Pile Groups: Design Considerations.

This chapter provides several hand calculation methods for a quick estimate of the capacity and movement characteristics of a selected group of driven piles or drilled shafts for given soil conditions.  Recommended factors of safety for pile groups are also given in Table 3-2

a.  Driven piles.   Driven piles are normally placed in groups with spacings less than 6B where B is the width or diameter of an individual pile.  The pile group is often joined at the ground surface by a concrete slab such as a pile cap, Figure 5-1a.  If pile spacing within the optimum range, the load capacity of groups of driven piles in  cohesionless soils can often be greater than the sum of the capacitites of isolated piles, because driving can compact sands and can increase skin friction and end-bearing resistance.

b.   Drilled shafts.   Drilled shafts are often not placed in closely spaced  groups, Figure 5-1b, because these foundations can be constructed with large diameters and can extend to great depths.

Exceptions  include using drilled shafts as retaining walls or to improve  the soil by replacing existing soil with multiple drilled shaf ts.  Boreholes prepared for construction of drilled shafts reduce effective stresses in soil adjacent to the sides and bases of shafts already in place.  The load capacity of drilled shafts in cohesionless soils spaced less than 6B may therefore be less than the sum of the capacities of the individual shafts.  For end-bearing drilled shafts, spacing of less than 6B can be used without significant reduction in load capacity.

Figure 5-1.  Groups of deep foundations
Table 3-2 Factors of Safety for Bearing capacity.

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