CLASSIFICATION OF SOILS
(IMPORTANT STATEMENT: These lecture notes are based upon BS5930:1999 and BS1377:1990 but, where appropriate, reference is made to “Eurocode 7 related documents”. These are namely BS EN ISO 14688-1:2002, BS EN ISO 14688-2:2004 and BS EN ISO 146891:2003). During this transitional stage as the full recommendations of the Eurocode are being implemented during 2010 students are advised to be aware that published text books are likely to make little reference to the Eurocode 7 (ie. EN 1997) and there are some major differences in the way that soils are described. Students should be aware that the final National Annex to BS EN 1997 was published on 31st December 2009 and that sections of BS5930 are currently being rewritten to comply fully with the Eurocode). See: http://www.eurocodes.co.uk
Purpose of soil classification 1. 2. 3. 4. Provides a concise and systematic method for designating various types of soil. Enables useful engineering conclusions to be made about soil properties. Provides a common language for the transmission of information. Permits the precise presentation of boring records and test results.
Object of soil classification Is to provide a soil NAME and symbol, e.g. GRAVEL is G, based on the results of simple and quick to perform (therefore economic) key tests; 1. Particle size distribution (P.S.D.) or sieve analysis. 2. Plastic properties; Liquid limit test Plastic limit test Soil is initially classified into either coarse or fine soil on the basis of particle size. Coarse soil (Granular) Physical characteristics and appearance are influenced by the distribution of particle sizes within the soil, i.e.>0.063mm (1/16mm) A granular soil is classified according to its Particle Size Distribution. Fine soil (Cohesive) Physical characteristics and appearance influenced by cohesion and plastic properties (plasticity) associated with mineral composition and water content. The fine soil is sub-grouped according to its plasticity. CLASSIFICATION OF COARSE SOIL Is classified on the sand (symbol S) and gravel (symbol G) content plus the amount of fines present. Particle size distribution by sieve analysis This test involves passing soil through a series of sieves of decreasing mesh size and recording the weight of soil retained on each. Soil which passes through the finest sieve (63 microns or 0.063mm) collects in a base tray and is also weighed. -1Classification v1.00 September 2010
The results are processed and plotted on a semi-log chart of cumulative percentage passing (y-axis) verses log of particle size in mm (x-axis). NB Where a soil contains fines, a relatively small proportion may cause the soil to form a cohesive mass and dominate the properties of the soil. It is considered that a soil containing 35% fines or more will behave as a cohesive soil (ie silt or clay). The fines content is critical and is indicated with an extra symbol; Symbol G S G-F S-F GF SF % Fines 0–5 5 – 15 15 - 35 Terminology Clean gravel Clean sand Gravel with some fines Sand with some fines Fines content Little influence Some influence
Gravel with much fines Considerable influence Sand with much fines
How the laboratory test data is processed and plotted will be explained during the lecture. Grading The shape of the Particle Size Distribution curve indicates the range of particle sizes within a soil. Coarse soils are sub-grouped on whether a soil is well graded or poorly graded. A well graded (symbol W) soil has approximately equal proportions of particles sizes and the curve is usually smooth. Note the Till is a well graded soil and the well graded gravel GW in the PSD chart below. A poorly graded (symbol P) soil may contain a high proportion of material within a limited particle size band or bands. Poorly graded soil may be further sub-divided into uniform soil and gap graded soil: A poorly graded soil (Pu, uniform or closely graded) has a major proportion of the particles lying between narrow...
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