Civil Engineering Reference
In-Depth Information
the ultimate load capacity of each pile in the group to the ultimate load capacity of a single
isolated pile. If the spacing between piles in the group are at a distance that is about 7 times
or greater than the pile diameter, then the group efficiency is equal to one (i.e., no reduc-
tion in pile capacity for group action). The group efficiency decreases as the piles become
closer together in the pile group. For example, a 9
9 pile group with a pile spacing equal
to 1.5 times the pile diameter has a group efficiency of only 0.3 to 0.4. Figure 11.16 can be
used to determine the ultimate load capacity of a pile group in cohesive soil.
Similar to pile groups in cohesionless soil, the settlement of the strata underlying the
pile group can be evaluated by using the 2:1 approximation to calculate the increase in
vertical stress
v
for those soil layers located below the pile tip. If the piles in the group
develop their load-carrying capacity principally by end-bearing in cohesive soil, then the
2:1 approximation starts at the tip of the piles (
L
length of the pile group,
B
width of
the pile group, and
z
depth below the tip of the piles). If the pile group develops its load-
carrying capacity principally through cohesive soil adhesion along the pile perimeter, then
the 2:1 approximation starts at a depth of 2/3
D
, where
D
depth of the pile group. See
Fig. 11.14 for examples.
FIGURE 11.16
Ultimate capacity of a pile group in cohesive soil. (
Developed by Whitaker 1957, reproduced
from NAVFAC DM-7.2, 1982.
)
