Metro gives extra attention to pillars

As a precautionary measure,the DMRC has set up metal structures around and under the piers for support. But while piers on the Noida line are being fattened,those on the Gurgaon line are likely to undergo “pre-stressing” — a process in which concrete structures are placed under compression in those regions where load causes stress.

Work has already started on the Central Secretariat-Badarpur section — which witnessed a mishap in Zamrudpur on July 12 due to a cantilever pier collapse. Of the eight piers found to have cracks,six have been identified for pier cap demolition and reconstruction with a different design. All six pier caps were similar in design to that of pier 67 which gave way.

The new design will have additional couplers — joints between piers and pier caps — to be tied with more re-bars to consolidate the concrete structure,said a DMRC official.

The process is being done with sophisticated mechanical hammers,so the adjoining areas are not damaged. Metro officials had also announced that the cantilever load on a few of the piers on the Badarpur stretch adjoining Pier no. 67 will be reduced. The cantilever width will be reduced from 3.88 m to 2.5 m.

The design for rectification of cantilever piers on the Central Secretariat-Gurgaon line,meanwhile,is being considered by design experts in DMRC.

Metro officials said a process of pre-stressing is being considered,the main purpose of which is to induce desirable strains and stresses in structure and its components.

The design,however,is yet to be cleared and as a precautionary measure,metal shuttering (erection of metal structures around the cantilever piers) has been done on three of the eight piers detected with cracks,said a DMRC official.

“After an inspection by the technical auditor,three cantilever piers on the Gurgaon stretch from Arjangarh to HUDA City Centre were identified for strengthening. Though unlike the piers on the Noida section,these do not require fattening,a design proposing pre-stressing is under consideration,” said a DMRC spokesperson. “As the cantilever piers on all three lines have been designed by different design consultants and constructed by different contractors,the methodology for corrective measure has to suit design specifications,” he said.

On the Noida line,there will be no demolition of piers,even in part,and a jacket is being created around the cantilevers. As part of this,another layer of concrete will be created around the cantilever and the part of the pier it rests on,for added support. With this,the length of the cantilever portion will also get reduced.

The Noida corridor was scheduled for opening in August,but has been delayed till early December due to the corrective measures.

The DMRC officials,however,maintained there will be no such delay in the Gurgaon corridor,which is scheduled to open in January 2010. Earlier Badarpur,which is slated for commissioning later next year,was delayed by three months after the Zamrudpur mishap.

Contractors
NOIDA corridor : Gammon India Limited

Central Secretariat-Badarpur : Gammon India Limited

Central Secretariat-Gurgaon : (Elevated section from Qutab Minar to Gurgaon is elevated and has cantilever structures) ITD ITD CEM JV

What is a cantilever pier?
A beam supported rigidly at one end to carry a load along the free arm or at the free end.