Soil classification

1. Soil Classification

2. 2 Soil Texture

3. 3 Soil Texture The texture of a soil is its appearance or “feel” and it depends on the relative sizes and shapes of the particles as well as the range or distribution of those sizes. Coarse-grained soils: Gravel Sand Fine-grained soils: Silt Clay 0.075 mm (USCS) 0.06 mm (BS) (Hong Kong) Sieve analysis Hydrometer analysis

4. 4 1.2 Characteristics (Holtz and Kovacs, 1981)

5. 5 Grain Size and Grain Size Distribution

6. Take note!!! T he sizes of particles that make up soil vary over a wide range. Soils are generally called gravel, sand, silt, or clay , depending on the predominant size of particles within the soil. To describe soils by their particle size , several organizations have developed particle-size classifications.

7. Particle Size Distribution boulders > 60mm 60mm > gravel > 2mm 2mm > sand > 60  m 60  m > silt > 2  m 2  m > clay Each class may is sub-divided into coarse, medium and fine. for sand: 2mm > coarse sand > 600  m 600  m > medium sand > 200  m 200  m > fine sand > 60  m Classification boundaries either begin with a '2' or a '6' .

8. Soil-Particle Size Classification

9. 9 Grain Size Gravel Sand Silt Clay 4.75 Unit: mm (Holtz and Kovacs, 1981) USCS BS 0.075 2.0 0.06 0.002 USCS: Unified Soil Classification BS: British Standard

10. Data often presented as Particle Size Distribution Curves with logarithmic scale on X-axis Particle Size Distribution (continued) S - shaped - but some conventions of curves going left to right, others, the opposite way around sand silt clay

11. A Problem clay is used both as a classifier of size as above, and also to define particular types of material. clays exhibit a property known as cohesion (the "stickiness" associated with clays). General Properties Gravels ----- permeability is of the order of mm s -1 . Clays ----- it is 10 -7 mm/s or less. Compressibility of the soil increases as the particle size decreases. Permeability of the soil decreases as the particle size decreases. Particle Size Distribution (continued)

12. 12 Sieve Sizes (Das, 1998) (Head, 1992)

13. 13 Grain Size Distribution (Cont.) Coarse-grained soils: Gravel Sand Fine-grained soils: Silt Clay 0.075 mm (USCS) 0.06 mm (BS) (Hong Kong) Experiment Sieve analysis Hydrometer analysis (Head, 1992)

14. 14 Grain Size Distribution (Cont.) Log scale (Holtz and Kovacs, 1981) Finer Effective size D 10 : 0.02 mm D 30 : D 60 :

15. 15 Grain Size Distribution (Cont.) Describe the shape Example: well graded Criteria Question What is the C u for a soil with only one grain size?

16. 16 Answer Question What is the C u for a soil with only one grain size? D Finer Grain size distribution

17. SAMPLE PROBLEM 1. For a soil with D 60 =0.42mm, and D 30 =0.21mm, and D 10 =0.16, calculate and the coefficient of gradation. Sol’n ; C U =D 60 /D 10 =0.42mm/0.16mm=2.625 C C = (D 30 ) 2 /(D 10 )(D 60 ) =(0.21) 2 / (0.16)(0.42)=0.66

18. 2. The following are the results of a sieve analysis : US Sieve No. Mass of Soil Retained on Each Sieve (g) Opening Commulative Mass % Passing 4 10 20 40 60 100 200 PAN 18.5 53.2 90.5 81.8 92.2 58.5 26.5 4.75 2.0 .850 .425 .250 .150 .075 18.5 71.5 162.2 244 336.2 394.1 421.2 100 95.608 82.977 61.491 42.070 20.180 6.434 a.) Determine the percent finer than each sieve size and plot a grain-size distribution curve. b.) Determine D 10 , D 30 and D 60 from the grain-size distribution curve. c.) Calculate the uniformity coefficient, C u . d.) Calculate the coefficient of graduation, C c .

19. Grain-size distribution curve

20. Solution c.) C u = D 60 /D 10 =0.4/0.12= 3.33 d.) C C = (D) 2 /(D 60 )(D 10 ) =1.01

21. 3.The particle-size characteristics of a soil are given in the table. Sieve no. Opening % Passing 4 10 20 40 80 200 4.75 2.00 0.850 0.425 0.180 0.075 100 90 64 38 18 13 Calculate the uniformity coefficient ( C u ) and coefficient of gradation ( C c ).

22. Sol’n : C u = D 60 / D 10 = 0.73/0.019= 38.421 C c = (0.29) 2 / (0.73)(.019)=0.063

23. 23 Grain Size Distribution (Cont.) Engineering applications It will help us “feel” the soil texture (what the soil is) and it will also be used for the soil classification (next topic). It can be used to define the grading specification of a drainage filter (clogging). It can be a criterion for selecting fill materials of embankments and earth dams, road sub-base materials, and concrete aggregates. It can be used to estimate the results of grouting and chemical injection, and dynamic compaction. Effective Size, D 10 , can be correlated with the hydraulic conductivity (describing the permeability of soils). (Hazen’s Equation).( Note: controlled by small particles) The grain size distribution is more important to coarse-grained soils.

24. 24 Particle Shape Important for granular soils Angular soil particle  higher friction Round soil particle  lower friction Note that clay particles are sheet-like. Rounded Subrounded Subangular Angular (Holtz and Kovacs, 1981) Coarse-grained soils

25. Thank you!!!