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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
direction measured in the large-scale model experiment
k
was
compared with the design value for the upper layer of composite
ground with solidified improved columns
k’
. In Case 1 with
natural ground whose
N
value was 10 for the entire layer, the
measured value
k
roughly corresponded to the design value
k’
when the permissible horizontal displacement was 1.0% of the
pile diameter, which is the standard value set in existing design
methods (Japan Road Association 2002, Architectural
Association of Japan 2001). In Case 2 where the improvement
depth was 1/
β
and the unconfined compressive strength
q
u
was
used as the standard strength, the
k
and
k’
values were similar
for pile displacement at the ground surface when the
displacement was 0.5% of the pile diameter (
y
= 0.5 mm).
However, when displacement at the ground surface was 1% or
more of the pile diameter, the measured value
k
did not satisfy
the design value
k’
. While the measured value satisfied the
design value at the same displacement in Case 4 where the
improvement depth was 1/2
β
, pile strain increased in the bottom
layer of the solidified improved columns because no binding
effect could be expected from them in the deep section, and the
measured bending moment of piles tended to be underestimated.
In Case 3 where the unconfined compressive strength of
solidified improved columns
q
u
was extremely high, the
measured value did not satisfy the design value at all
displacement levels. In other words, although a certain degree
of reaction effect can be expected, the elastic subgrade reaction
design method for piles is not feasible if solidified improved
columns are very strong.
Accordingly, to enable application of the basic design
method for composite ground pile foundations, the
improvement depth should always be 1/
β
, and
q
u
= 200 kN/m
2
should be set as the standard value for the unconfined
compressive strength of solidified improved columns. At the
same time, the permissible horizontal displacement of piles used
for composite ground pile foundations should be reduced to
0.5% of the pile diameter instead of 1% (or 15 mm) for natural
ground.
Next, the measured horizontal subgrade reaction
P
H
in Cases
2, 3 and 4 was compared with the design value
P
HU
. It can be
seen from the table that the measured
P
H
in Cases 2 and 4
satisfied the design value
P
HU
when pile displacement at the
ground surface was up to around 2.5% of the pile diameter (
y
=
2.5 mm). However, the measured
P
H
in Case 3 was less than
half of the design value
P
HU
at all displacement levels,
indicating that the elastic subgrade reaction design method for
piles is not feasible when the strength of solidified improved
columns is extremely high as seen in the examination of the
modulus of subgrade reaction in the horizontal direction.
4 CONCLUSION
Based on the results of a large-scale model experiment, the
following findings were obtained in regard to a design
verification method for pile foundations used in combination
with solidified improved columns (i.e., composite ground pile
foundations):
(1) In the basic design method for composite ground pile
foundations, specifications for solidified improved columns
should be based on engineering grounds, the improvement
depth should be based on the characteristic pile length 1/
β
,
and
q
u
= 200 kN/m
2
, with which constitutive laws of soils
(Public Works Research Center 2004) can be followed,
should be applied as the standard value for the unconfined
compressive strength of solidified improved columns.
(2) While the limit state of damage to solidified improved
columns in composite ground pile foundations is assumed to
be reached within the range of pile deformation to around
2.5% of the pile diameter, it should be verified that the
design horizontal subgrade reaction
P
HU
calculated as a
product of the design pile displacement
y
and the design
value
k
’ is smaller than the passive earth pressure strength
of composite ground given by solidified improved columns
to provide inner stability and ensure column soundness.
The use of this index is expected to help prevent cracking in
improved columns and other types of damage caused by pile
behavior in normal conditions and during Level-1
earthquakes.
(3) To sustain the reaction effect and ensure the external
stability of solidified improved columns in composite
ground pile foundations, the permissible horizontal
displacement of piles in normal conditions and during
storms and Level-1 earthquakes should be reduced to 0.5%
of the pile diameter instead of 1% (or 15 mm) for natural
ground. The elastic subgrade reaction design method for
piles can be considered feasible when the above design
verification method is applied.
In this paper, a new design verification method for
composite ground pile foundations with consideration for the
limit state of solidified improved columns was presented based
on past results from research on pile foundations used in
combination with solidified improved columns. Using these
results and the outcomes of discussions by a technical
exploratory committee consisting of foundation engineering
experts from the government, universities and industries,
guidelines on design and construction methods for composite
ground pile foundations have been established (Civil
Engineering Research Institute for Cold Region 2010).
5 REFERENCES
Architectural Institute of Japan 2001:
Recommendation for the Design
of Building Foundations
, pp. 173 –326.
Civil Engineering Research Institute for Cold Region 2010:
Guidelines
on design and construction methods for composite ground pile
foundations in Hokkaido
, pp. 1 – 189.
Japan Road Association 2002:
Specifications for Highway Bridges V –
aseismic design
, pp. 4 – 118.
Japan Road Association 2002:
Specifications for Highway Bridges IV –
substructure
, pp. 348 – 433.
Japanese Geotechnical Society 2010:
Method of horizontal loading test
of piles and instruction manual
, pp. 22 – 28.
Public Works Research Center 2004:
Design and construction manual
for the deep mixing method in work on land
, pp. 76 – 127.
Tomisawa, K. and Nishikawa, J. 2005: A design method concerning
horizontal resistance of piles constructed in improved ground,
16
th
International Conference on Soil Mechanics and Geotechnical
Engineering
, pp. 2187 – 2192.
Tomisawa, K. and Nishikawa, J. 2005: Pile design method in composite
ground formed by deep mixing method,
Journal of the Japan
Society of Civil Engineers
No. 799/III-72, pp. 183 – 193.
Tomisawa, K., Nishimoto, S. and Miura, S. 2009: Evaluation of
mechanical behavior and examination of seismic performance of
pile foundation in composite ground,
Journal of Structural
Engineering
Vol. 55A, pp. 1182 – 1195.
Tomisawa, K. and Miura, S. 2007: Mechanical behavior of pile
foundation constructed in composite ground and its evaluation,
Soils and Foundations
, Vol. 47, No. 5, pp. 961 – 972, 2007.
Tomisawa, K. and Miura, S. 2010: Method of evaluating the seismic
performance of piles in composite ground,
Society of Materials
Science, Japan, Material
Vol. 59, No. 1, pp. 26 – 31.
Tomisawa, K., Miura, S. and Watanabe, T. 2008: Influence of the
improved strength and the range of improvement of composite
ground on the seismic behavior of piles,
Journal of the Japan
Society of Civil Engineers
, C, Vol. 164, No. 1, pp. 127 – 143.
