Introduction to Temporary Works in Metro Rail Projects

Introduction

In Part-I of this paper (published in April 2024 issue of NBM&CW), we discussed temporary works in underground metro rail projects (including the ramp portions); TBM trust frame, cradle unit, strut & walers, top-down/bottom-up, soft-eye etc. were lucidly presented (authors did not claim any originality or research content in the data presented). In the present Part-II, the authors have presented key points to ponder in temporary works required in elevated metro projects.


Elevated metro projects mainly involve two types of structures – (a) viaduct/bridge portion which runs longitudinally to the alignment and (b) the elevated station buildings. Important temporary works required for these are listed here and described briefly:

1. Temporary works involved in construction of elevated viaduct portion include:
   1. Roadside barricades.
   2. Shoring system for construction of pile cap or open foundation if pier is located on a busy road.
   3. Pile load test / plate load test assembly.
   4. Pier reinforcement cage supporting frames.
   5. Formwork for pile cap / pier / pier caps & friction clamp for installing precast pier cap.
   6. Casting yard & stacking yards for superstructure girders / segments.
   7. Bridge erection cranes / launching girders (LG), lifter beams, temporary PT frames.
   8. Span load test assembly.
      I. Miscellaneous units, like
      1. Formwork for pedestal / seismic arrestor / lateral restrainer.
      2. Formwork for precast crash barriers & installation trolleys.
      3. Superstructure inspection trolley / carts.
2. Temporary works involved in construction of station building:
   1. Formworks for pile caps / piers / portals.
   2. Access ladders.
   3. Post tensioning access cabin / bucket.
   4. Sacrificial corrugated sheets for concrete slabs (sometimes used in lieu of conventional formwork).
   5. Formwork / staging for upper floor slabs.
   6. Formworks for staircase, water tanks, platform level slab.
   7. Temporary steel stubs at platform level (along station building) – if the LG from viaduct spans is required to rollover over station.

The terms above are briefly explained below:

Pile load test assembly:- ( Fig.1) Usually, elevated metro rail viaduct piers are supported on pile foundations; if good strata like weathered rock etc. is available, then open foundations can also be given (provided there are no adjoining structures and open excavation for foundation is possible). In case of piles, the safe load bearing capacity of piles for an available strata, must be estimated in advance, as per guidelines of IS-2911 code or IRC.78 code; this estimated capacity has to be established / verified at site with a physical load test.


Pile load tests are classified into two types:- Ultimate load tests and routine load tests. Generally, for a single project / single type of strata one ultimate load test is performed on a pile which is not a part of permanent foundation; whereas routine tests are performed on actual usable piles (called working pile); the number of routine tests is generally 2% of total number of working piles. Readers may refer to IS-2911 (part.4) for more information.


Depending on the direction of the load applied, PLT are classified as – (a) compression test, (b) pull out tests, (c) lateral load test. Above mentioned options are static / cyclic in nature. Now a days, advance options like dynamic load tests, osterberg load cell are frequently used to save the testing time.


Plate load test assembly:- As explained earlier, if good strata is available then open footing can be proposed. In such case, the assessment of safe bearing capacity at site shall be done in advance using plate load test. Plate load test can be done using heavy kentledge (steel platform loaded with sand bags) or simply using a reaction below heavy vehicle or using steel truss with its supports anchored down/buried below sand bags. Load-settlement curves are plated; entire test is performed as per guidelines of IS:1888 code.


Formwork for pile cap/pier/piercaps & friction clamp for installing precast piercap:- Various sub-structure components are cast at site using conventional formwork (made out of timber/steel/aluminum/heavy duty plastic or even composite materials). Figures ahead show the various types of formworks used in elevated metro construction works. Now a days option of using precast components is becoming more popular (due to congested city roads, limited space at sites, noise restrictions, limited working hours etc.), in such case the conventional formwork is replaced by sophisticated shutters and used in casting yard. All types of formworks /shutters are designed as per guidelines of IS:14687/ IRC:87 / ACI-347 / CIRIA-108 report or other equivalent norms and project requirements. The ‘concrete surface finish’ obtained using steel formwork is often better than other materials; also steel formwork has better repetitions as compared to other materials (but it is very costly); hence the choice of formwork material shall be done carefully under guidance of experienced engineer.


Usually, the piers are cast monolithically at site; the reinforcement cage is assembled at sites at locally supported using steel framework. In past, many mishaps have happened at sites due to negligence in assembling the rebar cages (the rebar cage snapped off from starter bars and collapsed on adjoining busy road); hence utmost care is needed in this small step even. After pier construction is over the pier caps can be cast at site or even precast in yard and installed later at site using friction collar/clamp as shown ahead in this paper. Formworks for portal type pier-cap should be designed–erected and safeguarded carefully; because in certain cases the supporting steel towers were hit by the fast move dumpers/trailers in night times (better to place temporary barricades/heavy concrete blocks, around these steel towers to protect them for vehicle impact); if possible the traffic to be diverted through other route or speed to be restricted to 20kmph or less.


Casting yard & stacking yards :- In busy, congested metro cities (having space constraints) usually the superstructure of elevated viaduct portion (bridge portion) is precast in casting yards located far away from city boundaries and then brought to site using low bed trailers and then erected using a crane or launching girder (as explained ahead). The segments are cast using – ‘long-line bed formwork’ or ‘short-line formwork’ and after sufficient hardening they are lifted and stored in the stacking yard in one or two layers or rarely in three layers (as per available SBC of soil).


Bridge erection cranes/launching girders (lg)/lifter beams :- Safety, speed, economy as well as public convenience are the key requirements in any mega metro project. Considering these, all the construction operations are planned and designed. The Superstructure if one piece span then can be erected using heavy capacity cranes. If the span is segmental type then it can be erected using launching girder (overhead or under-slung type LG) or using ground supported staging system (GSS). If a long span proposed by DDC (viz. balance cantilever or an extradosed bridge) then a traveling formwork is required. Reactions caused by the self-weight of launching system shall be accounted for in the permanent design. During heavy gusty wind conditions (>30kmph speed), no launching is performed; also all the moving parts of LG / LT / GSS / form-traveler are locked and whole setup is anchored firmly to the stable permanent structure.


Span load test assembly :- After sufficient number of viaduct spans are constructed, the structural adequacy of a typical span is checked using full scale bridge load test (as per guidelines of IRC:SP-51 code or equivalent specs). Bridge load test can be performed either using static loading (concrete blocks) placed over desired span or using a moving loaded trucks; applied in incremental fashion (viz. 25% - 50% then 75% of loading at 1 hour interval and finally 100% load sustained for 24hrs); the deflection of loaded span is recorded at L/4, L/2 and 3L/4 distance of that span (where ‘L’ is length of the loaded span) using strain gauges or sensitive stationary theodolite etc. For more details about span load test, readers may refer to technical paper on bridge load test mentioned in references at the end of this paper. Figure ahead shows typical span load test.


Temporary works in elevated metro station :- Typical temporary works (formwork) required for pile cap/piers/ pier caps are similar to the viaduct portion (as covered earlier); now a days due to congested city roads contractors prefer more and more pre-casting and assembling them at site using post-tensioning systems. Another important aspect of temporary works is the reactions caused by them on the partly constructed permanent structure could govern the entire design; hence the temporary works designer should assess the loads / reactions (due to launching girder movement / staging cribs etc.) in advance and provide them to the designer of permanent structure (DDC).


Miscellaneous temporary works in elevated metro construction works :- Apart from above main temporary works there are several minor items required in the construction of elevated metro rail project. A few of them are indicated in the following photographs. At the start of any metro project, experienced designer & planner prepare the list of temporary structures needed for that project to avoid last minute rush/delays or extra expenses.

A word of caution :- The write up gives only outline description about the temporary works, one shall not underestimate or misunderstand about their importance. A few of the following photos show the mishaps that took place in several places in India due to careless attitude towards temporary works.

References :-

• Chapter-18 – ‘Design of Steel Enabling Structures’ contributed by Er. Vivek Abhyankar, in book ‘Design of steel structures – by Dr. N. Subramanian, published by Oxford University Press
• IngIABSE Journal Volume.47 No.2 June 2017 – paper ‘Various Hurdles in Design and Construction Of Metro-Rail Projects In India’ by Er. Vivek Abhyankar
• Structural Engineer’s World Congress (SEWC-2015) – paper ‘Bridge Erection Techniques and influence on design’ - by Er. Vivek Abhyankar
• Paper ‘Load test on bridges’ by Er. Vivek Abhyankar published in NBM&CW
• Various websites for source of information & images.

About the author :-

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 underwater metro rail project), Delhi Metro, Kanpur metro, Surat metro, and a few international metro projects. He holds a PG degree in Project Management from NICMAR and a PG 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 of 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); Fellow of Institute of Engg (India), Fellow of IaSTRUCE, Licensed Structural Engineer (MCGM), and life member of various professional Institutes (IRC, ISSE, IIBE, ACI, ICI, ACCE, ISRMTT, INSDAG, ASCE, NICEE, SEFI), is a gold medalist from University of Mumbai in PG-Structures. He has over 25 years of experience in planning and design of various civil engineering structures (roads / metros / buildings / temporary works etc.). He was a visiting faculty for Structural Engineering at VJTI, SPCE; has written more than 30 technical papers on engineering and contributed 3 chapters in top rated books, guided more than ten M.Tech, AMIE thesis, and contributed to various professional initiatives in corporate sectors 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.)