1378
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2
DUMPING CONCEPT
In order to reduce the deleterious effects on the future use of
the mining area that may arise from dumped soils, the RWE
Power AG developed a concept introducing restrictions
regarding the content of fine particles (d < 0,06 mm) of the soils
dumped below the future motorway. Figure 2 illustrates the
cross section of the dump with requirements on the soils.
sand and gravel
content of fines < 5 M.-%
surface
60°
possible: mixed grained soils with
a content of fines < 20 M.-%
50 m (A 44 alignment)
not allowed: fine grained soils or
soils saturated with water
-10 m
-90 m
max. -185 m
no restrictions regarding the
dumped materials
150 m
possible: mixed grained soils with
a content of fines < 30 M.-%
not recommended: fine grained
soil or soils saturated with water
Figure 2. Dumping concept
At depths below - 90 m under the planned surface on which
the motorway will be constructed it is not recommended to
dump fine-grained or saturated soils. In this bottommost part of
the dump only soils with fines contents < 30 mass-% should be
deposited. The width of this lower region is 3 times the width of
the planned motorway. Between -90 m and - 10 m more
stringent restrictions are set. Only soils with a fines content
< 20 mass-% may be used to construct this part of the dump.
The top part of the dump from - 10 m to the ground surface is
made up of sand and gravel with a fines content < 5 mass-%.
To verify the described concept, soil samples were taken at
different depths and positions from the dump along the A 44
route since the year 2008. A total of 716 samples were classified
by the year 2011. Considering the total soil masses dumped into
the areas described in figure 2 and the capacity of about
240000 m
3
per day and per large scale spreader, sampling may
be considered as a random test method. Despite this, the
evaluated data provides essential information about the
adherence to the defined dumping concept.
The evaluation is based on a macroscopic classification
scheme of every single soil sample. The classification divides
the grain-size fractions gravel, sand and fines. This gives a
quick classification without conducting laboratory-based tests
such as sieving. The macroscopic classification was verified by
calibration phases at different points between 2008 and 2011. In
this calibration the outcome of the macroscopic classification
was compared and finally adjusted to the results of the grain
size distribution gained from laboratory sieving. During the
calibration phases a total of 106 grain size distributions shown
in figure 3 were investigated.
0
10
20
30
40
50
60
70
80
90
100
0,001
Diameter of the particle [mm]
Mass percentage [%]
0.002
Silt
Sand
Gravel
Clay
Stones
fine medium coarse fine medium
fine medium
.
0.2
0.06
0.02
2
63
0.006
0.63
6.3
20
coarse
coarse
Figure 3. Grain size distribution (106 soil samples)
The results show that for 91 % of the soil samples the sand
fraction is dominant. 92 % of the soil samples have a content of
fines < 15 mass-%. Only 2.5 % of the samples are classified as
fine-grained soils with a fines content > 40 mass-%. In the top
layer of the dump sand and gravel is dominant.
3
TIME-DEPENDENT DEFORMATION OF THE DUMP
3.1
Geodetic measurements along the autobahn alignment
It is well known that large dumps within the Rhenish lignite
mining area exhibit significant time-dependent deformations
(Nehring 1968, Kothen and Knufinke 1990). This is mainly due
to the comparably low densities resulting from the dumping
method without compacting of the soil masses. As noticed in
Section 2 the soil type and the water content mainly influence
the magnitude of the time dependent deformations. Typically
the rate of deformation decreases strongly according to the
elapsed time since the end of the dumping process.
Nevertheless, due to the large dump depths of by far more than
100 m settlement rates of the surface in a magnitude of several
centimetres per year may be observed even a decade after
completing the dump (Lange 1986).
For providing detailed information about the time-dependent
deformation of the Garzweiler dump along the planned A 44
route a series of measurements were undertaken and analyzed.
The survey department operated by RWE POWER AG provides
a main database for surface deformations. The changing
geometry of the dump resulting from the advancing dumping
progress is monitored continuously throughout the entire dump.
Additionally along the A 44 route discrete survey points are
installed every 50 m to get as precise data as possible. In two
sections (at station 4900 m and station 5900 m) the intervals of
the survey points were reduced to 10 m apart in order to
examine differential deformations over short distances.
Figure 4 plots calculated strains of the dump at the survey
points against time. The diagram shows data collected from
survey points near station 4900. Zero time is set at the end of
the dumping process. The first measurement was taken on day 1
and then on day 7 day after the completion of the dump.
0.0
0.1
0.3
0.4
0.5
1
10
100
1000
10000
Time [d]
Strain according to the height of the landfill [%]
Deformation [m]
0.00
0.15
0.45
0.60
0.75
Area of observation at station +4900 m
Height of landfill: app. 150 m
Time of first measurement:
End of dumping
0.2
0.30
Figure 4. Time-dependent strain and deformation of the dump body
In the logarithmic time scale used in figure 4 the relationship
between strain and time show a nonlinear behaviour during the
first 100 day period. This shape as given in figure 4 is strongly
dependent on the method by which the data is evaluated. The
longer the interval between the end of dumping process and the
first measurement, the stronger the curvature is of the data line.
To ensure a uniform evaluation, the onset of the time dependent
deformation is set by the end of dumping process for each
survey point. In reality time dependent deformations already
occur during dumping process. So start of the time-dependent
deformation is a function of the speed of the dumping progress
itself. Furthermore, the deformation rates decrease rapidly
shortly after completion of the dump. This means that even once
the dumping process is completed within a period of 1 day a
significant curvature appears at the beginning of the time
dependent deformation curve.
For the serviceability of the autobahn, deformations that
occur once the superstructure of the roadways is built are of