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Construction of a 170m Long Cripple Sided Tunnel Using Variable Geometry
Hydraulic Formwork in DTL 3, C927
Conference Paper · September 2018
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Massimo Marotta
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Singapore Land Transport Authority
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Underground Singapore 2018
Construction of a 170m Long Cripple Sided Tunnel Using
Variable Geometry Hydraulic Formwork in DTL 3, C927
M. Mohiadeen, M. Marotta
Land Transport Authority, Singapore
ABSTRACT: This paper describes the construction of a 170 m long cripple sided mined tunnel located east of Bedok Town Park Station, part of Downtown Line 3 Contract C927. The length of the tunnel is 170m, the longest for a rail tunnel in soil constructed by Sprayed Concrete Lining (SCL) in Singapore. The tunnel has variable profile section in order to allow the installation of ventilation jet-fans.
The project also included the construction of five contiguous ventilation passages connecting the
cripple sided tunnel to the bored tunnel. Each of the five ventilation tunnels has a profile section similar to the one normally used for the escape Cross Passages, however the construction of five contiguous opening has required special design and construction consideration. The relatively long length of
the tunnel has allowed the Contractor to use special mining equipment which are rarely seen in Singapore. In fact only for tunnels of certain length it is feasible (in term of space and constructability) and
economically worth (in term of investment) to use those special equipment. In particular, the paper
describes the permanent lining done with a hydraulic formwork designed for single stage wall/arch
casting, and for the variable tunnel profile.
1 INTRODUCTION
The cripple sided tunnel is between the two bored tunnel driven from the launching shaft located
within Bedok Town Park Station (BDT), built by civil Contract C928 and handed over to contract
C927 for the duration of tunneling works. The total length of the tunnel is 170 m. The SCL tunnel is
connected to the Expo Bound tunnel by 5 ventilation passages and to the Bukit Panjang Bound tunnel
by 1 cross passage, as shown in Figure 1.
Cripple Siding
Ventilation
Passages
Bored tunnels
BDT
Station
Figure 1. Cripple sided tunnel plan view
Cross
Passage
2 GENERAL SITE MANAGEMENT AND SITE ORGANIZATION
Table 1. Site Organization
Organization
Client
Contractor
QP(S)
QP(D) Temporary Works (Cripple Siding)
QPD – CP19, CP20 and CP21
QPD – CP22 and VP A-E
SCL Design Representative
Instrumentation Contractor
Waterproofing Contractor
Name
Land Transport Authority
CMC Ravenna – Singapore Branch
CPG Consultants Pte Ltd
Geoconsult Asia Pte Ltd
Meinhardt Infrastructure Pte Ltd
T.Y.Lin International Pte Ltd
Mr. Lee Young Zoo
Tritech Engineering and Testing Pte Ltd
Guan Tech Construction 2000 Pte Ltd
3 EXCAVATION OF THE CRIPPLE SIDING TUNNEL
3.1 Geological Condition
SCL cripple sided tunnel located in between the two bored tunnels and the geological conditions
found were predominantly in competent Old Alluvium soil of grade OA(A) with SPT-N value approximately over 100 as shown in the Figure 2. The tunnel overburden ranges from 20.5 m to 25.5 m. The
competent old alluvium geological profile provides a stable ground condition for the SCL method of
construction.
Tunnel
Cross Passage 1
LEGEND
FILL
KALLANG FORMATION
O(A)
TUNNEL
CROSS PASSAGE
TBM SHAFT
Figure 2. SCL mined tunnel geological profile
3.2 Excavation and temporary works installation
The construction was carried out with the excavation of the top heading for the entire length of the
tunnel, followed by the excavation of bench and invert. Being the tunnel of a suitable length, the
bench & invert excavation was carried out from the end of the completed heading towards the tunnel
portal, to allow for concurrent activities and in particular the start of the waterproofing for the permanent lining. The Land Transport Authority regularly engages a panel of local and international experts, named the International Panel of Advisors (IPA). During a pre-construction presentation, IPA
advised, in consideration of the considerable height of the tunnel of over 10 m at the jet-fan section, to
keep a constant height for the top heading stage, of approximately 5.8 m. This mitigation reduced
risks by minimizing the the exposed soil during the more critical top heading jet fan stage, and in addition address constructability issues by having a constant section for the construction equipment to
work within. The balance bench/invert excavation, ranging from 2 m to 3 m, was excavated in one
stage for the circular section and in 2 stages for the transition and oval Sections, as shown in Figure 3.
Figure 3: SCL tunnel variable geometry profile and longitudinal profile of top heading and bench/invert.
Excavation for the cripple sided tunnel was carried out by means of hydraulic excavator. In view of
the very good geological conditions, there was no pre-support (fore poling) and soil improvement
(soil nail) designed for the entire length of the SCL cripple sided tunnel. The typical round length of
the heading excavation started with 1m and it was increased to 1.4m due to excellent ground conditions. Benching excavation typical cycle varies from 2m to 3m on various sections of tunnel. Figure 4
and 5 shows the top heading and benching excavation on site.
Figure 4. Top Heading Excavation
Figure 5. Two stages Bench/Invert Excavation
3.3 Instrumentation and monitoring of SCL tunnel
A comprehensive instrumentation and monitoring plan was implemented during the construction of
SCL cripple sided mined tunnel. Different types of deformation monitoring were done to monitor the
SCL tunnel behavior. Other than surface and subsurface instrumentation, convergence monitoring was
done inside the tunnel (for shotcrete shell deformation). Total of sixteen arrays of convergence were
installed to monitor the tunnel behavior. The same prisms were also used to monitor the settlement
measurement inside the tunnel. For the entire tunneling process the instruments readings were stable
and the maximum convergence recorded within 5mm (AL - 15mm / WSL - 25mm). Figure 6 and 7
shows the monitoring plan, installed convergence prisms and convergence trend graph recorded in
SCL mined tunnel.
Figure 6: Convergence monitoring plan and prisms installed inside tunnel.
Figure 7: Convergence monitoring trend graph
4 PERMANENT LINING
4.1 Waterproofing
The general sequence of permanent lining included the installation of PVC waterproofing membrane.
Figure 8 shows typical scheme of the waterproofing system design for SCL cripple siding tunnel. The
waterproofing system includes a first protection layer by geotextile, which was fixed by HILTI nails
to the shotcrtete. The same nail also anchor a PVC roundel, which was later used to hot weld the
membrane. Thus, no perforation or puncture to the water proofing membrane.
Figure 6: Water Proofing scheme for SCL mined tunnel
The waterproofing system was installed using FLAGON PVC membrane, and PVC water-stop in order to create compartments. Those compartments, with provision of grout ports, would eventually
confine the water propagation in case of leakages and facilitate the tracking of water leakage source
and the remedial grouting. The water stop, embedded in the concrete at every construction joints,
would mitigate and eventually stop water migration by imposing a tortuous path to any water. The
completed waterproofing system with compartmentalization is shown below in Figure 9.
Figure 9. Compartmentalized Waterproofing System
4.1.1 Welding of the Sheets
Hot air manual welding was carried out using a hot air gun for sheet-sheet & sheet-water stop welding. However automatic welding mechanism using hot wedge welders were performed for longer
stretch for sheet-sheet welding. The welding temperature varies according to type of material
(FLAGON PVC), to the thickness and type of nozzle used, ranging between 350ºC and 600ºC.
After the final inspection of the installation, the water stops of the compartmenting system were welded by hot air gun to the existing waterproofing membrane at every CJs, the construction joints, both in
the horizontal and the vertical plane. Figure 10 to Figure 12 shows the welding equipment and processes during installation.
Figure 10: Hot Air Manual Welding Equipment for the Water Proofing System
Figure 11: Automatic Welding Equipment for Water Proofing Sheets
Figure 12: Installation of PVC Roundels
4.2 Reinforcements Installation
As the tunnel was provided with a PVC waterproofing membrane, the installation of the rebar need to
be done without puncturing the membrane. For this reason Lattice girders where used to supports the
reinforcement, refer to the Figure 14 and were designed and checked for structural capacity and for
maximum deflection. This method have been used in some of the previous mined tunneling projects
under LTA (Marotta, 2013)
Controlling the deflection of the lattice girder after reinforcement installation is important to ensure it
does not exceed the limit which might affect the final concrete cover. Reinforcement consist of an
outer layer of T20@200 mm and an inner layer of T12@200 mm, resulting in a load of approximately
0.20 kN/m2 (Han, 2014). Using a 230 mm x 230 mm lattice girders, spaced at 2.5 m center to center,
the design indicates a maximum deflection (at tunnel crown) of 6 mm for the tunnel Circular Section
and of 20 mm for the Jet-Fan Oval Section as shown in the Figure 13. (Han, 2014)
Figure 13. Lattice girder deflection for normal section (left) and Jet-Fan section (right)
Figure 14. Rebar installation with guidance from lattice girders
4.3 Permanent lining – formwork and concreting works
A main challenge for the Permanent Lining was the variable geometry of the tunnel section to facilitate the installation of ventilation equipment. The tunnel geometry consists of a standard section and a
higher section or ventilation section which were linked by a transition zone, where the roof of the tunnel was at a slope. Permanent lining casting carried out in two stages (i) Invert casting and (ii) Wall
and crown casting – using CIFA hydraulic formwork.
Invert zone casting were carried out in few stages as the length of the tunnel was 170m and it was
done concurrently with the tunnel excavation operation in the tunnel invert zone. Sidewalls and crown
concrete lining were carried out using a steel system form, 6m length CIFA formwork in single stage
which reduced the number of constructional joints (CJ) which could be potential water path of the
concrete lining. System form movement and opening/closing functions were hydraulically controlled.
Figure 15 and 16 illustrate the variable geometry and two stage casting of the SCL tunnel.
Figure 15. SCL tunnel longitudinal profile with variable cross section
Figure 16. Two stage casting using system form in SCL tunnel to minimize the constructional joints
4.3.1 Design of system formwork
The travelling formwork was designed for ease of modification in order to cast the different tunnel
sections including the challenging transition zone where the tunnel roof was inclined, Figure 17 below
shows the 3D model of the system form. The formwork was designed for a total of 10 configurations
(8 configurations for the 2 transitions, 1 configuration for the standard section and 1 configuration for
the ventilation section).
Figure 17. CIFA Hydraulic System Form used in 3D view
The system formwork was designed using the FEM program ANSYS. There were twenty four different load cases considered for the most stressed formwork configuration, the worst load case was considered for the design. The design difference in concrete height between the two sides was 500 mm
and it was very important to maintain this maximum difference in level while casting wall & crown
section. Figure 18 below reported the scheme of formwork and the applied loads in the worst state of
stress. C.M.C (2013)
Figure 18. Scheme of formwork and applied load in the transition zone
In compliance with the formwork design specifications, the erection of the tunnel lining were carried
out in layers with a maximum level difference between the two sides of the formwork of 500mm. The
maximum pour rate was not exceeded 1m/h or 40 cubic meter/h. The raise of the concrete level within
the forms was controlled from the inspection portholes which was provided for this purpose. The said
portholes were also used for concrete pouring purposes when required.
4.3.2 System formwork setting and casting
CIFA formwork was assembled at the surface and lowered down to the shaft bottom as shown in the
Figure 19.
Figure 19. Lowering down and setting of CIFA system formwork
Casting started with circular section. Positioning of formwork at block B1 and adjustment of formwork alignment in both the tunnel longitudinal and transverse directions were carried out after the rebar installed. Formwork anchors were tightened and stop-end shutter formwork were installed with
windows to monitor the casting process. Concrete pouring done in one single stage for both side walls
and crown. There were total of 20 circular sections casting followed similar procedure as shown in
Figure 20.
Figure 20. Casting of circular section
Casting of two inclined transition sections were followed after the completion of circular section. The
formwork vault was lifted up of an angle matching the inclination of the tunnel lining along the transition section. Additional intermediate shield pier elements and panels were added to match the slope
and the formwork anchors were tightened. Stop end formwork were installed and casting carried out.
There were 3 transition sections, on each side, casting followed similar procedure as shown in Figure
21 and 22.
Figure 21. Casting of inclined transition section
Figure 22. Transformation of CIFA system form & casting of oval section
Casting of the enlarged oval section was the last to be erected since the formwork, in such a configuration, could only be operated inside that section of the tunnel alignment. For this purpose the formwork was re-configured similar to that of the transition sections with the only difference constituted
by the removal of the intermediate shield pier elements with the sloped profile, replaced by standard
elements with rectangular cross-section. There were 3 enlarged oval sections casting followed similar
procedure as shown in Figure 23.
Figure 23. Casting of Enlarged Oval Section
At an average of 3 casting per week (each casting 6m) was achieved for the entire permanent lining
works using CIFA formwork. Figure 24 to 26 below shows the striking of formwork, which was done
when the concrete achieved 2MPa strength in compression, concrete port & external vibrator used in
the system form and the final concrete lining of the SCL cripple siding tunnel.
Figure 24. Striking of system form after casting (right), retracted by hydraulic jacks
Figure 25. Concrete port and hydraulic external vibrator in CIFA system form
Figure 26: Completed permanent lining for circular and oval section with Jet Fan Slab for SCL tunnel
5 CROSS PASSAGE AND VENTILATION PASSAGE
There are total of four cross passages and a contiguous ventilation passage (5nos) VP- “A to E” along
the bored tunnels. Cross passage CP22 connects the Bukit Panjang and SCL mined tunnel & 5 ventilation passages connects the expo tunnel with the SCL mined tunnel, as shown in Figure 27.
Figure 27. Location plan for cross passages and ventilation passages connecting SCL tunnel
Figure 28. Ventilation passage temporary & permanent work sequence
5.1 Excavation & Permanent Lining Sequence of Ventilation Passages
The construction of the ventilation passages were carried out by mechanical excavation SCL method.
Construction operations followed a procedure and work sequence established for the construction activities for the temporary & permanent works for the ventilation passages as shown in the Figure 28
and Figure 29.
Figure 29. Ventilation passages connecting bored tunnel and SCL mined tunnel
Excavation and permanent lining completed in the centre ventilation passage B at first. Ventilation
passage A and C at both ends were completed with excavation and permanent lining followed by ventilation passages D and E. Temporary bracings were installed as a support for the bored tunnel segments before the commencement of excavation works. To expedite the excavation process, the contractor used a mini robotic excavator to fit in the opening of the ventilation passage. Robotic
excavation helped significantly in the progress rate and played an enhanced safety environment.
Figure 30 and 31 shows the temporary bracing, excavation using mini robotic excavator and the completed structure of the ventilation passages connecting bored tunnel.
Figure 30. Mini robotic excavator used in ventilation passage construction
Figure 31. Temporary bracing installed for the 5 ventilation passage openings.
Figure 32. Completed permanent linings for all five ventilation passages
6 CONCLUSION
The excavation and supports installation for the 170 m long cripple siding tunnel and ventilation passages have been successfully completed on schedule, with high productivity being achieved by the usage of specific equipment and technology as remote controlled excavator, robotic shotcrete machine,
and special mining equipment.
For the permanent lining construction, particular care was taken to ensure no puncturing of waterproofing membrane due to reinforcement installation. This was done by suing girders as a support system for the reinforcement, to be installed prior the formwork installation. The use of the hydraulic
formwork, specifically designed and built for this contract, allowed the casting to be completed in only two stages, the invert and the full bench/heading of up to 11 m, thus minimizing the number of construction joints which are potential path for water ingress. Additionally, the specially designed formwork, designed to be adjusted for variable geometry, allowed high quality and high productivity in
casting of the challenging transition zone.
The authors would like to thank Mr. Giovanni Matta of CMC Singapore Branch, for the information
provided in writing this publication. The authors would also like to thank all the other parties involved
in the successful completion of this work.
REFERENCES
C.M.C (2013). Report calculation for cripple tunnel formwork.
Han (2014). Design Calculation Report for Lattice Girder Support of Steel Reinforcement bar.
Marotta (2013). Singapore Land Transport Authority: 15 years of experience of innovative use of Sprayed Concrete Lining. Singapore International Transport Conference and Exhibition SITCE 2013. Singapore: Land
Transport Authority.
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