SHALLOW FOUNDATIONS Spread footings Square Rectangular Circular Continuous

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 > 2 ft 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 = c. Nc +s’D Nq + 0. 5 g. BNg strip footing q ultimate = 1. 3 c. Nc + s’D Nq + 0. 4 g. BNg square footing q ultimate = 1. 3 c. Nc + s’D Nq + 0. 3 g. BNg 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) – Case 1 - DW < D (high water table; use buoyant unit weight) – Case 2 -D<Dw<D+B (intermediate water table; prorate unit weight) – Case 3 -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 = Sp. Lf 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)