In engineering, a foundation is the element of a structure which connects it to the ground, and transfers loads from the structure to the ground. Foundations are generally considered either shallow or deep.Foundation engineering is the application of soil mechanics and rock mechanics (Geotechnical engineering) in the design of foundation elements of structures.

1. P R E S E N T E D B Y
A . B H A R AT K U M A R
B . T E C H , M . T E C H
SHALLOW
FOUNDATIONS

2. SHALLOW FOUNDATIONS
 Spread footings
 Square
 Rectangular
 Circular
 Continuous
 Mat (Raft) foundations

3. SPREAD FOOTINGS
 Made from reinforced concrete
 Square (B x B)-Usually one column
 Rectangular (B x L)-When large M is needed
 Circular (D/B<3, 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.

4. 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

5. 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 < 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 > 2ft or
0.60 m in diameter) and filled with concrete and steel
reinforcement

6. SHALLOW FOUNDATIONS
Bearing Capacity
 Gross Bearing pressure
q = (P+Wf)/A – u
where Wf =gc*D*A, u = pore water pressure
 Net Bearing pressure = Gross Bearing pressure –
Effective stress
 q = P/A + gc*D– u SQUARE FOOTINGS
 q = P/(B*b) + gc*D– u CONTINUOUS FOOTINGS

7. SHALLOW FOUNDATIONS
Bearing Capacity (Cont’d)
 FS bearing capacity = q ultimate / q allowable = 2 to 3
 q allowable= Gross bearing pressure
 q ultimate = cNc +s’
D Nq + 0.5gBNg
strip footing
q ultimate = 1.3cNc + s’
D Nq + 0.4gBNg
square footing
q ultimate = 1.3cNc + s’
D 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, s’
D= vertical effective
stress at foundation base level, D (surcharge), g=unit weight of soil
below foundation base level, B=width (diameter) of footing
 Effect of Groundwater table (Page 624)
 Case1- DW
< D (high water table; use buoyant unit weight)
 Case2-D<Dw<D+B (intermediate water table; prorate unit
weight)
 Case3-D+B <Dw (Deep water table; use moist unit weight)

8. SHALLOW FOUNDATIONS
Design-Cohesive soils
1. End-of-construction (short term) analysis
2. Calculate q ultimate
3. q allowable = q ultimate / FS bearing capacity
4. Area allowable = P/ q allowable
5. Calculate setllement-
d <d allowable- DESIGN OK
d >d allowable- Consider soil
improvement, deep foundation.
Increasing area will not help, cause
more settlement

9. SHALLOW FOUNDATIONS
Design-Cohesionless soils
1. Drained (long term) analysis
2. Calculate q ultimate
Assume B to calculate q ultimate
3. q allowable = q ultimate / FS bearing capacity
4. Area allowable = P/ q allowable will give you B.
Iterate until B assumed = B computed
5. Check if q allowable is OK for settlement case (usually
at most 1 inch)

10. Deep Foundations Design
 Static Analysis:
Qultimate= QEB+QSR (end bearing + shaft resistance)
QEB = qult Ap where Ap is the area of pile tip
qult = c Nc* + s’
D
N
q
*
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 s’
v tand + ca where K=lateral earth coefficient, s’
v = vertical
effective stress at given depth, d=pile-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 FS=2 for deep foundations.

11. Bearing Capacity Factors for Deep Foundations (Meyerhof, 1976)