PORTLAND -- A landslide like the one that occurred Wednesday morning typically follows a period of heavy rain. But geologists point out that was not the case this week.

 Background: Six families still evacuated

In fact, KGW Meteorologist Dave Salesky says Portland has gotten less than 1½ inches of rain this month.

So geologists are testing the soil in the area to determine what triggered the landslide.

Bill Burns, an engineering geologist for the state says Wednesday’s slide was likely caused by a combination of water in the soil, unstable ground and the steep slope the house was erected upon.

The state has also determined that 6438 SW Burlingame Place sat on an old drainage area.

Burns warns that the danger may not be over, yet, in the area. He says two adjacent homes, along with the road itself, are at risk of sliding, too.

But the big question is why Kathy Hendrickson’s house? What about it and the soil underneath made it give way?

A massive, gaping hole was left behind where the Hendrickson house had stood, prior to it sliding 300 feet down from Burlingame Place to Terwilliger Boulevard below.

Debris is all that’s left of what was once a beautiful home.

Engineers are puzzling over how it happened and what caused all this damage.

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In the hours following the slide geologists tested the soil, looking for clues as to why this home tumbled down the hillside.

“The soil is wet but it doesn’t appear to be any more or less than normal,” Burns said. “It’s a little unusual that we're seeing slides like this, this time of the year.”

Burns is an expert in landslides and has been studying them for more than a decade.

“There’s some wind-blown silt that sits on top of bedrock that is prone to sliding,” he said.

Burns has more than 700 Portland metro-area slide maps in his office. The last major landslide in the west hills occurred in 1996, triggered by massive flooding.

But there hasn’t been any recent flooding in the area that would have caused Wednesday’s catastrophe.

The area remains closed off to ensure public safety. Six families remain evacuated from their homes along Burlingame and Terwilliger.

KGW Reporter Keely Chalmers contributed to this report.