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Technology Journal
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RESEARCH ARTICLE
Clogging Potential of Earth-Pressure Balance Shield Driven Tunnels
1
2
1
3
1,*
Alireza Rashiddel , Fatemeh Amiri Ramsheh , Asma Ramesh , Daniel Dias and Mohsen Hajihassani
1
Department of Mining, Faculty of Engineering, Urmia University, Urmia, Iran
Mining and Metallurgical Engineering department, Amirkabir University of Technology, Tehran, Iran
3
Department of Civil Engineering, Grenoble Alpes University, Laboratory 3SR, Polytech Grenoble, France
2
Abstract:
Background:
Nowadays, the construction of urban tunnels for rapid transportation in metropolises is necessary. Since these tunnels are excavated at low depths,
they are often associated with different problems and hazards. Some of them can reduce the efficiency of the tunnel boring machines and
sometimes will stop the project. Among these problems the clogging can cause problems at the cutter head, in the chamber, and in other sections
where the material transference occurs.
Objective:
The main purpose of this paper is to evaluate and determine the risk of clogging in the tunneling boring machine in Line 6 of the Tehran Metro. It
includes stations: Amirkabir, Shohada Square, Emam Hossein Square and Sayyadeh Shirazi. This phenomenon induces an adhesion of the shield
with the soil, increasing the necessary shear forces and it can eventually leads to the project interruption.
Methods:
Due to the fact that the criterion for the behavior of fine soils against moisture is Atterberg Limits, therefore, Atterberg Limits and the water
content were utilized. For this purpose, the new method proposed by Hollman and Thewes (2013) was used. In this study, in addition to the
Atterberg limits, the amount of free water resulting from the machine and from the underground water inflow was included in the calculations.
Results:
It was found that the water content should be increased carefully as the soil is very sensitive to this parameter. An increase of 15% of the water
content permits to reduce the risk of clogging. If the added free water amount 15%, the probability of clogging becomes high. Whereas, in case
where the added free water amount reaches 20%, the risk of clogging decreases significantly.
Conclusion:
According to the performed assessments, it was found that critical areas for the clogging aspect are both the cutter head and the chamber. The
sensitivity of the soil is very important to the free water amount. Therefore, due to the behavior of sticky and plastic of clay soils against increasing
water, it is necessary to determine the percentage of allowable water used in mechanized excavation projects.
Keywords: Shield tunneling, Clayey soil, Clogging, Subway, Atterberg limits, Water content.
Article History
Received: February 19, 2020
1. INTRODUCTION
Tunnel Boring Machines (TBM) permit to increase the
excavation speed and decrease the ground displacement. In
urban environments, these machines face challenges such as
* Address correspondence to this author at the Department of Mining, Faculty of
Engineering, Urmia University, Urmia, Iran; Tel: +989122735914;
E-mail: M.hajihassani@urmia.ac.ir, mohsen_hajihassani@yahoo.com
Revised: May 11, 2020
Accepted: May 12, 2020
soil cohesion, soil abrasion, large rock pieces, ground
settlement and instability. To identify geological hazards, it is
necessary to define the different geological features, the soil
parameters and the groundwater level along the tunnel route.
Table 1 presents these geotechnical and engineering-geological
features and parameters as well as their degree of importance to
identify each geological hazard.
DOI: 10.2174/1874836802014010185, 2020, 14, 185-195
186 The Open Construction and Building Technology Journal, 2020, Volume 14
Rashiddel et al.
Table 1. Geotechnical and geological parameters influencing the occurrence of geological hazards in urban environments [1].
Geological Hazards
The influencing geotechnical or engineering-geological parameters (high importance ■ low importance □)
Petrography of
Grains
Soil
Grading
Soil Cohesion
Soil Abrasion
Large Rock Pieces
Percentage and
Type of Clay
Minerals
Deformability
Properties
Strength
Characteristics
■
■
Water
Content
Groundwater
Condition
■
■
■
Ground Subsidence
Excavation Face Stability
■
■
■
■
Fig. (1). Clogging potential- four effective mechanisms [2].
Some materials, especially those containing a high
percentage of plastic clay, tend to stick to metal surfaces and
contribute to clogging. The clogging potential of clay
formations, as shown in Fig. (1), is defined by four effective
mechanisms [2]:
evaluate the sticky behavior of clay soils. Hollman and Thewes
(2013) presented new diagrams to investigate the clogging
potential in clay soils for all mechanized excavation machines.
In addition, Thewes and Hollman (2016) proposed a new test
to assess a blocked cutter head in sedimentary rocks with clay
minerals.
Sticking of clay particles on the components’ surface,
Bridging of clay particles on the transfer route of
excavated materials,
Adhesion and cohesion of clay particles,
Low tendency of clay particles to be dissolved in
water.
The ability of clay to develop an adhesive behavior that
leads to the clogging phenomena depends on several factors,
including [4]:
Among these four mechanisms, the sticking of clay
particles to the components’ surface is the most important and
effective mechanism.
Type of soil and its grain-size distribution
Type of clay minerals
Plasticity of soil
Water content of the soil and availability for free water
in the area
During a tunnel excavation by shield machines, severe
clogging may occur in clay formations. This can cause
problems for the excavation process and may lead to clogging
in the cutter head, in the machine cutter head disks, in the
chamber behind the cutter head, and in the spiral conveyors. It
can also prevent shield progress due to friction. If the clogging
is not successfully inhibited, it can lead to a performance
decline due to reduced progression rates and time required for
cleaning [3].
This paper investigates the clogging potential between
chainages of 7 to 11.7 km from the tunnel route of the Tehran
Subway Line 6. The proposed method by Hollman and Thewes
(2013) was used in this study.
Recently, extensive research works were conducted on the
phenomenon of clogging. Anonymous (1995), Thewes and
Burger (2004), Marinos et al., (2008), Sass and Burbaum
(2008), Hollman and Thewes (2013) are among the major
investigators of this issue. Thewes and Burger (2004) presented
a diagram based on the consistency index and plastic index to
clogging makes the cutter head heavier and thus leads
to machine deviations during the excavation operation.
clogging at the cutter head leads to an increase of the
torque required by the machine, in some cases where
the maximum torque will not be sufficient to proceed,,
the TBM will be stopped.
2. IMPACTS OF CLOGGING ON TUNNELING
In general, the consequences of adhesive grounds are the
following [5]:
Clogging Potential of Earth-Pressure
clogging and blocking of the excavated materials inlet
scrapers prevent the excavated materials from leaving
the excavation face. This will increase the necessary
driving force of the machine by maintaining a
predetermined penetration rate, and can lead to a
higher abrasion of the cutter head.
As it can be seen in Fig. (2), the adhesion of the
excavated materials results in an agglomeration. It is
then difficult to transfer the material in the conveyor
belt.
3. DESCRIPTION OF THE PROJECT
3.1. Tehran Subway Line 6
Line 6 of the Tehran Metro is one of its most important and
longest lines, stretching from Tehran’s southeast to its
northeast. In its original design, line 6 extends for over 30 km
and incorporates 27 stations, of which, 9 mark the intersection
with other subway lines: 1, 2, 3, 4, 7, 8 and 9. Recently, with
the development of the southern part of Line 6, the length and
number of stations of this line have been extended to 38 km
and 31 stations [6].
According to earlier studies of the profile, route plan, depth
and height of the subway line, and also considering the high
groundwater level, the only safe and reliable method for
tunneling in the southern (first) section of the Tehran subway
line 6 is the use of a TBM in terms of speed, cost and safety.
As shown in Fig. (3), the length of the first line 6 section is
approximately 11.7 km and extends to the Sayyadeh Shirazi
Fig. (2). Soil sticking on the conveyor belt of TBM [5].
The Open Construction and Building Technology Journal, 2020, Volume 14 187
station in which the tunneling operation has been conducted
using the TBM method. The rest of the tunnel is excavated by
the NATM method. Metro Line 6 starts from the shrine of
Abdul Azim and after passing a south-north route, it intersects
with line 4 at Shohada Square Station, with line 2 at Emam
Hussein Station, line 1 at Hafteh Tir Station, and with line 3 at
Valiasr Square Station, and then continues to the west and
reaches the northwestern area of Tehran.
3.2. Engineering Geology
According to the existing topographic maps, the project
area is approximately located at level in the range from 1085 to
1391 m above the sea level. Tehran's alluvial sediments are
mainly the result of rivers and seasonal floods originating from
the northern highlands, located in central Iran, which is about
650 meters above the sea level. The changes in climate and the
tectonic regime have directly affected the status of the surface
water inflows and their sediment transport over time. It has
induced deposition of coarse-grained and fine-grained soils.
More coarse-grained soils can be found in the northern parts of
Tehran rather than its southern parts. The result of the feldspar
alteration and weathering are clay minerals, i.e. Illite and
Kaolinite. The crushing and erosion of quartzes mostly result in
sandy elements and thus, tuffs provide fine-grained materials
and alluviums [7].
The Physical soil parameters of boreholes on the route of
the Tehran subway line 6 are presented in Table 2. The depth
of the identification boreholes is selected according to the type,
conditions, and depth of the tunnel and station [7].
188 The Open Construction and Building Technology Journal, 2020, Volume 14
Rashiddel et al.
Fig. (3). The whole route of the Tehran subway line 6 [6].
Table 2. Physical properties of soil in some of the investigated boreholes [7].
Borehole
Percentage of soil passing through a
sieve
No. 200
Liquid limit
Percentage of water content
Plastic limit
BH7
36-43
BH8
14 - 19
BH9
23 - 34
27 – 29
7-8
11 - 12
SC
28 – 34
16 - 18
11 - 16
GC,GM,GW-GM,GP-GM,GW
24 – 28
9 - 11
15 - 16
GC,GM,GW-GM,GP-GM,GW
4. METHODOLOGY
Soil type
experiments suggesting that clogging can also occur in stiff
clays.
4.1. Definition of Clogging
4.1.2. Free Water Content
4.1.1. Plasticity and Consistency
Fine-grained plastic soils can be defined by their water
content, Wn, liquid limit WL and plastic limit, Wp. With the
increase in the water amount, the state of the soils changes
from very stiff to stiff and can reach a plastic state. At the
liquid limit, their consistency changes from very soft to a fluid
state without significant adhesion. The plastic index (IP) and
consistency index (IC) are defined as follows [4]:
I P WL WP
(1)
IC (WL Wn ) / I P
(2)
The tendency of adhesive soils for clogging can be
assessed using plastic and consistency indexes. To illustrate the
clogging risk in tunneling with TBM, Thewes (1999) provided
a diagram to estimate the degree of clogging tendency based on
experimental studies [8]. According to this diagram, the soil
with a plastic index of more than 20% and a high stiff
consistency index has the highest potential for clogging.
However, the diagram was corrected because of some site
In tunneling when using a TBM, in addition to pore water,
free water (groundwater inflow, machine cleaning water and
ventilation water) should also be included in the calculations.
Any adhesive soil with less than 10% clay has a potential for
the clogging phenomenon occurrence. Its occurrence speed
depends on the natural consistency and availability of
additional free water. Free water inflow during the drilling
operation increases the soil adhesion probability. Besides, the
availability of free water depends on the soil hydrological
conditions and the tunneling operating conditions. To ensure
the tunnel face stability, tunneling in soft soils is often done
using a slurry shield, in which water is used for maintenance.
Under these conditions, the phenomenon of clogging is more
likely to occur. Groundwater inflow is often associated with the
machine excavation face, where the ratio of the water volume
to the excavated soil depends not only on the water inflow rate
but also on the inflow time (time necessary for components
replacement and shift change). Finally, it can be stated that this
factor is mostly dependent on the excavation diameter. After a
time period, soil consistency may change [9]. Moreover, in
closed-mode boring machines, water is also required, so it is
Clogging Potential of Earth-Pressure
important to determine the risk of the clogging phenomenon in
this type of excavation.
4.2. Clogging Diagram of Thewes (1999)
The first clogging diagram (Fig. 4) was obtained from the
analysis of a large number of excavations in clayey soils.
Therefore, this diagram is suitable for slurry machines. A new
clogging diagram is developed in this work. It attempts to
consider the changes in the materials’ consistency in the
drilling chamber. So, it is suitable for all types of machines [4].
4.2.1. Development of the Basic form of the Diagram
In the first step of making this diagram, the method
proposed by Thewes (1999) is reviewed to find out where it
can and cannot be applied to define the clogging potential
diagrams for different types of machines. The principle of
using the plastic index can be extended to all types of machines
since this parameter is based on the plastic and liquid limits. As
these parameters are intrinsic, water content change does not
affect these limits. In contrast, changes in the consistency index
depend on the amount of free water, which in turn depends on
the type of machine system. As defined, the diagram in Fig. (4)
can be used for slurry tunneling projects.
In open-mode machines without underground water
inflow, clogging only occurs when the soil’s natural
consistency has already shown a tendency for adhesion.
Therefore, the diagram of potential for clogging (Fig. 4) can be
modified with the consideration of the clogging material
consistency (Fig. 5). In this diagram, open-mode machines will
not be exposed to clogging, whereas they are likely to be
clogged based on the diagram shown in Fig. (4). This
classification is consistent with the results of Schlick (1989),
who investigated the behavior of clay clogging during ground
movements without the presence of underground water inflow
[10].
Fig. (4). Modified clogging diagram for slurry excavation machines [4].
The Open Construction and Building Technology Journal, 2020, Volume 14 189
In the presence of groundwater inflows, the modified
diagram cannot be used for open-mode machines. In these
cases, the soil, with its natural non-critical consistency turns
into an adhesive consistency. The intensity of the groundwater
inflows considering different boundary conditions (availability
of water) will create new conditions. Therefore, in these cases,
it is not possible to develop a specific model for each boundary
condition. If the groundwater inflow or the water from the
cleaning process during tunneling is anticipated, a comparison
of diagrams 4 and 5 can be used.
A new diagram with the possibility of expressing the soil
consistency is required during the excavation to show potential
soil changes. Therefore, an accurate estimate of how the
materials are critically transferred outside the chamber can be
obtained. To assess the impact of water during the excavation,
the amount of water should be measured . While the water
content is indirectly included in the diagram of clogging
potential ( Figs. 4 and 5), changes in water content cannot be
shown. According to the plastic and liquid limits, the defined
consistency changes depend on the changes in the water
amount.
A new diagram is presented based on the same parameters,
as stated above. As previously mentioned, issues during the
excavation are related to plastic or liquid consistency.
Therefore, the resultant diagram is based on the difference
between the liquid or plastic limit and water content (x-axis:
the difference between the plastic limit and water content, yaxis: the difference between the liquid limit and water content).
Very hard clay with a high plastic property and high clogging
potential (Type A soil) is used as an example. The data pairs
are proportionally transferred in the diagram as the water
content increases (Fig. 6). Each soil specified in Fig. (6) has a
specific plastic index, so the line values considering different
water content are related to the plastic index contours. By
changing the plastic index equation, contour lines of the
defined plastic index can be shown in this diagram:
190 The Open Construction and Building Technology Journal, 2020, Volume 14
Rashiddel et al.
Fig. (5). Potential of clogging for open-mode machines without ground water inflow [4].
Fig. (6). Original shape of the new diagram with a soil specimen and its changes in water content [4].
I P WL Wn Wp Wn
(3)
W
(4)
Wn I P Wp Wn
L
The overall results can be expressed by:
y x IP
(5)
The relationships between the values are linear, with the
slope of the line being equal to unity. This line intersects the
vertical axis at the plastic index value. The diagram (Fig. 7) is
stopped by the plastic index Ip = 0, which intersects the zero
point (because the plastic index must be positive).
The Open Construction and Building Technology Journal, 2020, Volume 14 191
Clogging Potential of Earth-Pressure
Fig. (7). The original shape of the new diagram with plastic index lines [4].
Fig. (8). Original shape of the new diagram with plastic index lines (in black) and consistency index lines (in red) [4].
According to the definitions of the axes, a soil with a water
content equal to the liquid limit intersects the horizontal axes
([WL-Wn] = 0) and a soil with a water content equal to the
plastic limit intersects the vertical axis ([WP-Wn] = 0).
Rewriting the consistency index equation (equation 2), the
result is shown as follows [4]:
W
L
Wn Wp Wn * I C ( I C 1)
(6)
Consistency limits can be defined as follows [4]:
IC 0.00
WL Wn 0 liquid lim it
(7)
IC 0.50
WL Wn Wp Wn
(8)
IC 0.75
WL Wn 3* Wp Wn
(9)
IC 1.00
WP Wn 0 Plastic lim it (10)
IC 1.25
WL Wn 5* Wp Wn
(11)
By definition, it would be possible to scale the consistency
on the diagram (Fig. 8). Soils with different plastic indexes but
with the same water content will not be placed in the same
water content contour line. Whereas, a specific change in water
192 The Open Construction and Building Technology Journal, 2020, Volume 14
content provides the same absolute value for each soil at each
position on the diagram. Therefore, it is possible to define a
scale for the diagram that indicates the water content changes
(Fig. 9).
The final diagram is shown in Fig. (10). In this diagram,
Rashiddel et al.
each soil moves downward and to the left direction parallel to
the plastic index lines as the water content increases. Whereas
for coarse-grained soils, this may decrease the plastic index
(i.e., sand materials appear as part of the clogging). To evaluate
soils with higher plastic indexes, additional lines with an index
greater than 70% can also be added to the diagram (Fig. 11).
Fig. (9). Original shape of the new diagram with a scale to estimate the water content (the distancee between two blue lines illustrates 5% of
difference in terms of water content) [4].
Fig. (10). General classification diagram for the critical consistency changes with respect to clogging [4].
The Open Construction and Building Technology Journal, 2020, Volume 14 193
Clogging Potential of Earth-Pressure
Fig. (11). Extended classification diagram for critical consistency changes for the Aalbeke clay with step increments in water content [4].
Table 3. Results of the clogging pretension for the four stations of the Tehran subway line 6.
Potential for Clogging by
Adding 5% Water in Each Step
Stations
Amirkabir
Shohada Square
Imam Hossein Square
5%
Lumps
Lumps
Lumps
Lumps
10%
Medium clogging
Medium clogging
Medium clogging
Medium clogging
15%
Strong clogging
Strong clogging
Strong clogging
Strong clogging
20%
Little clogging
Little clogging
Little clogging
Little clogging
5. DISCUSSION AND INTERPRETATION OF THE
RESULTS
In this paper, the data from four stations (Amirkabir,
Shohada Square, Emam Hossein Square and Sayyadeh Shirazi)
are studied in terms of the risk of clogging in both the cutter
head and machine chamber (Table 3).
The plastic index, consistency index and also the
difference between the plastic limit/liquid limit and the water
content were used. As mentioned before, one of the optimal
methods to calculate the clogging rate of a drilling machine is
with the help of the diagram developed by Hollman and
Thewes (2013); this diagram includes four parameters: the
difference between the plastic limit and water content, the
difference between the liquid limit and water content, the
plastic index and the consistency index. It permits to obtain the
consistency index of the soil adhesion. According to this index,
the diagram is divided into various states, including very high
adhesion to the liquid state.
The diagram process first requires the use of the difference
between the plastic limit or the liquid limit and the water
Sayyadeh Shirazi
content is obtained; then, using the positioning method, the
location of the soil is determined on the diagram using these
parameters. Then, only the adhesion condition of the soil with
its natural water content according to the value of the
consistency index is determined. Subsequently, in order to
express the addition of free water during excavation, a line
parallel to the plastic index lines is drawn. It passes through the
gravity center of the points. The difference between each blue
lines on the diagram represents 5% of difference in terms of
water content. In fact, in this way, the effect of free water is
also included.
The diagram can be split from the top right to the bottom
left, which means that at the left bottom, the soil becomes more
sensitive to the amount of water and as a small amount of water
is added, it can changes from the hard adhesion zone to the
very soft adhesion zone, while the same amount of water in the
upper right of the diagram cannot affect the consistency of the
soil. In other words, the studied soils are highly sensitive to
water additions and a slight difference in the amount of water
can alter their consistency, so extreme caution must be taken
when adding water.
194 The Open Construction and Building Technology Journal, 2020, Volume 14
Rashiddel et al.
Fig. (12). Results obtained from the borehole information on the clogging determination diagram for four stations of Tehran subway line 6.
CONCLUSION
CONFLICT OF INTEREST
The main purpose of this paper was to evaluate and
determine the risk of clogging in the tunneling boring machine
in Tehran (Line 6 of the Tehran Metro) (Fig. 12). This
phenomenon induces adhesion of the shield with the soil,
increasing the necessary shear forces and it can eventually lead
to the project interruption. For this purpose, the new method
proposed by Hollman and Thewes (2013) was used. In this
study, in addition to the Atterberg limits, the amount of free
water resulting from the machine and from the underground
water inflow was included in the calculations. According to the
performed assessments, it was found that critical areas for the
clogging aspect are both the cutter head and the chamber. The
sensitivity of the soil is very important with respect to the free
water amount. If the added free water amount is 15%, the
probability of clogging becomes high. Whereas, in case where
the added free water amount reaches 20%, the risk of clogging
decreases significantly. Therefore, extreme caution should be
taken to add the free water amount.
The author declares no conflict of interest, financial or
otherwise.
CONSENT FOR PUBLICATION
Not applicable.
ACKNOWLEDGEMENTS
Declared none.
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© 2020 Rashiddel et al.
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