2 SPREAD FOOTINGS Made from reinforced concrete Square (B x B)-Usually one columnRectangular (B x L)-When large M is neededCircular (D/B<3, Rounded)-Flagpoles, transmission linesContinuous (Strip)-Support of bearing wallsCombined (Cantilever)-Provides necessary M to prevent failure. Desirable when load is eccentric and construction close to property line.
3 MAT (RAFT) FOUNDATIONS Necessary when the soil is weaker and more compressibleSince large area is needed from a spread footing, mat foundation is more economic.AdvantagesSpread the load in a larger area-Increase bearing pressureProvides more structural rigidity-Reduce settlementHeavier-More resistant to upliftDistributes loads more evenly
4 DEEP FOUNDATIONS When shallow foundations cannot carry the loads Due to poor soils conditionsWhen upper soils are subject to scourPiles-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 (Cont’d) FS bearing capacity = q ultimate / q allowable = 2 to 3q allowable= Gross bearing pressureq ultimate = cNc +s’D Nq + 0.5gBNg strip footingq ultimate = 1.3cNc + s’D Nq + 0.4gBNg square footingq ultimate = 1.3cNc + s’D Nq + 0.3gBNg circular footingfSee 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 footingEffect 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)
7 SHALLOW FOUNDATIONS Design-Cohesive soils End-of-construction (short term) analysisCalculate q ultimateq allowable = q ultimate / FS bearing capacityArea allowable = P/ q allowableCalculate setllement-d <d allowable- DESIGN OKd >d allowable- Consider soil improvement, deep foundation.Increasing area will not help, cause more settlement
8 SHALLOW FOUNDATIONS Design-Cohesionless soils Drained (long term) analysisCalculate q ultimateAssume B to calculate q ultimateq allowable = q ultimate / FS bearing capacityArea allowable = P/ q allowable will give you B. Iterate until B assumed = B computedCheck if q allowable is OK for settlement case (usually at most 1 inch)
9 Deep Foundations Design Static Analysis:Qultimate= QEB+QSR (end bearing + shaft resistance)QEB = qult Ap where Ap is the area of pile tipqult = 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 foundationf= 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 surfacePile 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.
10 Bearing Capacity Factors for Deep Foundations (Meyerhof, 1976)