SHALLOW FOUNDATIONS

1
SHALLOW FOUNDATIONS

• Spread footings
• Square
• Rectangular
• Circular
• Continuous
• Mat (Raft) foundations

2
SPREAD FOOTINGS

• Made from reinforced concrete
• Square (B x B)-Usually one column
• Rectangular (B x L)-When large M is needed
• Circular (D/Blt3, Rounded)-Flagpoles, transmission
  lines
• Continuous (Strip)-Support of bearing walls
• Combined (Cantilever)-Provides necessary M to
  prevent failure. Desirable when load is eccentric
  and construction close to property line.

3
MAT (RAFT) FOUNDATIONS

• Necessary when the soil is weaker and more
  compressible
• Since large area is needed from a spread footing,
  mat foundation is more economic.
• Advantages
• Spread the load in a larger area-Increase bearing
  pressure
• Provides more structural rigidity-Reduce
  settlement
• Heavier-More resistant to uplift
• Distributes loads more evenly

4
DEEP FOUNDATIONS

• When shallow foundations cannot carry the loads
• Due to poor soils conditions
• When upper soils are subject to scour
• Piles-prefabricated small-size (usually lt 2 ft or
  0.6 m diameter or side) poles made from steel (H
  or pipe piles), wood or concrete and installed by
  a variety of methods (driving, hydraulic jacking,
  jetting, vibration, boring)
• Drilled shafts-Drilled cylindrical holes (usually
  gt 2ft or 0.60 m in diameter) and filled with
  concrete and steel reinforcement

5
SHALLOW FOUNDATIONSBearing Capacity

• Gross Bearing pressure
• q (PWf)/A u
• where Wf gcDA, u pore water pressure
• Net Bearing pressure Gross Bearing pressure
  Effective stress
• q P/A gcD u SQUARE FOOTINGS
• q P/(Bb) gcD u CONTINUOUS FOOTINGS

6
SHALLOW FOUNDATIONSBearing Capacity (Contd)

• FS bearing capacity q ultimate / q allowable
  2 to 3
• q allowable Gross bearing pressure
• q ultimate cNc sD Nq 0.5gBNg strip footing
• q ultimate 1.3cNc sD Nq 0.4gBNg square
  footing
• q ultimate 1.3cNc sD Nq 0.3gBNg circular
  footingf
• See Table 17.1, page 623 for bearing capacity
  factors (Nc , Nq , Ng) as a function of friction
  angle, f. c cohesion, sD vertical effective
  stress at foundation base level, D (surcharge),
  gunit weight of soil below foundation base
  level, Bwidth (diameter) of footing
• Effect of Groundwater table (Page 624)
• Case1- DW lt D (high water table use buoyant unit
  weight)
• Case2-DltDwltDB (intermediate water table
  prorate unit weight)
• Case3-DB ltDw (Deep water table use moist unit
  weight)

7
SHALLOW FOUNDATIONSDesign-Cohesive soils

• End-of-construction (short term) analysis
• Calculate q ultimate
• q allowable q ultimate / FS bearing capacity
• Area allowable P/ q allowable
• Calculate setllement-
• d ltd allowable- DESIGN OK
• d gtd allowable- Consider soil improvement, deep
  foundation.
• Increasing area will not help, cause more
  settlement

8
SHALLOW FOUNDATIONSDesign-Cohesionless soils

• Drained (long term) analysis
• Calculate q ultimate
• Assume B to calculate q ultimate
• q allowable q ultimate / FS bearing capacity
• Area allowable P/ q allowable will give you B.
  Iterate until B assumed B computed
• Check if q allowable is OK for settlement case
  (usually at most 1 inch)

9
Deep Foundations Design

• Static Analysis
• Qultimate QEBQSR (end bearing shaft
  resistance)
• QEB qult Ap where Ap is the area of pile tip
• qult c Nc sD Nq
• QSR SpLf where p is the pile perimeter, L
  pile length, and f unit shaft resistance (skin
  friction) in a layer of soil on the side of the
  deep foundation
• f K sv tand ca where Klateral earth
  coefficient, sv vertical effective stress at
  given depth, dpile-soil interface friction
  angle, ca pile-soil adhesion in a given soil
  adjacent to lateral pile surface
• Pile load test, dynamic formulas, and wave
  analysis during driving are also used to arrive
  at a reliable pile capacity, Qu.
• Qallowable Qultimate /FS typically FS2
  for deep foundations.

10
Bearing Capacity Factors for Deep Foundations
(Meyerhof, 1976)