Soil Classification

Soil Classification2 Soil Texture3Soil TextureThe 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 SandFine-grained soils: Silt Clay0.075 mm (USCS)0.06 mm (BS) (Hong Kong)Sieve analysisHydrometer analysis3Talk about the difference between the clay-size particle or clay minerals.4

1.2 Characteristics(Holtz and Kovacs, 1981)4Please remind students about the oxymoron of the cohesion and cohesionless.Change this table5Grain Size and Grain Size DistributionTake note!!! The sizes of particles that make up soil varyover 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.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'.

Soil-Particle Size Classification9Grain Size

GravelSandSiltClay4.75Unit: mm(Holtz and Kovacs, 1981)USCSBS0.0752.00.060.002USCS: Unified Soil ClassificationBS: British Standard9Mention sieve analysis and hydrometer analysis for different size of soilsThere is not distinguish for silt and clay in the USCS system.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

sandsiltclayA 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 PropertiesGravels ----- 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)12Sieve Sizes

(Das, 1998)(Head, 1992)12It is not necessary to use the full set of sieves, but the particle size should be distinguished.13Grain Size Distribution (Cont.)Coarse-grained soils:Gravel SandFine-grained soils: Silt Clay0.075 mm (USCS)0.06 mm (BS) (Hong Kong)

Experiment

Sieve analysisHydrometer analysis(Head, 1992)13Wet sieving:According to the British standard, dry sieving may be carried out only on materials for which this procedure gives the same results as the wet-sieving procedure. This means that it is applicable only to clean granular materials, which usually implies clean sandy or gravelly soils-that is, soils containing negligible amounts of particles of silt or clay size. Normally the wet-sieving procedure (section 4.6.4) should be followed for all soils (Head, 1992).14Grain Size Distribution (Cont.)

Log scale(Holtz and Kovacs, 1981)FinerEffective size D10: 0.02 mmD30: D60:14Effective 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 an ddrainage through soils.15Grain Size Distribution (Cont.)Describe the shapeExample: well graded

Criteria

QuestionWhat is the Cu for a soil with only one grain size?

16AnswerQuestionWhat is the Cu for a soil with only one grain size?

DFiner

Grain size distributionSAMPLE PROBLEM1. For a soil with D60 =0.42mm, and D30 =0.21mm, and D10 =0.16, calculate and the coefficient of gradation. Soln; CU =D60 /D10 =0.42mm/0.16mm=2.625 CC = (D30 )2 /(D10 )(D60) =(0.21)2 / (0.16)(0.42)=0.66 2. The following are the results of a sieve analysis:

US Sieve No.Mass of Soil Retained on Each Sieve (g)OpeningCommulative Mass% Passing410204060100200PAN018.553.290.581.892.258.526.54.752.0.850.425.250.150.075018.571.5162.2244336.2394.1421.210095.60882.97761.49142.07020.1806.4340a.) Determine the percent finer than each sieve size and plot a grain-size distribution curve.b.) Determine D10 , D30 and D60 from the grain-size distribution curve.c.) Calculate the uniformity coefficient, Cu.d.) Calculate the coefficient of graduation, Cc.Grain-size distribution curveSolution c.) Cu = D60 /D10 =0.4/0.12= 3.33 d.) CC = (D)2/(D60)(D10) =1.01

3.The particle-size characteristics of a soil are given in the table.Sieve no. Opening% Passing4102040802004.752.000.8500.4250.1800.0751009064381813Calculate the uniformity coefficient ( Cu) and coefficient of gradation ( Cc).Soln: Cu= D60/ D10= 0.73/0.019= 38.421 Cc = (0.29)2 / (0.73)(.019)=0.063

23Grain Size Distribution (Cont.)Engineering applicationsIt 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, D10, can be correlated with the hydraulic conductivity (describing the permeability of soils). (Hazens Equation).(Note: controlled by small particles)The grain size distribution is more important to coarse-grained soils.2324Particle ShapeImportant for granular soilsAngular soil particle higher frictionRound soil particle lower frictionNote that clay particles are sheet-like.

RoundedSubroundedSubangularAngular(Holtz and Kovacs, 1981)Coarse-grained soilsThank you!!!