Shallow Foundations

1
Shallow Foundations

• Ch. 9

2
Introduction

• Definition of a foundation
• shallow (footings)
• deep (pier, caisson or piles)
• Footings - Fig. 9-2
• individual
• continuous or wall
• strap
• mat foundation

3
Foundation Design

• Resistance to bearing capacity failure
• Resistant to excessive settlements
• Table 9-1
• Footing design procedure
• p.264

4
Loads on Foundations

• dead load
• live load
• wind load
• snow load
• earth pressure
• water pressure

5
Depth and Location

• Frost action
• effect of expansion and contraction
• Soil Volume change
• depth of 5 to 10 ft. below surface
• Groundwater
• construction (dewatering)
• reduced strength
• hydrostatic uplift
• frost action
• waterproofing problems

6
Bearing Capacity Analysis

• Ultimate bearing capacity
• allowable bearing capacity
• factor of safety
• Terzaghi equations
• Continuous or wall footing(width B)
• Circular footing(radius R)
• Square footing(width B)

7

• Terzaghi bearing factors
• Figure 9-7 bearing capacity factors for both
  cohesive and cohesionless soils
• cohesion (C) and angle of internal friction (?)
• cohesive soils, ? is equal to zero
• cohesionless soil, c is equal to zero
• dense sand and stiff clay
• loose sand and soft clay
• c and ? values
• Figure 9-9 bearing capacity factors for
  cohesionless soil

8
Effect of groundwater table on bearing capacity

• Case 1 - groundwater at or above base of footing
• Case 2 - groundwater is at a distance, B, below
  the base of the footing
• Case 3 - groundwater is between the base of the
  footing and a distance B below the base of the
  footing

9
Structural Design of Footings

• Size of Footing
• Section 9-5
• Required base area of a footing is obtained by
  dividing the Column Load, Q, by the allowable
  bearing capacity, q(a).