Index Properties of Soil
Soil Index Properties
Q: Explain with sketch phase diagrams for the partially saturated soil and dry soil.
Q: Explain soil as three phase system
Soil as three phase system
A soil mass consist of the solid particles and the voids in between them. These voids are filled with air or/and water. So there is a three phase system, but when the voids are only filled with air, or only filled with water then soil becomes a two phase system. Three phase system can be represented with a diagram as shown below. When the voids are only filled with water, it is said to be saturated.
Figure 1 Soil as three phase system
In above figure,
Wa= weight of air
Ws = weight of soil solids
Ww= weight of water
Wv =weight of voids = Ww + Wa
W= weight of soil mass = Wv + Ws = Ww + Wa + Ws
Va= Volume of air
Vs = Volume of soil solids
Vw= Volume of water
Vv =Volume of voids = Vw + Va
V= Volume of soil mass = Vv + Vs = Vw + Va + Vs
This three phase system converted in to two phase system when soil becomes saturated or dry as below.
Figure 2 Soil as two phase system- saturated soil
Wv =weight of voids = Ww + Wa = Ww
W= weight of soil mass = Wv + Ws = Ww + Ws
Vv =Volume of voids = Vw + Va = Vw
V= Volume of soil mass = Vv + Vs = Vw + Vs
Figure 3 Soil as two phase system- dry soil
Wv =weight of voids = Ww + Wa = Wa = 0
W= weight of soil mass = Wv + Ws = Ww + Ws = Ws
Vv =Volume of voids = Vw + Va = Va
V= Volume of soil mass = Vv + Vs = Va + Vs
We have the mass and weight relationship;
Weight of soil = Mass of soil x 9.81
Hence all above weight related quantities can be expressed in terms of mass of soil.
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Q: Explain following index properties
Q: Define and explain following properties
Density
It is the ratio of mass of soil to volume of soil
Unit 🡪 gram per cubic centimeter (g/cc)
ρ = MV
Dry Density
It is the ratio of dry mass of soil to volume of soil
Symbol 🡪d
Saturated Density
It is the ratio of saturated mass of soil to volume of soil
Symbol 🡪sat
Submerged Density
It is the ratio of submerged mass of soil to volume of soil
Symbol 🡪'
'= sat- w= sat- 1 …….density of water 🡪w=1 g/cc
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Unit weight
It is the ratio of weight of soil to volume of soil
Unit 🡪 kilogram per cubic meter (kg/m3) or kN/m3
γ= WV
Dry Unit Weight
It is the ratio of dry weight of soil to volume of soil
Symbol 🡪d
Saturated Unit Weight
It is the ratio of saturated weight of soil to volume of soil
Symbol 🡪sat
Submerged Unit Weight
It is the ratio of submerged weight of soil to volume of soil
Symbol 🡪'
'= sat- w= sat- 9.81 …….unit weight of water 🡪 w=9.81 g/cc
Relation between density and unit weight
γ= ρ ×g= ρ ×9.81
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Water content
It is the ratio of mass of water in soil to mass of soil solids
Unit less quantity/ expressed as a percentage
w = MwMs= WwWs
Water content experiments formulae
Oven drying formula, w=M-MsMs
Pycnometer method, w=M2-M1M3- M4 ×G-1G-1 ×100
Where, M1 is mass of empty pycnometer, M2 is mass of pycnometer + most soil, M3 is mass of pycnometer + most soil + water, M4 is mass of pycnometer +water only, G is specific gravity
Specific gravity
It is the ratio of density of soil solid to density of water at standard temperature.
Unit less quantity/ expressed as a decimal
G = soil solidswater= sw
Apparent/mass/ bulk Specific gravity
It is the ratio of density of soil to density of water at standard temperature.
Unit less quantity/ expressed as a decimal
Gm = soilwater= w
Specific gravity experiments formula
Pycnometer method, G=M2-M1(M3- M4)-(M3-M4)
Where, M1 is mass of empty pycnometer, M2 is mass of pycnometer + dry soil, M3 is mass of pycnometer + most soil + water, M4 is mass of pycnometer +water only
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Void Ratio
It is the ratio of the volume of voids to the volume of solids. It is denoted by ‘e’.
Unit less quantity/ expressed as a decimal
e = VvVs
Porosity
It is the ratio of the volume of voids to the total volume of soil.
Unit less quantity/ expressed as a percentage
n = VvV
Degree of saturation
It is the ratio of the volume of water to the total volume of voids.
Unit less quantity/ expressed as a percentage
S = VwVv
S is maximum (100%) when soil is saturated
S is minimum (0%) when soil is dry
Air content
It is the ratio of the volume of air to the total volume of voids.
Unit less quantity/ expressed as a decimals
ac = VaVv
ac is maximum (100%) when soil is dry
ac is minimum (0%) when soil is saturated
Relationship between S and ac
S + ac = 1
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Percentage of air voids
It is the ratio of the volume of air to the total volume of soil.
Unit less quantity/ expressed as a percentage
na = VaV
Density Index/ relative density
DI=RD=ID = emax- eemax- emin
emax = voids ratio when soil is in loosest state
emin = voids ratio when soil is in densest state
e = voids ratio when soil is in natural state
ID is minimum; equals to 0 (zero) when is at loosest state
ID is maximum; equals to 1 (one) when is at densest state
Hence ID varies between 0 to 1
Q: Derive relation between voids ratio and porosity
Q: prove that,1-n= 11+e,
Q: Prove that, e= n1- n
Q: Prove that, e= n1- n
Q: Derive relation between degree of saturation, voids ratio, water content and specific gravity
Q: prove that, Sr e=w G
Q: Derive relation between unit weight/density, dry unit weight/density and water content.
Q: prove that, d= 1+w
Q: Derive relation between degree of saturation, voids ratio, bulk unit weight and specific gravity
Q: prove that, = G+e S w1+ e
Q: prove that, sat= G+e w1+ e
Q: prove that, d= G w1+ e
Q: Explain IS soil classification system (as per IS 1498-1970)
Classification Based on Grain Size: The range of particle sizes encountered in soils is very large: from boulders with dimension of over 300 mm down to clay particles that are less than 0.002 mm. Some clays contain particles less than 0.001 mm in size which behave as colloids, i.e. do not settle in water.
In the Indian Standard Soil Classification System (ISSCS), soils are classified into groups according to size, and the groups are further divided into coarse, medium and fine sub-groups.
Description:
Coarse-grained soils are those for which more than 50% of the soil material by weight has particle sizes greater than 0.075 mm. They are basically divided into either gravels (G) or sands (S).
Fine-grained soils are those for which more than 50% of the material has particle sizes less than 0.075 mm. Clay particles have a flaky shape to which water adheres, thus imparting the property of plasticity.
Gravel are those for which more than 50% of the soil material by weight has particle sizes greater than 4.75 mm and less than 80mm. it is divided into two part coarse and fine gravel as shown in table.
Sand are those for which more than 50% of the soil material by weight has particle sizes greater than 75 micron and less than 4.75mm. it is divided into three part coarse, medium and fine gravel as shown in table.
Fine grained soil classified according particle size as clay (< 0.002mm) and silt (2 -75micron). As the sizes are very less to do sieve analysis, the plasticity chart is used for subclassification as shown in chart and table below.
Based on liquid limit values and compressiblity characteristics the soil is classified as ML, OL, CL, etc.
Based on gradation of different soil elements further classsfication is made as shown in chart.
GW, GP, GM, GC, SW, SP, SM, SC- here W stands for well graded soil, P- stands for poorly graded soil, M is silt and C is clay. Hence the name indicates the classifition and grading of soil.
Soil classification using group symbols is as follows:
Q: Explain Plasticity chart
A plasticity chart, based on the values of liquid limit (WL) and plasticity index (IP), is provided in ISSCS to aid classification.
The 'A' line in this chart is expressed as IP = 0.73 (WL - 20).
Depending on the point in the chart, fine soils are divided into clays (C), silts (M), or organic soils (O).
The organic content is expressed as a percentage of the mass of organic matter in a given mass of soil to the mass of the dry soil solids.
Three divisions of plasticity are also defined as follows.
The 'A' line and vertical lines at WL equal to 35% and 50% separate the soils into various classes.
For example, the combined symbol CH refers to clay of high plasticity.
Q: Explain Soil gradation system
In the Indian Standard Soil Classification System (ISSCS), soils are classified into groups according to size, and the groups are further divided into coarse, medium and fine sub-groups.
The grain-size range is used as the basis for grouping soil particles into boulder, cobble, gravel, sand, silt or clay.
Gravel, sand, silt, and clay are represented by group symbols G, S, M, and C respectively.
Physical weathering produces very coarse and coarse soils. Chemical weathering produce generally fine soils.
Coarse-grained soils are those for which more than 50% of the soil material by weight has particle sizes greater than 0.075 mm. They are basically divided into either gravels (G) or sands (S).
According to gradation, they are further grouped as well-graded (W) or poorly graded (P). If fine soils are present, they are grouped as containing silt fines (M) or as containing clay fines (C).
For example, the combined symbol SW refers to well-graded sand with no fines.
Both the position and the shape of the grading curve for a soil can aid in establishing its identity and description. Some typical grading curves are shown.
Figure 4 Soil gradation curves (experiment: sieve analysis)
Coefficient of uniformity (Cu)
It is a measure of particle size range.
This term identified in mechanical Sieve analysis (experiment)
Cu = D60D10
For uniform soil Cu =1
For well graded soil Cu > 4 for gravels (along with Cc condition)
For well graded soil Cu > 6 for sands (along with Cc condition)
Coefficient of curvature (Cc)
It is a measure of shape of particle size.
This term identified in mechanical Sieve analysis (experiment)
Cc = D302D60D10
Here D10, D30 & D60 are the “particle sizes (let’s say A, B, C)” such that 10%, 30% & 60% of particles in soil are smaller/finer than that (A, B & C respectively) sizes respectively. These values are identified from gradation curve (experiment: - mechanical Sieve analysis)
For well graded soil Cc should be within 1 to 3 (along with Cu conditions)
Q: Explain consistency of soils
Figure 5 states of soil consistency
The consistency of a fine-grained soil refers to its firmness, and it varies with the water content of the soil.
A gradual increase in water content causes the soil to change from solid to semi-solid to plastic to liquid states. The water contents at which the consistency changes from one state to the other are called consistency limits (or Atterberg limits).
The three limits are known as the shrinkage limit (WS), plastic limit (WP), and liquid limit (WL) as shown. The values of these limits can be obtained from laboratory tests.
Two of these are utilised in the classification of fine soils:
Liquid limit (WL) - change of consistency from plastic to liquid state
Plastic limit (WP) - change of consistency from brittle/crumbly to plastic state
The difference between the liquid limit and the plastic limit is known as the plasticity index (IP), and it is in this range of water content that the soil has a plastic consistency. The consistency of most soils in the field will be plastic or semi-solid.
Liquid limit (wL)
It is minimum water content at which soil is in liquid state.
Plastic limit (wP)
It is minimum water content at which soil can be remould (shaped) without any crack.
Shrinkage limit (ws)
It is maximum water content reduction in water content will not affect total volume of soil mass.
Additional formulae
Plasticity Index = Ip = wL- wP
Consistency Index = Ic = wL-wIp
Liquidity Index = IL = w - wPIp
Consistency Index + Liquidity Index = 1 🡪 IC + IL = 1
Flow index = slope of liquid limit curve (line) from logarithmic plot
Flow index = IF = w1- w2n2n1
Where w1 and w2 are water contents corresponding to number of blows n1 and n2 (in liquid limit test: experiment) respectively
Toughness Index = IT = IpIF
Activity of clays (Ac) = Ip% by weight of clay
Ac < 0.75 🡪 Inactive soil
0.75 < Ac < 1.40 🡪 Normal soil
Ac > 1.40 🡪 Active soil
Determination of index properties
Note: Refer laboratory manual for all experiments
Soil Structure
Single grained
Honeycomb
Flocculent
Dispersed Coarse grained skeleton
Cohesive matrix
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