Construction work of Phase–II of Delhi Metro is in progress. It presents many civil engineering challenges at sites involving greater heights and deeper depths than those of Phase-I. NBMCW through its interaction with contractors, engineers and workers at construction sites finds out some of these challenges met successfully through deployment of new experties, sophisticated machinery and equipment. Cranes here are playing pivotal role to meet tight deadlines.
A visit to different metro job sites, where men and machines are working round the clock under challenging situations to meet the tight deadlines, revealed entirely a different story. The ongoing Phase II project appears to be a complex one as the elevated tracks are being built above road crossings, railway intersections and flyovers at about two dozens locations across Delhi. So much so that at certain locations spans are more than 34 meter long.
During the course of executing different phases, the DMRC deployed top class modern technology, competent contractors with world–class cranes and trained crew. They include Snebo Engineering, ITD-ITDC and Gammon India, Larsen & Toubro, CEC SOMA JV,MTG, DV Deck, IRCON, Samsung and Shimizu.
The longest distance of about 21.16 km long is coming up between Central Secretariat and a highly dense railway line on the outskirts of the city. The span is 100 meter long with supporting spans of 75 meter on either side of the track. Another important component of the project is the construction of 19 meter tall pier which so far has been the tallest piers of Delhi metro project.
Carrying out the metro construction job in a city, with more than 14 million population, where mounds of construction material have to be transported and placed on roadsides minimizing the space for crane crew and creating traffic bottlenecks at innumerable places, is in fact a tough task. But with world– class project management, technology and expertise attained from top class global consultants during the first phase, DMRC is brimming with confident to complete the work much ahead of its scheduled time.
Commenting on the job sites working scenario, Larsen & Toubro Phase-II Construction Manager said, “Since the project is located in the heart of the city, the construction sites have space constrains and the crawler cranes have to be operated with utmost care so as they may not hit adjoining buildings or damage roads during slew and lifting operations. This is really a challenging task since the lifting job is being carried out on busy roadsides.”
Cut & Cover Systems
Parts of the project are close to buildings built during the pre–independence era and are currently housing central government offices. There is constant danger to their damage due to underground vibrations. In such areas the DMRC uses cut and cover systems to avoid cracks, mishaps and damage to the building structures.
During the cut and cover operations constructing tunnels require cranes in good strength. While carrying out such operations engineers dig open excavation, wherein 13t Universal beams are fixed horizontally and on the edges of the tunnel, piles are placed and then more Universal beams vertically placed on the top.
The entire process of laying beams is being undertaken using crawler cranes. At a given underground section with 5.16 km stretch, at least three Telcon Tata TFC 280 crawlers are found carrying out operations. The cranes have a lifting capacity of 75t each at 24.39m basic boom. Under the 14 km underground link, at the Central Secretariat, three Telcon Tata ALC 955 crawlers lattice boom cranes each with a capacity of 75t with boom extended double to normal length of 30.5m, are also found carrying out operations.
Crawler cranes are also in action on DMRC projects on the elevated stretches for unloading 20t and 23t pre-cast concrete girders from truck trailers and used in building superstructures. The girders are lifted by 70t and 72t capacity hoists, ready to be installed on to the superstructures. In addition to this, the cranes are also put to use to install steel formworks used constructing piers.
It is only because these cranes possess advantages over RT otherwise they are not the first choice of the contractors. First, operators jerk sticky concrete out of the bucket they lift by releasing the brakes momentarily so that their hook ensures free falls. Only crawler cranes are built for this kind of stress. Similarly, the 4 to 5 meter crawler track spread takes a less space than RT on outriggers.
Apart from this, their swing radius is greater than RTs, hence extra loads needed to be slewed and crawler tracks can damage the work site surface during their movements. Manpower engaged at the cut and cover work site have to deal with the constraints and consequences of site congestion routinely. The work site measures 25m across, from one barrier to another. There is a 20m hole inside for laying strutting beams, with a gap of about 3m on either side. One side of the site is housing for government officials; on the other side there is a road. For easy access, the crane works from the roadside. In order to cover the entire job site, the crane needs a working range of 18m to the midpoint of the strutting works. That means that the crane hangs 5m outside the work site on to the road. The crane’s work including lifting and placing the beams takes place only after workers close the road for vehicular traffic. Stopping traffic is another tough task as most of vehicles belong to VIPs sporting their signs. Roadside trees, buildings, lanes and by-lanes also hamper smooth functioning of cranes.
Cranes are also functioning in and around the fabrication yards for different contractors along the job sites extensions. A new model Kobelco RKE-500 rough terrain, with a lifting capacity of 51t at 10.2m, and a new 70t Tadano GR-700 EX rough terrain under the 5.16 km stretch are being used to load and unload Universal beams at contractor Senbo Engineering Limited’s fabrication yard.
Larsen & Toubro’s 12t ACE 12XW cranes are also functioning at the company’s fabrication yard accomplishing jobs like lifting of reinforcement rods, construction equipment, concrete pumps, and generator sets. The company is constructing two underground stations in the 22km long stretch. Another use of the yard cranes is to place construction site barricades, segregating the construction site from the public access areas.
DMRC has put a tight schedule and strict stipulations for cranes so as to adhere to the norms laid down by the government of Delhi. Cranes found wanting on this score will be banned from further extension.
As per the contract, the DMRC could punish contractors overshooting their deadlines by deducting money under the ‘Liquidity Damage’ (LD) clause. Otherwise the corporation prefers to keep cordial relations with contractors. Ensuring timely completion of the project, chief project managers hold weekly meetings with contractors and consultants. The DMRC also advises contractors in advance to strictly adhere to the deadlines.
DMRC has appointed International Engineering Co., as the testing agency to examine cranes before they are allowed to enter job sites. From time to time, the agency conducts proof load tests ensuring the cranes are fitted with safe load indicators and also carries out checks twice a month. It also checks the manufacturing date, quality of the wire ropes, brakes, winches, pulleys, and hoist.
Work at Night
Visit a construction site at night where Metro construction work is going and have a unique experience. At some sites on elevated sections one can see 20-30 workershanging on 20 feet above the road and working fearlessly, handling 40 tonnes concrete structuretransported on a trailers and doing the job with ease, joining these segments to create a span between two pillars. These workmen are guided by the color of their helmets, the one with yellow are workers who carry out civil work and operate machines, those with blue and red helmets are the supervisors. Those with green hats are safety workers to ensure safety norms and the one with White helmet is the boss of the project from DMRC. As the crane lifts the segment, other workersapply a glue (Epoxy) to paste the segment. Explains a site engineer. All this work is time bound, the workers have only 60 minutes to lift the giant structureand paste the segment and then prestress the entire span using hydraulic machines. Each segment is joined to the other to create a span between pillars, the piers. The tracks are then laid on the span. On a construction site 20 feet below the ground, workers in yellow helmets could be seen clinging on the ceiling of the undergroundstructure through a giant lattice anddoing work in perfect synchronization and discipline.
To make the Metro structures earthquake proof, all the piers have a pile foundation going about 10-15 feet below ground level to make them strong. Further, bearings with rubber have been put along the structure to make it more elastic. Seismic concrete retainers have also been incorporated in the design to prevent dislodgement during tectonic movements. In addition, DMRC is installing seismic meters at two important locations to make the network more secure. The gadget will raise an alarm as soon as an earthquake begins and within seconds also measure its intensity on the Richter scale. DMRC network has been designed to withstand earthquakes which measure up to 7 points on the Richter Scale. But, when the intensity lies between 4 and 7, the staff have been instructed to take a set of listed measures to prevent any untoward incident.