Md. Adil SHAIKH, Mangesh SAWANT & Ajay TANK, Sr. Design Engineers at Spectrum Techno Consultants present the various aspects of planning, design and construction considered for this project keeping in view its complexity with respect to location, space and time constraints.
The success story of Delhi’s metro network has shown how a well planned and executed infrastructure project can change the functioning of a city. Mumbai city, for a long time has been relying on the suburban train network for transporting majority of workforce to and from the city and suburbs. The suburban rail network as well as the road network have reached its maximum capacity due to which there is a need of a major infrastructure overhaul for Mumbai city. Mumbai Metro line 2A is one such project being executed by Mumbai Metropolitan Region Development Authority (MMRDA). This elevated line is planned from Dahisar in the northern boundary of the suburban city to Andheri in the heart of the suburbs. The overall length of the elevated corridor is about 20.6km with 17 stations. The proposed metro line shall provide interconnectivity among the existing Western Express Highway, Western Railway, other existing and proposed metro lines. It is expected to reduce the traffic on highly congested suburban road network as well as the Western Express Highway. It is also expected to reduce the passenger load on the western line of the suburban railway network.
Mumbai metro line 2A was planned and conceptualised by Delhi Metro Rail Corporation (DMRC) for MMRDA and the tender was awarded to M/s. J Kumar – CRTG JV in May 2016. M/s. Spectrum Techno Consultants Pvt Ltd, were appointed as the design consultants and M/s. AECOM were appointed as general consultants for the project. The works started on ground in August 2016 on the total length of 20.6km including 17 stations. Execution of a project of such magnitude in a city like Mumbai presents many challenges in design and execution due to space constraints, existing utilities, huge traffic etc. Considering the tight schedule and quantum of work, it was clear from the very beginning that the project cannot be completed on time by traditional construction methods and there was a need to innovate in design as well as in construction methodology. Further, considering the space and traffic constraints, it was decided to have maximum precast elements in the project. In the viaduct portion, precast U-girder superstructure rests on precast piercap. In stretches where U-girder type superstructure is not feasible, precast I-girder superstructure with cast-in-situ slab has been provided. In the station buildings, entire concourse and platform floor including cross arms, longitudinal girders and slab have been made precast. The launching activity of precast elements governed by space constraints and heavy weight of precast components is an additional challenge for execution.
Alignment
The alignment of Mumbai metro line 2A passes through new link road between Andheri and Dahisar which is one of the densest corridors in Mumbai. There are total of 17 stations proposed on this line namely Dahisar East, Anand Nagar, Rushi Sankul, IC Colony, Eksar Nagar, Don Bosco, Shimpoli, Mahavir Nagar, Kamraj Nagar, Charkop, Malad Metro, Kasturi Park, Bangur Nagar, Goregaon Metro, Adarsha Nagar, Shastri Nagar and DN Nagar. The alignment of Mumbai metro line 2A is shown in figure 1.
Merto line 2A merges with proposed line 7 at Dahisar and with the proposed line 2B at DN Nagar. It also connects with the existing line 1 at DN Nagar. Metro car depot has been proposed at Charkop between Kamraj Nagar and Charkop Stations.
Structural Features
Station Buildings
All station buildings for metro line 2A have been conceptualised as cantilever structure. The tracks, platform and concourse are supported on a single pier on median. The entry exit structures have been proposed on off road location on the footpath. In general, the rail level has been kept at 13m above ground level. The difference between platform and concourse level for all stations in 6.45m. A typical architectural section of the station building is shown in figure 2 & structural framing section is shown in figure 3.
The grid spacing of stations is 15m + 17m x 9 + 15m = 183m. Total length of station is 185m. The overall width of station is 20.5m. All the stations have two tracks except for Kamraj Nagar station which has three tracks to facilitate the easy train movement to & from metro car depot at Charkop. At DN Nagar, the metro line 2A crosses the existing metro line 1. Due to this the rail level at this location is at approximately 21m above ground level. Therefore, an additional floor has been introduced in the station which will act as a property development floor to be used for retail purposes later.
Concourse level
The concourse level of the station comprises of transverse cross arm and longitudinal girders. One of the most time-consuming activity in station construction is the casting of slab which required preparation of staging for shutter, tying of reinforcement and casting in-situ concrete. To minimise this activity, precast pretensioned T-girders have been provided in longitudinal direction which are pretensioned. The length of girders varies between 13.1m to 15.3m. RCC L-girders have been provided as edge beams. Figure 4 shows the cross section of L-girder and T-girder. At the joint of flanges 300mm wide in-situ stich has been provided for transverse continuity of slab. Figures 5, 6 & 7 show casting and stacking activities of T & L-girders at casting yard.
The cross section of transverse cross arm is shown in figure 8. Inverted T-section has been provided for cross arm. Articulated supports of T-girders rest on the flange of cross arm. This helps in reducing the overall depth of the station. The cross arms are precast and cast in two pieces each of length 9.1m. The pier is cast up to bottom of platform cross arm as shown in figure 9. Shear keys are provided on the transverse face of pier at concourse level. Two pieces of precast cross arms are launched and rested on steel trestles at 150mm from face of pier as shown in figure 10. The 150mm gap between the pier and cross arm is stitched with non-shrink concrete and the first stage transverse prestressing of the cross arm is applied. The detailed construction/launching sequence of the station building is discussed in chapter 3.1.3.
Platform & Track level
For supporting the tracks, pretensioned U-girder have been proposed with a similar section to viaduct but with a modified flange. The flange of U-girder partially acts as a part of the platform for the station. The rest of the platform is made up of Precast RCC PI-Girder. The section of the PI-girder and modified flange of U-girder is shown in figure 12. The platform PI-girder and U-girders rest on the precast prestressed platform cross arm. The length of the precast cross arm is 20.5m in transverse direction and the depth is 1.7m. The piercap has a central prismatic hollow section for in-situ stitch and integration with the RCC pier. The elevation and plan of the precast piercap is shown in figure 22.
Launching sequence of Station building
The construction & launching sequence of the station can be summarised in the following points.
- Casting of foundation and pier upto bottom of platform beam (Figure 18).
- Launching of concourse cross arm in 2 pieces on staging erected from ground. Casting in-situ stitch and stressing of first stage transverse cables (Figure 18).
- Launching of platform cross arm and resting it on the staging erected over concourse cross arm. Casting in-situ stitch and stressing of second stage transverse cables (Figure 19).
- The longitudinal components viz., T-girders, L-girders, PI Girders and U-Girders are launched in a specific sequence as shown in figure 20. The sequence has been decided based on crane positioning and boom movement.
- The same sequence is repeated for all the grids.
- At locations where the above sequence could not be followed due to space constraints or unavailability of precast elements, the sequence was changed ensuring that the eccentricity of the loads is minimum.
- After launching of the above elements, the in-situ stitches for connecting the various elements is done. The second stage stressing of concourse cross arm and third stage stressing of platform cross arm is carried out.
- The works on internal walls, floor screed and internal staircases is carried out after which the third stage prestressing of concourse cross arm is done.
- The works on roof and external staircases completes the construction of the station building.
Viaduct
Superstructure
U-girder was adopted for superstructure since majority of the route has mild radius of curvature in horizontal alignment. U-girder is pretensioned and easy to cast in casting yard. The design of U-girders has been provided by M/S. Systra for standard spans. For optimised section and handling criteria, maximum span of 28m was adopted for U-girder. U-girder being cast as straight element with rotated bulkheads for horizontal curvature, there is a limitation horizontal radius.
For sharp radius below 300m and at the location of scissors and crossovers, precast I-girders with cast-in-situ deck slab has been adopted for superstructure. The typical cross section of two track viaduct is shown in figure 21 & 23. Overall width of the superstructure is 10.55m at two track locations. The width of superstructure has been increased at locations where there are 3 & 4 tracks as required. Other constraints with U-girder are transportation and launching. Due to congested space in a city like Mumbai, it is very important to carefully study the route along which the U-girder is to be transported from the casting yard to site. Launching the girder at site is another important aspect since it needs two heavy cranes to be stationed parallel to the two piers. The decision regarding possibility of launching the U-girder at a pier location is decided after carefully studying the space requirement of cranes, strata below position of outriggers and presence of drains /culverts at the location. The span arrangement and/or superstructure type had to be changed based on the feasibility of launching.
Substructure
The substructure of viaduct comprises of circular RCC pier of 1.8m dia and prestressed precast piercap to support the superstructure. The precast piercap has a flanged section in the cantilever arms with a hollow cone at the centre to facilitate the in-situ stitch with pier. Once the pier is cast and concrete achieves strength, a steel collar with hydraulic jacks is fixed to the pier as shown in figure 24. The launched precast piercap is placed on the hydraulic jacks & the in-situ concrete is poured. Once stitch concrete achieves strength, steel collar is released and piercap is now monolithic with pier. Before launching of superstructure, two cables are stressed in the piercap. The remaining three cables are stressed after the launching of superstructure. For piercap with U-girder, the width of piercap is 10.56m and for piercap with I-girder, the width is 7.7m. Elevation of piercap for U-girder and I-Girder is shown in figures 21 & 23 respectively.
Eccentric piers or portals have been proposed at locations where concentric piers could not be provided due to constraints like road width. Portals are also proposed at locations where there are more than two tracks between Kamraj Nagar & Charkop station where third and fourth tracks facilitate the train movement to and from the depot. Eccentric piers pose a different challenge with respect to design. The challenge was further enhanced by the requirement of having the eccentric piercap in precast.
Moreover, the eccentricities of the pier vary between 0.5 to 2.0m. The success of precasting lies in repeating the same moulds multiple number of times by keeping the number of moulds as minimum as possible. With so many variations in the eccentricities of pier, having different mould for each eccentricity was not feasible. To solve this issue, the shape of the piercap was adopted in such a way that all the piercaps with varying eccentricities are cast in a single mould. This was achieved by changing the location of end shutters as per eccentricity requirement keeping the length of piercap constant i.e. 10.56m. The shape of cantilever piercap is shown in figure 26 & 27. The detailing of the stitch portion of the cantilever piercap with RCC pier had to be designed carefully to make sure that proper force transfer is ensured. In figure 26 & 27, the blue coloured portion having length of 1.5 m on either side indicates the variable portion where the end shutters will be shifted based on eccentricity of pier. With this arrangement, the piercaps with eccentricities from 0.5m to 2.0m could be casted easily in the same precast mould. The launching activity of precast eccentric piercap is shown in figure 28.
Conclusions
Excellent quality and speed of construction was achieved by use of precast construction technology. The components of station were conceptualised and designed as per the requirements of launching team incorporating their ideas and considering their constraints. For smooth functioning of a project of this magnitude, it is important that there is minimum variation in precast elements so that there is constant feeding of elements to the site from the casting yard. Also, this allows to have an easy inventory of elements at casting yard. This aspect leads to increase of cost of material, but it helps in saving time-based costs and overheads.
Acknowledgements
- Mumbai Metropolitan Regional Development Authority
- Delhi Metro Rail Corporation
- J Kumar Infraprojects
- AECOM
