2527
Technical Committee 211 /
Comité technique 211
the sewage sludge are developing slowly and are not exposed to
the erosion processes. This kind of material is hazardous when
disposed but when treated by vegetation and additives it is
safely absorbed and utilized by plants. Additionally it has to be
mentioned that sewage sludge supply is free of charge. The
mixture is applied by hydraulic seeders supplied with high
pressure pumps, which enables spraying on different
soil/material types. The advantage of using sewage sludge is
that seeds are protected from the erosion and excessive drying.
The viscosity of the sludge and its mixing ability with other
components, assure even and smooth protection cover, and
moreover, high adhesion to the sprayed surface. The most
significant advantage of using the sewage sludge is the nutrition
content, essential for the vegetation cover establishment.
Especially the undrained sludge is rich in microelement,
nitrogen, phosphorus, potassium and organic matter. Some of
them are highly valuable for plants (Koda 2011), however
cannot exceed normative values of dry mass. The usability of
ashes and sewage sludge for the geotechnical purpose is
determined by several physical and mechanical properties such
as: capacity index in saturated conditions, grain-size
distribution, maximum dry bulk density, swelling, internal
friction angle, and passive capillarity.
4 VEGETATION COVER AS A RELIABLE METHOD
OF SLOPE STABILITY IMPROVEMENT
Beyond described activities for the slope stability and erosion
control improvement purpose on the Radiowo landfill, there
were also bio-engineering techniques applied with additional
use of geosythetics. Due to the usability assessment of the
compost, from organic waste as an enhancing material for the
grass carpets, an experimental plot was established within the
compostory plant area (Koda, 2012). A composite (grass carpet)
consisting of three elements was constructed: reinforcing
material, substrate and grass seeds mixture was prepared. As a
reinforcing material the geotextile (G) and geogrid (Gs) was
used. A reinforcing material task was to connect particular
elements of the carpet, improving the shear strength and
hydraulics conditions, and also an increase of erosion control on
landfill slopes. A porous structure of geotextile and geogrid
enhances establishment of the root zone deeper into the surface.
During the selection of reinforcing material the mechanical
properties and the stock was considered. The polypropylene
materials guarantee long term durability and resistance to
aggressive environmental conditions. A seeding suspension
consisted of a mixture of three types of grass seeds: lawn type
(MT), pasture type (MP) and “gazon” type grass seeds (G). A
substrate consisted of sand and compost mixture in three
different volumetric proportions: 1P/1K- 1:1 (1 measure of sand
+ 1 measure of compost), 1P/2K- 1:2 (1 measure of sand + 2
measures of compost), and K- pure substrate (100% compost).
The scheme of experimental plot is presented in Figure 2.
Additionally an application of already described fly-ash and
sewage sludge suspension on such slopes to accelerate the
establishment of a green cover was also provided. The grass
carpets were introduced in order to maintain the observation and
to conduct further research on how does such solution influence
conditions of slopes. The assessment of the effectiveness of bio-
engineering activity on landfill slopes were undertaken after 1,
2, 6, and 10 years of the experiment duration. The result of the
observation confirms the reinforcing purpose of the method, as
even after 10 years of grass carpets establishment the slopes are
evenly covered with plants, while on the slopes where only
traditional method of planting was applied, the slope conditions
are significantly worse. Additionally, the numerical analyses
involving the influence of reinforcing layer also proved the
correctness of applied method on slope of section I-I marked on
Figure 1 where location map is provided. For the results please
refer to Table 3.
25.0 m
48.0m
21.0 m
15.0 m
15.0 m
21.0 m
12.5 m
12.5 m
12.5 m
12.5 m
25.0 m
2.0m
2.
0m 2.0m 2.0m 2.0m 1.3m 1.3m
1.3m 1.3m
1.3m 1.3m
1.3m
1.3m
2.0m
1P/1K/1
1P/1K/2
1P/2K/1
1P/2K/2
MT MT
MT
MT
MT
MT
MT
MP
MP MP
G
G
G
K/2
MT
MP
G
- Lawn type mixture
- Pasture type mixture
- „Gazon” type mixture
MP
1/2
1/1
2/1
2/2
3/1
3/2
4/1
4/2
5/4
5/3
5/2
5/1
6 7
8
9
10
11
12/1
12/2
13/1
13/2
14/1
14/2
Volum etric content of substrate:
1 sand measure
1,2 compost measure
1,2 layers of substrate
1P/1K/1
1P/2K/2
MP
GEOTEXTILE
width 2.0 m
GEOGRID
width 1.3 m
14/2
1/1 1/2 3/2 8 11
-
Numbers of variants
Figure 2. Scheme of the experimental plot established at Radiowo
landfill slopes (Koda 2011).
The additional solution improving the slope stability is a
proper establishment of high trees and shrubs on slopes (Coppin
and Richards 1990, Norris and Greenwood 2003, Clark et al.
2003). Such activity was also conducted for Radiowo landfill
site. Comprehensively analysed plant species were selected in
terms of root system characteristics and assimilation ability in
such specific ecosystem as contaminated land (Coppin and
Richards 1990, Greenwood 2006).
In the present study, slopes where the vegetation cover was
applied, have been assessed to see whether implementation of
plants affected the resulting stability significantly. Firstly,
however the geotechnical parameters of waste had to be
determined. For such purpose back analyses, CPT and WST
tests were conducted on site. The back-stability analysis by the
Bishops', Swedish (GEO-SLOPE program) and FEM (Z-SOIL
numerical program) methods were performed for three chosen
cross-sections of Radiowo landfill slopes and were applied for
the shear strength parameters verification. The results are listed
in Table 2.
Table 2. Shear strength parameters for municipal solid waste (Koda,
2011)
Material
[kN/m
3
]
[
]
c
[kPa]
Method
non-composted
waste
11.0 20
25
failure tests,
CPT, WST
non-composted
waste + sand
12.0 25
23
failure tests,
CPT, WST
old municipal
waste
14.0 26
20
back-analysis
CPT, WST
The computations of factor of safety including vegetation
cover influencing slope stability were conducted with use of
General Greenwood Method. Greenwood (2006) developed an
equation, based on the limit equilibrium method, where
parameters of plants existing on the slope are considered. These
parameters are: root reinforcement forces, wind forces, or the
mass of vegetation, or related to these, changes in the pore
water pressure. In Slip4EX the Factor of Safety can be
calculated by using several equations developed by Greenwood
(2006), however in this study the Greenwood General Method
was used, as it presents similar characteristics to other methods
used in this study. A powerful equation of FOS concerning a
vegetation influence, proposed by Greenwood is as follows:
