01/03/2012 1 SOIL COMPOSITION SOIL COMPOSITION

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SOIL COMPOSITION

SOIL COMPOSITION� Generally soil can be divided to three components in

natural occurrence

� This separation know as three phase systems or three phase diagram

� It consists of soil solids, water and air

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Soil components in natural occurrence

Volume-Weight relationships� As a result from 3 phase diagram several relationship

were produce

� This relationship known as volume weight relationships.

� Refer to 3 phase diagram

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Relationship equations

V = Vs +Vv ( eq. 1)= Vs + Vw + Va

whereVs = volume of soil solidsVv = volume of voidsVw = volume of water in the

voidsVa = volume of air in voids

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Relationship equations

W = Ws + Ww ( eq. 2)

where,Ws = weight of soil solidWw= weight of waterWa = zero (negligible)

Water content@ Moisture content

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Void ratio

The ratio of the volume of voids Vv to the volume of soil solid Vs

Porosity

The ratio of the volume of voids, Vv to total volume, V expressed as either a decimal or a percentage.

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Relationship between void ratio and

porosity

e = Vv = Vv = (Vv/V ) = nVs V – Vv 1 – (Vv/V) 1- n

n = e1 + e

Degree of saturation

The ratio of the volume of water, Vw to the volume of voids, Vv. It’s commonly in percentage

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Unit weight

The weight of soil per unit volume

Moist unit weight

Dry unit weight

Relationship between dry unit and bulk unit weight

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Derivation –cont’d

� So what is the different between unit weight and density?

� Unit Weight is influence by gravity forces while mass isn’t (Unit Weight=Weight/ Volume while Density= Mass/Volume)- Weight = Mass x Gravity acceleration

� So we can use the similar equations on related calculation if there is only density

Specific Gravity

Specific gravity can be determined as a ratio of the unit weight of the soil material to the unit weight of water (9.81 kN/m3 or density 1000 kg/m3)Most of the specific gravity of soil fall within a range of 2.6 – 2.9

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Relationships between Unit weight, Void ratio, Moisture Content and Specific

gravity

let us consider a volume of soil in which the volume of the soil solids is one

Derivation of equationsTherefore Gs will be substituted as

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Derivation –cont’d

γγ = W = WS +WW =GS γγ W + wGs γγ W = (1+w) Gs γγ w

V V 1+e 1+e

γγ d= Ws = Gs γγww

V 1+e

S= Vw = wGsVv e

Therefore

How Vw become wGS???

Vw = Ww = wGs γγ w =wGsγγ w γγ w

Derivation –cont’d

γγ sat = W = WS +WW = Gs γγ w + e γγ w = (Gs+e) γγ wV V 1+ e 1+e

So if S = 1 (100%)

Se = wGs

e = wGs

Beside bulk unit weight, dry unit weight and

saturated unit weight there is another type of

unit weight that is submerge unit weight, γγ’=

γγ sat- γγ w

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Relationships between Unit weight, Porosity, Moisture Content

and Specific gravity

Relationships between Unit weight, Porosity, Moisture Content

and Specific gravity

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Relationships between Unit weight, Porosity, Moisture Content

and Specific gravity (Saturated Case)

Relationships between Unit weight, Porosity, Moisture Content

and Specific gravity

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Air Content� The air content, A can be expressed as

� A = e- wGs / 1+ e

� or A = e/ 1+e ( 1- wGs/e)

� Since n = e/1+e

� Therefore, A = n (1-Sr)

� Beside using symbol A as air content sometimes authors use Na

Relative Density

� Dr= relative density (%)

� e = in situ void ratio of the soil

� emax = void ratio of the soil in the loosest state

� emin = void ratio of the soil in the densest state

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Relative Density

� Dr= relative density (%)

� ρd = in situ dry unit weight of the soil

� ρd(max) = void ratio of the soil in the loosest state

� ρd(min) = void ratio of the soil in the densest state

Relative Density

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Relative Density� For given sandy soil, emax = 0.75 and emin = 0.4. Gs =

2.68. In situ moist density = 1797.4 kg/m3 , wc = 12%. Determine the Dr of compaction.

Relative Density