more piles to increase capacity and loading resistance. The angle of

inclination should rarely exceed 20 degrees from the vertical for

normal construction and should never exceed 26 degrees.

Battered piles should be avoided where significant negative skin

This chapter provides several hand calculation methods for a quick

friction and downdrag forces may occur. Batter piles should be

estimate of the capacity and movement characteristics of a selected

avoided where the structure's foundation must respond with

group of driven piles or drilled shafts for given soil conditions. A

ductility to unusually large loads or where large seismic loads can be

computer assisted method such as described in Chapter 5,

transferred to the structure through the foundation.

paragraph 4, is recommended for a detailed solution of the

performance of driven pile groups. Recommended factors of safety

for pile groups are also given in Table 3-2. Calculation of the

the loading capacity of the pile group. Fixing the pile rather than

distribution of loads in a pile group is considered in paragraph 2b,

pinning into the pile cap usually increases the lateral stiffness of the

Chapter 2.

group, and the moment. A group of fixed piles can therefore support

about twice the lateral load at identical deflections as the pinned

group. A fixed connection between the pile and cap is also able to

with spacings less than 6*B *where *B *is the width or diameter of an

transfer significant bending moment through the connection. The

individual pile. The pile group is often joined at the ground surface

minimum vertical embedment distance of the top of the pile into the

cap required for achieving a fixed connection is 2*B *where *B *is the

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

pile diameter or width.

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.

influence the distribution of structural loads to the individual piles.

The thickness of the pile cap must be at least four times the width of

an individual pile to cause a significant influence on the stiffness of

spaced groups, Figure 5-1b, because these foundations can be

the foundation (Fleming et al. 1985). A ridgid cap can be assumed

constructed with large diameters and can extend to great depths.

if the stiffness of the cap is 10 or more times greater than the stiffness

Exceptions include using drilled shafts as retaining walls or to

of the individual piles, as generally true for massive concrete caps.

improve the soil by replacing existing soil with multiple drilled

A rigid cap can usually be assumed for gravity type hydraulic

shafts. Boreholes prepared for construction of drilled shafts reduce

structures.

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 6*B *may therefore be less than the sum of the

loads affect the ability of the pile group to resist the applied forces.

capacities of the individual shafts. For end-bearing drilled shafts,

Cyclic, vibratory, or repeated static loads cause greater

spacing of less than 6*B *can be used without significant reduction in

displacements than a sustained static load of the same magitude.

load capacity.

Displacements can double in some cases.

greater than that of an isolated pile driven in cohesionless soil

Piles are normally constructed in groups of vertical, batter, or a

because the density of the soil within and around a pile group can be

combination of vertical and batter piles. The distribution of loads

increased by driving. The pile group as a whole may not reflect this

applied to a pile group are transferred nonlinearly and

increased stiffness because the soil around and outside the group

indeterminately to the soil. Interaction effects between adjacent

may not be favorably affected by driving and displacements larger

piles in a group lead to complex solutions. Factors considered

than anticipated may occur.

below affect the resistance of the pile group to movement and load

transfer through the pile group to the soil.

may change the stress distribution in the soil and influence the

group load capacity. The length of the cutoff should be

modulus of subgrade reaction *E*1s relate lateral, axial, and rotational

determined from a flow net or other seepage analysis. The

resistance of the pile-soil medium to displacements. Water table

net pressure acting on the cutoff is the sum of the

depth and seepage pressures affect the modulus of cohesionless soil.

unbalanced earth and water pressures caused by the

The modulus of submerged sands should be reduced by the ratio of

the submerged unit weight divided by the soil unit weight.

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