Aerial view of the final closure of the
municipal solid waste landfill
Once the Cerro Maggiore waste disposal site had come to the
end of its useful life, it was necessary to design a landfill
cover system (known as Capping) that proved problematic due
to the geometry of the existing site.
A capping system is designed to guarantee the impermeabilty
of the landfill sides slopes, allowing for proper gas venting,
with an adequate drainage system on top to avoid ground water
infiltration, with a vegetated cover system to minimize environmental
impact and to protect the geosynthetic liner.
basic landfill profile was particularly long (60 m) and steep
(between 20° and 37°), separated in three distinct
sections 20 m long separated by horizontal berms having widths
varying from 2.50 m to 5.00 m.
The slope was waterproofed using a bentonite geocomposite (GCL),
which guaranteed a performance equivalent to 1 m of clay soil with
a permeability of 10-12 m/s (alternatively polymeric geomembranes
have been used).
It is essential to protect the waterproofing system
against mechanical puncturing and damage that may occur both during
compaction of the various soil layers that cover Municipal Solid
Waste and during anchorage of the capping system to the upper berms.
This protection is achieved using a geocomposite layer above and
below the waterproofing system and covering all with a minimum of
0,20 m thickness of topsoil (see cross section drawing). To act
as a gas venting layer (below the GCL) and as a rainwater drainage
layer (on top of the GCL), TENAX TNT 600 geocomposite was selected
for its high drainage capacity and long-term compressive resistance.
This is a combination of a TENAX CE geonet (drainage and load distribution
functions) and two layers of nonwoven geotextiles (filtration action)
heat bonded to the drainage net.
The TENAX TNT geocomposite offers a complete system of "filter-drainage-protection":
the geotextile acts as a water/soil filter for the geocomposite
and prevents intrusion within the drainage core of the bentonite
migrating from the GCL during hydratation and intrusion of the loose
fill material laid directly above, thus allowing for a long term
hydraulic flow. A geocomposite layer was installed below the GCL
liner to allow the biogas to be vented off avoiding excessive gas
A geocomposite layer
was deployed above the GCL liner to drain the filtered rainwater
that, if left flowing in contact with the bentonite geocomposite,
drastically reduces the shear resistance (angle of friction) between
the soil and GCL.
The excess water, that produces soil interstitial water pressures
and the loss of shear resistance, may lead eventually to a catastrophic
The choice of a geonet manufactured using the most stable polymer
known, HDPE, is recommended due to the aggressive chemical nature
of the leachates that are a by-product of urban waste.
The design parameters for the slope drainage were a hydraulic gradient
of 0.5 (i=0.5) for an average slope angle of 26°, a slope length
between drainage collection points of about 20 m and a soil fill
depth over the capping of 0.20 m equivalent to about 3 kPa.
From these input parameters and given the typical rainfall distribution
data of the area; it was established that a hydraulic flow rate
of 1.00 E-04 m3/s/m was required allowing an adequate safety factor.
Thus TENAX TNT 600 geocomposite was selected since it satisfies
all requirements, as shown in the flow rate chart below. Furthermore,
slope veneer stability analysis were also conducted, showing that
it was necessary to use above the geocomposite layer a reinforced
geomat for soil retention, this would be achieved due to the materials
tensile properties and high interlock with the surrounding soil.
The design followed the principles of long term stability and performance
even during periods of prolonged heavy rainfall, and also allowed
for the protection and growing of grass on steep slopes.
The project designers selected TENAX MULTIMAT 110 R that is a three-dimensional
geomat composed of extruded and bi-oriented grids, with a high tenacity
PET geogrid as reinforcement medium.
The geomat is thus capable
of withstanding long-term tensile loads and is able to distribute
the stresses and minimise surface erosion.
Soil filling of the TENAX MULTIMAT 110R geomats
1: TENAX geocomposites for rainfall drainage
2: TENAX geocomposites for biogas drainage
Landfill slope after geomat installation and fill soil covering
The final capping cross section was, starting from the bottom of
the cross-section (see drawing):
- Geocomposite for biogas collection: TENAX TNT 600;
- Bentonite geocomposite, GCL for water proofing;
- Geocomposite for surface water drainage: TENAX TNT 600;
- Three-dimensional reinforced geomat: TENAX MULTIMAT 110 R
for veneer stability;
- 0.20 m thickness of hydro seeded topsoil.
This challenging project proved that:
- The use of TENAX TNT geocomposites (both for biogas drainage
and rainfall water drainage) gives engineers the opportunity
to design very long and steep slopes over impermeable and smooth
faced waterproofing systems, that could otherwise prove impossible
using traditional techniques such as gravel and/or sand layers
that are more expensive to install and require higher factors
- To guarantee adequate veneer stability of the imported vegetated
topsoil layer, it is necessary to add a reinforcement geosynthetic,
such as the three-dimensional reinforced geomat TENAX MULTIMAT
Geosynthetics installation procedures
With reference to the below typical cross section drawing, after
excavating the 0.50 m x 0.50 m anchor trench, the gas venting
geocomposite was installed and connected to the gas venting system,
then the GCL and the geocomposite for surface water drainage were
installed on the slope.
The complete geosynthetic system was held in place by using 0.60
m long large headed anchor pins at a 2.00 m offset spacing.
GCL liner was installed with a 0.20 m overlap taking care to assure
The geocomposite above the liner, was wrapped
around a perforated drainage pipe that had been installed inside
the anchor trench and space 0.20 m apart.
Finally the geomat was
placed in the anchor trench which was then back filled with a
granular fill and compacted.
The same granular material soil was placed and compacted using
a vibrating roller to a depth of 1.00 m on the top of the berm.
A 0.20 m layer of organic topsoil was carefully placed on the
completed slopes and was lightly compacted by roller.
prevent surface erosion prior to the establishment of vegetation,
a straw/coconut fibre biomat was installed prior to hydro seeding
Slope stability analysis
The Geosynthetics Division of TENAX SpA carried out the slope
stability analysis for the landfill capping system. The design
evaluated the geometry of the covered slope, all the interface
shear coefficients for the different materials used and for all
the geosynthetics installed.