Soil Mechanics

1. CE6405 SOIL MECHANICS
V.Nageshwaran, M.E.,
Assistant Professor,
Department of Civil Engineering,
UCET

2. COURSE CONTENT
 General and Introduction
 Unit I – Soil Classification and Compaction
 Unit II – Soil Water and Water Flow
 Unit III – Stress Distribution and Settlement
 Unit IV – Shear Strength
 Unit V – Slope Stability

3. GENERAL AND INTRODUCTION

4.  Soil – Naturally occurring material – obtained from
Weathering/Decomposition/Disintegration of rocks.
 Soil – Unaggregated/Uncemented deposits of minerals and organic
particles covering large portion of Earth crust.
 Soil – Heterogeneous mixture of Fluids (Air and Water) and Particles
(Clay, Sand, Silt and Gravel) – Sometimes it may contain Organic Solids,
Liquids, Gases and other matters.

5. SOIL MECHANICS
 Soil Mechanics – Branch of Engineering Mechanics –
describes behaviour of Soils.
 Soil Mechanics – Branch of Civil Engineering – Deals with
Properties, Behaviour and Performance of Soil as Structural
Material.
 A sub-discipline of Civil Engineering, and Engineering
Geology, a sub-discipline of Geology.
 Used to analyze the deformations of soils and flow of fluids
within natural and man-made structures that are supported
on or made of soil, or structures that are buried in soils.

6. HISTORY OF SOIL MECHANICS
 Knowledge and Use of Soil – Prehistoric times – When Man
started Constructing Dwellings for living and roads for
transportation.
 Primitive Civilizations – Soil used by Man – Construction
material for Foundation and for Structure themselves.
 Knowledge of Soil for Foundations, Bunds and Roads –
gained by Trial and Error Experiences.
 Recent Knowledge – Still – Empirical.
 Use of Both Timber and Stone Caissons of Soft Ground
Shaft Construction was known in Egypt in 2000 B.C.
 Hanging Garden of Babylon – King Nebuchadnezzar – Big
Retaining Walls to support the Terraces of the Garden –
Knowledge of Earth Pressure (Jumikis, 1962).
 Technical Literature of during the Roman Empire – Evidence
they paid attention to some properties of Soils and to the
Stability of Foundation.

7. CONT…
 Romans built notable Engineering Structures, such as Harbours,
Moles, Break-waters, Aqueducts, Bridges, Large Public Buildings,
Sewage Lines and a Vast network of durable and excellent Roads
– requiring solutions of Earthwork and Foundation Design.
 Vitruvious – Roman Engineer – wrote 10 books on Architecture –
1st Century B.C. – discussing the Stability of Buildings – “Greatest
Care must be taken in the substructure, because immense
damage is caused by Earth piling against them. (Summer &
Winter alters.)
 Vitruvious – recommends the use of piles to be driven close
together by machinery and intervals between them to be filled with
charcoal in the case of Loose or Marshy Land.
 India – Mansar, Mayamata, Viswakarma, Agastya, Santakumara,
Mandana, Srikumara, etc. wrote books laying down rules of
construction – among these, Mansar’s “Silpa Sastra” – 6th/7th
century – recommended compaction of soil by cows and oxen and
dewatering of foundations.

8. CONT…
 Medieval Period (400 – 1400 A.D.) – many Structures were
built – main problems – compression of soil and consequent
Settlement of Buildings.
 Leaning Tower of Pisa – 1174 to 1350 A.D. – example.
 India – Taj Mahal – 1632 to 1650 – unique foundation
problems because of proximity of the River Jamuna –
Terrace and Mausoleum Building & Minarets, rest on one
fimr compact bed of mansonry supported on masonry
cylindrical wells sunk at close intervals.
 Earth Dams – Mudduk Masur Dam – South India – 33 m
height – 1500 A.D.
 1661 – France – Improvement of Highways and Building
Canals.
 Later part of 17th century – French Military Engineers –
Empirical and Analytical Data pertaining to Earth Pressure
on Retaining Walls – Design of Revetment for Fortification.

9. CONT…
 1715 – France – Dept. of Roads and Bridge.
 1776 – 1st contribution to the present study of soil behaviour
– Frenchman – Coulomb – Wedge theory of Earth Pressure.
 Coulomb – 1st introduced the Shearing Resistance of Soil –
2 Components (Friction and Cohesion).
 1788 – 1867 – Poncelet – Extended Coulomb’s theory –
Graphical Method of finding the Magnitude of Earth Pressure
on the Wall (Vertical & Inclined Wall surface on the backfill
side & for arbitrary broken polygonal surfaces.
 1866 – K.Culmann – Extended Coulomb-Poncelet theory –
Geometrical formulation.
 1871 – Rebhann & 1878 – Weyrauch elaborated the Earth
Pressure theory using Graphical Analysis.

10. CONT…
 1856 – Darcy’s Law for Flow of Water through soils &
Stoke’s Law for settlement of solid particles in liquid were put
forward.
 1857 – Rankine – Theory for calculating Earth Pressure and
Safe Bearing Capacity of Foundation.
 Rankine and others of his time – did not take cohesion of
clay soil into calculation – even though they knew its
existence.
 1885 – Boussinesq – Analysis for Stress Distribution in a
Semi-infinite Elastic Medium under Surface Point loads.
 1906 – Muller-Breslau – performed extensive experiments
with a large scale model retaining wall to test Earth Pressure
theories.
 1871 – O. Mohr – Graphical representation of Stress at a
point – Mohr’s Stress Circle – used in Analysis of the
Shearing Strength of Soils.

11. CONT…
 1911 – Under the Chairmanship of Dr.Fellenius – Atterberg a
Swedish Soil Scientist proposed – different Stages of
Consistency of Clay Soil depending on Water content.
 1885 – Shear Box was 1st developed in France – Leygue to
measure Shear Strength of Sand.
 1918 – Shear Box was improved – Krey (Germany) &
Terzaghi and Casagrande (USA) individually.
 1910 – Resal & 1915 – Bell – extended Rankine’s Analysis
of Earth Pressure – including both friction and cohesion.
Also, Bell suggested – Method of calculating Bearing
Capacity of Cohesive soils.
 1916 – Petterson & Hultin – proposed circular Sliding theory
– Friction Circle Theory – Stability calculations.
 1926 – Fellenius further developed Friction Circle theory –
Swedish Method of Slope Analysis.

12. CONT…
 1920 – Prandtl – proposed theory of Plastic Equilibrium –
basis for various Bearing Capacity theories.
 1923 – Dr. Terzaghi – Theory of Consolidation.
 1925 – Dr. Terzaghi – published a German Titled book
“Erdbaumechanik” – also coined a term “Soil Mechanics”.
 Dr. Terzaghi – “Father of Soil Mechanics”.
 1933 – Proctor – proposed the Principles of Soil
Compaction.
 1922-23 – Pavlovsky – Russia – solved complex problems of
Seepage below Hydraulic Structures – proposed Electrical
Analogy method for Seepage computation.
 1934 – Weaver & Khosla – solved some of Seepage
problems independently.
 1939-45 – during World war – various scientists and
Engineers of different countries Worldwide contributed
towards the establishment of branch of Engineering.

13. CONT…
 1930 – Soil Mechanics established as a branch of Civil
Engineering.
 1st Scientific Study of Soil Mechanics – French Physicist –
Charles-Augustin de Coulomb – Published – Earth Pressure
theory – 1773.
 1857 – William Rankine – Scottish Engineer – Published
Coulomb’s work and Theory of Earth Masses – Used as
Primary Tools to Quantify Earth Stresses.
 20th Century – Recently – Influence of Cohesion – Causes
Soil to behave differently under Stress than Rankine and
Coulomb predicted.

14. APPLICATIONS OF SOIL MECHANICS
 Foundation Design and Construction
 Pavement Design
 Design of Underground Structures
 Design of Earth Retaining Structures
 Design of Embankments and Excavations
 Design of Earth Dams
 Seepage Problems, etc.

15. SOIL COMPOSITION
The ideal composition of soil,
 25% Air,
 25 % H2O,
 45% Mineral Matter
 5% Organic Matter.

16. CONT…
 Carbonates – Calcite & Dolomite – Used to make Cement
 Oxides
 Hydrous Oxides – Gibbsite & Brucite – Sheets in Clay
Minerals
 Phosphates – Mining for Fertilizer
 Silicates – > 90% of all Soil
Note:
 Major Composition of Soil
 Minerals
 Organic Matters

17. MINERAL COMPOSITION IN SOIL
 Minerals – Inorganic (non-living) substances, definite
composition, characteristic physical properties
 Melting temp, shape, color, hardness
 Primary minerals – formed by the cooling of molten rock
 Secondary minerals – precipitated or re-crystallized from
solutions that contains elements from dissolutions from
other minerals
 Rocks – mixtures of minerals
 Igneous – cooled molten rock
 Sedimentary – sediments deposited in water &
consolidated materials
 Form from one time rock, minerals, soil particles, and
soluble substances cemented into hard masses
 Sandstones
 Shales – consolidated clays & silts

18. CONT…
 Limestones – clays, silts, and sands cemented in mixtures
of calcium carbonates and magnesium carbonates (50%
mass is carbonates)
 Dolomites – magnesium carbonates
 Quartzites – silica-cemented sands
 Metamorphic – igneous or sedimentary rock hardened/altered by
heat, pressure, reactions with other chemical solutions may be as
hard or harder than other forms, weather to produce similar soils
 Gneiss – minerals form segregated light/dark bands (granites)
 Schist – fissile/foliated (flaky/layered), composed of many
minerals
 Slate – hardened shale or siltstone, very hard (pool tables,
chalkboard)
 Quartzite – re-crystallized quartzic sandstone, formed by heat
& pressure, slow to weather, produces sandy & shallow soils
 Marble – hardened limestone or dolomite (easily decomposed)

19. CONT…
 Mineral soils
 Develop from minerals and rocks
 Mostly quartz, feldspars, dark minerals, lime, gypsum
 Weather to sands, clays
 Also provide the majority of soil mineral nutrients
 Mineral matter is composed of inert solids, gravel, coarse
and fine sand, and silt and clay particles. These different
names come about because of the size of each is different.
Soil Texture
Gravel > 2mm
Coarse Sand ~ 2mm
Fine Sand < 0.2mm
Silt < 0.02mm
Clay <0.002mm

20. Gravel Sand Silt Clay
Coarse Medium Fine Coarse Medium Fine Coarse Medium Fine Coarse Medium Fine
60 20 6 2 0.6 0.2 0.06 0.02 0.006 0.002 0.0006 0.0002

21. ORGANIC MATERIALS IN SOIL
 98% of all soils are mineral soils
 Organic soils form from plant residues in ponded or cold, wet
areas
 Decomposition is slow
 Referred to as: peat or muck
 Anaerobic decomposition is slow
 Organic soils can be deep if allowed to decompose naturally
 Florida Everglades, Stockton Delta in CA
 Common materials: grasses, mosses, leaves, cattails, reeds
 All soils contain significant organic matter
 Organic soils are very rich when excavated
 Less sites for organic soils today as many marshes, wetlands,
etc. have been drained for development or agriculture

22. SOIL TEXTURE
Particles
 The texture of a soil depends on the relative mixture of sand, silt and
clay particles.
 The most common method of classifying soils is based on the
percentage clay in the oil.
 E.g. Soils that contain 0 – 5 % clay are known as sandy soils.
 The particles in the soil are classed on their size.
 Anything over 2mm in diameter is referred to as gravel, pebbles or
stones.
 Particles from 2 mm to 0.5 mm are called sand particles.
 From 0.5 mm to 0.002 mm are called silt particles.
 Any particle under 0.002 mm is referred to as clay.
 Sand and Silt are similar in composition and are formed by physical
breakdown of rocks.
 Clay particles are formed by both physical and chemical breakdown
of rocks.

23. SOIL TEXTURE
 As mentioned before soils are classified by the amount of clay in the soil.
 0 – 5 % Clay Sandy Soil (களிமண் கலப்பு பெருமணல் மண்)
 5 – 10 % Clay Sandy Loam (களிமண் கலப்பு பெருமணல் ெசளள மண்)
 10 – 20 % Loam (ெசளள மண்)
 20 – 30 % Clay Loam (களிமண் கலப்பு ெசளள மண்)
 30 – 40 % Clay Soil (களிமண்)
 40 % & above Heavy Clay Soil (செறிவுற்ற களிமண்)
 A more common and accurate way at looking at the type of soil is by using a soil
triangle.
Loam is soil composed mostly of sand
(particle size > 63 µm), silt (particle
size > 2 µm), and a smaller amount of
clay (particle size < 2 µm).
Its composition is about 40%-40%-20%
concentration of sand-silt-clay,
respectively.
Silt – வண்டல் மண்

25. TYPES OF SOIL (BASED ON SIZE)
(ெரல் கற்கள்)

27. GENERALIZED SOIL TYPES
Sandy Soils
 Have large air holes.
 Free Draining soils
 Is easy to work with (light)
 Dries out quickly
 Minerals are easily leached.
 Poor soil with little or no nutrients.
 Is a warm soil.
Clay Soils
 Holds water easily
 This protects from leaching of minerals.
 Is naturally fertile soil.
 Very poor drainage, which can lead to water logging
 Is a cold soil.

28. CONT…
Loam Soils
 Intermediate characteristics of both clay and sandy soils.
 More advantages and fewer disadvantages than sandy or
clay soils.
 A good mixture is 40 % Sand, 40 % Silt and 20 % clay.
 While the nature of soil depends on the particle composition,
the amount of humus in the soil is also a major factor.

29. CONT…
Property Sand (0.05-
2mm)
Silt (0.002-0.05mm) Clay(<0.002
mm)
1. Means of observation Naked eye Microscopic Electron
Microscope
2.Dominant minerals Primary Primary and
Secondary
Secondary
3.Attraction of particles for each
other
Low Medium High
4. Attraction of particles for water Low Medium High
5.Ability to hold chemical nutrients
and supply them to plants
Very low Low High
6.Consistency properties when wet Loose, gritty Smooth Sticky,
plastic
7.Consistency properties when dry Very loose,
gritty
Powdery, some
clods
Hard clods

30. UNITS AND CONVERSIONS
 Density – Mass/Volume – g/cm3
 Unit Weight – Weight/Volume – kN/m3
 Pressure – Force/Area – kN/m2 or kPa
 g, Gravitational Constant – 981 cm/s2 or 9.81 m/s2
 1 kN/m3 = 9.81 × 1 g/cm3
 1 g/cm3 = 10-3 kg/m3
 1 g/cm3 = 1g/ml = 1 Mg/m3