SHALLOW FOUNDATIONS by pengxiang

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									      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.   End-of-construction (short term) analysis
2.   Calculate q ultimate
3.   q allowable = q ultimate / FS bearing capacity
4.   Area allowable = P/ q allowable
5.   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
1.   Drained (long term) analysis
2.   Calculate q ultimate
        Assume B to calculate q ultimate
3.   q allowable = q ultimate / FS bearing capacity
4.   Area allowable = P/ q allowable will give you B. Iterate
     until B assumed = B computed
5.   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)

								
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