10/08/2009 Lecture: 8 Index properties Sub-topics Particle - - PowerPoint PPT Presentation

IIT Bombay

CE 303 8 Instructor: AJ

10/08/2009 Lecture: 8 Index properties Sub-topics

• Particle shape & size
• Atterberg limits

IIT Bombay

CE 303 8 Instructor: AJ

IIT Bombay

CE 303 8 Instructor: AJ

Particle size distribution curves

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 Classification test 2

Contd. Percent finer by weight Diameter (mm)

IIT Bombay

CE 303 8 Instructor: AJ

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

Percent finer by weight Diameter (mm)

IIT Bombay

CE 303 8 Instructor: AJ

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

Percent finer by weight Diameter (mm)

IIT Bombay

CE 303 8 Instructor: AJ

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

Percent finer by weight Diameter (mm)

IIT Bombay

CE 303 8 Instructor: AJ

Can we define the gradation curve using some parameters??

D10, D30 and D60 are grain diameters that are used Certain grain diameters D which correspond to equivalent “percent passing” on PSD curve

D60 = grain diameter (in mm) corresponding to 60% passing by weight; in

• ther words 60% of particles are smaller than the diameter D60

D30 = grain diameter (in mm) corresponding to 30% passing D10 = grain diameter (in mm) corresponding to 10% passing

IIT Bombay

CE 303 8 Instructor: AJ

10 60 u

D D C = For well-graded soils, 1 < Cc < 3, and Cu > 6 (for sand) and > 4 (for gravel)

( )

60 10 2 30 c

D D D C = Coefficient of uniformity Cu Coefficient of curvature Cc

IIT Bombay

CE 303 8 Instructor: AJ

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

IIT Bombay

CE 303 8 Instructor: AJ

Atterberg noted consistency of soil depends on its water content

The broad regimes of soil behaviour with changing water content

Liquid limit Plastic limit Shrinkage limit Soft, ductile solid Hard, brittle solid Slurry No volume change Decreasing water content Liquid limit Plastic limit Shrinkage limit Soft, ductile solid Hard, brittle solid Slurry No volume change Decreasing water content Liquid limit Plastic limit Shrinkage limit Liquid limit Plastic limit Shrinkage limit Soft, ductile solid Hard, brittle solid Soft, ductile solid Hard, brittle solid Slurry No volume change Slurry No volume change Decreasing water content Decreasing water content

IIT Bombay

CE 303 8 Instructor: AJ

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

• ne 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

IIT Bombay

CE 303 8 Instructor: AJ

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

• f the standard groove. This method tends to be rather subjective

BS 1377 recommends use miniature fall cone test Weight of cone = 80g and Vertical (apex) angle = 30

• LL defined as the water content at which

penetration is 20mm

Weight

30

• Weight

30

• Weight

30

• Weight

30

• Weight

30

• Weight

30

• d

Thus liquid limit test is either a miniature slope stability test or a cone penetration test

IIT Bombay

CE 303 8 Instructor: AJ

IIT Bombay

CE 303 8 Instructor: AJ

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

IIT Bombay

CE 303 8 Instructor: AJ

PI PL w PL LL PL w LI − = − − =

100x

IIT Bombay

CE 303 8 Instructor: AJ

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 Quick > 15 Extra-sensitive 8 to 15 Sensitive 4 to 8 Normal 1 to 4 Classification Sensitivity

IIT Bombay

CE 303 8 Instructor: AJ

(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

min max

• max

D

e e e e I

• r

RD − − =

Very dense > 85 Dense 65 to 85 Medium 35 to 65 Loose 15 to 35 Very loose <15 Classification RD (%) granular soils RD ↑ granular soil more dense high strength

IIT Bombay

CE 303 8 Instructor: AJ

(4) Activity m 2 than finer weight by % PI soil

• f

Activity μ = Soils (that contain clays) can be grouped depending upon their activity:

Active > 1.25 Normal 0.75 to 1.25 Inactive < 0.75 Classification Activity