01 Jul 97
b. Batter. Battered piles are used in groups of at least two or
more piles to increase capacity and loading resistance. The angle of
inclination should rarely exceed 20 degrees from the vertical for
1. Design Considerations
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
c. Fixity. The fixity of the pile head into the pile cap influences
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
group, and the moment. A group of fixed piles can therefore support
about twice the lateral load at identical deflections as the pinned
a. Driven piles. Driven piles are normally placed in groups
group. A fixed connection between the pile and cap is also able to
with spacings less than 6B 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 2B 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
d. Stiffness of pile cap. The stiffness of the pile cap will
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
b. Drilled shafts. Drilled shafts are often not placed in closely
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
effective stresses in soil adjacent to the sides and bases of shafts
e. Nature of loading. Static, cyclic, dynamic, and transient
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
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 6B can be used without significant reduction in
displacements than a sustained static load of the same magitude.
Displacements can double in some cases.
f. Driving. The apparent stiffness of a pile in a group may be
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
g. Sheet pile cutoffs. Sheet pile cutoffs enclosing a pile group
transfer through the pile group to the soil.
may change the stress distribution in the soil and influence the
a. Soil modulus. The elastic soil modulus Es and the lateral
group load capacity. The length of the cutoff should be
modulus of subgrade reaction E1s 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.