SOIL, GEOTECHNICAL ENGINEERING AND FOUNDATION ENGINEERING

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SOIL, GEOTECHNICAL ENGINEERING AND FOUNDATION
ENGINEERING

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SOIL

• Natural aggregates of mineral grains, loose or
  moderately cohesive inorganic or organic in
  nature that have the capacity of being separated
  by means of simple mechanical processes.
• Structures are built with soil
• Dams , embankment
• Structures are built in soil
• Structural foundations footings, piles, rafts,
  tunnels

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Geotechnical engineering

• A unique combination of science, experience,
  judgment and a passion for understanding the
  uniqueness and variability of ground conditions
  resulting from the forces of nature.
• It is the art of determining the properties of
  unseen and variable materials to provide a
  facility that perform as expected at acceptable
  level of risk and at an optional cost.

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• Geotechnical engineering involves investigation
  and engineering evaluation of earth materials
  including soil, rock, ground water and man-made
  materials and their systems, structural
  foundations and other civil engineering works.
• The practice involves applications of the
  principles of the soil mechanics and knowledge of
  engineering principles, formulas, construction
  techniques and performance evaluation of civil
  engineering work influenced by earth materials.
• The base up on which knowledge structure is built
  in Geotechnical Engineering is a through
  comprehension of the elements of geologic
  environment.

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Foundation Engineering

• In a broad sense, foundation engineering is a art
  of selecting, designing and constructing the
  elements that transfer the weight of structure to
  the underlying soil or rock.
• The role of engineer is to select the type of
  foundation, its design and supervision of
  construction.

Before the engineer can design a foundation
intelligently, he must have a reasonably accurate
conception of the physical properties and the
arrangement of the underlying materials. This
requires detailed soil explorations.
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General Observation

1. Soil does not posses a unique or linear
   stress-strain relationship.
2. Soil behavior depends up on the pressure, time
   and environment.
3. Soil at every location is essentially different
4. Nearly in all the cases, the mass of soil
   involved is underground and cannot be seen
   entirely, but must be evaluated on the basis of
   small size samples, obtained from isolated
   locations.
5. Most soils are very sensitive to disturbance from
   sampling and thus the behavior measured by a lab
   test may be unlike that of in situ soil.

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• The foundation engineer should posses the
  following information
• Knowledge of soil mechanics and background of
  theoretical analysis
• Composition of actual soil strata in the field.
• Necessary experience-precedents-what designs have
  worked well under what designs have worked well
  under what conditions-economic aspects
• Engineering judgment or intuition - to find
  solutions to the problems.

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Definition of foundation
Function of foundation

• The lowest part of a structure is generally
  referred to as foundation.

Requirements (Functional)
To transfer load of the superstructure to the
soil on which it is resting.
A properly designed foundation is one that
transfers the structural load throughout the soil
without overstressing of soil which can result in
either excessive settlement or shear failure,
both of which can damage the structure.
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Classification of Foundations

• Shallow foundations
• Deep Foundations

Shallow foundations located just below the lowest
part of the superstructure they support deep
foundations extend considerably deeper in to
earth.
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Shallow Foundations
PLAN
ELEVATION
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Shallow Foundations
PLAN
ISOMETRIC VIEW
Combined Trapezoidal Footing
ELEVATION
Wall Footing
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Shallow Foundations
Raft Foundation
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Loads on foundation
Dead Load Refers to the overall weight of the
structure. Includes weight of the materials
permanently attached to the structure (such as
flooring) and fixed service equipment (such as
air conditioning) Live load Refers to the
weight of the applied bodied that are not
permanent parts of the structure. Applied to the
structure during part of its useful life (e.g.
people, warehouse goods). Specified by
code. Wind loads Acts on all exposed parts of
the structure. Calculated using building
codes. Earthquake Forces Building code is
consulted.
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Depth and location of foundation

• Depth and location of foundation depends
  on
• Zone of significant volume changes in soil.
• Adjacent structures and property lines.
• Ground water
• Underground defects

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Depth and location of foundation
Zone of significant volume changes in soil
Clays having high plasticity shrink and swell
considerably up on drying and wetting
respectively. Volume change is greatest near
ground. Decreases with increasing depth. Volume
changes usually insignificant below a depth from
1.5-3.0 m and does not occur below volume changes.
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Depth and location of foundation
Structures may be damaged by the construction
of new foundations, as a result of vibrations,
undermining by excavation or lowering of the
water table. After new foundations have been
constructed, the (new) loads they place on the
soil may cause settlement of previously existing
structures as a result of new stress pattern in
the surrounding soil.
Adjacent structures and property lines.
Part extending property line
Property line
In general, deeper the foundations and closer
to the old structure, greater will be the
potential for damage to old structures.
New Footing
Existing Footing
Limit for bottom of deeper Footing
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Depth and location of foundation
Ground water Presence of water reduces soil
bearing capacity, larger footing size more cost.
During construction pumping is necessary adds
to the cost of construction. Underground
defects Footing location affected by underground
defects Faults, caves, mines, sewer lines ,
underground cables and utilities.
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Bearing Capacity Modes of Failure
Strip footing in dense soil
Load q (kN/m2)
Settlement (mm)
Sudden appearance of a clearly defined distinct
failure shape
General shear Failure
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Bearing Capacity Modes of Failure
Strip footing in Relatively loose soil
Load q (kN/m2)
qu (1)
qu (2)
Settlement (mm)
Local shear Failure
When Load reaches qu(1) further settlement takes
place with jerks At q qu(1) Not so
distinct failure surface develops does not reach
ground surface
At q qu(2) Failure surface finally
reaches ground surface not distinct Settlement
are more in this case as compared to earlier.