This is a pdf explaining the soil mechanics in civil engineering

1. PART II: SOIL MECHANICS
TEXT BOOKS/REFERENCES:
1. Kyulule, A.L. (1994): Introduction to Soil Mechanics. Dar es
Salaam University Press (DUP)
2. Wilun, Z. & Strazewsk, K. (1975): Soil Mechanics in
Foundation Engineering (Vol.1), 2nd Edition.
Surrey University Press.
3. Craig, R.F. (2004): Craig’s Soil Mechanics. 7th Ed. Spon
Press (Taylor & Francis Group). London.
4. Barnes, G.E. (2000): Soil Mechanics (Principles & Practice).
2nd Ed. Palgrave Macmillan. Hampshire, UK.
5. Bowles, J.E. (1929): Physical and Geotechnical Properties of
Soils. McGraw-Hill Int.

2. PART II  Soil Mechanics
• Soil Formation and Classification
• Soil Structure and Index Properties
• Soil Water, Capillarity and Permeability

3. 1.0 INTRODUCTION
Concerned with behaviour and performance of soil as a
construction material or support for engineered
structures
Concerned with
• Soil identification & properties: sampling & testing
• Soil strength/stability: ability to support load without
failure/rapture
• Deformation & compressibility: load-deformation
characteristics (no excessive deformations)
• Water flow (into, out of, and through soil): effects of H2O
on soil properties, effects of water flow, etc

4. Why Soil Mechanics?
“Virtually every structure is supported by soil or rock.
Those that aren’t - either fly, float, or fall over.”
-Richard Handy, 1995

5. 2. SOIL FORMATION AND CLASSIFICATION
• In engineering: Soil is the natural products of
(chemical / biological) weathering and mechanical
disintegration of the rocks which form the crust of the
earth.
• It is the loose/unconsolidated materials overlaying
bedrock produced by weathering.
• Depending on mode of formation, can be classified
as residual, transported (sedimentary), volcanic,
organic or fills :

6. 2.1 Types of Soil (based on formation)
1. Residual soils
- were formed in their present location by weathering of
parent rock or bedrock.
- Are common in the tropics (e.g. laterites)
- Are usually stiff and stable, but consist of highly
compressible materials surrounding blocks of less
weathered rock.
2. Sedimentary/transported soils
- Transported from the weathering site, deposited at current
location
- Transportation agents: water, glacier, wind and gravity
3. Volcanic Soils
- Deposits ejected from volcanic eruptions (e.g. volc ash, tuff)

7. Types of Soil (based on formation) cont…
4. Organic Soils
- Formed mainly in-situ by growth and subsequent decay of
plants or by accumulation of fragments of organic skeleton or
shells of organisms.
5. Fill
- Man-made fill deposits
- May vary from homogeneous and compacted to
heterogeneous and loose.
- E.g. embankments and earth dams, waste and refuse pits,
etc

8. 2.2 Granulometric Composition of Soils (PSD)
Classified according to the particle sizes, as:
• Boulders > 200 mm
• Cobbles 60 – 200 mm
• Gravel-sized 2.0 – 60 mm
• Sand 0.06 – 2.0 mm
• Silt 0.002 – 0.06 mm
• Clay < 0.002 mm
• Organic matter- of organic origin, at diff. stages of
decomposition

9. 2.3 MINERALOGICAL COMPOSITION OF SOILS
• Boulders, cobbles and gravel-sized particles have the
same mineralogical composition as the parent rock.
• Sand grains are generally composed of minerals
resistant to weathering, such as quartz & silica.
• Silt particles (rock flour) have similar mineralogical
composition as sand grains (by grinding action)
• Clay particles consist of clay minerals, which are the
product of chem. weathering of feldspars and micas.
- Are hydrates of Al, Fe, and Mg silicates, combined in
a form of sheet-like structures, only few molecules
thick (main dimension < 0.002 mm)

10. and Silicon
and Oxygen
Silica tetrahedron Silica sheet
and Hydroxyls Aluminium, magnesium
Alumina octahedron Alumina sheet
(Tarentino & Jommi, 2007)
BASIC UNITS OF THE CLAY LATTICE STRUCTURE

11. Clay Mineralogy
• Clay Formation – Chem. weathering of parent rocks
(feldspars, micas, lime stone)
• Most important groups of clay minerals:
- Kaolinite – Are very stable and absorbs little water
(saturation ~90% of dry weight)
- Have low swelling and shrinkage potential
- Montmorillonite – high water absorption (~300-700%!!)
- high swelling and shrinkage potential
- Illite – Water absorption and swelling/shrinkage
potential half-way between kaolinites and
montmorillonites.

12. EXPANSIVE CLAYS
Access to water, w↑ Volume increases (swell, heave)
Decreasing water, w↓  Volume decreases (shrink)
Constant moisture cont. Δw = 0  NO swell, NO shrinkage
Potential for swelling & shrinking  Clay mineralogy

13. Three-layer (2:1) lattice structure of
Montmorillonite (smectite)
T
O
T
Clay
particle
(2 or 3
sheets)
Patterns of
Clay Minerals
Minerals differ mainly in the type of ‘glue’
that holds the successive layers together

14. Montmorillonite (Smectite) - Facts:
• Parent rocks – Ferromagnesium minerals,
calcic feldspars, volcanic rocks
• Formation – Extreme disintegration, strong
hydration and restricted leaching
(accum. of Mg2+, Ca2+, Na+ & Fe2+ cations)
• Favourable conditions:- Semi-arid regions with
low seasonal rainfall
- Evaporation exceeds precipitation
(→unsaturated soil!)

15. MAJOR SOIL DEPOSITS
• Alluvial Deposits
The thickness of alluvial deposits varies from a few metres to
more than one hundred metres. The distinct characteristics of
alluvial deposits is the existence of alternating layers of sand,
silt and clay. The thickness of each layer depends on the local
terrain and the nature of floods in the rivers causing deposition.
The deposits are generally of low density and are liable to
liquefaction in case of earthquake or seismic loading
• Black cotton soils
These are residual deposits formed from basalt rocks. They are
quite suitable for growing cotton. These are clays of high
plasticity. They contain essentially the clay mineral
montmorillonite. They have high swelling and shrinkage
characteristics. Their shearing strength is extremely low. They
are highly compressible and have very low bearing capacity.

16. Major Soil Deposits (contd…)
• Lateritic Soil
These are formed by decomposition of rocks, removal of bases
and silica and accumulation of iron oxide and aluminium oxide.
The presence of iron oxide gives these soils the characteristic
red or brown to dark brown colour. These are residual soils.
Lateritic soils are soft and can be cut with a chisel when wet.
However, these harden with time. A hard crust of gravel size
particles, known as laterite, exists near the ground surface.
These soils, especially those which contain iron oxide, have
relatively high specific gravity (3.0 and above)
• Marine Deposits
These are mainly confined along a near belt near the coast.
They mainly appear as thick layers of sand above deep
deposits of soft marine clays. They have low shear strength and
are highly compressible and highly plastic. They contain a large
amount of organic matter.

17. 2.4 CLASSIFICATION OF SOILS
• To obtain a consistent and internationally recognized
description of soil sample
• To sort out soils into groups of similar behaviour
• NB: Properties and behaviour of soils mainly governed by
mineralogical composition of particles, water, inorganic
cementing materials and organic component
• Two classification types:- Field & Laboratory
• Field Classification – To classify soils on site, visually
without performing any lab test.
- Done during soil investigations
- Guideline for grain size: 0.063mm ≈ flour particle,
2.0mm ≈ match point; 63mm ≈ hen-egg, 200mm ≈ football

18. Composition of Soils
• Saturated soils  TWO phases: solids + liquid
• Unsaturated  THREE phases: solids + liquid + gas
Soils  Granular: Grains + Voids
 Saturated / Unsaturated
Soil grain
Void
Unsaturated

19. Laboratory Classification of Soils
• Based on grain (particle) size distribution (PSD), plasticity
(Atterberg limits) and organic content.
2.4.1 Particle Size Distribution:
Based on size of particles. Determined by sieve analysis.
- Particles > 0.063 mm (sand & gravel) → sieve analysis (dry, wet)
- Particles < 0.063 mm (silt & clay) → sedimentation analysis

20. Dry Sieving
• The sample is dried in oven at 105±5°C for 24 hours
• Soil is passed through a series of sieves of known
aperture (sieve size decreasing from top to bottom)
• Advantage: does not have to be dried before weighing
• Disadv: Small particles (flour) stick to coarser grains
Wet Sieving
• Applicable for particles between 10 and 0.063 mm,
unless the whole sample has been washed
• Water is flushed through sieves and sample dried
• Advantages: Clean particles obtained, → less error
• Disadv.: Needs drying, takes longer time

21. Particle Size Distribution (PSD)
• The percentage passing any sieve is given by:
Si = [(Ms-Msi) /Ms]*100%
Where: Si = %ge mass passing sieve i (smaller than d)
Mi = Total dry mass of the soil sample
Msi = Cum. dry mass retained on sieves of aperture ≥ di
Example: Soil sample, 120 g, sieved (dry and wet).
Sieve
Size (mm)
Retained
(g)
Cumulative
retained (g)
%ge
Retained
%ge
Passing
60
20
6
2
0.06
0
24
15
17
26
0
24
39
56
82
0
20
32.5
46.7
68.3
100
80
67.5
53.3
31.7

23. Sedimentation Analysis (Hydrometer Analysis)
• For PSD analysis of particles below 0.063 mm diam.
• A suspension of uniform concentration of soil particles
in water is prepared
• Changes of suspension density is observed as the
particles in water sediment (settle).
• A hydrometer is used
• Density of suspension depends on quantity & specific
gravity of solid particles in suspension
• Quantity of solid particles in suspension depends on
sinking velocity and time elapsed
• Sinking velocity depends on their weight (diameter)
and the viscosity of water (according to Stokes’ law)

24. Hydrometer Analysis

25. Hydrometer Analysis cont…
• Density of suspension:
ρzi = Msi + ρw - (Msi/ρs)*ρw
where: Msi = mass of solid particles per unit volume at time t
ρs = density of solid particles [g/m3]
ρw = density of water at that temperature [g/m3]
According to Stokes’ law:
vi = (ρs- ρw)gdi
2 If t = time taken by particle thru H
180 x η
vi = Hi/ti → Hi/ti = (ρs- ρw)gdi
2 → di = √[(180ηHi)/(ρs- ρw)gt]
180 x η
where di = diameter of biggest particle still in suspension

26. Hydrometer Analysis cont…
• For a given mass of total sample Mst, the percentage
by weight (mass) si, of particles smaller than di can
be found by:
si = (Msi/Mst) * 100% = ρs*Ri *100%
(ρs - ρw)Mst
where Ri is the hydrometer reading
A dispersive agent (anticoagulant) is normally added to
suppress coagulation (flocculation) of the fine soil
particles

28. 2.4.2 Plasticity Properties
- Concept

29. 2.4.2 Plasticity Properties (→ Consistency Limits)
• Applicable to fine-grained soils only
NB: course-grained soils are non-plastic or non-cohesive
• Consistency of cohesive soil depends on its water content
• Can change from solid (hard/stiff/firm) state to a
plastic (moldable) state to a liquid (fluid) state
• Three consistency limits can be defined:
- Shrinkage limit, ws
- Plastic limit, wP
- Liquid limit, wL
• From these we can determine Plasticity Index, Ip or PI,
Liquidity Index, IL and Consistency Index, IC

30. Consistency limits

31. Consistency limits cont…
1) Shrinkage Limit, wL
Defined as the water content (in %ge) of the soil at
the state when on drying it ceases to decrease in
volume (ceases to shrink)

34. Consistency limits cont…
2) Atterberg Limits → Plastic limit & Liquid limit
(a) Plastic Limit, wP
Is the water content of a cohesive soil where soil changes
from the plastic to the solid state and starts to show small
cracks when deformed
Determined by repeatedly rolling small balls into threads
until the soil crumbles when the threads are ~ 3 mm in diam.
Water content is then determined by heating in oven at 105°C
for 24 hrs

35. Atterberg Limits cont…
(b) Liquid Limit, wL
Is the water content (%) at which the soil changes from plastic
to liquid state and starts to behave like a liquid (to flow)
• Determined by using an empirical test → the Casagrande test.
• A soil paste is placed on the Casagrande apparatus, grooved
and subjected to standard blows against the base of the appar.
• Water content at which ~13mm (0.5“) of the grove closes when
the number of blows is 25 is recorded as the liquid limit.
• Normally, three to four points are established and plotted; water
content at which the # of blows is 25 can be determined.
• In some cases, the Fall-cone method is used, whereby the
penetration of a cone falling by gravity gives the liquid limit.
Standard charts are used

37. Atterberg Limits cont…
(c) Plasticity Index, IP = wL – wP
Indicates the quantity of water that a given soil
absorbs in changing from the solid to the liquid
consistency
It depends on the amount of clay particles and
the type of clay minerals present in the soil.
(d) Liquidity Index, IL
Is given by IL = [w - wP]/[wL - wP] = [w - wP]/IP
(e) Consistency Index, IC
Is given by Ic = [wL - w]/[wL - wP] = [wL - w]/IP

41. 2.5 Soil Classification Systems & Methods
• Several systems exist, including
- The American Society for Testing Materials (ASTM)
- The British Soil Classification System (BSCS)
- The Unified Soil Classification System (USCS)

42. 2.5.1 Classification Methods
It should be noted:
• PSD → BSCS adopted
• Classification method → USCS adopted
E.g. Fine soil → > 50% fines (USCS), [Not >30%, BSCS]
Coarse soil → < 50% fines (not < 30%)
SOIL GROUPS
G = Gravel S = Sand M = Silt
C = Clay O = Organic silt /Organic clay Pt = Peat
Soil may be well-graded (W) or poorly-graded (P)
Well-graded (W) → PSD extending evenly over a wide range of
particle sizes, w/out excess or deficiency of any particle size
Poorly-graded (P) → with excess/deficiency of some particle sizes

43. 2.5.2 Classification of Fine-grained Soils
• Based on Plasticity
• Generally classified as … of high plasticity (H) or … of
low plasticity (L). E.g. CH, CL, MH, ML

44. Can be subdivided into: …of Low plasticity (L) [wL<35];
Intermediate plasticity (I) [wL= 35-50]; High plasticity (H)
[wL= 50-70]; Very high plasticity (V) [wL= 70-90]; and
Extremely high plasticity (E) [wL> 90]

45. 2.5.3 Classification of Coarse-grained Soils
• More than 50% with particle size > 0.06 mm (BSCS)
• Classification uses Coefficient of uniformity (Cu) and
Coefficient of curvature (Cc); defined as:
Cu = d60/d30; Cc = d30
2/(d10d60)

46. Course-grained Soils cont…
• For GW (well graded gravel): Cu > 4 and 1<Cc< 3,
otherwise → GP (poorly graded gravel)
• For SW (well graded sand): Cu > 6 and 1<Cc< 3,
otherwise → SP (poorly graded sand)
• Examples:
Next slide
Kyulule, 1994; page 21

48. 2.5.4 Mixtures of Coarse- and Fine-grained Soils
• Fines content < 5%, ignore fines → clean sand/gravel
• Fines content 5-12% → dual symbols, e.g.
GW-GM → Well graded silty GRAVEL
GP-GM → Poorly graded silty GRAVEL
SW-SC → Well graded clayey SAND
SP-SC → Poorly graded clayey SAND, etc.
• Fines content > 12% → GM, GC, SM, SC
(→ gravel-silt mixture, gravel-clay mixture, sand-clay mixture...)

49. 2.5.5 Other Soil Mixtures (examples)
A. Mixtures of Fine-grained Soils
Silty CLAY (CM)
Clayey SILT (MC)
B. Mixtures of Coarse-grained Soils
Sandy GRAVEL (GS)
Gravelly SAND (SG)
C. Mixtures of more than two components
Silty sandy GRAVEL
Clayey silty gravelly SAND

53. More than 50% of the soil sample is smaller than 0.06mm
→ Fine grained soil
• Liquid limit, wL = 25.9%,
• Plastic limit, wP = 13.3%
• → Plasticity index, Ip = wL-wP = 12.6%
• Using the Plasticity Chart, the soil sample is classified as CL

54. 3.0 SOIL STRUCTURE & INDEX PROPERTIES
Unsaturated
3.1 Basic Concepts
-Granular material consisting of solid particles and voids
May be dry (voids filled with air only), unsaturated
(air + water in voids), or saturated (filled with water)

55. Basic Concepts cont…

60. Thus, γ΄ = γsat – γw
And ρ΄ = ρsat -ρw