Lecture 12 - BC1 BEARING CAPACITY: (BC) Recall the...

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BC1 BEARING CAPACITY: (BC) Recall the requirements for a satisfactory foundation: 1. Future influences (frost, shrink-swell clay, sinkholes, etc.) 2. Bearing capacity failure (catastrophic, not very common in modern times) 3. Excessive settlements (long term, still a too-frequent problem) Bearing Capacity Design Criteria: o Bearing Stress (q s ) - the bearing stress actually applied to the soil by a foundation, force per unit area (Q s /A). o Allowable Bearing Stress (q a ) - the bearing stress used as a design limit after consideration of stability, failure criteria, soil layering and variability, influence of other structures/footings, and risk tolerance - typically divide the ultimate bearing capacity by a factor of safety (F.S. 3). o Local Shear Bearing Capacity (q ls ) - the bearing stress at which local shear failure occurs, typically where the bearing stress vs. movement plot becomes significantly nonlinear. o Ultimate Bearing Capacity (q u ) - the bearing stress at which there is catastrophic movement, usually a general shear failure Factors Affecting Mode of BC Failure: o Depth of embedment, D f o Stiffness or relative density, % 100 x e e e e D min max max r - - = o Geometry of foundation (B/L), shape o Inclination or eccentricity of applied load BC Failure Modes: o General shear failure (q u ) - abrupt, sudden. Failure surface extends to ground surface (dense sand). o Local shear failure (q ls ) - occurs slowly, with substantial settlement. Failure surface does not extend to ground surface. Progresses to general shear failure (medium compacted sand, clayey soil) Dense or Stiff Soil Loose or Soft Soil Δ q s q a Settlement, Δ Bearing Stress, q s q u q ls Failure Surface Q q s q u Δ Failure Surface Q Δ q s q u q ls
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BC2 o Punching shear failure (q ps ) - continuous punching failure and settlement with gradual increase in q s due to compaction (loose sands), more likely to occur at depth. o
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Lecture 12 - BC1 BEARING CAPACITY: (BC) Recall the...

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