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Foundation Engineering Overview

Soil as Foundation Material

  • Essential for supporting structures.

Introduction to Foundation

  • Definitions:
    • Structural member supporting a structure.
    • System affected by loading (soil/rock).
  • Common materials: Concrete, Steel, Wood.

Types of Foundations

Shallow Foundations
  • For firm soil and light loads.
  • Types include:
    • Spread Footings
    • Mat Foundations
Deep Foundations
  • Include:
    • Driven Piles
    • Drilled Shafts
    • Auger Cast Piles

Factors Affecting Foundation Choices

  • Primary Factors:
    • Sub surface soil
    • Groundwater conditions
    • Structural requirements
  • Secondary Factors:
    • Building codes/regulations
    • Impact on surrounding structures
    • Construction risk/schedule

Types of Shallow Foundation

  • Square and Rectangular Footings.
  • Includes Circular and Continuous Footings.

Advantages and Disadvantages of Shallow Foundations

Advantages:
  • Simple construction procedure.
  • Cost-effective.
  • Low skill required for labor.
Disadvantages:
  • High settlement risk.
  • Subjected to various forces (pullout, torsion).
  • Unsuitable for irregular/sloped surfaces.

Types of Structural Settlements

  • Uniform Settlements
  • Tilting Settlements: heta=8Lheta = \frac{8}{L}
  • Differential Settlements: heta=8Lheta = \frac{8}{L} (Exterior Damage).

Reducing Settlement

Design Considerations:
  • Modify structure (increase stiffness or flexibility).
  • Lighten structural loading.
  • Employ construction joints.
Practical Considerations:
  • Modify foundation (larger/more footings).
  • Use mat foundations.
  • Employ deep foundations.

Determining Bearing Capacity of Soils

  • Methods include:
    • Meyerhof’s Theories
    • Terzaghi’s Theories
    • Hansen’s Theories
    • Vesic’s Theories

Terzaghi Bearing Capacity Equation

  1. For general shear failure:
    • Strip: q=1.0cN′+DY+0.5BN′q = 1.0cN' + \frac{D}{Y} + 0.5BN'
    • Square: q=1.3cN′+DY+0.4N′Bq = 1.3cN' + \frac{D}{Y} + 0.4N'B
    • Circular: q=1.3cN′+DY+0.3N′Bq = 1.3cN' + \frac{D}{Y} + 0.3N'B
  2. Incorporate bearing factors using tables/charts.

Bearing Factors

  • Coefficients for General and Local Shear Failure:
    • Nc, Nq, Ny based on friction angle.
    • Plotting factors against friction angle aids in calculations.

Example Calculation

  • Ultimate Bearing Capacity: Calculate for a square footing: width = 2.1m, depth = 1.2m (water table not influencing).
  • Use general shear failure condition for determination.