Lecture (4)

Mechanical Analysis of Soil is the determination of thesize range of soil particles, expressed as a percentage oftotal dry weight.The particle size distribution of soil can be fined by twomethods.• The sieve analysis technique is applicable for soilgrains larger than No. 200 (0.075 mm) sieve size.• For fine-grained soils the procedure used fordetermination of the grain-size distribution ishydrometer analysis. This is based on the principle ofsedimentation of soil grains.• Particle Size Classification. (Amercian Association –(AASHTO )

Mechanical Analysis of SoilSieve Analysis

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For a basic understanding of the nature of soil, the distributionof the grain size present in a given soil mass must be known.The grain-size distribution of coarse-grained soils (gravellyand / or sandy) is determined by sieve analysis. the openingsize of some U.S. sieves. Below,

GRAVEL%

SAND% CLAY&SILT SAND %

GRAVEL%PASSINGSIEVE 200COARSE MEDIUM FINE

4.75-75 mm 2.0-4.75 mm 0.42-2.0 mm 0.075-0.42mm < (0.075 mm)

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U.S. standard sievesSieve no. Opening size (mm)4 4.758 2.3810 2.0020 0.8430 0.5940 0.42550 0.29760 0.2570 0.21100 0.149140 0.105200 0.075

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Sieve Analysis consists of shaking the dried soil samplethrough a set of sieves having progressively smaller openings.• First the soil sample is oven dried.• All lumps are broken into small particles• The standard sets of sieves is placed in the sieveshaker and the sample is passed through the sieves.( the duration of sieve shaker is about 10 minutes).• The portion of soil sample retained on each sieve isweighted.• Percentage passing are calculated, the resultingvalues are plotted on a semi-log. Scale.• The cumulative percent by Dry weight of a soil passing agiven sieve is referred to as the percent finer

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Figure below shows the results of a sieve analysis for a sandysoil. The grain-size distribution can be used to determine someof the basic soil parameters, such as the effective size, theuniformity coefficient, and the coefficient of gradation.

Grain-size distribution of a sandy soil.5/16

• The effective size of a soil is the diameter throughwhich 10% of the total soil mass is passing and isreferred to as D10.Effective Diameter = D10Cu Uniformity coeff. = D60 / D10Cc Gradation coeff. = (D30 )2/ (D60 * D10 )

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A soil is called a well-graded soil if the distribution ofthe grain sizes extends over a rather large range. In thatcase, the value of the uniformitycoefficient is large. Generally, a soil is referred to as wellgraded if Cu is larger than about 4–6 and Cc is between 1and 3.When most of the grains in a soil mass are ofapproximately the same size—i.e., Cu is close to 1—thesoil is called poorly graded.A soil might have a combination of two or more well gradedsoil fractions, and this type of soil is referred to as agap-graded soil.

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I. Poor graded Soil – Most of the soil grains are the Same SizeII. Well graded Soil - Soil particle sizes are distributed over a wide range.III. Gap graded Soil – Soil might have a combination of two or more uniformly graded fractions.

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PARTICLE-SIZE ANALYSISASTM D 422

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D10% = 0.101D30% = 0.266D60% = 0.555Effective Diameter = 0.101cu uniformity coeff. = 5.471Cc gradation coeff. = 1.260Effective Diameter = D10Cu Uniformity coeff. = D60 / D10Cc Gradation coeff. = (D30 )2/ (D60 * D10 )

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PARTICLE-SIZE ANALYSISASTM D 422

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Pharos UniversityFaculty of Engineering CV 256: Soil Mechanics Architectural Dept. Dr. Hassan Abd Ellatif

Sheet (2)Grain Size Analysis

Sieve analysis was made for four soil samples and the results are

as shown in the following table:

Sieve (D) in mm

Sample (A) Sample (B) Sample (C) Sample (D)

(W retained) in gm (W retained) in gm (W retained) in gm (W retained) in gm

4.75 0.00 28 0 0

2.00 21.60 42 44 0

0.840 49.50 48 56 0

0.425 102.6 128 82 249.1

0.250 89.10 221 91 179.8

0.149 95.60 86 136 22.7

0.075 60.40 40 92 15.5

Pan 31.2 24 45 23.5

* Determine the percent finer than each sieve and plot a Grain -Size Distribution Curve for all the samples.* Calculate D10, D30 & D60 form the Grain -Size Distribution Curve for all the samples* Calculate the uniformity coefficient, Cu * Calculate the coefficient of gradation, Cc.* Comment about each sample gradation.* Classify each sample.