PORTLAND, Ore. -- It's not just the rain causing dozens of landslides across the Portland area. A Portland State University geologist says there's another culprit.

PSU geology professor Scott Burns has been studying landslides in Portland's West Hills for nearly three decades.

This year he has noticed something happening much more frequently than years past: landslides caused by old and failing retaining walls.

The failing walls are anywhere from 50 to 100 years old.

They were built with holes at the bottom, commonly called weep holes, to help drain water that builds up behind them.

The problem: many are no longer draining.

Burns said in studying the dozens of landslides this season, he noticed the trend.

"The big issue is that these old walls are getting clogged, the water is not able to seep through them, the water is building up in back of them and that additional weight and pressure is causing the walls to blow over,” he explained.

Burns said an aging infrastructure is causing another rain-related problem city wide: sink-holes.

"We're seeing more and more of them probably this year,” he said. “We've had maybe ten-plus large ones and it’s all directly related to age of the pipes and the amount of water coming through."

Burns anticipates we're going to start seeing a lot more sinkholes as well as landslides due to failing retaining walls.

And while it's up to the city to fix the pipes, it's up to homeowners, in many cases, to make sure their retaining walls are draining properly.

Burns said if you can't clean the drainage holes yourself, you should consider hiring a geo-technical engineer to do the work for you.