PALM DESERT, Calif. -

Work is underway to replace a pipeline that caused a sinkhole along Cook Street last year.

Heather Engel of the Coachella Valley Water District said, "We didn't know the pipe was deteriorating."

The three and a half mile stretch of pipeline was first installed in 2007 and was supposed to last for decades.  But just five years later, it failed opening up a 10-foot deep hole in the road.

The 20-foot wide sinkhole surprised the water district because Engel said, "It was a relatively new pipe, those pipe typically last for about 75 years and it had only been in place a couple of years."

The pipe's interior walls are supposed to be smooth.   But a radar survey showed the pipe's walls were not smooth, and were thinning at the top.

"It was so unusual that we really needed to take a step back," said Engel, "and analyze the entire 3.5 miles of pipe that was put in and really try to figure out what went wrong and make sure that we have a full understanding of what happened before we started to make repairs."

1400 feet of pipeline needs to be replaced or more sinkholes could follow.

Engle said, "At some point if this pipe wasn't fixed there would be homes and businesses without sewer service."

The Coachella Valley Water District said it's confident it's found all of the questionable pipeline, but there could be other pipes elsewhere around Southern California also being weakened. 

"Unfortunately we still to this day do not understand why the pipe deteriorated why it did," said Engle.  "We took out a couple of sections and sent it off to a lab for special analysis, we haven't heard back yet, hopefully that will provide a little bit of insight as to what happened," she continued.

ARB Engineering installed the pipe in question and it was manufactured by Mission Clay with offices in Corona and Los Angeles.

Work along Cook Street should be finished by April at a cost of $1 million.  

More work will also need to be done later this year to shore up the remaining pipe.