WORCESTER - Work to repair a massive water main break not far from the Saint-Gobain factory complex is nearing completion, but city officials are still not sure why a 24-inch water main split lengthwise along the bottom.

The pipe burst Tuesday, causing flooding in the Norton and Shore drives area, leaving a traffic mess, and many in the city without water. One motorist who tried to drive through the flood waters ended up nose first in a large sink hole.

The broken water pipe has been replaced, but the city will be flushing it with chlorine into next week to disinfect it, according to Philip D. Guerin, director of water and sewer operations for the city. The only place still affected is one business in the Saint-Gobain complex, which has rerouted its water system until service is restored.

The northbound lane of Norton Drive remained closed Friday, and will remain so until work is completed. Drivers can now use the southbound side of the road if they are headed from Shore Drive to West Boylston Street.

Mr. Guerin said he is not sure what caused the pipe to crack. It is 63 years old, not that old by water-line standards. He said it is unusual but not unheard of for a pipe to crack lengthwise. Normally, water lines break at the joints. But, he said, the pipe is cast iron, which can corrode or break.

"Because it is cast iron, when a crack starts, it will tend to continue," he said.

Mr. Guerin said the city is contemplating having a company do a forensic study of the pipe to try to learn how it cracked.