2912
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
4 IMPLIED INTERACTION COEFFICIENTS
This interaction between piles is expressed through the concept
of the coefficient of interaction
ij
, which is equal to the ratio of
the settlement of pile
i
to the settlement of pile
j
when pile
j
is
loaded. Using this concept, the settlement of any pile
i
in a
group with a rigid cap is expressed through (Salgado 2008)
n
j
= ij
j=1
tj
Q
w i
K
(1)
where
w
i
is the settlement of pile
i
,
ij
is the influence factor
between
i
and
j
.
Q
j
is the load acting on pile
j
, and
K
tj
is the
stiffness of pile
j
(in the sense of how much load is required to
have unit pile head stiffness). Details on how to obtain the
interaction coefficients using these equations by linear
optimization can be found in Dai et al. (2012).
The influence coefficients versus pile group settlements are
shown in Figure 12. In general, with the increase of group
settlement, the interaction coefficient increases, with an
inflection point for small settlements (marking the transition
from little interaction for small settlements to a higher level of
interaction) and later a tendency of stabilization at large
settlements, which is consistent with more intense localization
of shear strain around the piles at large settlements, which leads
to a reduction in the interaction for incremental settlement.
The results for the 2-pile groups QZ2 are inconsistent with
the other results, with the interaction coefficient being
practically zero. This may be because of spatial variability of
the soil or other variability in the pile installation or pile cap.
For the four-pile groups, the pile spacing has a larger effect on
interaction than pile length, which is to be expected. The
interaction coefficient in group QZ4 with
s
p
= 2.5
B
and
L
= 20
m is on average larger than that of group QZ4L with
s
p
= 3.0B
and
L
= 24m. For the nine-pile group, the interaction
coefficients are distributed proportionally to pile center-to-
center spacing. The interaction coefficients for the piles in the
four-pile group are larger than comparable coefficients (at the
same spacing) for the nine-pile group. The presence of
additional piles around interacting piles likely interferes with
load or settlement transmission between the interacting piles.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 30 35 40 45 50 55 60
α12
Interactioncoefficient
Pilegroup settlement (mm)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 30 35 40 45 50 55 60
α12
Pilegr
lement (mm)
Interactioncoefficient
oup sett
(b)
(a)
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
0 5 10 15 20 25 30 35 40 45 50 55 60
α12
α14
Pilegroup settlement (mm)
Interactioncoefficient
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
0 5 10 15 20 25 30 35 40 45 50 55 60
α12
α14
Pilegroup settlement (mm)
Interaction coefficient
(c)
(d)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 30 35 40 45 50 55 60
α1
α2
α3
α4
α5
Pilegroup settlement (mm)
Interactioncoefficient
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 30 35 40 45 50 55 60
α1
α2
α3
α4
α5
Pilegroup settlement (mm)
Interactioncoefficient
(a)
(b)
Figure 12.Interaction coefficients versus settlement in each pile group:
(a) QZ2; (b) QZ2L; (c) QZ4; (d) QZ4L; (e) QZ9; (f) QZ9L.
5 CONCLUSION
A field pile load testing program was carried out on isolated
bored piles and bored pile groups (with two, four, and nine piles
with different pile lengths and pile spacing) installed in a soil
profile with mixed layers of clay and silt. Based on the analysis
of the field test results, the following conclusions can be
reached:
(1) Based on the traditional 0.1
B
relative settlement criterion,
the two single piles DZ1 (
L/B
= 50) and DZ1L (
L/B
= 60)
mobilized essentially only shaft resistance, with loads measured
at the strain gauge level closest to the pile base accounting for
only 2.2% of the total load for the 20-m-long pile and 4% of the
total load for the 24-m-long pile.
(2) The general response of an individual pile in the 2-pile
groups was observed to be very close to that of the
corresponding single pile, suggesting minimal interaction
between piles in the two-pile groups.
(3) The values of settlement ratio of both the four-pile and nine-
pile groups tended to increase with settlement. The single pile
settlement was observed to be generally smaller than the
corresponding pile group settlement at the same average load
per pile when the load was relatively large.
(4) Group effect was more pronounced for QZ4 than for QZ4L
and for QZ9 than for QZ9L, showing that the impact of the pile
spacing is greater than that of the pile length on group load
response.
(5) The load at the top of the corner piles was observed to be the
largest, followed by side piles and then center piles. However,
the load differences were not large, particularly for side versus
corner piles.
(6) The interaction coefficient was seen to be a function of
settlement and the size of the group. With the increase of group
settlement, the interaction coefficient was observed to increase.
6 ACKNOWLEDGEMENTS
The research was supported by the National Natural Science
Foundation of China (Grant No.50908048) and a project funded
by the Priority Academic Program Development of Jiangsu
Higher Education Institutions (PAPD).
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