow or nonexistent. Plasticity test: If a sample of moist soil can be manipulated between the palms of the hands and fingers and rolled into a long thread of about 3 mm diameter, the soil then contains a significant amount of clay. Silt cannot be rolled into a thread of 3 mm diameter without severe cracking.
  2. 2. Soil Classification Dispersion test: This test is useful for making a rough estimate of sand, silt and clay present in a material. The procedure consists in dispersing a small quantity of the soil in water taken in a Soil Phase Relationships, Index Properties and Soil Classification 69 glass cylinder and allowing the particles to settle. The coarser particles settle first followed by finer ones. Ordinarily sand particles settle within 30 seconds if the depth of water is about 10 cm. Silt particles settle in about 1/2 to 240 minutes, whereas particles of clay size remain in suspension for at least several hours and sometimes several days. Organic soils: Organic material in soil is usually derived from plant or root growth and consists of almost completely disintegrated matter, such as muck or more fibrous material, such as peat. The soils with organic matter are weaker and more compressible than soils having the same mineral composition but lacking in organic matter. The presence of an appreciable quantity of organic material can usually be recognized by the dark-grey to black color and the odor of decaying vegetation which it lends to the soil. Table 3-1 Field Identification Test Test ML CL OL MI CI OI MH CH OH a. Dilatancy Quick None to slow Slow Quick to slow None Slow Slow to none None None to Very Slow b. Toughness None Medium Low None Medium Low Low to medium High Low to medium c. Dry strength None of low Medium Low Low Medium to high Low to medium Low to medium High to very Hign Medium to high 3.2 Soil Classification Systems Descriptive classification of soils Under this classification system, soils are designated as a. Boulders: Most of the systems recognise any material larger than 7.62 cm (3 inches) size as a boulder. b. Gravel: gravel is the intermediate size between boulder and sand c. Sand: particle size between 2mm-0.06mm is sand. d. Silt: organic silt (0.002-0.06mm) which is usually dark to brown colour, highly compressible. Similarly, non plastic silt ( < 0.002mm) are plate like bulk grains and incompressible. e. Clay: soil particles smaller than 200mm size and capable of showing plasticity when wet. f. Peat: peat is partly carbonized organic matter and is fibrous in nature. It is very light compared with other soils. g. Black-cotton soil: it contains high percentage of montmorinollite clay and it has very high value of swelling and shrinkage. When dry this soil is very hard but become very soft in wet. It is black in colour and not good for foundation. Textural classification
  3. 3. Soil Classification The visual appearance of the soil is called texture. The texture depends upon particle size, shape of the particles and gradation of particles. The textural classification incorporates only the particle size, as it is difficult to incorporate the other two parameters. The triangular classification system suggested by U.S. Bureau of Public Roads in commonly known as the textural classification system. Unified Soil Classification System (USCS) The Unified Soil Classification System was first developed by Casagrande in 1948. It is based on the recognition of the type and predominance of the constituents considering grain-size, gradation, plasticity and compressibility. It divides soil into three major divisions: coarse-grained soils, fine grained soils, and highly organic (peaty) soils. Table 3-2. Symbols used in USCS Division Symbols Description Primary G S M C O Pt Gravel Sand Silt Clay Organic Peat Division Symbols Description Secondary W P M C L H Well-graded Poorly graded Non-plastic fines Plastic fines Low plasticity High Plasticity
  4. 4. Soil Classification Table 3-3. Unified Soil Classification System Major Division Group Symbols Typical Names Classification Criteria Coarse- Grained Soils [More than 50% retained on No 200 (0.075mm)] Gravel [50% or more of coarse fraction retained on No. 4 sieve ( 4.75mm)] Clean Gravels GW Well graded gravels Cu > 4 Cc = 1 to 3 GP Poorly graded gravels Not meeting both criteria for GW Gravels with fines GM Silty gravels Atterberg Limits below A-line or plasticity index less than 4 Atterberg Limits in hatched GM-GC GC Clayey gravels Atterberg Limits above A-line or plasticity index greater than 7 Sand [50% or more of coarse fraction passing through No. 4 sieve ( 4.75mm)] Clean Sands SW Well graded Sands Cu > 6 Cc = 1 to 3 SP Poorly graded Sands Not meeting both criteria for GW Sands with fines SM Silty Sands Atterberg Limits below A-line or plasticity index less than 4 Atterberg Limits in hatched SM-SC SC Clayey Sands Atterberg Limits above A-line or plasticity index greater than 7 Fine- Grained Soils [50% or more passing No 200 Sieve (0.075mm)] Silts and clays Liquid limit 50% or less ML Inorganic silts of low plasticity See Plasticity chart CL Inorganic clays of low plasticity OL Organic silts of low plasticity Silts and clays Liquid greater than 50% MH Inorganic silts of high plasticity CH Inorganic clays of high plasticity OH Organic silts of high plasticity Highly organic Soils Pt Peat, muck and other highly organic soils Visual-manual identification
  5. 5. Soil Classification Coarse-grained soils: The coarse-grained soils are designated as gravel (G) if 50% or more of coarse fraction is retained on No.4 (4.75 mm) sieve otherwise it is termed as sand (S). If the coarse-grained soils contains less than 5% fines and are well-graded(W), they are given symbols GW and SW, and if poorly graded (P), symbols GP and SP. If the coarse-grained soils contain more tan 12% fines, these are designated as GM, GC, SM or SC, as per criteria given. If the percentage of fines is between 5-12% dual symbols such as GW-GM, SP-SM, are used. Fine grained Soils: Fine grained soils are classified on the basis of plasticity index (PI) and liquid limit (LL). If liquid limit is less or equal to 50% then soil of low compressibility (L), these are given the symbols ML, CL and OL. If liquid limit is more than 50% then denoted by symbols MH, CH and OH (Fig. 3.1 ) Fig. 3.1 Plasticity chart (USCS) 3.3 Practical Implications of the Soil Classification System The individual constituents of a soil mixture can be separated and identified as gravel, sand, silt and clay on the basis of mechanical analysis. The clay mineral that is present in a clay soil is sometimes a matter of engineering importance. According to the mineral present, the clay soil can be classified as kaolinite, montmorillonite or illite. The minerals present in a clay can be identified by either X-ray diffraction or differential thermal analysis. A description of these methods is beyond the scope of this book. Buildings, bridges, dams etc. are built on natural soils (undisturbed soils), whereas earthen dams for reservoirs, embankments for roads and railway lines, foundation bases for pavements of roads and airports are made out of remolded soils. Sites for structures on natural soils for embankments, etc, will have to be chosen first on the basis of preliminary examinations of the soil that can be carried out in the field. An engineer should therefore be conversant with the field tests that would identify the various constituents of a soil mixture. The behavior of a soil mass under load depends upon many factors such as the properties of the various constituents present in the mass, the density, the degree of saturation, the environmental conditions etc.
  6. 6. Soil Classification If soils are grouped on the basis of certain definite principles and rated according to their performance, the properties of a given soil can be understood to a certain extent, on the basis of some simple tests.

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