IIT Bombay 10/08/2009 Lecture: 8 Index properties Sub-topics � Particle shape & size � Atterberg limits CE 303 8 Instructor: AJ
IIT Bombay CE 303 8 Instructor: AJ
IIT Bombay Particle size distribution curves Classification test 2 Contd. Results of particle size analysis represented in form of semi-log graph Per cent of total weight of all particles smaller than any given diameter is plotted on the ordinate using arithmetic scale Size of all particles is plotted on the abscissa using logarithmic scale Percent finer by weight Diameter (mm) CE 303 8 Instructor: AJ
IIT Bombay Percent finer by weight Diameter (mm) Curve 1 represent well-graded soil with particle ranging from gravel to fines It represents particle size over wide range its gradation curve is smooth and generally convex upwards CE 303 8 Instructor: AJ
IIT Bombay Percent finer by weight Diameter (mm) Curve 2 represent uniformly graded soil. The soil has either excess or deficiency of certain particle sizes, or has most of particles about the same size CE 303 8 Instructor: AJ
IIT Bombay Percent finer by weight Diameter (mm) Curve 3 represent gap-graded soil. The soil has some missing particle sizes (e.g. particles between 0.1 and 1mm are relatively low in Curve 3) Uniformly and gap-graded are together termed as poorly-graded soils CE 303 8 Instructor: AJ
IIT Bombay Can we define the gradation curve using some parameters?? Certain grain diameters D which correspond to equivalent “percent passing” on PSD curve D 10 , D 30 and D 60 are grain diameters that are used D 60 = grain diameter (in mm) corresponding to 60% passing by weight; in other words 60% of particles are smaller than the diameter D 60 D 30 = grain diameter (in mm) corresponding to 30% passing D 10 = grain diameter (in mm) corresponding to 10% passing CE 303 8 Instructor: AJ
IIT Bombay D C = 60 Coefficient of uniformity C u u D 10 ( ) 2 D C = Coefficient of curvature C c 30 c D D 10 60 For well-graded soils, 1 < C c < 3, and C u > 6 (for sand) and > 4 (for gravel) CE 303 8 Instructor: AJ
IIT Bombay Consistency of clays: Atterberg limits Consistency : used to describe degree of firmness of soil (e.g. soft, firm, stiff or hard) It is only associated with clays Atterberg's Limits - Liquid and Plastic Limits - were proposed by Atterberg, a Swedish Agricultural Scientist in ~ 1911 CE 303 8 Instructor: AJ
IIT Bombay Atterberg noted � consistency of soil depends on its water content The broad regimes of soil behaviour with changing water content Decreasing water content Decreasing water content Decreasing water content Decreasing water content Liquid Liquid Liquid Liquid Plastic Plastic Plastic Plastic Shrinkage Shrinkage Shrinkage Shrinkage limit limit limit limit limit limit limit limit limit limit limit limit Slurry Slurry Slurry Slurry No No No No volume volume volume volume change change change change Soft, ductile solid Soft, ductile solid Soft, ductile solid Soft, ductile solid Hard, brittle solid Hard, brittle solid Hard, brittle solid Hard, brittle solid CE 303 8 Instructor: AJ
IIT Bombay 3 limiting water contents to separate the 4 regimes of soil behaviour : LL, PL and SL These are boundary water contents at which soil undergoes change from one state to another → collectively called “consistency limits” In geotechnical engineering, LL and PL are much more often used that SL The soil remains completely saturated up to shrinkage limit CE 303 8 Instructor: AJ
IIT Bombay Liquid limit is also flowing limit: determined by 2 methods on soils passing 0.425mm sieve : Casagrande method : A standard-sized groove is cut in pad of soil in a standard dish. The dish is then given series of blows to close the groove LL is defined as water content at which 25 blows just close 1-cm segment of the standard groove. This method tends to be rather subjective Weight Weight Weight Weight Weight Weight BS 1377 recommends � use miniature o o o o o o 30 30 30 30 30 30 fall cone test Weight of cone = 80g and o Vertical (apex) angle = 30 LL defined as the water content at which penetration is 20mm d Thus liquid limit test is either a miniature slope stability test or a cone penetration test CE 303 8 Instructor: AJ
IIT Bombay CE 303 8 Instructor: AJ
IIT Bombay Plastic limit (PL) test : Small pad of soil is rolled out into thin threads using open palm of hand Repeated rolling and remoulding action, together with heat from the hand gradually dries out the soil, thereby reducing its water content PL is defined as water content at which the thread just cracks up when the diameter reduces to 3mm Plastic limit (PL) is also the rolling-out limit Plasticity index (PI) is numerical difference between liquid limit and plastic limit PI = LL – PL PI is range of water content over which soil exhibits plasticity CE 303 8 Instructor: AJ
IIT Bombay − − w PL w PL = = LI − LL PL PI 100x CE 303 8 Instructor: AJ
IIT Bombay Other soil properties…… (1) Sensitivity Some fine grained soils lose part of their strength upon remoulding …due to breakdown of the original soil structure and disturbance caused to water molecules in the absorbed layer Ratio of undrained shear strength of undisturbed soil specimen to the undrained shear strength of remoulded soil specimen (at unaltered water content) is called sensitivity Sensitivity Classification 1 to 4 Normal 4 to 8 Sensitive 8 to 15 Extra-sensitive > 15 Quick CE 303 8 Instructor: AJ
IIT Bombay (2) Thixotropy (pronounced : thiks-aa-tropy) It is property of some clays by virtue of which they regain part of strength lost due to remoulding � with time at constant water content Increase in strength is attributed to tendency of clays to regain their equilibrium with reorientation of water molecules in adsorbed layer (3) Relative density or density index RD (%) Classification granular <15 Very loose − e e soils = max o RD or I D − 15 to 35 Loose e e max min 35 to 65 Medium 65 to 85 Dense > 85 Very dense RD ↑ granular soil more dense � high strength CE 303 8 Instructor: AJ
IIT Bombay (4) Activity PI = Activity of soil μ % by weight finer than 2 m Soils (that contain clays) can be grouped depending upon their activity: Activity Classification < 0.75 Inactive 0.75 to 1.25 Normal > 1.25 Active CE 303 8 Instructor: AJ
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