Saturday, November 17, 2012

Capacity - Selection of Deep Foundations.

Deep foundations transmit structural loads to deep strata that are capable of sustaining the applied loads.
Accurate predictions of load capacity and settlement are not always possible.  Adequate safety factors are therefore used to avoid excessive movement that would be detrimental to the structure that is supported and to avoid excessive stress in the foundation.   Driven piles or drilled shafts are often used to resist vertical inclined, lateral, or uplift forces and overturning moments  which cannot otherwise be resisted by shallow footings.   These foundations derive their support from skin friction along the embedded length and by end bearing at the tip (base).  Both factors contribute to the total ultimate pile capacity, but one or the other is usually dominant depending on the size, load, and soil characteristics.  The capacity of deep foundation is influenced by several factors:

(1)  Design limits.  The limiting design criterion is normally influenced by settlement in soft and moderately stiff soil, and bearing capacity in hard soil or dense sand, and by pile or shaft structural capacity in rock.

(2)  Skin resistance mobilization.  Full skin resistance is typically mobilized within 0.5 inch of displacement, while end bearing may not be fully mobilize d until displacements ex ceed 10 to 20 percent of the base diameter or underream for drilled  shafts, unless the tip is supported by stiff clay, dense sand, or rock.  Figure 1-5 illustrates an example of the vertical axial load displacement behavior of  a single pile or drilled shaft .  The load-displacement behavior and displacements that correspond to ultimate  load are site specific and depend on the  results of analyses.

(3)  La teral loads.  Lateral load capacity of a pile or drilled shaft is directly related to the diameter, thus increasing the diameter increases the load-carryin g mcapacity.   For a drilled shaft that sustains no axial load, the cost of constructio n may be optimized by the  selection of rigid shafts  without underreams and with length/diameter  ratios less than 10. The selected shaft dimensions should  minimize the volume of concrete required and maximiz e constuction efficiency. T he lateral load capacity of driven piles may be increased  by increasing the number of piles and  battering piles in a pile group.  Batter piles are efficient in resisting lateral loads but significantly reduce ductility of the pile group in  the lateral direction, resulting in a brittle failure.  Vertical piles , though less efficient in resisting lateral loads, are also less stiff and do not fail suddenly.  These con flicting characteristics need to be balanced in  design, and they are considered critical where seismic or dynami c lateral loads are involved.

Figure 1-5.  Axial-load deflection relationship

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