Soil Properties & soil grained structure

1. Types of soil Structure:
• Single-grained structure
Single-grained structure is characteristic of coarse-grained soils,
Gravitational forces predominate the surface forces and hence grain to
grain contact results.
• Honey-comb structure
This structure can occur only in fine-grained soils, especially in silt and
rock flour. The structure has a large void space and may carry high
loads without a significant volume change. The structure can be
broken down by external disturbances.
• Flocculent structure
This structure is characteristic of fine-grained soils such as clays. Inter-
particle forces play a predominant role in the deposition, Mutual
repulsion of the particles may be eliminated by means of an
appropriate chemical; this will result in grains coming closer together
to form a ‘floc’. Formation of flocs is ‘flocculation’

3. COMPOSITION OF SOIL
• Soil is a complex physical system. A mass of soil includes accumulated
solid particles or soil grains and the void spaces that exist between the
particles. The void spaces may be partially or completely filled with
water or some other liquid. Void spaces not occupied by water or any
other liquid are filled with air or some other gas.
• Since the volume occupied by a soil mass may generally be expected to
include material in all the three states of matter—solid, liquid and gas,
soil is, in general, referred to as a “Three-phase system”.
fig: soil structure

4. Fig: three phase soil system

5. Weight volume relationship:
Fig: Two phase diagram
Fig: Three phase diagram

6. • Porosity
‘Porosity’ of a soil mass is the ratio of the volume of voids to
the total volume of the soil mass.
It is denoted by the letter symbol n and is commonly
expressed as a percentage:
n = 𝑉𝑣
𝑉
×
100 Here, Vv = Va + Vw ;
V = Va + Vw + Vs
• Void Ratio
‘Void ratio’ of a soil mass is defined as the ratio of the
volume of voids to the volume of solids in the soil mass. It is
denoted by the letter symbol e and is generally expressed as
a decimal fraction :
e =
𝑉𝑣
𝑉𝑠
×100

7. Degree of Saturation
‘Degree of saturation’ of a soil mass is defined as the ratio of the
volume of water in the voids to the volume of voids. It is
designated by the letter symbol S and is commonly expressed as a
percentage :
S=
𝑉𝑤
𝑉𝑣
× 100 ...Here, Vv = Va + Vw
For a fully saturated soil mass, Vw = Vv.
Therefore, for a saturated soil mass S = 100%.
For a dry soil mass, Vw is zero.
Therefore, for a perfectly dry soil sample S is zero.

8. Percent Air Voids
‘Percent air voids’ of a soil mass is defined as the ratio of the
volume of air voids to the total
volume of the soil mass. It is denoted by the letter symbol na and
is commonly expressed as a
percentage :
na =
Va
V
× 100
Air Content
‘Air content’ of a soil mass is defined as the ratio of the volume of
air voids to the total volume
of voids. It is designated by the letter symbol ac and is commonly
expressed as a percentage :
ac =
Va
Vv
× 100

9. Unit Weight of Solids
‘Unit weight of solids’ is the weight of soil solids per unit
volume of solids alone. It is also
sometimes called the ‘absolute unit weight’ of a soil. It is
denoted by the letter symbol γs:
γs =
Ws
Vs
× 100
Unit Weight of Water
‘Unit weight of water’ is the weight per unit volume of water.
It is denoted by the letter symbol γw :
γw =
Ww
Vw
× 100

10. • Mass Specific Gravity
• The ‘Mass specific gravity’ of a soil may be defined as the ratio of
mass or bulk unit weight of soil to the unit weight of water at the
standard temperature (4°C). This is denoted by the letter symbol
Gm and is given by : Gm= 𝜸
𝜸𝒘
This is also referred to as ‘bulk specific gravity’ or ‘apparent specific
gravity’.
Specific Gravity of Solids
The ‘specific gravity of soil solids’ is defined as the ratio of the unit
weight of solids (absolute unit weight of soil) to the unit weight of
water at the standard temperature (4°C). This is
denoted by the letter symbol G and is given by : G=𝜸𝒔
𝜸𝒘
This is also known as ‘Absolute specific gravity’ and, in fact, more
popularly as ‘Grain
Specific Gravity’. Since this is relatively constant value for a given
soil, it enters into many
computations in the field of soil mechanics.

11. WEIGHT-VOLUME RELATIONSHIPS:

15. Classification Criteria for Fine-grained Soils
Fig. Plasticity chart

16. Particle size distribution curve

17. Coefficient of uniformity:
ratio of D60 to D10 is called coefficient of uniformity.
Cu = D60 /D10
D10 represents a particle size in mm such that 10% of the
particles are finer than this size.
D60 means 60% of the particles are finer than the size of the
particle at 60% point on the curve.
Coefficient of curvature:
The shape of the particle size indicated by coefficient of
curvature (Cc)
Cc = (D30)2
(D10 x D60)
D30– Particle size corresponding to 30% finer

18. Cu must be > 4 for gravels, and > 6 for sands
If Cc = 1, the particles are of same size.
For well graded soil, Cc lies between 1 and 3
 The larger the numerical values of Cu, the more
are the range of particles.
 Soils with a value of Cu less than 2 are uniform
soils.
 Sands with a value of Cu of 6 or more are well
graded.
 Gravels with a value of Cu of 4 or more are well
graded.

19. Consistency Limits:
• It is used to denote the degree of firmness of a soil.
Consistency of a soil is indicated by such terms as soft,
firm or hard. In 1911, a Swedish agriculture engineer
Atterberg mentioned that a fine grained soil can exist in
four states, namely, liquid, plastic, semi-solid and solid
state. The water content at which the soil changes from
one state to another are known as Consistency limits or
Atterbergs limits.

20. Consistency limits or Atterbergs limits:
liquid limit:
The water content at which the soil changes from the
liquid state to plastic state is known as liquid limit (WL).
plastic limit:
The water content at which the soil becomes semisolid is
known as the plastic limit. (WP).
shrinkage limit:
The water content at which the soil changes from a
semisolid state to the solid state is known as the shrinkage
limit (WS)

21. • Plasticity Index (Ip): The numerical difference between
the liquid limit and the plastic limit is known as plasticity
index.
PI = LL – PL
• liquidity index is also known as water plasticity ratio.
L.I. = w - wp x 100
IP
Where, w - water content of the soil in natural condition.
Consistency Index (C.I.):
Ic = (wL –w) x 100
Ip

22. Flow index: (If) is the slope of the flow curve obtained
between the number of blows and the water content in
Casangrande’s method of determination of the liquid limit.
If = w1 - w2
Log10 (N2 /N1)
Where; N1= number of blows required at w. c. of w1
N2= number of blows required at w. c. of w2
Toughness index: (It) of a soil is defined as the ratio of the
plasticity index (Ip) and the flow index (If).
It = Ip/ If

23. • ACTIVITY OF CLAYS
‘Activity (A)’ is defined as the ratio of plasticity index to the percentage
of clay-sizes:
A =
Ip
% clay Fraction(C)
where c is the percentage of clay sizes, i.e., of particles of size less than
0.002 mm.
• Sensitivity of Soil (St):
Sensitivity (St)’ of a clay is defined as the ratio of the its unconfined
compression strength in the natural or undisturbed state to that in the
remoulded state, without any change in the water content;
St =
𝐪𝐮 (𝑼𝒏𝒅𝒊𝒔𝒕𝒖𝒓𝒃𝒆𝒅)
𝐪𝐮 (𝐑𝐞𝐦𝐨𝐮𝐥𝐝𝐞𝐝)

24. Sensitivity Classification Remarks
2 to 4 Normal or less
sensitive
Honeycomb structure
4 to 8 Sensitive Honey or flocculent
structure
8 to 16 Extra-sensitive Flocculent structure
> 16 Quick Unstable

27. Indian Standard Soil Classification System:
IS: 1498-1970 describes the Indian Standard on Classification and
Identification of Soils
Coarse-grained Soils: More than 50% of the total material by
weight is larger than 75-μ IS Sieve size.
Fine-grained Soils: More than 50% of the total material by weight
is smaller than 75-μ IS Sieve size.
Highly Organic Soils and Other Miscellaneous Soil
Materials:
These soils contain large percentages of fibrous organic matter, such as
peat, and particles of decomposed vegetation.