1. SHALLOW FOUNDATIONS
BY, Babariya Ashish Gondaliya Ronak Gondaliya akshay Javiya hardik

2. SHALLOW FOUNDATIONS Spread footings Mat (Raft) foundations 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
End-of-construction (short term) analysis
Calculate q ultimate
q allowable = q ultimate / FS bearing capacity
Area allowable = P/ q allowable
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
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)

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 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 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)