The document discusses a project to separate the existing Green and Red underground metro lines in Baku, Azerbaijan. This is a complex project due to constraints like limited surface access, unstable soil conditions with artesian water, and the need to complete the work within a 5 week shutdown period to minimize metro disruptions. The chosen design solution involves installing a steel pipe jacking frame to excavate two large caverns and separate the existing lines while constructing new tunnels, with soil treatment to control geotechnical risks and impacts on existing structures.

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SEPARATING UNDERGROUND
METRO LINES UNDER OPERATION
IN BAKU
SYSTRA FR: Federico VALDEMARIN, Mott MacDonald CZ: Jan CENEK,
Self-employed prof.: Elena CHIRIOTTI

3. SEPARATING UNDERGROUND
METRO LINES
UNDER OPERATION IN BAKU
SYSTRA FR: Federico VALDEMARIN, Mott MacDonald CZ: Jan CENEK,
Self-employed prof.: Elena CHIRIOTTI
SUMMARY
The “28 May Station” is the only transfer station between the
existing Green and Red metro lines in Baku. As part of the huge
development strategy of the metro network, the complete
separation of the existing underground lines has been planned
at this location, in order to simplify the operation of the future
metro system.
The extreme challenge of the project is represented by the
need of achieve the separation of such lines over a breakdown
of the operation of just five weeks. The lack of direct access
from the surface, a complex geology with artesian water
layers, a congested underground space and sensible buildings
represent additional constraints.
The chosen design solution consists in creating a complex
steel pipe jacking frame around the operating tunnels and then
excavating under such frame two large caverns where the
separation of the existing lines and the construction of the
new metro tunnels will be achieved. Soil treatments have been
conceived to mitigate the geotechnical risks and to reduce
possible impacts on the existing structures.
The paper illustrate the challenges of this extraordinary project,
one of the most complex underground structures under study,
the design solutions and the risk management approach used
to manage the complexity and to design the mitigations
measures.
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4. 4
The Project
Prior to the extension of the southern part
of the existing Red Line and Green Line, and
in order to improve the current transport
capacity, a key activity is the removal of
the “Y” operation at the 28 May Station and
the separation of the Red and Green Lines’
tunnels. The 28 May Station in fact, connects
today the arrivals on the Red Line in the
direction Icharishahar (Old Town) and the Green
Line in the direction Darnagul. Passengers
wishing to continue along the Green Line,
must first go through a complex system of
galleries to transfer at Cefer Cabbarli station
where the trains are dispatched to the Khatai
Station (Figure. 1).
The new project aims to simplify the operation
of the May 28 Station and to completely
separate the tunnels of the two lines. Figure 2
presents the existing and the future layout
of the tunnels and station’s platforms at the
28 May and Cefer Cabbarli Stations.
The Separation of the Red and Green lines at
the 28 May Station consists of the construction
of two tunnels of 190meters in total as shown
in Figure 3. They are created by partially
demolishing the existing garage and storage
terminals of the “Xatai shuttle” and making
available the connection to Nizami station
trough the Green Line.
INTRODUCTION
The metro network in the capital of
Azerbaijan is rapidly growing together with
its metropolitan area. The beginnings of
the existing metro system started in Baku
in 1967 in Soviet times. To date, only
two lines (Green and Red) are in operation
with a total length of 34km, including
23 stations. The future network will be
much wider: besides the construction
of three completely new lines and the
extension of two existing lines, the whole
metro project includes the development
and modernization of the existing
metro sections.
A consortium formed by SYSTRA (leader),
Mott MacDonald (Praha) and Saman (Korea)
awarded the contract for the preliminary
and detailed design of infrastructures for
the extension of Baku Metro Network.
This extension consists in 83km of new
lines and extension of existing lines.
As part of this modernization project,
the connection of the Green Line at the
28 May/Cefer Cabbarli Stations, focus of
this paper, represent also it’s “Achilles’
heel” given that the already very congested
system have only one transfer station.
Figure 1. Existing Baku metro network and location of the 28 May Station

5. 5
of clays, sands, and some banks of limestone
and sandstone.
Based on the geological data made available
from previous project in the area at the
feasibility stage, the entire geological profile
was considered a huge layer of relatively
stiff clay.
A first investigation campaign composed by
9 core recovery borehole and 8 SPT vertical
logs was launched in 2012 in order to confirm
these assumptions. The very first boreholes of
the campaign detected loose soils and artesian
water under the stiff clays starting 25-30m BGS.
After performing laboratory analysis (more than
200 identification tests, 30 shear tests and
16 compressibility tests) the local contractor
was not able to satisfactorily characterize
the lower layer whose characteristics appeared
contradictory and inconsistent with the
description of the borehole logs.
The designer requested additional geological
surveys, which were supposed to satisfactorily
identify and characterize the lower strata.
For this purpose, 13 cone penetration tests
THE INPUT DATA
AND CONSTRAINTS
To get accurate and reliable data for planning
infrastructure and underground works in post-
Soviet countries is quite complex because
for secrecy, confidentiality, topographical
information was deliberately distorted or shifted
by tens of meters, mostly unavailable as built
documents for existing structures. In addition,
the local suppliers for exploratory work do
not follow international standard methods and
their output can be unclear.
The Geological Investigations
and their Uncertainties
Baku urban area lays in the middle of a large
syncline responsible for the amphitheatre
morphology of the city and the differences
between the formations. The folded
geological formations present in Baku subsoil
are sediments from the Quaternary, and
Neogene. These sediments are mainly made
Figure 2. Existing (left) and future (right) metro network at 28 May station.
Figure 3. 3D model – 28 May station after separation

6. 6
Based on the above characterization,
the distribution of the geotechnical units
has been studied and a geotechnical 3D
model of the studied zone was produced.
The study has been developed using the
GDM software of BRGM that allows to build
an interpretative 3D model of the geology,
the existing and the future structures,
and to represent such interpretation along
oriented sections.
The Geotechnical
Characterization
It must be mentioned that geotechnical
campaigns (both in situ and laboratory tests),
performed in Baku by local companies,
did not comply with international standards
in terms of methodology, equipment
implementation and calibration, recovery,
testing, and interpretation. Specifically,
the sampling method (simple core barrel)
led to significant sample disturbance and
affected the reliability of mechanical test.
For such reasons many parameters were
derived more from CPTUs results or from
designer’s experience on Baku’s soils than
from Lab tests results. Also taking in to
account the intrinsic variability of the Units,
a set of worst credible values for each Unit
has been defined further to the reference
values (see Table 1), in order to develop
the design by scenarios.
CPTu (the firsts ever conducted in the country)
and 2 pumping tests were recommended and
carried out. Based on the results, the bottom
layer was characterized and classified as a
sandy silt.
During the implementation of the additional
geological survey, the presence of artesian
groundwater was confirmed, which increased
the concerns about the underground works
being executed in the layer of sandy silt.
The pump tests also confirmed a continuous
water horizon under a confined pressure of
about 3 bars.
The Geological Model
Based on the results of the CPTu it was
possible to clearly identify and distinguish
the following Geotechnical Units:
• Unit 1 (0 to 4m depth): man-made ground
unit
• Unit 2 (4 to 26-30m depth): stiff Silty Clay
unit. The unit is composed mainly by silty
clay, with subunits of sand and sandy
silt. These sand lenses are almost always
recognized trough the CPTUs. The connection
between such lenses, although not surely
established based on the actual data, was
considered in the implementation of risk
analysis.
• Unit 3 (26-30 to 60m depth): Silty Sand unit,
consisting of silty fine sands laminated with
thin to very thin interbedded silty clay and
sandy silts.
Table 1. Geotechnical characterization for the geotechnical Units

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• The existing running metro tunnels: circular
shaped, lined by cast iron segments of
5.1m internal diameter with pressure gate
chambers with a cast iron segmental lining
of 6.2m diameter;
• an underground shopping passage consisting
of a reinforced concrete box which extend
foundations to about 5m below the surface;
• utilities such as a water pipeline (D 500mm),
a medium pressure gas pipeline, 0,4 kV
electric cables and multiple unspecified
manholes, which need to be diverted before
any works may take place;
• the Azerbaijan State Oil Academy: a five
storey masonry structure, with 4,5m deep
strip foundations considered a sensitive
structure due to its public use.
The Existing Structures
One of the biggest constraints of such project
is represented by the surrounding existing
structures. The presence and the sensitivity
of such structures limits the jobsite access
and space at the surface, potentially interferes
with the designed temporary and permanent
structures, and requires particular attention
to the effects of induced settlement. Figure
4 present a 3D view of the main existing
structures at the location of the future works.
The main existing structures considered as
design constants and for risk analysis are:
• the 28 May Station: a deep mined station
consisting of a central tunnel, two side
tunnels and transversal galleries for the
connection with the Cefer Cabbarly station;
• the Cefer Cabbarli Station for the shuttle line
to Xataï: a deep mined station consisting
of two twin tunnels, two dead-end galleries
and a network of adits for ventilation
and drainage connected at the end;
Figure 4. Existing surrounding structures at project location

8. 8
collisions with the network of existing tunnels,
and the possible effects of the artesian
waters. This was done in close collaboration
with the contractor selected by the Client.
The completed design was submitted
for comments to the Azerbaijani ‘state expert
analysis’ and the Independent Checker selected
by the Client.
The Basic Design
The challenge consisting of excavating two
relatively short tunnels (about 100m long)
which encountered a section of an existing
tunnel made of cast iron segments and
performing this work as quickly as possible to
minimize interruptions to the metro operation.
The work also has to be done in proximity
to a network of existing tunnels without easy
access from the surface.
The whole design was developed in considering
the following key-issues, with taking into
consideration the project’s risks and constraints:
1. To develop a design based on a systematic
risk analysis process shared with the Employer
and his representative: the design choices were
driven by risk assessment and this process has
been traceable and explicit.
2. To develop the design by scenarios:
• a reference scenario based on a
deterministic set of parameters (most likely
soil and groundwater characteristics and
behaviour, volume loss, deconfinement
ratio, works performance, geometry, etc.);
• a scenario based on “worst credible
parameters” quantitatively considering the
impact of a certain number of geotechnical
DESIGN PROCESS
AND SOLUTIONS
As stated at the previous chapter, the seemingly
simple task that involved the construction
of two 100m long track tunnels is further
complicated by the results of the investigations
and by the time constraints imposed by
the Client. Those constrains mainly consist of:
• Topography – there is no direct surface
access; access located immediately below
university building precluded the use
of a large excavated open pit, and in
proximity to a complex system of existing
underground structures;
• Geology - the risk of encountering saturated
sand lenses, soft clays, artesian aquifers;
• Time - the client requested the work be
completed in an extremely short period
(during shutdown of the Green Line –
5 weeks).
During the Feasibility Phase, three different
options were analysed, including the direct
re-excavation of the existing tunnels (Fig. 5a),
the excavation of a cavern preceded by simple
side adits (Fig. 5b) or by adits combined with
diaphragm walls (Fig. 5c).
The client selected the third variant, despite
to the longer period of construction and higher
costs, because it resulted in the shortest
shutdown period.
The selected option was subsequently
developed to the preliminary design level
and then further modified to account for the
topographical and geological surveys that
allowed to better estimate the possible
Figure 5. The three options from the Feasibility Study

9. 9
The access shafts are excavated to the level
of an intermediate slab. From there, the upper
level adits are excavated. At the same time,
the shafts are completed to the bottom level.
If necessary, ground treatment (e.g. jet grouting,
ground freezing) from the upper level adits
is performed to support the excavation phase
of the lower adits.
Between the lateral adits, vertical and horizontal
pipe jacks are installed. These pipes are then
filled with concrete and reinforced at their ends
(corner connections), within the adits, to ensure
a stiff frame around the caverns excavated at
a later time.
Longitudinal adits will be kept close to the
future and existing Green Line tunnel to form
a square cavern shape which has 3 adits in
3 corners of the rectangle. The distribution of
the longitudinal adits and pipe jacking frame
can be seen in Figure 7.
The access to the cavern excavation is opened
from the shaft with an approximately 6.0x6.5m
window to facilitate the excavation progress.
From there, the first level of the cavern
(5.5m under pipe jacking roof) is excavated as
well as the head walls. The drainage system
is also installed.
After this phase, the operation of the green
line needs to be interrupted. Then, the caverns’
excavation is completed and the existing
running tunnel demolished. The invert and
construction elements for the new running
tunnel are installed in sequence. Once
the connecting structures are built, the caverns
are filled and traffic operation is resumed.
uncertainties and their variability to test
the robustness of the proposed technical
solution, and to incorporate the necessary
corrective measures in the design.
3. To define mitigation and contingency
measures to control the effects induced by
the installation, excavation, and completion
of the underground work, for both the
reference (mitigation measures) and the worst
(contingency measures) scenarios.
4. To define a set of driving parameters with
their respective operational ranges (when
applicable), and attention and alert thresholds
to be used during construction to monitor
the work and assist in decision making.
5. To predefine the sequence of actions when
attention or alert thresholds are reached.
6. Given the multiple excavations and mutual
interferences, the correct design method is
a 3D approach. In particular, numerical, coupled
analyses have been considered (combining
mechanical and hydraulic challenges).
7. To select in a traceable way the most
adequate construction methods and ground
treatment, taking into consideration the soil
and groundwater conditions and required space
for jobsite logistics.
8. To cope with time and environmental
constraints.
The Basic Design’s Solution
The separation of the red and green lines is
to be performed mainly the under protection
of two underground caverns, both constructed
below the Oil Academy Building. The depth
of the new tunnel alignment is approximately
18 – 22m below the surface. The overburden
of the upper lateral adits is around 12 – 15m.
The caverns themselves are planned to have
a maximum width of approximately 17m and a
height of approx. 10m.
The ground bearing structure of the caverns
are to be built before the excavation of the
caverns. The 3D model of Figure 6 gives an
overview of the project at the 28 May station.
The magenta show the new separation tunnels,
while the objects in green, yellow and pink
represent all the preliminary works needed to
install the new separation tunnels in the cavern.
Figure 6. 3D model of the basic design’s solution

10. 10
of the excavation and the application of
soil support.
The geotechnical hazards above mentioned
have been thoroughly considered in the
Risk Register Matrix. The process of minimizing
of such risks has been an integrated part
of the design phase and will be completed
and implemented in the future stages of this
project, together with:
• Non deterministic approach in design
(most likely and worst credible conditions)
• Identification of mitigation measures,
where and when needed
• Definition of predefined countermeasures
(to be fully shared with the Contractor)
• Observational method during construction
(role of monitoring)
A preliminary Risk Register has been drafted
in accordance with ISO: 3100 and other
relevant standards [1-4] by the designer
in the basic design phase of the 28 May
Station Upgrade project. This has allowed to
properly communicate and share the risks
among the project’s actors, establish shared
geotechnical baselines to access the impact
of geotechnical uncertainties, and to define
the provisions for risk.
The risk related to the uncertainty of certain
geotechnical parameters and to the response
of the soil to excavation and soil treatment,
has pushed the designer and the Owner
to propose that the access shafts be used
as the location for the field trial tests for
the soil treatment work and to confirm/
update as much as possible the geotechnical
characterization by means of additional vertical
and inclined boreholes and in situ tests.
Consequently, the contractor has the
responsibility of updating the design based
on the results of such tests and on the surveys
carried out through excavation of the shafts.
This has led the project’s actor to select
a design -build form of contract in which
the steps of the design update are formalized,
together with the procedure in which such
results are used to update the project.
This solution allows reducing the traffic
interruption on the green line for a period
not to exceed 5 weeks.
RISK MANAGEMENT
The main geotechnical hazards identified for
the construction phases are:
• The presence of the artesian groundwater
during excavation of the lower adits and the
connection between the pipejackings tubes
may result in the instability of the excavation
and unsafe conditions for the workers.
• Although the low permeability values
registered during the tests, the presence
of artesian groundwater in the Silty Sand Unit
may result in water inflows and in stability
problems at the base of the caverns.
• The presence of sand lenses interbedded
in the Silty Clay Unit may contain water
or, if thick enough and continuous, may
be connected to the artesian aquifer,
results in possible water inflows and
instability during the excavations phases.
• The uncertainties due to poor recovery of
the samples and questionable reliability
of lab results oblige to carry out the design
through parametric studies, analyzing
the influence of key input parameters in
the engineering solutions.
The identified hazards have also to been
considered in the context of a densely
urbanized environment and heavily utilized
underground space, which strongly limit
the investigations distribution and that present
a significant constraints for the geometry
Figure 7. The section type for basic design’s solution

11. 11
• two huge caverns supported by rigid closed
water resistant pipe jacked frame completely
jacked from small underground access
galleries,
• the massive ground treatment measures
(in alternatives, also installed from the
underground) and taking in to account
existing and operating tunnels and buildings
in the close proximity,
• replacement of old by new metro tunnels
including rail and MEP in extremely short
time.
Where the contractor’s experience has been
involved as early as possible in the study of
the construction method. This was combined
to a systematic risk management approach and
to the definition of the most adequate form
of contract considering the project constraints
and characteristics.
The project which consists in separating the
running tunnels of the Red and Green Metro
Lines in Baku is particularly challenging for
many reasons: the Client’s requirement of
interrupting the metro traffic for no longer
than 5 weeks; the complexity of the existing
underground structures; the sensitivity of the
buildings above the future work; the limited
access space on the surface and the limitations
in placing and dimensioning access shafts to
support the excavation of the underground
work; the existing condition of the subsoil and
the pressurized aquifer; the uncertainties related
to the ground characterization.
This has led to develop a very challenging
design involving innovative technical solutions
requiring high accuracy and latest technologies
for the first time used not only in Azerbaijan
such as:
REFERENCES
[1] The Code of Practice for Risk Management of Tunnel Works (2006). International Tunnelling
Insurance Group (ITIG), presented at the ITA World Congress, Seoul, April 2006.
[2] Guidelines for Tunnelling Risk Management: International Tunnelling Association, Working
Group n°2 (2004). Tunnelling and Underground Space Technology, N.19, 2004, pp. 217-237.
[3] The Joint Code of Practice for Risk Management of Tunnel Works in the UK (2003).
Published by the British Tunnelling Society (BTS), prepared jointly by BTS and the
Association of British Insurers.
[4] A Guide to the Systematic Management of Risk from Construction (1996), CIRIA,
Special Publication 125.
CONCLUSIONS

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