PARTICLE SIZE OF COURSE-GRAINED SOILS

The distribution of particle sizes or

average grain diameter of coarse-

grained soils—gravels and sands—is

obtained by screening a known weight of

the soil through a stack of sieves of

progressively finer mesh size.

It is virtually always useful to quantify the size of the grains in a type of soil. Since a given soil will often be made up of grains of many different sizes, sizes are measured in terms of grain size distributions. Grain size distribution (GSD) information can be of value in providing initial rough estimates of a soil’s engineering properties such as permeability, strength, expansivity, etc. A subject of active research interest today is the accurate prediction of soil properties based largely on GSDs, void ratios, and soil particle characteristics. At this point in time, though, such research has not yet produced results that are usable in standard engineering practice. In this period, we will look at methods of measuring GSDs of soils, and also different measures of soil grain shapes.

When measuring GSDs for soils, two methods are generally used:

−> For grains larger than 0.075mm sieving is used; −> For grains in the range of .075mm > D > 0.5µm, the hydrometer test is used.

PARTICLE SIZE OF FINE-GRAINED SOILS

FINE GRAINED SOIL

The fine-grained soils are not classified on the basis of grain size distribution, but according to plasticity and compressibility.

In the course analysis (Sieve Test) anything finer than 63 m was recorded as clay and silt.

The Plasticity of a soil has a marked effect on the engineering properties of a soil-shear strength, compressibility etc.

As the particles which make up the clays and silts tend to be “flaky” in nature, this together with changes in the water content gives rise to an inherently variable material.

Laboratory classification criteria are based on the relationship between the liquid limit and plasticity index as designated in the plasticity chart. This chart was established by the determination of limits for many soils, together with an analysis of the effect of limits upon physical characteristics. Examination of the chart shows that there are two major groupings of fine-grained soils. These are the L groups, which have liquid limits less than 50, and the H groups, which have liquid limits equal to and greater than 50. The symbols L and H have general meanings of low and high compressibility, respectively. Fine-grained soils are further divided with relation to their position above or below the A-line of the plasticity chart.

CHARACTERISTICS OF FINE-GRAINED SOIL

Fine grained soils are identified on the basis of its plasticity. Individual particles are not visible by naked eye.

Fine grained soils are also divided in two groups:

-Silt -Clay.

Particles having diameter:

-between 75 micron to 2 micron are called Silt -smaller than 2 micron is called Clay.

Verbal description of fine grained soil is done on the basis of its:

-dry strength -Dilatancy -dispersion and plasticity.

Fine grained soil exhibit a poor load bearing capacity.

Fine grained soil is practically impermeable in nature because of its small particles size.

Volume change occurs with change in moisture content.

Strength changes with change in moisture condition.

Fine grained soil is susceptible to frost action.

Engineering properties are controlled by mineralogical factors.

When touched by hand it feels smooth, greasy and sticky.

The screening process cannot be used for fine-grained soils—silts and clays—because of their extremely small size. The common laboratory method used to determine the size distribution of fi ne-grained soils is a hydrometer test The hydrometer test involves mixing a small amount of soil into a suspension and observing how the suspension settles in time. Larger particles will settle quickly, followed by smaller particles. When the hydrometer is lowered into the suspension, it will sink into the suspension until the buoyancy force is sufficient to balance the weight of the hydrometer.

HYDROMETER TEST

ATTERBERG LIMITS

The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil, such as its shrinkage limit, plastic limit, and liquid limit.

can be used to distinguish between silt and clay, and it can distinguish between different types of silts and clays.

limits were created by Albert Atterberg, a Swedish chemist.

These distinctions in soil are used in assessing the soils that are to have structures built on. Soils when wet retain water and some expand in volume.

These tests are mainly used on clayey or silty soils since these are the soils that expand and shrink due to moisture content.

Clays and silts react with the water and thus change sizes and have varying shear strengths. Thus these tests are used widely in the preliminary stages of designing any structure to ensure that the soil will have the correct amount of shear strength and not too much change in volume as it expands and shrinks with different moisture contents.

CHARACTERIZATION OF SOILS BASED ON PARTICLE SIZE

Soils will be separated into two categories. One category is coarse-grained soilsthat are delineated if more than 50% of the soil is greater than 0.075 mm (No. 200sieve). The other category is fine-grained soils that are delineated if more than 50%of the soil is finer than 0.075 mm. Coarse-grained soils are subdivided into gravelsand sands, while fine-grained soils are divided into silts and clays. Each soil type—gravel, sand, silt, and clay—is identified by grain size, as shown in Table 2.1. Clayshave particle sizes less than 0.002 mm. Real soils consist of a mixture of particlesizes.

Sand Silt Clay

Soil gradation is a classification of a coarse-grained soil that ranks the soil

based on the different particle sizes contained in the soil. Soil gradation is an important aspect of soil mechanics and geotechnical engineering because it is an indicator of other engineering properties such as compressibility, shear strength, and hydraulic conductivity. In a design, the gradation of the in situ or on site soil often controls the design and ground water drainage of the site. A poorly graded soil will have better drainage than a well graded soil.

The selection of a soil for a particular use may depend on the assortment of particles it contains. Two coefficients have been defined to provide guidance on distinguishing soils based on the distribution of the particles. One of these is a numerical measure of uniformity, called the uniformity coefficient, Cu, defined as

where D60 is the diameter of the soil particles for which 60% of the particles are finer, and D10 is the diameter of the soil particles for which 10% of the particles are finer. Both of these diameters are obtained from the grading curve.

Coefficient of Uniformity

The other coefficient is the coefficient of curvature, Cc (other terms used are the coefficient of gradation and the coefficient of concavity), defined as

where D30 is the diameter of the soil particles for which 30% of the particles are finer. The average particle diameter is D50

Coefficient of Curvature

Poorly graded soils have uniformity coefficients <4 and steep gradation curves. Well-graded soils have uniformity coefficients >4, coefficients of curvature between 1

and 3, and flat gradation curves. Gap-graded soils have coefficients of curvature <1 or >3, and one or more humps on the gradation curves.

References:

http://engineeringtraining.tpub.com/14070/css/14070_389.htm

http://www.slideshare.net/feride/soil-classification

http://slideplayer.com/slide/220544/

https://www.google.com.ph/search?q=Clay+and+Silt&biw=1093&bih=564&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjeg7-bo9jJAhXFMGMKHYEVBD0Q_AUIBigB#imgrc=FdWjP0c0RW-1sM%3A