Soil Mechanic I

Lecture 3

Grain Size Disribution and

Mechanical Analysis of Soil

Mechanical Analysis of Soil• Determination of the size range of particles

present in a soil, expressed as a percentage of the total dry weight.

• Two methods are generally used to find the particle-size distribution of soil: (1) sieve analysis-for larger than particle sizes 0.075 mm in diameter, and (2) hydrometer analysis-for particle sizes smaller than 0.075 mm in diameter.

Sieve analysis• Sieve analysis consists of

shaking the soil sample through progressively a set of sieves that have smaller.

Hydrometer analysisHydrometer analysis is based on the principle of sedimentation of soil grains in water. When a soil specimen is dispersed in water, the particles settle at different velocities, depending on their shape, size, and weight, and the viscosity of the water.

R = hydrometer reading

Hydrometer analysis (finer than 0,075 mm)

TRGLd

)(98030

d = diameter of soil particles (mm) = viscosity of water (poise = dyne.det/cm2)L = distance of sedimentation (cm)T = sedimentation time (menit)G = soil specific gravity (gr/cm3)R = hydrometer reading (density of solution-gr/cm3)

Given :Specific gravity of water = 1 gr/cm3

Volume of solution = 1000 mlMass of soil sample = w gramThe percent finer =

wGGR

)1()1(

x 1000 x 100 %

Sieve analysis (greater than 0,075 mm)• oven-dry the soil and break all lumps into small particles.• Shake the soil through a stack of sieves with openings of

decreasing size from top to bottom• After the soil is shaken, calculate the % finer as the following

procedures:1. Determine the mass of soil retained on each sieve( i.e.,M1,

M2, . . . Mn,)and in the pan (i.e., Mp).2. Determine the total mass of the soil : M1 + M2 + . . . + Mi . . . +

Mn + Mp = M3. Determine the cumulative mass of soil retained above each

sieve for the ith sieve it is : M1 + M2 + …+ Mi

4. The mass of soil passing the ith sieve is : M - (M1 + M2 +…. + Mi)5. The percent of soil passing the ith sieve (or percent finer) is:

6. Plot on semilogarithmic graph paper with percent finer as the ordinate (arithmetic scale) and sieve opening size as the abscissa( logarithmic scale). This plot is referred to as the particle size distribution curve

3 inch = 75 mm

2 inch = 50 mm

1 inch = 25 mm

No.10 = 2 mm

No.4 = 4,75 mm

No.40 = 0,425 mm

No.200 = 0,075 mm

Pan = tutup

M1 gram

… gram

….gram

…. gram

Mn gram

M2 gram

M3 gram

Mp gram

Sample the particle size distribution curve

Parameter on grain size distribution of soil

D60 = diameter corresponding to 60% finer

D10 = diameter corresponding to 10% finer

D30 = diameter corresponding to 30% finer

Uniformity of soil :Cu < 5 : uniformCu = 5-15 : medium uniformCu > 15 : well graded

Types of gradation (grain size distribution)

Uniform graded

Well graded

Gap graded

Example 2.1

Solution

Particle size distribution curve

Determining the gravel, sand, silt, and clay-size particles

0

10

20

30

40

50

60

70

80

90

100

0,001 0,01 0,1 1 10 100

Grain size (mm)

% P

assi

ng/F

iner

clay silt ( lanau ) Sand pasir) Gravel (kerikil)

Clay and silt ( lanau ) Sand pasir) Gravel (kerikil)

AASHTO

USCS

Problem 1

Problem 2

Problem 3

Problem 4