1. foundations are generally grouped into two categories:
Bearing Capacity
foundations are designed to transmit load from the structure they support to the soil
foundations are generally grouped into two categories:
A. Shallow Foundations
B Deep Foundations

2. Shallow Foundations
the most common (and cheapest) type of shallow foundations are
SPREAD FOOTINGS
square spread footings to support individual columns (also circular)
McCarthy, 6th Ed.

3. Strip Footings to support wall loads
McCarthy, 6th Ed.
Rectangular and Trapezoidal Footings for two columns (combined footing) or machine base
McCarthy, 6th Ed.

4. RAFT or MAT Foundations
McCarthy, 6th Ed.
To lower the bearing pressure and reduce differential settlement on soils with low bearing capacity or erratic or variable conditions

5. FLOATING Foundations
McCarthy, 6th Ed.
where deep deposits of compressible, cohesive soil are present and piles are impractical
building’s substructure is a combination mat and caisson to create a rigid box
weight of earth displaced by foundation is equal to total weight of structure, thereby minimizing settlement from consolidation

6. Deep Foundations
used when soil near surface has poor load-bearing capacity
they transmit load through weak soil strata (overburden) to stronger, load-bearing stratum (eg., bedrock, dense sand and gravel, etc.)
loose soil
bedrock

7. Types of Deep Foundations
PIERS
where load-bearing stratum no more than 5 m deep
not used much any more
McCarthy, 6th Ed.
CAISSONS
where over-burden no more than m thick
replacing piers
McCarthy, 6th Ed.

8. deep over-burden more than 8 - 9 m thick
PILES
deep over-burden more than m thick
Various types and placement methods
Craig, 6th Ed.

9. Structural Requirements
Factor of Safety against General Shear Failure of supporting soil is normally required to be in the range 2.5 – 3.0
Tolerable amount of settlement; in particular, differential settlement should not cause significant damage to structure nor interfere with function
Secondary to these, during construction, there should be no adverse affect on adjacent structures or services

10. Ultimate Bearing Capacity, qf
The least pressure that would cause shear failure of supporting soil immediately below and adjacent to a foundation
Craig, 6th Ed.

11. modes of failure: General Shear Failure
on low compressibility (dense or stiff) soils
plastic equilibrium throughout support and adjacent soil masses
heaving on both sides of foundation
final slip (movement of soil) on one side only causing structure to tilt

12. Local Shear Failure
on highly compressible soils
only partial development of plastic equilibrium
only slight heaving on sides
significant compression of soil under footing but no tilting

13. Punching Shear Failure
on loose, uncompacted soils
vertical shearing around edges of footing
high compression of soil under footing, hence large settlements
no heaving, no tilting

14. Terzaghi’s Theory strip footing of infinite length and width B
Craig, 6th Ed.
strip footing of infinite length and width B
uniform surcharge, q0 on surface of isotropic, homogeneous soil
Rankine active wedge, ABC: forces ¯
Passive zones, ADE (­¬) & BGF (®­)

15. above EDCGF: plastic equilibrium below EDCGF: elastic equilibrium
Craig, 6th Ed.
transition between ¯ & «­: ACD & BCG (zones or radial shear or slip fans)
above EDCGF: plastic equilibrium
below EDCGF: elastic equilibrium

16. the more general case is a footing at depth D
Craig, 6th Ed.
Neglecting the shear strength of the soil above depth D implies that this soil is a surcharge: q0 = gD
Terzaghi’s general equation:
qf = 0.5gBNg + cNc + gDNq
Soil Self Weight
Shear Strength
Surcharge
Contribution of:

17. Bearing Capacity Factors
Ng, Nc and Nq are bearing capacity factors and are derived from various sources
Craig, 6th Ed.

18. General Shear Failure of Footings (Ultimate Bearing Capacity)
theory was developed for strip footings
to adapt to square, circular and rectangular shapes, Terzaghi & Peck developed shape factors here which are still widely used today:
FOOTING TYPE Sγ Sc
Strip
1.0
Square
0.8
1.2
Circular
1.6
Rectangular

19. Allowable Bearing Capacity
the allowable bearing capacity, qa is the value used in the design of footing size
in North America, a factor of safety against general shear failure, F is applied to the ultimate bearing capacity, qf:
in Britain, F is not applied to the surcharge:

20. Skempton’s Nc Values
if undrained shear strength parameters are used for the design then a special case arises:
since fu = 0, Nq = 1 and:
values of Nc are acquired from Skempton’s Chart®
Craig, 6th Ed.