V B Gadgil elaborates on the methodologies adopted by L&T Metro Rail (Hyderabad) Ltd in the execution of the Hyderabad Metro Rail Project.
Construction equipment forms the backbone of any Metro project. All Metro projects are equipment-intensive in comparison to residential projects which are more labour-intensive. Metro projects are constructed in an urban environment right in the middle of the road. It is therefore imperative that we use the right set of equipments to erect the structures taking into consideration safety and quality, thereby causing minimum inconvenience to the general public.
In general, the superstructure works of any Metro project is done using precast technology. There are designated yards where these segments are cast under strict supervision. After the segments are cast, they are lifted using gantry cranes of high capacity and transported to the erection site using trailers. These segments are then lifted and placed in position using custom-built erection tackles (called Launching Girders). An alternate method would have been constructing the entire superstructure using staging and formwork, followed by cast-in-situ construction. This would have caused a great deal of hindrance to the general traffic and public. Further, this would require huge quantity of structural steel for staging works and additional workmen would be required for cast-in-situ construction.
Hyderabad Metro Rail Project (HMRP) is a 72-km elevated Metro project including 64 stations and three depots crisscrossing through the heart of the city. Since the project is an elevated one, we have ensured that the inconvenience to the general public is minimised. We have adopted the concept of pre-cast pre-stressed technology in this project. We have been provided land on temporary basis in Uppal and Qutubullapur by Hyderabad Metro Rail Ltd (PSE) wherein we have established our precast yards for both the viaduct as well as stations. It is the largest precast yard in India.
The precast segments are cast in these yards and then transported to the site and erected at place using Launching Girders (LGs). Since the traffic is peak during day time, we transport these segments only during night (between 11 pm and 6 am). The LGs are designed considering the sharp curves across the city. We have introduced a hinge box also in the LGs so as to negotiate through these curves. We have also introduced under-slung LGs to take care of erection of station viaduct construction activity. We are taking utmost care by testing the LGs 25 per cent more than the allowed maximum load. All the safety and quality checklists are verified before transporting the LGs to the designated viaduct location.
Another unique thing we have adopted is for station construction. Stations in almost all Metros, are being constructed by cast-in-situ method. This has caused a lot of inconvenience to the general public. The method of construction is generally slow due to sequencing of operations.
In HMRP, we have introduced the concept of Spine and Wing for Concourse level of the stations. This is the first time this concept has been adopted in India. The station box rests on a row of single piers (10 units) and is of balanced cantilever type. The segments which are casted at the precast yards are transported and erected using custom-built launching girders. The segments are then pre-stressed/post tensioned in both directions, thus making the whole box act as a single system. The very concept of this system has eliminated the construction of additional columns at station locations which would have caused great inconvenience to the public.
The precast/pre-stressed technology adopted in this project has helped in reduction in total construction time, thereby reducing the overall cost while ensuring strict adherence to quality and safety standards.
Other important technical features
The trains will be using Regenerative Electric Braking, thereby converting the momentum into electrical energy and feeding back to power supply system while braking. As a contribution towards the CDM, this will reduce the energy requirement from the grid.
The advanced signalling and train control technology, Communication Based Train Control (CBTC), is adopted for Hyderabad Metro to control the trains. Hyderabad Metro would be first in India to claim train control by CBTC technology.
HMRP is using ´Track Master´, a unique instrument for checking the parameters of track and to enhance the quality of track installation. This has been procured from Trimble, USA. This instrument is being used in India for the second time after Chennai; L&T is the first to introduce this technology in India, both in Chennai and Hyderabad.
Simulation study-based track design: This type of designing the track is being adopted for the first time in the country. In this process, prior to the track design, the probable speed of a train in a particular stretch is determined by carrying out a simulation study. Based on this data, the track is accordingly designed only for that particular speed in that particular stretch. This avoids designing the track for much higher speeds than required.
This process has the following advantages:
The Hyderabad Metro Rail Project is of huge magnitude and requires efficient and technically viable solutions for construction to complete the project in the desired time period, the concessionaire- L&T Metro Rail (Hyderabad) Ltd has adopted pre-cast type of construction thereby ensuring the fastest operations within desired time lines.
Segmental Type of Bridge construction is predominant in most countries since long and the same method is operational in India since 1995. Delhi Metro Rail Corporation has also adopted the same system of bridge construction and succeeded in implementing the project with minimal time losses. The segments are manufactured to comply with Indian Standard Codes as well as some International codes.
The Hyderabad Metro Rail Project is also one of those projects where latest technology of construction is adopted by using Precast Techniques and erection of these Precast segments with the help of purpose-made erection machines. These Launching Tackles are custom made to suite particular applications under tight safety and quality norms. Selection of material in fabrication is the most important aspect and chosen from suitably assessed vendors. The Launching girder is fabricated in modules at the fabrication yard and assembled at site in their sequence and connected to each other with the help of High Strength Friction Grip Bolts. The other accessories required for erection of precast elements are also assembled at the site and the total unit is lifted on top of the piers with the help of hydraulic cranes of 500 MT capacity.
The required precast elements of the viaduct are casted at the designated places near the project site in a scientific way. The segments are casted in match-cast condition replicating the arrangements that should be at the actual site by the most feasible technical manner.
Each segment is fabricated with high strength concrete matching each other in all respects, i.e., both size and shape in the pre-casting yard. Necessary care shall be taken to match the original geometry in both longitudinal and vertical planes. The LG is erected on top of the pier at the site to its true line and level.
The segments which are already fabricated and kept ready at the precast yard are loaded into a custom made heavy duty trailer and transported to the site with utmost care. Since safety is the foremost in the operation, the segments are predominantly transported to the site at nights where the pedestrian traffic is almost nil and the regular vehicular traffic is at its minimum.
The segments are brought underneath in a sequential order and lifted to the top with the help of powerful electrical winches hooked from both sides. These are then transferred to the LG with the help of hanger bars and the winch is then released. Meanwhile, the empty segment trailer is sent back to the precast yard and the next trailer is put into position.
Similarly, all the segments are erected in their particular sequence and lifted to the top and then hooked to the LG. The segments are then dry-matched to each other to ascertain the correctness of their casting with the help of moving trolleys which are on the top of the LG.
After all, the segments are dry-matched, they are separated with the help of top trolleys, now epoxy glue is applied on the cross-sectional surface of the first two segments and joined together with the help of temporary pre-stressing force.
Similarly, all the segments are joined. High strength post-tensioning steel wires of required diameter are then inserted through the ducts, which are already provided in the match-cast segments from one end to the other.
These high-tension cables are stressed by giving jacking force from the ends to their design levels and locked with the help of hydraulic jacks. Bearings are now placed underneath the span on the pier locations and the span is released on to these bearings by loosening the top hangers of the LG.
Now the span acts as a monolithic structure forming a part of the bridge. Similar methodology is adopted for erection of segments in all the spans.
The author is CE & MD, L&T Metro Rail (Hyderabad) Ltd.