Bearing Capacity - Shallow Foundations Foundation...

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Shallow Foundations
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Foundation Challenges: Type of Loading Static loading w at e r Settlement (consolidation) Bearing Capacity (extra material added into a silo)
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Foundation Challenges: Type of Loading Dynamics loading: e.g. wind and waves
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Definitions and Key Terms Foundation : Structure transmits loads to the underlying ground (soil). (e.g. Rhinoceros's foot…) Footing: Slab element that transmit load from superstructure to ground (e.g. Skier's skis…). Embedment depth, D f : The depth below ground surface where the base of the footing rests. Bearing pressure q: The normal stress imposed by the footing on the supporting ground. Ultimate bearing capacity q ult : The maximum bearing pressure that the soil can sustain (i.e it fails). Ultimate net bearing capacity (q unet ): The maximum bearing pressure that the soil can sustain above its current overburden pressure D f Footing Ground Surface
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Definitions and Key Terms (Cont.) Allowable bearing capacity: (q all ): The working pressure that would ensure an acceptable margin of safety against bearing capacity failure Factor of safety: The ratio between (q unet ) and (q all ). (F.S. = q unet /q all ) Ultimate limit state: A state that defines a limiting shear stress that should not be exceeded by any conceivable or anticipated loading during the life span of a foundation or any geotechnical system Serviceability limit state : A state that defines a limiting deformation or settlement of a foundation, which, if exceeded will impair the function of the supported structure.
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Types of Foundations Shallow Foundations Deep Foundations D f /B <= 2.5 D f /B > 2.5 B is the width of the footing, and D f is the foundation depth
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Examples of Shallow Foundations Examples of Shallow Foundations Isolated Footing Combined Footings Raft Foundation
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Examples of Shallow Foundations (Cont.) Examples of Shallow Foundations (Cont.) V H D d Skirted Foundations s u z kz s uo
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Design Requirements Load Load Settlement 1. The foundation must not collapse or become unstable under any conceivable load 2. Deformation (settlement) of the structure must be within tolerable limits [N.B. Special case: dense sand] Collapse Load
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Stages in load-settlement of shallow foundations Load Load Settlement Relatively elastic vertical compression The load-settlement curve is almost straight. Local yielding starts to affect Upward and outward movement of the soil with a possible surface heave. General shear failure Large settlements are produced as plastic yielding is fully developed within the soil. In dense sands: softening can occur after collapse . [N.B. Special case: dense sand] Collapse Load
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Collapse and Failure Loads Dense sand (say Dr > 0.7) 1 Q ult Load S e t l m n Q ult Collapse Failure Q ult 2 Medium sand (say 0.7>Dr>0.3) Q ult Failure Loose sand (say Dr<0.3) 3 Q ult Load Q ult Failure
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Characteristics of Each Failure Mode General shear (Dense sand): well defined failure mechanism continuous slip surface from footing to surface sudden catastrophic failure Local shear (Loose sand):
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This note was uploaded on 04/19/2010 for the course CIVL 306455301 taught by Professor Mudiliu during the Three '09 term at University of Sydney.

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Bearing Capacity - Shallow Foundations Foundation...

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