SHALLOW FOUNDATIONS
• Spread footings
–
–
–
–
Square
Rectangular
Circular
Continuous
• Mat (Raft) foundations
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.
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
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
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
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)
SHALLOW FOUNDATIONS
Design-Cohesive soils
1.
2.
3.
4.
5.
End-of-construction (short term) analysis
Calculate q ultimate
q allowable = q ultimate / FS bearing capacity
Area allowable = P/ q allowable
Calculate setllementd <d allowable- DESIGN OK
d >d allowable- Consider soil
improvement, deep foundation.
Increasing area will not help, cause more
settlement
SHALLOW FOUNDATIONS
Design-Cohesionless soils
1.
2.
Drained (long term) analysis
Calculate q ultimate
Assume B to calculate q ultimate
3.
4.
5.
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)
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.
Bearing Capacity Factors for Deep Foundations (Meyerhof, 1976)