Civil Engineering Reference
In-Depth Information
load tests of piles, have been developed for soft clays, stiff clays, and sands and have been
incorporated into computer programs that are used to obtain the pile defections as well as
the shear and bending moments in the piles (Matlock 1970; Reese et al. 1974, 1975). For
closely spaced piles in a group, it has been proposed that the
p
-
y
curves be reduced by using
“
p
-multipliers” to reduce all the
p
-values on a given
p-y
curve (Brown et al. 1987). For
more information on
p-y
curves, see Coduto (1994).
Pile Groups.
When considering the lateral resistance of a closely spaced group of piles,
the total lateral resistance cannot be calculated as the allowable lateral load value for a
single pile times the number of piles. The reasons are because of shadowing (overlapping
passive soil wedges) and soil gap formation. The formation of a gap results in a reduction
in passive resistance for the trailing rows.
Shadowing is the process where the passive soil wedge from the leading row overlaps
into the trailing row of piles, resulting in a reduction of passive resistance. The effects of
shadowing have led to the development of row reduction factors. The leading pile row may
have some reduction in lateral capacity, while the pile rows that trail behind the leading row
would have more reduction in lateral load capacity.
There have been lateral loadings of full-scale pile groups embedded in soil. For exam-
ple, results from a study by Brown et al. (“Lateral Load Behavior of Pile Group in Sand,”
1988) are as follows:
●
For the 3 by 3 pile group tested, results clearly showed the effect of shadowing, in which
the soil resistance of a pile in a trailing row was greatly reduced because of the pres-
ence of the pile ahead of it. The soil resistance of the piles in the leading row was only
slightly less than that of a laterally loaded isolated single pile. The key in the design of
pile groups subjected to lateral loads is to account for the loss of soil resistance for the
piles in the trailing rows.
●
When the pile group was subjected to two-way cyclic lateral loading (i.e., back and forth
lateral loading) at the pile head, the shadowing effect was not appreciably diminished.
●
Although the two-way cyclic lateral loading did not influence the shadowing effect, there
was densification of the soil adjacent the piles related to sand falling into the voids around
the pile as it was pushed back and forth. For 100 cycles of loading, about 9 in. (23 cm)
of ground surface settlement directly around the pile was observed due to this effect of
sand falling into the voids created by pushing the pile back and forth. This densification
effect of the sand caused by the back and forth lateral loads appeared to improve the soil
resistance at subsequently larger lateral loads.
●
Cyclic lateral loading in only one direction would not produce as much densification as
the two-way cyclic lateral loading and would result in a greater loss of soil resistance
with increasing cycles of load. In this respect, the results of the experiment show the
beneficial effects of back and forth cyclic loading but the worst case would be lateral
loading in only one direction. The results of the study clearly show the importance of the
nature of the loading.
There have been other lateral loading studies of full-scale pile groups and the results are
summarized in Table 11.8. 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 row reduction factor is equal to
one (i.e., no reduction in lateral pile capacity for group action).
Example Problem:
Use the data from the previous example problem. Assume a 3 by 3
pile group in clay with a spacing of 3 pile diameters. Using the data in Table 11.8, deter-
mine the allowable lateral load of the pile group. Also calculate the allowable lateral load
of the pile group for a spacing of 7 pile diameters.
