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Bearing Capacity of Shallow Foundations

Bearing Capacity of Shallow Foundations. Ch. 6. B.C. Failures. General shear Dense soils, Rock, NC clays Defined failure surf. Fast failure. Local shear Intermediate case +/- gradual failure. Punching Loose sands, weak clays (dr.) F. surf. not defined Gradual failure.

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Bearing Capacity of Shallow Foundations

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  1. Bearing Capacity of Shallow Foundations Ch. 6.

  2. B.C. Failures General shear Dense soils, Rock, NC clays Defined failure surf. Fast failure Local shear Intermediate case +/- gradual failure Punching Loose sands, weak clays (dr.) F. surf. not defined Gradual failure

  3. B.C. Failures Deep foundations Sand Circular foundations (Vesic, 1963 and 1973)

  4. We design for the general shear case (for shallow foundations)

  5. Bearing Capacity Theory LIMIT EQUILIBRIUM • Define the shape of a failure surface • Evaluate stresses vs. strengths along this surface

  6. Bearing Capacity Theory LIMIT EQUILIBRIUM BC Factor Ultimate bearing capacity = qult = ? (Bearing press. required to cause a BC failure) Moments about point A

  7. Terzaghi’s Bearing Capacity Theory Assumptions D < or = B Homogenous and isotropic s = c’ + s’tan(f’) level ground rigid foundation full adhesion between soil and base of footing general shear failure develops

  8. Terzaghi’s Bearing Capacity Theory

  9. Terzaghi’s Bearing Capacity Theory Terzaghi developed the theory for continuous foundations (simplest, 2D problem). From model tests, he expanded the theory to:

  10. Terzaghi’s Bearing Capacity Theory Nc= cohesion factor Nq = surcharge factor Nγ = self wt factor = fn (f’) See table 6.1 for values

  11. affects Shear strength Groundwater level effects groundwater by • Reduction in apparent cohesion - cap (sat. soil for lab tests) • Decrease in s’

  12. Groundwater level effects D

  13. Groundwater level effects Case I

  14. Groundwater level effects Case II

  15. Groundwater level effects Case III

  16. Groundwater level effects For total stress analysis: regardless of the case (gw effects are implicit in cT and fT)

  17. FS for BC uncertainty Allowable BC = qa FS = function of soil type structure type soil variability extent of site characterization

  18. BC of shallow foundations in practice (per Mayne ‘97) (Mayne, 1980) Undrained Nc* = 5.14 for strip footing = 6.14 for square or circular footing The value of su is taken as the ave. within a depth = to 1B to 1.5B beneath the foundation base

  19. BC of shallow foundations in practice (per Mayne ‘97) Drained Ng* = fn (foundation shape and f’) Consider gw cases (I, II, or III to determine g’)

  20. BC of shallow foundations in practice (per Mayne ‘97) Sands Perform drained analysis Clays Perform both

  21. Problem formulation – BC design Consider (drained vs. undrained) and methods for obtaining OCR and f’ ---- CPT? 1. Find B so that FS = 3 Get q Get q ult (by BC analysis) Set FS ratio and solve for B

  22. Problem formulation– BC design Important too: Foundation shape (cost and labor) Moment loads and eccentricity Weight of the foundations 2. Find B and D so that FS = 3 Get q Get q ult (by BC analysis) Set FS ratio and solve for B Determine this for various D values…

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