Actes du colloque - Volume 6

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Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013, volume 6, 2016
Presses de l'Ecole Nationale des Ponts et Chaussées (in French).
It contains 8 chapters drafted by a 12-member review committee.
4.6 Advances owed to the CLOUTERRE I and II NPs
It can be stated, yet without any hard quantitative justification,
that these two NPs have definitively contributed to the
widespread popularity of nailed soil walls in France as
permanent structures, thus making it possible to generate
considerable savings compared to more conventional wall
construction. Let's cite for example the nailed soil walls around
some of the piles on the Millau Viaduct. Initially designed as
temporary facilities, these walls were, at the time of restoring the
site upon project completion, transformed into permanent
structures and included in the comprehensive monitoring process
aimed at the various viaduct components, although they were
assigned an observational method of approach. The savings
relative to newer reinforced concrete retaining walls were
substantial. Moreover, let's note the 1998 "reference structure"
ranking produced by the IVOR (French acronym for Validated
Innovations on Reference Structures) Committee for the nailed
soil retaining walls on the A12 motorway, which was heavily
instrumented within the scope of the CLOUTERRE II project.
In the international arena, the CLOUTERRE I National
Project, along with the English language translation of the
CLOUTERRE 1991 Recommendations, was undeniably
responsible for the widespread renown of French technique.
More specifically, it was the primary motivation behind the
American FHWA Agency's decision to participate as a partner in
the CLOUTERRE II project and then later in the FOREVER
National Project. Let's also point out that that the Talren
software application, designed and developed by Terrasol
Company, was and still is widely used across many countries for
the design of nailed soil structures (walls, embankments, slopes).
For this purpose, the "CLOUTERRE 1991 Recommendations"
were translated into Korean.
At the very beginning of the 1990's, both the FHWA and the
American TRB (Transportation Research Board) had organized a
"scanning tour" in Europe to learn about the development of this
nailing technique. Their delegations were very favorably
impressed by the extent of nailing activities in France. In the
same manner that the Reinforced Earth technique experienced
tremendous development in the United States, soil nailing was
quickly adopted by American authorities and reached such new
heights of popularity that the cumulative benefit derived thanks
to use of this technique would, several years ago, be estimated
by these U.S. agencies in the hundreds of millions of dollars. At
present, soil nailing is practiced basically throughout the entire
world due to its simplicity, ease of implementation and lack of
patent protections.
5 THE “ FOREVER” NATINAL PROJECT ON MICROPILES
5.1 Objective and organization
A micropile is a pile with a diameter less than 250 mm, in most
instances bored, and containing a central metal reinforcement
rod, which quite often is a tube embedded into a mortar or
cement grout. The load-bearing capacity of a micropile is
basically provided by the micropile/soil skin friction, which can
be mobilised should the grout be injected under high pressure.
Four types of micropiles are to be distinguished on the basis of
the grout injection pressure value, i.e.:
- Type I: Bored and cased, fitted or not with a reinforcement
rod, filled with a cement mortar inside an injection pipe. The
casing is to be recovered;
- Type II: Bored, fitted with a reinforcement rod and filled with
a mortar or cement grout using an injection pipe by gravity or
subjected to very low pressure;
- Type III: Most often bored, fitted with both a reinforcement
rod and a grout injection system using a sleeved pipe (“tube à
manchettes”) within a grout sheath. The one-time injection
covers the entire installation, with a pressure at the top of at
least 1 MPa;
- Type IV: Identical to Type III, except for the fact that the
injection is repeated at selected levels with a single or double
valve (“packer”) option.
For many years, micropiles have offered a broad field of
application when used in groups (i.e. sets of vertical micropiles)
or in a network (inclined micropiles). Their primary purpose is to
support the foundation underpinning or they may be used for: the
foundations of newer structures built with difficult ground
conditions; slope and embankment stabilization; and retaining
walls, tunnels and protections of underground facilities.
Micropile networks also feature an exceptional capacity to resist
seismic forces.
The objective of the NP labeled FOREVER (French
acronym for Vertically Reinforced Foundations) was to specify,
through a study and full-scale testing program, the behavior of
micropiles, whether isolated, in groups or in networks, and then
establish recommendations along with a set of design methods to
allow extending their field of application.
Experimental groups and networks were built and
instrumented at the CEBTP's St Rémy-lès-Chevreuse site.
The supervisory team for this NP consisted of a President, a
Scientific Director and a Technical Director. The project
encompassed 22 partners and was conducted between 1993 and
2001. Its budget amounted to €5,091,000, with €754,000
awarded as a DRAST subsidy and the remainder through partner
support (dues and in-kind contributions). The participation of
three foreign partners in Forever is acknowledged: Federal
Highway Administration (U.S.), University of Canterbury (New
Zealand), and Polytechnic University of New York (U.S.).
5.2 Micropile groups: Experimental results
Based on a wide array of tests conducted on a reduced-scale
model (calibration chamber, centrifuge) and a full-scale model,
as part of the Forever project, it could be confirmed that the
spacing S between micropiles of a given group in sand is one of
the most influential parameters on load-bearing capacity under a
vertical loading. The coefficient of efficiency Ce, i.e. the ratio of
the average load-bearing capacity of a group micropile to that of
the isolated micropile, varies between 0.59 and 2.2.
For the same tests, the number N of group micropiles also
proves to be an influential parameter: for N < 10, Ce lies between
0.59 and 1.35, whereas for N > 10, the C e value ranges from 1.4
to 2.2.
The order of micropile installation also exerts an influence.
For a group of 5 micropiles driven into sand of average density,
the placement of a 5th micropile in the middle of the other 4
serves to increase the group's load-bearing capacity by 40%.
On the other hand, the load-bearing capacity of a group of
micropiles subjected to a horizontal load turns out to be quite
similar to that of a group of piles.
5.3 Micropile groups: Numerical computation methods
5.3.1 The GOUPEG Program
In 1994, Maleki and Frank developed the GOUPEG Program for
micropile groups, so as to extend the GOUPIL-LCPC Program
from 1989 that relied on axial loading transfer functions (t-z
mobilization curves for axial skin friction), and for transverse
loadings (p-y reaction curves). Their study entailed adding group
effects to GOUPEG in the case of axial forces. Their method
was considered a "hybrid", whereby Mindlin elasticity solutions
were used to automatically calculate the displacements induced
on adjacent piles and thus determine the "y" type factors (i.e.
displacements z) that correct the t-z skin friction mobilization
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