Arabian Journal of Geosciences (2020) 13: 41
https://doi.org/10.1007/s12517-019-5030-8
CAJG 2018: TOPIC 3
Assessment of design parameters and the slope stability analysis
of weak clay-bearing rock masses and associated spoil piles
at Tunçbilek basin
Zeynal Abiddin Erguler 1
&
Hüseyin Karakuş 1 & İ. Göktay Ediz 2 & Cem Şensöğüt 2
Received: 17 June 2019 / Accepted: 19 December 2019 / Published online: 7 January 2020
# Saudi Society for Geosciences 2020
Abstract
Tunçbilek lignite basin situated in western part of Turkey has been increasingly excavated by open-pit and underground mining
methods since 1940 to meet Turkey’s energy demand. The blasted and then the excavated stratified rock masses in this basin
consist mainly of clay-bearing rocks such as claystone, mudstone, siltstone, and marl. It was found that the strength and
deformation properties of these clay-bearing rocks decrease with the increase in water content. In addition, time-dependent
attenuations were also recorded in the mechanical properties due to disintegration caused by physical weathering processes.
Therefore, considerable time-dependent instabilities were observed in the slopes left after open-pit mining activities. In this study,
it was aimed to determine the design parameters of weak rock masses and spoil piles at Tunçbilek basin and then to recommend
safe slope geometry for open-pit mining activities. For this purpose, the geotechnical properties of rock masses and spoil piles
were obtained based on field tests as well as characterizations and back analyses of the instabilities. This study reveals that shear
strength parameters of undisturbed clay-bearing rock masses are 200 kPa and 30°. In addition, a cohesion of 150 kPa and an
internal friction angle of 25° should be used for slope stability analyses of both weakly cemented conglomerate-sandstone and
disturbed clay-bearing rock masses.
Keywords Instability . Open-pit mining . Slope stability . Spoil pile . Weak rock mass
Introduction
The identification of procedures for investigation stability of
slopes and spoil piles in an open-pit mining site involves very
challenging detailed geotechnical investigation. Since the
1970s, in parallel with the increase in demand for energy,
lignite has been widely produced in Neogene-aged
Tunçbilek basin (Kütahya,Turkey) by Turkish Coal
"This paper was selected from the 1st Conference of the Arabian Journal
of Geosciences (CAJG), Tunisia 2018"
Responsible Editor: Amjad Kallel
* Zeynal Abiddin Erguler
zeynal.erguler@dpu.edu.tr
1
Geological Engineering Department, Kütahya Dumlupınar
University, 43270 Kütahya, Turkey
2
Mining Engineering Department, Kütahya Dumlupınar University,
43270 Kütahya, Turkey
Enterprises (TKI). In addition to underground mining at few
locations, open-pit mining technique has been much more
preferred in this site for lignite production. The lignite bearing
sequences are found within the geological units composed of
weak, thinly bedded, stratified, and fractured rock masses.
Therefore, many geotechnical problems, particularly slope instabilities, were observed in various dimensions at this basin
due to the rather weak geotechnical characteristics of these
rock masses.
It is very challenging to determine representative physical
and mechanical properties of such rock masses or upscale
these parameters to the actual field condition by utilizing rock
mass classification systems and empirical models. In addition,
direct measurement of these properties by conventional laboratory tests is very difficult due to the fact that the overall
structure strength and deformability features of rock mass is
mainly controlled by both the strength of intact pieces and
geotechnical properties of discontinuities within the rock
masses. Therefore, various large-scale triaxial tests were carried out in the previous studies (such as Reik, 1979; Wichter
41 Page 2 of 11
and Gudehus, 1982; Natau et al., 1983; Mutschler and Natau,
1991) to achieve further understanding of failure behavior and
combined effect of intact rock and discontinuities in rock
masses. However, utilizing such a large-scale mechanical test
constitutes some limitations (such as being highly expensive
and time-consuming). For practical engineering purpose, rock
mass classification systems such as rock mass rating
(Bieniawski, 1989), Q classification system (Barton et al.,
1974), and approaches such as Hoek-Brown failure criterion
(Hoek et al., 2002) are generally used for predicting strength
and deformation properties of rock masses. Furthermore, up to
now various predictive empirical models have also been recommended by many researchers (e.g., Bieniawski, 1978;
Serafim and Pereira, 1983; Nicholson and Bieniawski, 1990;
Mitri et al., 1994; Barton, 2002; Hoek et al., 2002; Sonmez
et al., 2004; Hoek and Diederichs, 2006) to estimate deformation modulus of rock masses. In addition to incorporation of
rock mass classification systems and approaches, back analysis on slope instabilities (Öge, 2017; Tutluoglu et al., 2011;
Jiang and Yamagami, 2008), data mining, and observations
directly obtained at field conditions contribute significantly to
achieve design parameters for mining activities and also to
understand the effect of mining activities on failure behavior
of rocks masses. According to Öge (2017) and Hammah et al.
(2004), back analysis performed on an instability of open-pit
mine can be a significant source of experience and database
for the decision on design parameters of slope geometry.
Considering these results obtained from the previous studies on strength and deformation characteristics of rock masses,
a comprehensive research program providing the required
geotechnical parameters of rock masses to assess the stability
of slopes and spoil piles was carried out in Tunçbilek open-pit
coal mining site in western part of Turkey. The geological
units at Tunçbilek basin generally consist of clay-bearing
rocks such as claystone, mudstone, marl, etc., as well as thinly
bedded limestone layers. Based on the field and experimental
studies, it was identified that the strength and deformation
characteristics of rock masses were mainly controlled by these
clay-bearing rocks decreasing their mechanical properties
with increasing water content and time-dependent physical
weathering processes. Such a sensitivity in shear strength parameters of these rock masses against variations in water content and the highly intensive faulting feature of the geological
formations are thought to be the main reasons for the development of slope instabilities in this mining site.
In this study, field measurements and observations, rock
mass rating (Bieniawski, 1989), Hoek-Brown failure criterion
(Hoek et al., 2002) proposed for defining rock mass behavior,
and the results predicted from back analysis of previous slope
instabilities are all used in the accurate determination of shear
strength parameters for the rock masses and spoil piles. As a
result of the analyses made, the cohesion and internal friction
angle of clay-bearing rock masses were determined as 200 kPa
Arab J Geosci (2020) 13: 41
and 30°, respectively. However, a cohesion of 150 kPa and an
internal friction angle of 25° are considered to be more representative for both weakly cemented conglomerate-sandstone
layers and disturbed clay-bearing rock masses due to the disturbance effects originated from blasting and subsidence of
the underground mining activities in the region. Slope stability
analysis by the limit equilibrium method reveal that the overall
slope angles ranging between 32° and 35° depending upon
both of the level of disturbance and depth of mine should be
used for the open-pit excavation in clay-bearing rock masses.
Furthermore, it can be concluded that the overburden materials can safely be stockpiled at an overall slope angle of 23°
with a geometry of 40 m, 30 m, 36°, for bench width, bench
height, and angle, respectively.
Geological and tectonic settings of Tunçbilek basin
The lignite bearing levels are found inside Tunçbilek basin
having an extending of NE and SW direction. The Paleozoic
metamorphic rocks such as schists constitute the base rocks of
this basin. The basin has developed in the Cretaceous-aged
sequence of radiolarites and ultrabasic rocks such as ophiolites
and gabbro. These geological units unconformably overlay
the Paleozoic basement. According to Baş (1983), Neogene
units containing lignite levels unconformably overlie these
Cretaceous rocks.
The Neogene deposits in the Tunçbilek basin begin with
the Beke formation reaching a thickness of about 1000 m.
Çelik (2000) reported that the Beke formation consists of pebbles, sandstones, mudstones, and thin lignite levels. Tunçbilek
formation involving lignite layers, which can be economically
produced by open-pit and underground mining techniques,
conformably overlies Beke formation. This lignite containing
formation mainly consists of clay-bearing rocks such as marl
and claystone, which indicate very fast disintegration and decomposition behavior against physical and chemical
weathering processes, sandstone, conglomerate, and limestone levels. The geological units representing quaternary
are mainly alluvium and debris materials.
Normal faults, which are typical characteristics of the
stretched tectonic regime, and these faulting system-based deformations were obviously observed in Tunçbilek basin. The
vertical separations of the faults are usually between 1 m and
8 m; however, faults with displacements of several meters
were also detected. A typical example of such faults having
very high vertical separation is given in Fig. 1 as an example.
Tunçbilek basin is found in the second-degree seismic zone
according to the seismic hazard map of Turkey. This map was
mainly drawn based on active fault map of Turkey that updated in 2012 by the General Directorate of Mineral Research
and Exploration (MTA) considering strong and major destructive earthquakes occurred in last decade. Although microseismic activities having magnitudes lower than 4 have been
Arab J Geosci (2020) 13: 41
Page 3 of 11 41
Fig. 1 Normal fault-based displacement having a separation of about 50 m. observed in the study and its representative geological cross-section
recorded, strong destructive earthquake has not been recorded
during the period of instrumental observation in the close vicinity of the area based on data provided by AFAD-TEDC
(Republic of Turkey Prime Ministry Disaster and
Emergency Management Presidency – Turkey Earthquake
Data Center). Recently, various earthquakes having magnitudes of up to 1.8 have been recorded in the study site.
Despite of the fact that many of these earthquakes are considered to be originated from the excavation activities such as
blasting, it was thought that dynamic slope stability analyses
should be performed considering both high vertical separation
of faults and the previous seismicity observed around the
study site such as 1944 Gediz (M = 6.2), 1970 Gediz (M =
7.2), 2008 Simav (M = 5), and recently the 2011 Simav (M =
5.9) earthquakes.
Geotechnical properties of rock masses and spoil piles
The slopes of open-pit mining activities were excavated within weak rocks masses involving claystone, mudstone, siltstone, marl, clayey limestone, limestone, and weakly
cemented conglomerate and sandstone layers. Uniaxial compressive strength (UCS) is utilized as an input parameter in
rock mass classification system. However, it is very hard to
extract core samples from such weak clay-bearing rocks and
lignite. Thus, UCS of these weak geological units were predicted by performing needle penetrometer test (Ulusay and
Erguler, 2012). In addition to the strength of rock materials,
degree of weathering, geomechanical properties of discontinuities (type, dip, fracture frequency, roughness, aperture, and
infilling) are also very important for understanding failure
behavior of rock masses. These properties were measured
during field investigation. Furthermore, for quantitative description of fractures, rock quality designation (RQD) values
of rock masses were also determined as 20–93% with a mean
value of 63.1% by using core boxes of 20 different core runs
of a drilling program in Tunçbilek basin. A typical view of
extracted core samples having high total core recovery from
clay-bearing rocks is given in Fig. 2. In the same region, Köse
et al. (2012) specified the RQD value of rock mass as approximately 47.4%.
Rock mass rating (RMR) system recommended by
Bieniawski (1989) was also used for the engineering classification of the rock masses encountered within this study. By
utilizing charts and graphs proposed by Bieniawski (1989)
and using the data obtained from the field and laboratory studies (Table 1), the RMR values for clay-bearing rock levels
(e.g., marl, claystone, etc.) and other weakly cemented geological unit involving conglomerate-sandstone were approximately determined as 42 and 38, respectively. The shear
41 Page 4 of 11
Arab J Geosci (2020) 13: 41
Fig. 2 A typical image of the
cores used to determine the RQD
strength parameters (cohesion and internal friction angle)
were predicted from RMR values of associated geological
materials. When approaches recommended by Bieniawski
(1989) were applied to the rock masses, it was found that the
cohesion of clay-bearing rocks changes between 200 and
300 kPa and internal friction angle of same rock units ranging
from 25° to 35°. For geomaterials mainly consisting of weakly
cemented conglomerate and sandstone, the cohesion and internal friction angle are 100–200 kPa and 15° and 25°,
respectively.
In addition to RMR system (Bieniawski, 1989), HoekBrown failure criteria (Hoek et al., 2002), defined by below
given equations, was also used for understating failure behavior of rock masses in the present study site.
a
0
σ
0
0
σ1 ¼ σ3 þ σci mb 3 þ s
ð1Þ
σci
0
0
where σ1 and σ3 are major and minor effective principle
stresses at failure, σci is uniaxial compressive strength of rock
material, and mb, s and a are material constants for rock mass
defined by utilizing below equations:
Table 1 Assessment clay-bearing
rocks and weakly cemented
conglomerate-sandstone sectors
based on rock mass rating (RMR)
GSI−100
mb ¼ mi exp
28−14D
GSI−100
s ¼ exp
9−3D
1 1 − GSI
20
e 15 þ e− 3
a¼ þ
2 6
ð2Þ
ð3Þ
ð4Þ
where GSI, mi, and D are the geological strength index, material constant for the intact rock, and disturbance factor,
respectively.
The GSI value of rock masses was determined by using
general chart introduced by Marinos and Hoek (2000). During
site investigation, it was determined that the structure of rock
masses change between “blocky/disturbed” and “very
blocky.” The surface conditions of discontinuities were classified as ranging between “good” and “fair”. Based on these
input parameters, an average GSI value of 44 was applied to
accurately represent the studied rock masses. The other input
parameters such as mi for intact rock and D for this failure
criterion were determined as 4.9 and 0.5, respectively. The
results obtained by using RMR system (Bieniawski, 1989)
Input parameters
1.Uniaxial compressive strength (MPa)
2. Rock quality designation (RQD)
3. Spacing of discontinuities (mm)
4. Condition of discontinuities
Persistence
Separation
Roughness
Infilling
Weathering
5. Groundwater conditions
6. Orientation of discontinuities*
Adjustment for blasting
Adjusted RMR
Rock mass I
Rock mass II
10 (2)
63.1 (12.8)
217 (7.6)
1.9 (1)
47.4 (9.5)
217 (7.6)
> 20 m (0)
None (6)
Slightly rough (3)
None (6)
Slightly weathered (5)
Wet (7)
Fair (− 5)
0.94
42
> 20 m (0)
None (6)
Slightly rough (3)
None (6)
Slightly weathered (5)
Wet (7)
Fair (− 5)
0.94
38
Rock mass I, clay-bearing rocks; Rock mass II, weakly cemented conglomerate-sandstone; number in parenthesis
represents related rating; *dip of bedding planes changes between 6° and 14°
Page 5 of 11 41
Arab J Geosci (2020) 13: 41
Table 2 Input parameters used
for slope stability analyses of rock
masses and spoil piles
Parameters
Spoil pile
Rock mass I
Rock mass II
Cohesion (kN/m2)
Internal friction angle (°)
Effective cohesion (kN/m2)
Effective internal friction angle (°)
Unit weight (kN/m3)
20.5 (5–35.9)
32.3 (23.6–41)
–
–
20.1 (17.9–21.3)
200–300
25–35
80–237
24.6–40.6
22.6
100–200
15–25
27–94
12.8–26.1
22.6
Rock mass I, clay-bearing rocks; Rock mass II, weakly cemented conglomerate-sandstone
and Hoek-Brown failure criteria (Hoek et al., 2002) are summarized in Table 2. Because of the fact that Hoek-Brown
failure criterion is nonlinear, the instantaneous effective cohesion and internal friction angle values (Table 2) were determined for different slope heights ranging from 10 to 100 m.
Beside the rock masses, the geotechnical properties of
spoil piles produced after open-pit and underground mining activities are also significant to achieve safety condition for slopes excavated in the Tunçbilek basin.
Therefore, the wet unit weight of spoil piles was measured at field condition by digging trench in large sizes
(Fig. 3). After in situ volume and mass of collected samples based measurements, it was determined that the unit
weight of spoil piles changes between 17.9 kN/m3 and
21.3 kN/m3 with a mean value of 20.1 kN/m3. Since it
is very difficult to determine shear strength parameters of
spoil piles accurately by using small-size samples in laboratory, it was decided to utilize the average of previously
published cohesion and internal friction angle values
(Ulusay et al., 1994; Gerhard, 2000; Stormont and
Farfan, 2005; Kaşmer and Ulusay, 2006; Karakul, 2014)
given in Table 2 in slope stability analyses of these
geomaterials.
Fig. 3 Measuring unit weight of
spoil piles at field condition
In addition to the mechanical and physical parameters
of rock material, rock mass, and spoil piles, the details
related to the groundwater to be used as input parameters
should also be known to determine effective stresses in
the slope stability analyses. The decrease in the amount of
precipitation in the summer of 2017 and during the autumn months lowered groundwater level in the study site.
However, the possibility of rising groundwater level during sudden rainfall conditions is also required for slope
stability analyses. The hydraulic conductivities of rock
masses and spoil piles were determined as 1.50 × 10−6 1.12 × 10−5 m/s, 7.99 × 10−4-2.66 × 10−3 m/s, respectively,
by performing the inversed auger hole method. As described by van Hoorn (1979), this field test simply requires the opening of a boring hole to a depth of generally
less than 1 m. Then, the borehole is filled with water
several times for achieving water saturation condition,
and the rate of water level fall in the hole is measured.
These field tests based results indicated that the investigated rock masses and spoil piles had enough hydraulic
conductivity capacity to let a sudden increase in pore water pressure of slopes within very short time during
raining seasons. The presence of ponds in open pits
41 Page 6 of 11
Arab J Geosci (2020) 13: 41
Fig. 4 Comparison of the images taken at two different time periods in the region in terms of understanding the time-dependent slope instabilities: (a)
November 24, 2015, (b) September 29, 2016 (modified from Google Earth)
Page 7 of 11 41
Arab J Geosci (2020) 13: 41
Fig. 5 The bench slope angle of new (a) and old (b) spoil piles
located in the northwestern part of the excavation area
indicates that the groundwater level elevations change between 770 m and 780 m in this region.
Current slope geometries of rock masses and spoil
piles and time-dependent slope instabilities
During field investigation, it was found that the old slopes
were excavated by considering mainly dominant dip direction
of bedding planes of layers. The dip of bedding planes changes between 6° and 14°. The previous overall slope angles were
determined as 34°, 39°, and 45° for slopes generated in the
west-northwest, north-northeast, and east, respectively. The
precision digital elevation models produced by using data collected from drones (Fig. 4a) and aerial pictures (Fig. 4b) were
taken at different time to observe instabilities in the study site.
The width and length of the largest instability in the study area
(Fig. 4a) was approximately measured as 155 m and 253 m.
Except for this instability, other remaining instabilities were
found to have shallow depths. As it is obviously seen from
Fig. 4, despite of these slope failures, an important part of the
open-pit includes stable slopes since 2004. However, when
Fig. 4a is compared with Fig. 4b, which represents two different time periods in the region, in terms of understanding the
time-dependent slope instabilities, it can easily be concluded
that the number and sizes of slope instabilities increased with
the time. Such progressive instabilities having shallow depths
actually reveal time-dependent variations in the geotechnical
properties, particularly shear strength parameters of rock
masses. As it is well known from the previous studies
(Erguler and Shakoor, 2009; Erguler and Ulusay, 2009), the
clay-bearing rocks can easily disintegrate into pieces after being subjected to various physical weathering processes at field
condition. Thus, it was thought that such shallow instabilities
were recorded as a result of the effect of weathering processes
on mechanical properties of rock masses as well as water
content increase inducing decreases in the strength of this
rock.
The slope angles of the old and new spoil piles in the region
were measured by taking appropriately positioned photos. As
it is seen in Fig. 5a, the average slope angle values of newly
deposited spoil piles range from about 36° to 37°. However,
due to the time-dependent settlement and erosion processes,
these slope angle values decrease to approximately 34° for old
spoil piles (Fig. 5b). Karakul (2014) stated that the angle of
repose for spoil piles deposited in Milas-Sekköy (Muğla,
Turkey) lignite area changes approximately between 34° and
35°. It may be concluded from the data obtained in this study
and previous studies that spoil piles directly deposited as a
result of open-pit mining excavation of lignite in Neogene
basins have a stable condition at approximately similar bench
slope angle of 34°–37°.
Results and discussion
Limit equilibrium analyses of slopes and spoil piles were performed by using Bishop’s simplified method (Bishop, 1955)
implemented in Rocscience Slide v7. Considering discontinuity conditions of rock masses and heavily disturbed characteristics of the rock masses due to the previous longwall miningbased subsidence and the blasting operations used for mining
as well as old circular slope instabilities observed in this region, circular failure mechanism was taken into account for
slope stability analyses of both the rock mases and spoil piles
in the region.
As emphasized by Hoek and Bray (1981), it is recommended that the factor of safety ranging between 1.0 and
1.3 should be taken into consideration for long-term stability conditions at open-pit mining. However, these
values are proposed without considering potential seismic
loads. For this reason, the limit equilibrium analyses were
performed by using the data given in Table 2, considering
expected earthquake risk in the region. Based on Terzaghi
(1950) and Corps of Engineers (1982) recommendations
for “severe” and “major” earthquakes, a dimensionless
41 Page 8 of 11
seismic load coefficient of 0.1 was used to incorporate the
effect of pseudo-static earthquake loading in dynamic limit equilibrium analyses. In order to use in the limit equilibrium analysis, geological cross-sections with 19 different directions and 100 m. intervals were taken to provide
reliable representation of the slopes within the study site
(Fig. 6). Moreover, two more topographical cross-sections
were prepared from the spoil piles for associated analyses
in this geomaterial. The dynamic limit equilibrium analyses were performed on the slopes with different
Fig. 6 Cross-sections taken for
dynamic limit equilibrium
analyses
Arab J Geosci (2020) 13: 41
geometries having overall slopes angles ranging between
25° and 45° for all the geological cross-sections by fixing
the bench height to 9 m. As a result of these analyses, the
proposed design geometry for stable slopes excavated in
rock masses involving both sequences of clay-bearing
rock and weakly cemented conglomerate-sandstone unit
is illustrated in Fig. 7. The typical example of limit equilibrium analysis by considering the data given in Table 2
and the proposed slope geometry (Fig. 7) is shown in
Fig. 8. Dynamic limit equilibrium analyses involving
Page 9 of 11 41
Arab J Geosci (2020) 13: 41
Fig. 7 The geometry proposed
for the slopes excavated in
undisturbed rock masses
investigation on the slope stability of spoil piles were
performed by taking average shear strength parameters
given in Table 2, considering various slope geometries.
The results of these analyses on three different topographical cross-sections of the deposited spoil piles indicated
that any decrease in the factor of safety was mainly
controlled by an increase in bench height of the spoil
piles. When all the analyses are considered, the stable
condition for the deposition of spoil piles can be proposed
as 30 m, 40 m, and 36° for bench height, width, and
bench angle, respectively (Fig. 9). As it can be seen from
Fig. 9, the factor of safety for the most critical failure
Fig. 8 A typical limit equilibrium analysis result for geological cross-section of 18
41 Page 10 of 11
Arab J Geosci (2020) 13: 41
Fig. 9 A typical limit equilibrium analysis result for spoil piles having a geometry with a bench height, width, and angle of 30 m, 40 m, and 36°,
respectively
surface was found as 1.2 under dynamic limit equilibrium
analysis.
Conclusions
The following conclusions can be drawn from the abovedescribed study:
1. The time-dependent increases were identified in numbers
and dimensions of shallow slope instabilities. This change
of instabilities reveals the possibility of decreasing mechanical parameters of clay-bearing rock masses with
the time due to effective physical weathering processes
at shallow depth as well as water content increase inducing a decrease in the strength of these rocks.
2. Field tests, rock mass classification systems, failure
criteria proposed for rock masses, and the results of back
analysis after field measurements and observations were
all used to determine shear strength parameters of the
studied rock masses. As a result of these evaluations, the
representative cohesion and internal friction angle of rock
masses composed of clay-bearing rocks were specified as
200 kPa and 30°, respectively. However, relatively lower
shear strength parameters (150 kPa and 25°) were obtained for weakly cemented conglomerate-sandstone rich sequences and disturbed clay-bearing rock masses.
3. Based on the shear strength parameters given, it was
found that the overall slope angle might be 35° with a
haul road width of 30 m, bench height of 9 m, and bench
width of 8 m for undisturbed clay-bearing rock. However,
haul road width should be increased to 35 m for weakly
cemented conglomerate-sandstone rock masses and disturbed clay-bearing rock masses subjected to underground mining activities-based subsidence.
4. The unit weight of spoil piles ranging from 17.9 kN/m3 to
21.3 kN/m3 with a mean value of 20.1 kN/m3 was based
on in situ measurements. Dynamic limit equilibrium analyses results indicated that the spoil piles having a cohesion of 20.5 kPa and internal friction angle of 32.3° could
be safely deposited at an overall slope angle of 23° with
average bench height, width, and angles are kept as 30 m,
40 m, and 36°, respectively.
Acknowledgments The authors gratefully acknowledge the financial
support given by the Western Lignite Corporation (GLI) of Turkish
National Coal Board (TKI) to carry out the research.
Arab J Geosci (2020) 13: 41
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