CHENNAI: When construction of the outer walls for underground Central Metro started, engineers didn't expect that the mixed soil condition would impede work. Trenches were cut to build the walls that were inconsistent, the wall joints began to leak and lowering iron cages into the trench to pour concrete became difficult.
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“Had we adopted the conventional method, it would have taken us five years to build the box as the soil condition was mixed with loose soil on top and rock at the bottom. This was perhaps the first time in the country that overcut method was used to build an underground metro station,” an official said.
As the alternate concrete panels are built, excavation of the soil in between them begins. Engineers had to overcut into the already built adjacent panel for about 150mm to 250mm in width. The trench is then filled with concrete after reinforcement bars are fitted.
“In the overcut method, lowering and removal of stop-end tubes take less time and the cage is lowered only after trenching is complete. This was a cost effective as well as a time-saving method,” an engineer said.