Soil Mechanic

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  • Topic: Soil mechanics, Density, Hydrogeology
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SOIL MECHANICS 1 KAEA 2132 - Phase diagram & phase relationship -

Soil-Aggregate Basic Relationship

Dr. Meldi Suhatril
Department of Civil Engineering, Faculty of Engineering, University of Malaya.

Review : o Soil is inherently multiphase material
(Generally consists of three phases)
i. Solid phase ii. Liquid phase iii. Gaseous phase

3 – Phase system:


Va Vw Vs

Wa= 0 Ww Ws


o It can also be TWO PHASE material
i. With solid + Gaseous (DRY STATE) ii. With solid + Liquid (SATURATED STATE)


V = Vs + Vw + Va

W = Ws + Ww + Wa

Partially Saturated Soil



2 – Phase system:

Volumetric ratios



Ww W Ws

Volumetric ratios commonly used in soil mechanics are:
i. Void ratio,


Ws Vs



ii. Porosity, n iii. Degree of Saturation, Sr iv. Air Content, ac v. Air void ratio or Percentage air voids, na

Dry soil

Saturated soil

Void ratio e is defined as the ratio of volume of voids to the volume of solids

e=0 e>1

absence of voids (solid material) Vv >> Vs in the soil mass

e = Vv /Vs
• Volume of voids Vv refers to that portion of the volume of the soil not occupied by solid grains Since the relationship between Volume of air Va and Volume of water Vw usually changes with ground water conditions as well as imposed loads, it is convenient to designate all the volume not occupied by solid grains as void space Volume of voids Vv

Soil type Void ratio e Uniform sand, loose Mixed-grain sand, 0.85 dense 0.43 Soft glacial clay 1.20 Soft highly organic 3.00 clay 5.20 Soft Bentonite



• In nature, even though the individual void spaces are larger in coarse-grained soils, the void ratios of fine-grained soils are generally higher that those of coarse-grained soils. The ratios of volume of voids Vv to total volume V is defined as Porosity n.

• Porosity n of soil cannot exceed 100% 0 < n < 100 Porosity n of a natural deposit = f (Shape of grains, uniformly of grain size, and the conditions of sedimentation. • n = 25 - 50 % (natural sands) • n = 30 - 60 % (soft natural clays)

n = Vv /V
But V = Vv + Vs = (1+ Vv / Vs ) Vs = (1+e) Vs n = e/(1+e)

Out of void ratio e and Porosity n, void ratio is used frequently in soil engineering because:

Water content
The water content w is given as WW/Ws expressed as a percentage.

e = Vv /Vs

n = Vv /V

where Ww = weight of water Ws = Weight of solids (dry) Natural water content of fine-grained soils > coarse-grained soils. (No upper limit to w)

Any change in V is a direct consequence of a similar changes in Vv and while Vs remains the same.



Degree of saturation Sr
For a fully saturated soil-water system since all the voids will be completely filled with water: VVgw = WW where gw = unit weight of water For partial saturation: (VV - Va) gw = WW

Hence the relationship for Sr , the degree of saturation is given as:

Sr = ((VV - Va) gw)/Vv gw = Vw /Vv
Sr is the ratio of the volume of water to the volume of void space (Generally expressed as a Percentage) 0 < Sr < 100 • For completely dry soil, Sr = 0 • For fully saturated soil, Sr = 1 or (100 %)

Soil can be partially saturated (0 < Sr < 100)

Degree of Saturation of Sand in various states

Sr  100

Condition of Sand Dry Humid Damp Moist Wet Saturated

Sr , (%) 0 1-25 26-50 51-75 76-99 100

Thus, at a Sr of 100% all the voids are filled with water.

-Valid only for sands



Degree of saturation , Sr • Fine or silty sands are moist, wet or saturated Clays are always completely or nearly saturated except in the layer of soil subjected to seasonal variation of temperature and moisture.

Air content ac = Va/Vv
= =

(Va + Vw - Vw )/Vv
(Vv - Vw )/Vv 1 - Sr

ac = 0 for saturated soil ac = 1 for dry soil

Air void ratio or Percentage air voids na
= =

Unit weight g...
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