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**SHALLOW FOUNDATIONS Spread footings Mat (Raft) foundations Square**

Rectangular Circular Continuous Mat (Raft) foundations

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**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.

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**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

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**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

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**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

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**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)

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**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

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**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)

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**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.

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

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