Int. J. Min. & Geo-Eng.

69
IJMGE
Int. J. Min. & Geo-Eng.
Vol.48, No.1, June 2014, pp 69-79.
Uncertainties and Complexities of the Geological Model in Slope
Stability: a Case Study of Sabzkuh Tunnel
Abbas Eftekhari 1
, Majid Taromi 2*
, Mahdi Saeidi 3
1. Department of Geology, Tarbiat Modares University, Tehran, Iran.
2. Department of Civil Engineering, Islamic Azad University-South Tehran Branch, Tehran, Iran.
3. School of Mining Engineering, College of Engineering, University of Tehran, Tehran, Iran.
Received 5 April 2013; Received in revised form 27 September 2013; Accepted 20 May 2014
*Corresponding author: majidtarami@ymail.com.
Abstract
Slope stability analysis is a geotechnical engineering problem characterized by many sources of
uncertainty. In slope stability computations, some of these sources are encountered, such as geological
details missed in the exploration program, estimation of soil properties that are difficult to quantify
and many other relevant factors. Therefore, accurate reproduction of the spatial variability in the field
could be essential to decrease uncertainty. The Sabzkuh-Choghakhor water conveyance tunnel is
currently under construction using the NATM and TBM tunnelling methods in the Zagros Mountains
in south-western Iran. In the slope stability assessment of the Sabzkuh tunnel portal, despite adequate
geotechnical investigations, field studies have not been performed with appropriate accuracy. A
landslide and collapse has occurred in a part of the portal due to tunnel excavation. In this paper, the
importance of having a precise and predetermined schedule for selecting site location, monitoring,
complexities of the geological model, uncertainty and its effects on the stability of the trench were
investigated and the necessity of comprehensive slope management was emphasized.
Keywords: collapse monitoring, ground complexity, uncertainty, slope stability.
1. Introduction
Uncertainty is inherent in geotechnical design
[1, 2]. Geological anomalies, inherent spatial
variability of soil properties, scarcity of
representative data, changing environmental
conditions, unexpected failure mechanisms,
simplifications and approximations adopted in
geotechnical models and human mistakes in
design and construction are all factors
contributing to uncertainty [3]. Therefore,
geotechnical engineering almost always has to
deal with uncertainty, whether it is formally
acknowledged or not [4].
Slope stability is one of the most important
and delicate problems in civil engineering,
which particularly encountered in large and
important construction projects, such as dams,
highways and tunnels. Slope stability analysis,
an important research field of engineering

Eftekhari et al./ Int. J. Min. & Geo-Eng., Vol.48, No.1, June 2014
70
disaster prevention and mitigation, is used to
determine the most dangerous slip surfaces
(potentially the most dangerous sliding
surface) and the safety factor of the slope [5].
Therefore, the main aim of slope stability
analysis is to design a slope that is stable,
economical and has the least possible chance
of failure [6].
The Sabzkuh-Choghakhor water conveyance
tunnel under construction in the southwest of Iran
is 10,617 metres in length and 0.01% in gradient
(Figure.1). The tunnel is going to be excavated by
a double-shield tunnel boring machine (DS
TBM). The information obtained from the site,
including field reconnaissance as well as
geotechnical and geophysical studies revealed
that the tunnel from the beginning to 0+390 km
(T1) is located in the alluvium. So, taking into
consideration the application range of DS TBM
and its limitation for working in alluvial soils, this
part of the tunnel is being excavated by
conventional methods [7, 8, 9, 10, 11].
Obviously, a flatter slope in the geometric
design of a trench leads to more stable
conditions with regard to sliding or collapsing;
however, in the Sabzkuh tunnel project, it was
not the optimal and most economical solution
due to space limitations and not taking
(Demising) the land from its owner.
Therefore, despite these limitations to the
geometric design of the trench, the height of
the tunnel entrance has been considered at
over 32 metres with benches four metres wide
and eight metres high (2H: 1V & 1.5H:1V).
The stability of this portal was examined
by using SLIDE 5.0 and FLAC/Slope software
and it was found that the slope is stable in both
static (FS: 1.5) and pseudo-static (FS: 1.1)
conditions. An appropriate safety factor for the
stability of the trench was obtained in the
short-term (during construction) in accordance
with the assumption of saturated soils in the
static mode, as well as in the pseudo static
mode based on the assumption of a horizontal
peak acceleration of 0.251 g for return periods
of 75 years and a lifetime of 50 years. After
excavating 35 metres of tunnel and
confronting unexpected ground conditions, a
face collapse initiated and developed rapidly.
This problem caused a failure in the tunnel
portal area. After sliding and collapsing in the
portal of the tunnel we found that an error
occurred at one stage of the geological field
investigation, geotechnical data or in the
trench design. Thus, regardless of the primary
data and assumptions, all steps were reviewed
and soil sampling and laboratory tests were
performed again to identify the cause of the
collapse.
This collapsing is due to uncertainties in the
input parameters in stability analysis. Thus, the
present study is concerned with answering the
question of whether the assumptions used in
designing the construction process are matched
with observations.
2. Difference Between Geological and
Geotechnical Surveying
As slope stability analysis is not a simple task,
it requires knowledge from other disciplines.
Therefore, a joint effort from geologists,
geotechnical engineers and seismologists is
required to tackle this problem.
A comprehensive geological model is
absolutely fundamental to any slope design.
Without such a model, slope designers have to
resort to crude empiricism and the usefulness
of such designs, with the exception of very
simple pre-feasibility evaluations, is highly
questionable.
According to the site investigations and
two exploratory boreholes, the geology of the
portal included the contemporary alluvial and
debris. The boreholes log and the profile used
in the trench stability analysis are shown in
Figure 2a.
After geological mapping of the inside and
outside of the tunnel route, reinterpreting the
primary geotechnical reports and comparing
between these results and those obtained from the
boreholes logs and primary geological surveying at
the first stage was completed. Fundamental
differences were identified, particularly in the
diversity and the slope angle of soil layers. Then, in
accordance with the USCS, the new samples,
which were taken from the trench, were classified.
According to the new results of soil classification,
the number of soil categories increased from three
(ML, CL-ML, SC-SM), in the initial interpretation
(Fig. 2a), to six categories (CL, CH, ML, CL-ML,
SC-SM, GP-GM) (Fig. 2b).

71
Fig.1. Sabzkuh tunnel location (above) and longitudinal geological profile (below) [7].
(a) (b)
Fig. 2. Boreholes log: (a) initial; (b) modified [9].
The old sliding surface was then detected
at the both sides of the portal by surface
surveying (SS: 75-80 / 355-005). Due to the
existence of muddy soils at the surface, the
geometrical characteristics of alluvial layers
were immeasurable. Thus, on the waterway
path leading to the portal and also to the
collapsed zone in the trench, hidden old
alluvial layers under contemporary alluviums
were observed (50-55/005-010 (Dip/Dip
Direction)).
In order to complete the geotechnical
investigations, geoelectric studies using the
CRP method were performed in several
profiles within the trench. From these profiles
the movement of bed rock was easily
detectable as a result of sliding. The CRP
profile and the sliding surface of the trench are
shown in Figures 3 and 4, respectively.
IRAN
Chaharmahal va
Bakhtiari

72
Fig. 3. The CRP profile in portal [8]. Fig. 4. The trace of slide surface on the portal.
At the final stage, the physical and
mechanical parameters of soil samples were
reassessed by in-situ re-sampling from the soil
of cavity and in-situ field tests, and the new
geotechnical test results were interpreted by
statistical approaches.
The preliminary geotechnical properties of
the soil (highlighted in the primary trench
stability analysis), in accordance with the
previous geotechnical reports and performed
tests, as well as the results of the new
classification based on the USCS standard
(ASTM D-2487) with physical and
mechanical parameters of the trench soil, are
given in Table 1. Figure 5 shows the previous
modified profiles (Fig. 2a) and Figure 6 plots
the occurred collapse at the trench.
Table 1. Physical and mechanical parameters of the trench soil.
NO. SOIL UNIT γ (GR/CM3
) (DEG.) C (KG/CM2
)
1 CL 1.71 22 0.49
2 CH 1.64 19 1.1
3 ML 1.76 26.7 0.18
4 CL-ML 1.73 24 0.35
5 SC-SM 1.88 31.3 0.035
6 GP-GM 2.1 34 0.01
7 SLIDE SURFACE 1.8 65 0.0
Fig. 5. The modified geology profile [9]. Fig. 6. Tunnel collapsing and the failure of concrete
slab [7].
3. Monitoring
The monitoring data can provide a basis for
understanding the situation and understanding
warning signs in slopes [12]. Therefore, great
effort has been paid around the world every
year to monitoring and analysing the stability
of soil and rock slopes [13].
Wyllie and Mah noted that: “because of the
unpredictability of slope behaviour, slope
monitoring programs can be of value in managing
slope hazards, and they provide information that
is useful for the design of remedial work” [14].
Measurement of the surface displacement is
important in slope failure prediction models and
in conducting risk assessments [15]. Slope
monitoring using total station and in situ field
observations are common methods to investigate
landslides and monitor slope instability.
After completion of the trench construction
and before the beginning of the tunnel excavation,
Collapsed
Zone Sabzkuh Tunnel
Landslide

73
many cracks were observed on the concrete slabs,
as well as at the construction and expansion
joints. Therefore, an adequate monitoring scheme
was implemented before and during the tunnel
construction in order to monitor the interaction
between the tunnel and the trench movements.
The monitoring scheme around the study area
included the following key elements:
A) Monitoring of crack displacements
Major pre-existing cracks on the concrete slabs
were monitored using a series of glass slides to
review the development progress of these cracks.
Unfortunately, the veinlet structural
fractures were identified on the glass slides.
Consequently, due to the sensitivity of the
trench stability issue and propagation of
cracks, topographical control was conducted.
B) Topographical control
The use of total station surveying instruments
for monitoring the movement of structures
with good results have been reported by many
authors, such as Radovanovic and Teskey in
2001, Hill and Sippel in 2002, Kuhlmann and
Glaser in 2002, as well as Zahariadis and
Tsakiri in 2006 [16, 17, 18, 19].
Pins and control points fixed into the
potentially unstable ground were surveyed. A
total of nine bench marks were installed at the
various points of the trench (Fig. 7) and
monitoring of these points were inserted at the
work plan of the surveying unit.
Fig. 7. The location of the monitoring station and
overall tendency of displacement along X, Y and Z [8].
The preliminary estimation of the
displacement direction and assessment of the
trench stability can be identified as the two main
objectives for continuous trench monitoring.
After data acquisition, it was determined that the
pins at the seven stations had moved in one
direction of the X, Y, Z directions from north to
northeast, but there was no movement in the R3
and R6 stations (Fig. 8).
The soil of some parts of the trench
consists of a certain clay type, thus the
swelling is predictable as a result of increasing
saturation of the soil due to groundwater and
rain-fall that caused soil uplifting.
Fig 8. The displacement graphs of the installed pins along X, Y, Z. As observed in the Z direction despite the trench
subsidence, R1 station was moved upward. The soil at this location contains montmorillonite, which has the capability
of swelling (Modified [8]).

74
4. Process of the trench collapse
The displacement of installed pins was
recorded with millimetre precision before
collapsing. The first signs of sliding were
observed after the initiation of tunnel
excavation from 35 metres, where four metres
of the tunnel crown collapsed in eight hours
and 1500 m3
of soil collapsed six times in one
month (Fig. 9). The monitoring program was
simultaneously conducted in shorter, yet, more
accurate periods. The results showed that an
increasing movement of the trench ultimately
caused the failure of the concrete slabs.
It is obvious that bad engineering judgment
in the design of trench parametres will occur
as a result of inaccurate field investigations.
The high tunnel advance rate, stresses
releasing, existence of aqueous soils and
layers with high degree angles caused mass
soil sliding on the slide surface. Figure 10
offers a flowchart representation of the
collapsing factors for this trench.
Fig. 9. Stages of Trench failure [7].
Fig. 10. Flowchart of the trench failure mechanism [8].

75
5. Trench stability reanalysis
According to the results obtained from
geological and geotechnical studies as noted
earlier, geometric design and the trench
stability were reviewed and reanalysed based
on our new studies.
Evidently, the slope stability analysis is
based on simplified assumptions; consequently
slope designing depends heavily on field tests
and investigations.
Determining soil properties is the key
factor in stability analysis. On the other hand,
soil properties vary spatially even within
homogeneous layers due to depositional and
post-depositional processes that cause
variation in properties [20]. Nevertheless,
most geotechnical analyses adopt a
deterministic approach based on single soil
parameters applied to each distinct layer. The
conventional tools for dealing with ground
heterogeneity in the field of geotechnical
engineering have been applied under the use
of safety factors and by implementing local
experience and engineering judgment [21].
Calculations are performed based on the safety
factors for unstable (FS ≤ 1), boundary (FS
=1) and stable (FS> 1) states, in most
analyses, though with little attention to the
precision of soil properties and the geometric
of the layers.
Despite the uncertainties in the slope
stability problems, geotechnical engineers still
avoid adopting the probabilistic techniques in
their analyses; thus, most of the design
regulations such as Eurocode 7 (2004)
consider conservative properties for soils and
relatively large safety factors in the slope
stability analyses. In recent years, slope
stability analyses have widely utilized
numerical models that closely approximate
failure processes and more accurately
represent actual landslide conditions [22].
Simplified assumptions in 2D slope
stability methods have led to factors of safety
that differ from the more rigorous 3D slope
stability analysis methods. The 3D slope
stability analysis method establishes a
compatible velocity field and obtains the
factor of safety by the energy and work
balance equation [23].
Drawing on the above guidelines and by
using modified geological profiles and new
geotechnical data, a 3D numerical model of
the portal of the Sabzkuh tunnel with pervious
geometry was produced. The analysis methods
of slope stability include the rigid body limit
equilibrium method (RBLEM) [24, 25],
stereographic projection method [26], finite
element method (FEM) [27, 28, 29], discrete
element method (DEM) [30], discontinuous
deformation analysis (DDA) [31, 32, 33] and
numerical manifold method (NMM) [32, 33],
etc. These methods have both advantages and
disadvantages [34].
Numerical modelling software packages,
such as FLAC3D
(Fast Lagrangian Analysis of
Continua, which uses the explicit finite
differentiation formulation), can contribute to
solving problems that consist of several stages,
large displacements and strains, non-linear
material behaviour and unstable systems, and
even cases of yield/failure over large areas, or
total collapse.
The model geometry for the analyses using
FLAC3D
[35] is illustrated in Figure 11. The
model was 50 metres wide and 81 metres long.
The floor of the tunnel was located 15 metres
above the base of the model and the
overburden height varied between three metres
to 23.5 metres. A D-shaped tunnel with a
width of 5.6 metres and height of 5.51 metres
was excavated to the collapsing zone. The
unsupported tunnel length was one metre. For
each step, the excavation was advanced by one
metre and the pre-support shotcrete (25 cm
thick with two layers of welded wire mesh 8φ )
and steel arc support (2IPE140) were installed
at the specified distances from the tunnel face.
Fig. 11. The model geometry based on geology profile
for the analyses [8].

76
The tunnel was constructed in weak soil
conditions that are modelled as Mohr-
Coulomb materials with properties given in
Table 1. The initial stress state corresponds to
gravitational loading with the following
relation between vertical and horizontal
stresses: 2σzz = σxx = σyy.
The pre-support shotcrete was modelled
with zones. Note that the steel arch support
was not explicitly modelled and its effect was
combined with that of the shotcrete. Assuming
20GPa for modulus of elasticity and 0.2 for
Poisson's ratio and using the relations
presented by Hoek et al. in 2008, the
equivalent modulus of elasticity for the
support system would be 23.2GPa [36].
Before tunnel excavation, the factor-of-safety
(FOS) was calculated using the strength
reduction method. A value of 1.11 was
calculated for the FOS under wet conditions,
indicating that the slope was stable. However, as
observed after beginning the excavation of the
tunnel, displacements increased and in the
vicinity of the sliding surface the maximum
relative displacement was 3.358e-001m. Figure
12 plots the resulting failure surface due to
tunnel excavation by the displacement contour.
Before the sliding surface area, displacements
were below the permitted limit. Installation of a
support system, as soon as possible, leads to
preventing surface displacement during the
excavation stage. However, the trench collapsed
near the sliding surface before the support
system was installed. The velocity vectors
calculated during the excavation stage and the
maximum values are in the range of 7.823e-007
ms-1
. These values indicate that the soil
collapsed in wet conditions. Furthermore, field
observations confirmed that this phenomenon
may occur.
Figure 13 plots shear strain-rate contours,
which allow the failure surface to be identified.
In essence, the approach assumes that field
data (such as in-situ stresses, material
properties and geological features) will never
be completely known. It is futile to expect the
model to provide design data, such as
expected displacements, when there is massive
uncertainty in the input data. However, a
numerical model is still useful in providing a
picture of the mechanisms that may occur in
particular physical systems, but as it can be
observed, if extensive field data are available,
then these may be incorporated into a
comprehensive model that can yield design
information directly.
Fig. 12. Displacement contours and velocity vectors in the
model at the failure state [8].
Fig. 13. Shear strain-rate contours in slope model at the
last non-equilibrium state [8].
6. Slope management
In any economic slope, a variety of
instabilities may be present at various
locations in the slope at any time. A skilled
geotectonic engineer should be able to manage
these features successfully. The absence of
any failures is a sign of over-conservative
slope design, and hence, inefficient project
management. It is, therefore, an absolute
requirement that engineering geologists and
geotechnical engineers work together all the
time in order to ensure that:
1. the appropriate data are collected,
2. the appropriate analyses are carried out,

77
3. the slope designs are clearly conveyed
to, and understood by, the mine planners and
operators, and
4. well-conceived slope monitoring
programmers are established to monitor the
service performance of the slope throughout
its life. Contingency plans must also be
drawn-up to deal with the inevitable surprises
that will occur from time to time.
A slope design is based upon the best
possible evaluations of the soil types and
characteristics, the structural geology and the
groundwater conditions in the slope. Even the
best slope designs require some averaging of
all of this information and local variations in
geology or groundwater conditions are not
always incorporated into the design. These
local variations can have a significant impact
on slope stability and, depending on the
location of this instability, may have important
consequences for the performance of the
slope. Advance warning of these slope
instability problems is very important and
monitoring of slope movement has proved to
be the most reliable method for detecting slope
instability. The more accurate this
measurement the earlier the developing
problem can be detected.
Tools for slope displacement monitoring
are well developed and are used routinely on
many civil engineering projects in which large
slopes exist or are being excavated. These are
generally based upon observations on
numerous targets placed at carefully selected
locations on the benches of the slope.
Simple visual observation by geologists
and geotechnical engineers is a tool that is
frequently ignored. The development of
tension cracks and the appearance of bench
faces is due to slope instability. If these are
observed routinely and recorded
systematically, a ‘feel’ for the behaviour of the
slopes can gradually be developed. This is
important information that can be taken into
account when signs of significant slope
instability appear and when discussions on
remedial measures and contingency planning
take place [37].
7. Conclusions
This study employed the three methods of
field investigation, monitoring and numerical
modelling. The field survey, monitoring and
numerical simulation results revealed
landslide failure mechanisms, the area of the
potentially unstable slope and sliding depth,
which otherwise are difficult to obtain. The
conclusions are as follows:
1. using field investigation to estimate the
parameters of soil can compensate for the
deficiencies in overall representativeness of
mechanical tests, and
2. numerical simulation analysis
verification of the history of the disaster area
and estimated results can yield conservative
calculations of the potential landslide area.
Numerical simulation analysis can also
simulate the behaviour of the landslide mass
sliding through soil, which can be delineated
according to the scope and depth of the high
potential landslide.
In analyses and the design process of the
portal of the Sabzkuh tunnel, a geotechnical
model was planned, performed and analysed
independently, regardless of a geological
model. A lack of correlation between these
two models, as well as the complicated ground
situation and uncertainties provided some
inappropriate information for designers and
affected the analytical model. This problem
caused a failure in the tunnel portal area.
This case study demonstrates once again
the importance of early detection or prediction
of potentially problematic zones (via probe
drilling and monitoring) in slope stability,
particularly through mixed or difficult ground
conditions characterized by alternating layers,
faulting and landslides. As a result of the
inherent uncertainty in the slope design, the
presence of experienced geologists and
geotechnical engineers who can predict and
deal with such instability problems effectively
and promptly guarantees the successful
completion of civil engineering projects.
Acknowledgments
The authors are thankful to the Project
Manager of Sabzkuh water supply system, Mr.
A. Izadi of FTR Engineering Ins. for sharing
his vast experience and his guidance. Both of
the anonymous Reviewers are acknowledged
for their constructive criticisms and excellent
review comments.

78
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