Chapter 2 Origin of Soil and Grain Size Soil-Particle Size The sizes of particles that make up soil vary over a wide range. Soils generally are called gravel, sand, silt, or clay. Table 2.3 shows the particle- size classifications developed by several organizations.
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AASHTO Gravel (76.2-2)mm Sand (2-.075) Silt (0.075-.002) Clay (<0.002)
USCS Gravel (76.2-4.75)mm Sand (4.75-.075) Fines (<0.075) silt and clay
Specific Gravity (Gs) Specific gravity is defined as the ratio of the unit weight of a given material to the unit weight of water. The specific gravity of soil solids is often needed for various calculations in soil mechanics. It can be determined accurately in the laboratory.
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Mechanical Analysis of Soil Mechanical analysis is the determination of the size range of particles present in a soil, expressed as a percentage of the total dry weight.
Two methods generally are used to find the particle-size distribution of soil: (1) sieve analysis—for particle sizes larger than 0.075mm in diameter, and (2) hydrometer analysis—for particle sizes smaller than 0.075 mm in diameter.
(1) Sieve Analysis Sieve analysis consists of shaking the soil sample through a set of sieves that have progressively smaller openings. Table 2.5 Standard Sieve Sizes
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-Analysis Steps 1. Determine the mass of soil retained on each sieve. 2. Determine the total mass of the soil. 3. Determine the cumulative mass of soil retained above each sieve. 4. The mass of soil passing on each sieve. 5. The percent of soil passing. 6. plot the particle-size distribution curve on semilogarithmic graph paper as in (Figure 2.22)
(2) Hydrometer Analysis
Particle-Size Distribution Curve A particle-size distribution curve can be used to determine the following four parameters for a given soil: 1. Effective size (D10): This parameter is the diameter in the particle-size distribution curve corresponding to 10% finer. The effective size of a granular soil is a good measure to estimate the hydraulic conductivity and drainage through soil. 2. Uniformity coefficient (Cu): This parameter is defined as
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where D60 _ diameter corresponding to 60% finer. 3. Coefficient of gradation (Cc or Cz): This parameter is defined as
4. Sorting coefficient (S0): This parameter is another measure of uniformity and is generally encountered in geologic works and expressed as
D10, D25,…,D75 Can determine as in figure 2.26
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Problem 2.1
Solution:
Cu = =
Cz =
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Problem 2.5
Solution:
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Siev Mass e Mass cum. Of Size Siev Retaine Retaine Passin percent mm e No. d (g) d g passing % 4.75 4 0 0 551 100 92.0145190 2 10 44 44 507 6 81.8511796 0.85 20 56 100 451 7 66.9691470 0.425 40 82 182 369 1 57.7132486 0.25 60 51 233 318 4 38.4754990 0.18 80 106 339 212 9 21.7785843 0.15 100 92 431 120 9 6.35208711 0.075 200 85 516 35 4 pan 35 551 0 0 Total mass= sum. Of mass retained= 551g
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a)
Sieve Analysis 120
100
80
60 %Passing
40
20
0 10 1 0.1 0.01 Sieve Size (mm)
b)
c)
Cu = =
d)
Cz =
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Problem 2.9 Size Percent (mm) Finer 0.425 100 0.033 90 0.018 80 0.01 70 0.0062 60 0.0035 50 0.0018 40 0.001 35
Solution:
120
100
80
60
40
20
0 1 0.1 0.01 0.001
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a)According USCS % Gravel= 0% %Sand=% passing on 4.74mm- %passing on 0.075mm =100-93=7% % Silt& clay =% Fines=100%-%gravel-%sand = 100-0-7=93% b)According AASHTO % Gravel= 0% %Sand=% passing on 4.74mm- %passing on 0.075mm =100-93=7% % Silt=% passing on 0.075mm- %passing on 0.002mm = 93-40=53% % Clay=%passing on 0.002mm = 40%
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