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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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slide2

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

slide3

B.C. Failures

Deep foundations

Sand

Circular foundations

(Vesic, 1963 and 1973)

slide5

Bearing Capacity Theory

LIMIT EQUILIBRIUM

  • Define the shape of a failure surface
  • Evaluate stresses vs. strengths along this surface
slide6

Bearing Capacity Theory

LIMIT EQUILIBRIUM

BC Factor

Ultimate bearing capacity = qult = ?

(Bearing press. required to cause a BC failure)

Moments about point A

slide7

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

slide9

Terzaghi’s Bearing Capacity Theory

Terzaghi developed the theory for continuous foundations (simplest, 2D problem).

From model tests, he expanded the theory to:

terzaghi s bearing capacity theory
Terzaghi’s Bearing Capacity Theory

Nc= cohesion factor

Nq = surcharge factor

Nγ = self wt factor

= fn (f’) See table 6.1 for values

slide11

affects

Shear strength

Groundwater level effects

groundwater

by

  • Reduction in apparent cohesion - cap (sat. soil for lab tests)
  • Decrease in s’
slide16

Groundwater level effects

For total stress analysis:

regardless of the case

(gw effects are implicit in cT and fT)

slide17

FS for BC

uncertainty

Allowable BC = qa

FS = function of

soil type

structure type

soil variability

extent of site characterization

slide18

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

slide19

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’)

slide20

BC of shallow foundations in practice (per Mayne ‘97)

Sands

Perform drained analysis

Clays

Perform both

slide21

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

slide22

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…