Geosynthetics June/July 2021 - 24

Design of reinforcement geosynthetics in landfill piggyback expansion
on-site (average pressuremeter modulus
[EPMT=1,334 psi {9.2 MPa}]) and
the ISPM from data collected in the literature.
The estimation of the secondary
settlements was achieved by applying
the ISPM method with data resulting
from a retroanalysis carried out using the
topographic survey on the postoperation
settlements of the site. In the worst-case
scenario, the global settlements were estimated
at 4 feet (1.2 m): 3 feet (0.9 m) of
primary settlements and 1 foot (0.3 m) of
secondary settlements.
FIGURE 2 Typical cross sections
The guide " Recommandations
pour la conception des extensions
d'ISDND en appui sur des casiers
anciens " ( " Recommendations for the
design of nonhazardous waste landfill
extensions based on old cells " ) from
the Bureau de Recherches Géologiques
et Minières (BRGM) (Geological and
Mining Research Bureau) (2020)
FIGURE 3 Illustration of the behavior of a
homogeneous waste mass under loading
recommends the application of the
" Modèle Incrémental de Prédiction des
Tassements " ( " Incremental Settlement
Prediction Model " ) (ISPM) (Olivier
2003 and Agency for Environment and
Energy Management [ADEME] 2005).
This model, developed from field experience
in France and abroad, allows
the prediction of the evolution of the
primary (short-term) and secondary
(long-term) settlements of a waste
mass in the case of a vertical expansion
over it. It is recommended to apply this
model by retroanalysis (or calibration)
to improve accuracy, which has been
done on that project.
The estimation of the primary settlements
was carried out from the field
data by applying the pressiometric
method from modulus values measured
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Geosynthetics | June July 2021
Structural differential settlements
Structural differential settlements develop
in areas where there are significant variations
in the geometric parameters and the
nature of the support, such as:
* Variations in the thickness and nature
(age, composition) of the compressible
support or even the presence of
geotechnical structures (dikes, etc.)
* Variations of the load on the compressible
support (thickness and unit
weight of the material)
The consideration of the structural
differential settlements led to specific
constructive measures:
* Modification of the landfill gas collection
network, including the vertical
wells, to avoid the development of
localized hardpoints
* Specific layout with the installation of
two monodirectional reinforcement
geosynthetic layers orthogonally
crossed to have a homogeneous
reinforced soil structure at any point
under the new cell
Localized differential settlements
Localized differential settlements are difficult
to anticipate. The BRGM guide
suggests taking them into account by
considering a cavity with a diameter of
3-6 feet (1-2 m) within the waste mass,
as shown in Figure 3. It illustrates the
Geosynthetics June/July 2021

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