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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013, volume 6, 2016
- the effect due to mechanical interaction between micropiles,
which is to be added to the displacements.
5.6.4 Micropile networks
Despite the fact that studies and tests carried out as part of
FOREVER
are not sufficiently exhaustive, the following
conclusions can still be forwarded:
- A network, regardless of its number of micropiles, exhibits
a better behavior than the equivalent group.
- As regards the behavior when exposed to a vertical load, the
experimental results are, at the very least, contradictory.
- In order to obtain a positive network effect, the
recommendations issued for groups must naturally be followed,
especially as regards the number and length of micropiles as well
as soil confinement.
- In granular soils that are loose to moderately dense, which
have the most to gain from micropile reinforcement, it is
possible to obtain a positive network effect in comparison with
the equivalent group provided both an adequate soil confinement
has been achieved and the micropiles have been concentrated to
the greatest extent possible directly below the applied load. This
latter condition implies that the micropiles do not "exit" the
foundation surface, but instead line up towards the inside (
<
0°), so as to ensure maximum "nailing" of the soil. This notion is
quite similar to the concept proposed by Lizzi: a reinforced soil
foundation behaving like a monolith.
- For dense granular soils difficult to compact, it is impossible
to obtain a positive network effect.
- At the present time, it is not possible to design a micropile
network, with the exception of a simple layout (easel). However,
methods are currently being developed that make use of either
the transfer functions or homogenization techniques.
- From a pragmatic standpoint, the predominant idea at the
conclusion of
FOREVER
was that it became more advantageous
to solely seek a network effect in the case of micropiles either
bored or gravity injected. For those injected under high pressure,
of type IV (RSI i.e. repetitive and selective injection), it is
reasonable to assume they would function better when isolated
rather than in a group or a simple network.
5.6.5 Seismic behavior of micropiles
An analysis of damages caused by earthquakes, such as those
that occurred in Loma Prieta and Kobe, showed that the
foundations comprising steel piles of small diameter better
resisted the seismic loadings than large-diameter concrete piles.
This observation justifies the use of micropiles for foundations
in seismic zones since they display flexibility, ductility and
tensile resistance all at the same time. Micropiles prove to be
especially attractive for repairing structures that had undergone
damage during earthquakes. This technique in essence offers
engineers a multitude of possibilities in the area of design
(number, inclination and arrangement of micropiles) as well as
ease of placement, which makes its use highly advantageous,
particularly in more remote zones.
The use of micropiles as a reinforcement technique (for both
groups and networks) provides many additional advantages
inasmuch as such a technique serves to create a soil/structure
composite featuring special mechanical properties as regards
stiffness, strength and, above all, stability during earthquakes,
notably at sites with a soil liquefaction risk.
Research conducted as part of the
FOREVER
project on this
topic has included centrifuge tests, three-dimensional finite
element modeling and simple models built with springs and
dashpots (see Shahrour and Juran, 2004). It has also led to a
better understanding of micropile behavior when subjected to
seismic loading. The main set of results obtained are as follows:
a) The forces transmitted to micropiles stem from a kinematic
interaction along with an inertial interaction. The kinematic
interaction is more moderate for vertical micropiles used as
foundation elements. The considerable flexibility of micropiles
enables calculating the forces due to the kinematic effect, in
assuming that micropiles follow the free-field soil displacement.
b) Inertial forces, resulting from acceleration of the structure,
transmit a transverse force and an overturning moment to the
micropile group. These transverse forces and overturning
moments induce compressive and tensile forces inside the
micropiles. It thus becomes necessary to design micropiles, so
that they resist such forces, and then adopt the measures required
for the fastening between micropile and cap to resist tensile
forces. It should be noted that this phenomenon favors the use of
micropiles in seismic zones.
c) Micropile systems display a positive group effect that may
be ascribed to a structural effect derived from fastening
micropiles into the cap. This effect stems from both a reduction
in the bending moment within the micropiles and displacements
at the top as spacing between micropiles decreases. In the
absence of quantification, such an effect may be neglected given
that it is quite conservative.
d) The absence of damage observed in several earthquakes
demonstrates the favorable behavior produced by inclined and
flexible piles. Studies conducted during the
FOREVER
project
show that micropile inclination leads not only to an increase in
foundation stiffness relative to the seismic loading, but also to
stronger axial forces inside the micropiles.
e) The use of micropiles in liquefiable soils proves to be of
great interest. Results obtained in the centrifuge actually indicate
that micropiles confine the soil/micropile system, which serves
to: reduce soil movement, slow the rise in pore pressure, and
thereby lower the risk of liquefaction.
f) A comparison of centrifuge test results with those of both
the finite element model and the simplified calculation methods
based on Winkler's model reveals that these latest results may be
used for the seismic design of foundation micropiles.
g) Micropile design in seismic zones must take into account
all other project parameters, especially the frequencies (loading,
structures, soil layers, etc.).
6
THE “
VIBROFONÇAGE”
NATIONAL PROJECT
6.1
Introduction
The
Vibrofonçage
(vibratory pile driving) National Project was
supervised by IREX subsequent to an exploratory study (March
1998), followed by a feasibility study (January 1999). The
conclusions of this NP were presented in September 2006. The
day spent reporting results was coordinated with the 2006
TRANSVIB international symposium.
The total pre-tax project budget amounted to €1,152,000,
including a €246,000 (pre-tax) subsidy from the Research
Directorate of the Science and Technology Ministry, with the
balance made up of partners' in-kind contributions and dues. The
vast majority of this budget was devoted to experiments and
in
situ
measurements.
On the heels of the TUBA National Project, devoted to pile
driving by means of hammering, this
Vibrofonçage
NP focused
on the more recent technique of driving linear metal elements
(tubes, sheet piles) into the soil by means of vibration (Fig. 10).
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