 # SHALLOW FOUNDATIONS Spread footings Mat (Raft) foundations Square

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SHALLOW FOUNDATIONS Spread footings Mat (Raft) foundations Square
Rectangular Circular Continuous Mat (Raft) foundations

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
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

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)

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)