Enabling Works (Temporary Structures) in Metro Rail Projects

Introduction

Construction of metro rail projects is in full speed across India’s urban areas. Delhi, Mumbai, Kolkata, Hyderabad, Ahmedabad, Surat, Kanpur, Nagpur, Lucknow, Pune, Patna cities are leading the construction of Metro rail connectivity. As compared to the railways, the Metro rail is faster and more flexible, and when compared to road traffic, it is hinderance free. Due to the advantages of metro rails in increasing urbanization, India has made significant investment in this sector during the past ten years.

The design and construction of metro works is highly challenging in city areas where there is a lot of congestion, underground utilities, limited working hours, safety requirements etc. Metro projects are broadly classified as (a) elevated works (b) underground works and (c) ramp portions which connect elevated stretch of metro to an underground stretch. Each of these has its own set of requirements and challenges and involve three main disciplines: Design, Construction, Operations and Maintenance. The design discipline has further sub-disciplines such as (1.1) Architecture, (1.2) Structural design (1.2.a – design of permanent works, 1.2.b – design of temporary structures, 1.2.c – geotechnical), (1.3) Mechanical – Electrical – piping / plumbing (MEP), HVAC, signaling, drainage, lighting, (1.4) building information models (BIM), (1.5) Fire and safety, (1.6) CFD analysis, acoustic study and other higher researches (as per the requirements in individual projects).

Each of these disciplines have their own set of requirements / expertise and complexities. In this paper, the authors have elaborated mainly on Temporary Structures (also called enabling works). The paper is divided into two parts: temporary works in underground projects (including the ramp portions) and temporary works in elevated metro projects.

Underground metro projects involve mainly three types of structures: (a) bored tunnel tubes (b) station buildings (c) ancillary buildings and access underpass. The temporary works required for these are listed below and described briefly.

A. Temporary works involved in construction of diaphragm walls & stations:

1. Guide walls & Stop Ends.
2. Rebar Cage Lifters.
3. Shoring works (in bottom-up type construction) – steel Walers, supporting brackets, insert plates and struts (active / passive), temporary king posts / stanchions.
4. Temporary steel decking to facilitate surface vehicles movements.
5. Box pushing technique for subways (if applicable).
6. Plunge columns and barrettes (in top-down method) – this forms an integral part of the final structure.
7. Temporary bridges for crossing utility lines / mast supports etc.

B. Temporary works involved for TBM:

1. Temporary thrust frames for Tunnel Boring Machine (TBM).
2. Steel cradle to support TBM shield.
3. Temporary rails and walkway inside tunnel lining to support the muck carrying wagons.
4. Eye seal & temporary slurry piles at head wall.
5. Intervention Shaft for the TBM.
6. Muck bin, muck removing carts / chariot, muck bucket tilting device.
7. Temporary Ring support during cutting of Segment for Cross – Passage construction.
8. NATM / lining in cross passages between two tunnel tube lines (upward and downward lines).
9. Gantry arrangement for lowering of TBM, handling of precast Segment, removal of muck.
10. Crane foundation for lowering of TBM in Launching Shaft.

C. Temporary works involved for Tunnel Lining rings / pre-casting:

1. Casting yard & stacking yard sheds with EOT cranes.
2. Segment casting molds with steam curing facility.
3. Segment tilting device.
4. EOT Goliath cranes.
5. Steel sheds for office / workshop / testing labs.
6. Concrete Batching and crushing plants.

Each of the above items are briefly explained below:

Guide walls: (Fig.1) As the name suggests, guide walls are temporary concrete blocks / walls / structures constructed on either side of the D’wall, longitudinally, to predefine the footprints of the proposed diaphragm walls with some construction tolerance (Thickness of D-wall +50mm say 50mm) to facilitate the digging rig to enter the soil inside. Rarely do contractors construct guide walls using steel plate units; such walls are reusable but their sturdiness has to be ensured with proper design and detailing. The distance between two opposite face guide-wall units is maintained (equal to thickness of proposed wall + tolerance, say 50mm or so) using temporary wooden props or tailormade steel frames. Thickness, depth and reinforcement in the guide-wall, depend on the type of soil strata (loose / medium / dense / stiff etc.) and the depth of proposed wall and weight of rigging equipment. Before commencing the excavation in soil, surveyor shall ensure correctness on the coordinates of guide walls. After installation of D’wall is over, the guide wall can be broken / dismantled. For design of guide-walls the long-term and serviceability checks (like crack-width, shrinkage, creep etc.) are not applicable due to short life and temporary nature.


Stop Ends: (Fig.2) Longitudinal ends of two adjoining D’walls panels are connected using a grove shaped geometry, which also supports the water bar; this grove / or even a projection is facilitated using steel beam placed on the extreme edges of primary panels are called as stop ends. In case of diaphragm walls with greater depths, the stop ends are even provided with spice connection to optimize the lifting weight. The stripping time for stop ends matter a lot, which ranges from 8hrs to 10hrs; otherwise, the steel plates of stop ends may get firmly stuck to the hardened concrete. Bottom ends of each stop end unit may be tapered / chamfered. The width of stop end is kept 5mm lesser than the width of wall to facilitate easy removal (i.e. say if the thickness / width of wall is 600mm then the width of the stop end is kept 595mm or so). Steel stop ends are reusable; however now a days sacrificial precast concrete stop ends are also used in a few projects.


(L) freshly fabricated stop end units, (M) workers placing water stopper and applying lubricant / de-bonding agent to exposed surfaces, (R) chamfered bottom edge of stop end (Left & Middle) Precast concrete sacrificial stop end (Right) Typical spliced connection in stop ends.

Rebar Cage Lifters: The steel rebar cages are fabricated in the yard by the seasoned iron workers under guidance of experienced foremen / supervisor / shift engineer. For long length cage the bars are connected using laps / couplers. The final assembled cage is inspected by the clients (GEC) and after certification lifted using cranes as shown in the photo on left hand. To avoid the localized bending buckling of individual rebars a steel lifter / spreader beam / strong-back, is given at the top of the cage to which the wire ropes are connected at bottom and at top to the crane hook. Capacity of each lifter beam shall be properly designed and detailed; also to be marked with paint on respective lifter beam/s.

Shoring works (in bottom-up type construction), steel walers, supporting brackets, insert plates and struts (active / passive), temporary king posts / stanchions (Fig. 3): Steel shoring is required to keep the D’wall / soldier pile wall in proper position against the lateral soil load at the back of the wall. Accidental loads and additional stresses due to temperature changes shall be carefully considered while designing and detailing the struts. For struts having longer length intermediate steel column support may be given using kingpost to avoid sagging under self-weight and to maintain the effectiveness of this struts. In some cases, the struts are applied with a pre-compression using jacks or wedges – these are called active struts (else if preload is not applied then called as passive struts). Struts may be formed from - rolled joists / built-up truss / or simply steel liner pipes.


In certain open excavations to facilitate equipment movements the open struts may cause hindrance and hence are not permitted; in such case prestressed soil anchors are used at certain spacing to hold the waler beams in position.

In certain cases, the opposite side walls is located far away (which would lead too long length of a strut); in such cases inclined struts inside excavated pit are given (shown ahead in the photo). At walls bending 90degree inward, the diagonal struts are preferable. To increase the effectiveness of struts and to shorten its effective length in compression buckling check, short inclined ‘splay’ members are often provided. Hollow circular pipe sections / liners perform better in large compression and shorter lengths, than RSJ or built-up sections. Several software programs are available to plan and design the strutting and waling system.

Temporary Steel Decking: When underground construction goes on for any metro line, the surface operations (mainly the traffic movements) should go on without any disturbance; traffic diversion could be only a temporary solution. To enable the vehicle movement over trench area, steel decking is a good option. Such steel decking is nothing but a small span temporary steel bridge spanning over two opposite side diaphragm walls or temporary beams (as seen on the RHS photo).

Box pushing for subway: Often in urban areas, open trench cutting is not possible. In such cases to enable the public entry towards the metro station, underground subways are constructed using box pushing method.


Plunge columns and barrettes: In top down method of construction, after installation of D’wall the ground slab is constructed before and then after hardening of it, the excavation is done below slab; to support the load of this slab during construction rolled steel sections called plunge columns are driven in advance. At the bottom end of these steel columns a firm concrete plug / shoe called ‘barrette’ is also driven in advance. Proper connection between Plunge column and Barrette / Bored pile considering construction tolerance need to be checked. Vertical eccentricity of plunge column in both directions also need to be checked. Shear connector with Slab and Plunge column need to be checked.


TBM thrust frames and Steel cradle: For the initial few meters of length of boring in soil, the Tunnel Boring Machine requires external support with thrust of 10000kN to 14000kN depending on geological condition (thrust frame and back to generate forward marching force) and a cradle frame at base to facilitate leveling and avoid rolling of the circular shield. Once the TBM machine moves for about 60m length inside the soil mass then the friction between soil and concrete rings is adequate to sustain / facilitate forward push to TBM and then the thrust frames are removed.

Tunnel Eye/Soft Eye: At the starting location of penetration of TBM through D’wall and at the end point of receiving the TBM, a steel frame called Tunnel Eye is installed. Normally, GFRP bar is used for Tunnel Eye location considering 150mm all-round tolerance. All GFRP bars are tied with reinforcement by using 8mm dia four MS U-clamps of each bar. Eye seal is used when the water table is above the TBM. If the water table is below the TBM, Tunnel Eye seal may not be required.


Temporary Rails & Walkway: Mucking carts are used to remove the muck generated by TBM shield. These carts run horizontally from the TBM inside tunnel to the nearest vertical shaft / station, where the muck is lifted vertically to ground. To run these motorized carts, temporary rails are required (as shown ahead). During the tunneling operations. Crossover is required for crossing the muck car at certain intervals, but the first crossover shall be after 100m from the launching shaft.


Temporary slurry piles at head wall: Slurry wall or Pile of PCC concrete Grade M10, M15 or M20 are normally used if water table is high and GFRP bars are not used. If water table is low, GFRP bars are used at head wall and if there is no building around the influence zone of head wall, then slurry wall / pile may be avoided.


Intervention Shaft for TBM: Generally, the TBM assembly is lowered / retrieved from a designated area of UG station (called launching shaft). In some projects, the TBM is to be lowered / retrieved / or accessed (for repair and maintenance purpose) in between two UG stations. In such cases, a dedicated vertical intervention shaft (either in rectangular or circular shaped in plan) is bored / driven / sunk using either diaphragm walls / secant piles or well sinking method. Sometimes these shafts are of temporary nature and sometimes preplanned permanent structures. This intervention shaft is required to replace the parts of TBM, otherwise cutter head intervention can be done from inside of TBM with atmospheric pressure and location of intervention will be at open plot. For critical case of intervention at below building area, face pressure is to be maintained.


Muck removing carts / chariot, muck carrying conveyors. muck lifting / tilting device, muck storage bin: The daily TBM operation generates loads of soil muck, which is continuously collected from the TBM cutter using a screw conveyor and handed over to muck removing carts or chariots with the help of short belt conveyors. Muck carts are usually battery-operated wagons that run from TBM assembly towards the station / shaft area where the EOT unloads them into dedicated muck storage bins. Later, the trailers excavate the muck and dispose it off site to designated remote locations (Fig 9 shows the whole process).


Casting yard & stacking yard sheds with EOT cranes, segment casting molds with steam curing facility: As shown in the photos below, the tunnel ring segments are cast using inverted curved steel formwork and subsequently cured using steam curing chamber. Then the hardened segments are lifted from mould using vacuum pads and tilted upside down with a mechanical tilting device. All tilted segments in a single ring are clubbed together and then stacked in a stacking yard using EOT cranes. Later as and when site tunneling team requires rings the full set of segments is sent to site and lowered in the shaft area suing crane / EOT at site.


Steel sheds for office / workshop / testing labs, Concrete Batching and crushing plants: At the casting yard temporary steel sheds for offices, workshops, concrete testing laboratory are required to be set up. Sometimes, these sheds are made up of brick walls and steel roofing trusses covered with corrugated sheets and sometimes directly made from portable cabins. All the staff cabins, conference rooms, printers / plotters, pantry and toilet etc. (all of temporary nature for three-to-four-year duration) are provided.


Apart from these temporary structures, concrete batching plants / aggregate crushing plants / chilling plants etc. are also provided in the vicinity area. The foundations of these temporary structures have to be properly designed to sustain permanent and variable loads (like wind load); however, being temporary in nature, up to 30% overstress can be permitted in these structures (however many clients / proof checkers / GECs insist for a design that is same for permanent structure, which leads to wastage of national resources. One must keep in mind that permanent structures are designed to sustain for a life of 100 years whereas temporary structures are supposed to stay till maximum 5 years, so some relaxation in terms of stresses should be permitted as per authors personal opinion).

References

1. Analysis of stability of rock column between cut & cover Metro Station and NATM Tunnels: et. al. International Journal of Engineering Research and Applications, www.ijera.com , ISSN: 2248-9622, Vol. 13, Issue 7, July 2023, pp 214-224
2. Analysis of stability of rock column between cut & cover Metro Station and NATM Tunnel : ITA-AITES World Tunnel Congress 2024, Shenzhen, China, 19-25 April 2024
3. Challenges of TBM Tunnelling in DMRC Project, Phase – III, CC-04, Page No – 96 : INDIAN ASSOCIATION OF STRUCTURAL ENGINEERS
4. Chapter.18 contributed by Er. vivek abhyankar in the book Design of Steel Structures – by Dr. N. Subramanian published by Oxform IBH Publishers.
5. ‘Temporary works paves way to permanent profit’ – paper by Er. Raja Rajan, published in Ing. IABSE (Vol. 46, No.1 March 2016).
6. ‘Analysis of cracks in metro segment using FEM, rectification methods proposed and associated cost saving, points to ponder for young designers” – paper by Er. Vivek Abhyankar published in Indian Concrete Journal (vol. 92, no. 3 March 2018).
7. ‘Various hurdles in Design and Construction of Metro-Rail Projects in India’ – paper by Er. Vivek Abhyankar and Er. Raviteja Kilaparthi, and Er Rafik Basha - published in B& SE, Vol. 47, No. 2 June 2017.

And various websites for the source of information & images.


Note: In Part-II of this paper, the authors have covered Temporary works required in Elevated Metro Projects (viaduct portion and station areas) like - pile load tests; temporary barricades; formwork for pile-caps, piers and segments; rebar cage supports; bridge erection cranes and launching girders; segment lifter beams; segment gluing and temporary pre-stressing frame; span load tests; access ladders etc. Readers are advised to read both the parts together to understand the variety, uniqueness as well as importance of temporary works in metro projects. After Part-I and II, in upcoming literature series, authors are planning to present a few selected case studies of metro projects, which would benefit professionals to understand the subject, in holistic way (i.e. real life application of subject in part-I and II and pros & cons); so do not miss the upcoming issues in this series!

About the authors :-

*Er. Swarup Maiti – has more than 19 years of experience in the field of planning, design, execution and technical coordination of various metro projects like Kolkata Metro UG projects (India’s first metro rail project below River), Delhi Metro, Kanpur and Surat metro. Apart from Indian projects he has also participated in a few international metro projects. He holds a post graduate degree in Project Management from NICMAR and post graduate degree in Tunnel Engineering from MIT Peace University. He has worked with eminent companies like L&T, AFCONS, CEC and HCC. Currently, he is leading the technical / design team with Gulermak India (contact:- This email address is being protected from spambots. You need JavaScript enabled to view it.)

** Er. Vivek G. Abhyankar – Founder of SGAWings Civil Engg Consultants and Advisor (OPC) Pvt. Ltd.; Fellow of Institute of Engg (India), Fellow of IaSTRUCE, Licensed Structural Engineer (MCGM) and Life member of various other professional Institutes (IRC, ISSE, IIBE, ACI, ICI, ACCE, ISRMTT, INSDAG, ASCE, NICEE, SEFI). Gold medalist from University of Mumbai in PG-Structures degree, he possesses over 25 years of rich experience in planning and design, detailing of various civil engineering structures (roads / metros / buildings / temporary works etc.). Apart from professional works, author was also a visiting faculty for Structural Engineering at VJTI, SPCE and has acquired vast experience in technical training for site engineers. He has written more than 30 technical papers on practical aspects of engineering and contributed 3 chapters in top rated books and guided more than ten M.Tech, AMIE thesis. Contributed to various professional initiatives in corporate sector like E-Learning, Knowledge Management, Engineers’ Day, Standardization of Construction Inventory etc. (contact :- This email address is being protected from spambots. You need JavaScript enabled to view it.)