3. 2/22/2021 3
Soil Mechanics is defined as the branch of engineering
science which enables an engineer to know
theoretically or experimentally the behavior of soil
under the action of ;
1. Loads (static or dynamic),
2. Gravitational forces,
3. Water and,
4. Temperature.
Simply speaking it is the knowledge of engineering
science , which deals with properties, behavior and
performance of soil as a construction material or
foundation support.
4. Soil definition
• The word 'soil' has different meanings for
different professions.
• To the agriculturist, soil is the top thin layer
of earth within which organic forces are
predominant and which is responsible for the
support of plant life.
• To the geologist, soil is the material in the top
thin zone within which roots occur.
Introduction
2/22/2021 4
5. To engineering geologist
• The term soil according to engineering point of view
is defined as the material, by means of which and
upon which engineers build their structures.
• The term soil includes entire thickness of the earth’s
crust (from ground surface to bed rock), which is
accessible and feasible for practical utilization as
foundation support or construction material.
• It is composed of loosely bound mineral particles of
various sizes and shapes formed due to weathering of
rocks.
Introduction
2/22/2021 5
6. Why do you need to learn about soils?
Almost all structures are either constructed
of soil, supported on soil, or both.
Introduction
2/22/2021 6
7. Why do you need to learn about soils
V
arious reasons to study the properties of Soil:
1. Foundation to support Structures and
Embankments
2. Construction Material
3. Slopes and Landslides
4. Earth Retaining Structures
5. Special Problems
Introduction
2/22/2021 7
8. 2/22/2021 8
Why we study Soil Mechanics?
• Various reasons to study the properties of Soil:
1. Foundation to support Structures and Embankments
Effects of static loading on soil mass
• Shear failure of the foundation soil
• Settlement of structures
Stability criteria (Solution)
• There should be no shear failure of the foundation soil.
• The settlement should remain within permissible
limits.
• Firm Soil -> Spread Footing (Spread Foundation)
• Soft Soil -> Pile Foundation (Vertical members
transferring load of structure to ground i.e. rock)
9. Shear Failure-Loads have exceeded shear strength capacity
of soil!
Transcosna Grain Elevator, Canada Oct. 18, 1913
Problems in Geotechnical Engineering
2/22/2021 9
10. Problems in Geotechnical Engineering
Shear Failure-Loads have exceeded shear strength
capacity of soil!
2/22/2021 10
11. Problems in Geotechnical Engineering
Shear Failure-Loads have exceeded shear strength
capacity of soil!
2/22/2021 11
14. Teton Dam Failure
Dam Failure - Seepage
Problems in Geotechnical Engineering
2/22/2021 14
15. Soil subjected to dynamic load
2/22/2021 15
1. Foundation to support Structures and
Embankments
Effects of dynamic loading on soil mass
• For Design and construction of roads following must be
considered:
Compaction Characteristics
Moisture Variation
16. 2/22/2021 16
Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
2. Construction Material
• Subgrade of highway pavement
• Land reclamation
• Earthen dam
17. 2/22/2021 17
Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
3. Slopes and Landslides
Major cause is the moisture variation resulting in;
• Reduction of shear strength
• Increase of moisture
• Increase in unit weight
Excavation of trenches for buildings require
braced excavation.
18. 2/22/2021 18
Why we study Soil Mechanics?
Landslide of a parking area at the
edge of a steep
slope, mainly due to
increase in moisture
content.
19. 2/22/2021 19
Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
4. Earth Retaining Structures
• Earth retaining structure (e.g., Retaining walls) are
constructed to retains (holds back) any material
(usually earth) and prevents it from sliding or eroding
away.
20. 2/22/2021 20
Why we study Soil Mechanics?
Various reasons to study the properties of Soil:
5. Special Problems
i. Effects of river water on soil mass
Scouring
Causes:
• Increased flow velocity due to obstruction
• Fineness of river bed material
Stability criteria:
• The foundation of pier must be below the scour depth
ii. Land Erosion
24. 2/22/2021 24
Who must be concerned with soils?
Civil engineers (structural, environmental
and geotechnical) must have basic
understanding of the soil properties in order
to use them effectively in construction.
Introduction
26. How is soil formed?
• Soil is generally formed by disintegration and
decomposition (weathering) of rocks through the
action of physical (or mechanical) and chemical
agents which break them into smaller and smaller
particles.
• Weathering is the process of breaking down rocks by
mechanical and chemical processes into smaller
pieces.
2/22/2021 26
28. 2/22/2021 28
Physical (or mechanical) Weathering is the
disintegration of rocks into smaller particles
through physical processes, including:
The erosive action of water, ice and wind.
Opening of cracks as a result of unloading due to
erosion of overlying soil and rock.
Loosening through the percolation and subsequent
freezing (and expansion) of water.
Thermal Expansion and contraction from day to day
and season to season.
Landslides and rock falls.
Abrasion from the downhill movement of nearby rock
and soil.
• Common soil types formed by this process are coarse
grained soils such as Gravel, sand, cohesionless soil
30. In chemical weathering, the original rock minerals are
transformed into new minerals by chemical reaction.
Water and carbon dioxide from the atmosphere form carbonic
acid, which reacts with the existing rock minerals to form new
minerals and soluble salts.
Soluble salts present in the groundwater and organic acids
formed from decayed organic matter also cause chemical
weathering.
An example of the chemical weathering of orthoclase to form
clay minerals, silica, and soluble potassium carbonate follows:
The principal types of decomposition are hydration, oxidation,
carbonation, desilication and leaching.
2/22/2021 30
31. The weathering of granite
The results of chemical weathering are generally fine soils with
altered mineral grains.
2/22/2021 31
33. Controlling factors in soil formation
• Nature and composition of the parent rock
• Climatic conditions, particularly temperature and
humidity
• Topographic and general terrain conditions
• Density and type of vegetation
• Length of time related to particular prevailing conditions
• Interference by other agencies, e.g. earthquakes, action
of man etc
• Mode and conditions of transport
2/22/2021 33
34. Nature of parent rock - Bowen’s Reaction Series
-The reaction series are similar to the weathering stability series
More stable
Higher weathering resistance
2/22/2021 34
35. 2/22/2021 35
Soil types
1. Geological consideration:
Depending on the method of deposition,
soils can be grouped into two categories:
1-Residual soils:
The soils which remain at the place of
disintegration of parent rock.
2-Transported soils :
The soils, which carried away from their
place of disintegration to some other place
by transporting agencies.
36. 2/22/2021 36
The transported soils may be classified into several
groups, depending on their mode of transportation and
deposition:
1) Glacial soils—formed by transportation and
deposition of glaciers
2) Alluvial soils—transported by running water and
deposited along streams
3) Lacustrine soils—formed by deposition in quiet lakes
4) Marine soils—formed by deposition in the seas
5) Aeolian soils—transported and deposited by wind
6) Colluviam soils—formed by movement of soil from
its original place by gravity, such as during landslides
37. General types of soils
According to their grain size Grains diameters (mm)
–Cobbles > 76.2 mm
–Gravel 76.2 mm to 4.75 mm coarse grained soils
-Sand 4.75 mm to 0.075 mm cohesion less soils
–Silt 0.075 mm to 0.002 mm fine grained soils
–Clay < 0.002 mm (2μm) cohesive soils
2/22/2021 37
2. Engineering consideration:
38. Comparison of four systems for describing soils based on particle size
2/22/2021 38
39. Soil Characterization
Soil characteristic properties: 3 types
those of the constituents (nature )
those of the soil mass (state)
those related to the behavior of the soil mass
subjected to external stresses (mechanical properties)
Attention: disturbed / undisturbed soil ???
Undisturbed soil is obligatory to determine properties 2 & 3.
2/22/2021 39
40. Properties of soil constituents
• Mineral composition
• Unit weights of the grains and water
• Particle shape
• Specific surface
• Grain size distribution- granulometric anlaysis Index
• For fine grained soils – Atterberg limits properties
• (consistency limits)
• Mineral composition: the large majority of soils consist of mixtures of
different mineral particles
- Rock fragments
- Mineral grains 2/22/2021 40
50. Clay structures:
• 1) - Dispersed structure 2) flocculated structure
• Lower strength More strength
• permeability is less Permeability is higher
• higher compressibility Low compressibility
Formed by the deposition of
the fine soil fraction in water.
2/22/2021 50
51. Mechanical analysis of soil
• Mechanical analysis is the determination of the
size range of particles present in a soil, expressed
as a percentage of the total dry weight.
• Two methods generally are used to find the
particle-size distribution of soil:
• (1) sieve analysis—for particle sizes larger than
0.075 mm in diameter, and
• (2) hydrometer analysis—for particle sizes
smaller than 0.075 mm in diameter.
2/22/2021 51
52. 1) Sieve analysis (Direct method)
• Sieve analysis consists of shaking the soil sample through a
set of sieves that have progressively smaller openings.
• Sieve analysis is carried out by using a set of standard
sieves.
• -Sieves separate particles in the range between 75 mm and
75 μm
2/22/2021 52
62. Limitations of using the Stokes 'equation:
Soil particles are not spheres;
The fluid is not of infinite extent;
The specific gravity of individual particles may vary;
Turbulence caused by larger particles falling;
Disturbance due to insertion and removal of the hydrometer;
The test is actually used for diameters as large as 0.07 mm
2/22/2021 62
63. Plasticity of Fine Grained Soils
Plasticity is the ability of a soil to undergo unrecoverable
deformation at constant volume without cracking or
crumbling.
It is due to the presence of clay minerals or organic material.
Consistency limits (Atterberg limits):
Atterberg, a Swedish scientist developed a method for
describing the limit consistency of fine grained soils on the
basis of moisture content.
These limits are liquid limit, plastic limit and shrinkage limit.
2/22/2021 63
65. Liquid limit (L.L): is defined as the moisture content in
percent at which the soil changes from liquid to plastic
state.
Plastic Limit (P.L.): The moisture contents in % at which the
soil changes from plastic to semi solid state.
Shrinkage Limit (S.L.): The moisture contents in % at which
the soil changes from semi solid to solid state.
Plasticity Index (P.I.): it is the range in moisture content
when the soil exhibited its plastic behavior:
Plasticity Index = Liquid Limit – Plastic
Limit PI= LL – PL
2/22/2021 65
66. Liquidity Index (L.I. or IL) : a relation between the natural moisture contents
(ωn) and (L.L.) and (PI.) in form:
where; IL=liquid index;
wn= natural water/in
situ water ;
wp=plastic limit;
Ip= plastic index
2/22/2021 66
67. If LI > 1 Then the soil at Liquid state
If LI = 1 then the soil at L.L.
If LI< 1 then the soil below L.L.
where, wI = liquid limit, wn= in situ moisture
2/22/2021 67
68. Plastic limit
The soil is rolled on a glass plate with the hand, until it is
about 3 mm in diameter.
At the plastic state the soil rolled into threads of about 3 mm
diameter just crumbles
2/22/2021 68
