The mudslide that closed the Pali Highway in both directions also left the slopes above unstable, leaving worried state officials with few options other than to monitor the hillside and remove water and debris from the pavement.

It's too early to tell what crews can do to stabilize the hillside around the tunnel openings, a key to reopening the roadway, Scott Ishikawa, Department of Transportation spokesman, said last night.

The state will call in an engineer today to assess the condition of the slope and determine what should be done.

"It's a mess up here — one of the worst I've ever seen," Ishikawa said.

Last night, a waterfall was still flowing down the hillside, complicating efforts to shore up the slope. The ground was so soft that trees were leaning over and officials kept both tunnels closed.

Very little can be done until the rain abates and the ground dries up, Ishikawa said.

As of 8 p.m., crews had removed 80 truck loads, or 10 tons, of mud and debris from the collapsed hillside. The landslide carved a swath about 200 feet wide and 60 feet high from the mountainside, said Scott Naleimaile, of the state highways division, who is supervising the cleanup.

An excavator was to be brought in overnight to remove additional material, such as trees, to shore up the slope as much as possible.

All state highway maintenance crews, about 60 people, worked all day and through the night on the hillside.

The debris from the landslide was enough to cover all four lanes, and was 8 to 10 feet high.

The landslide was the result of heavy rains that pounded the highway area. More than a foot of rain fell during the previous two days — most of it in one intense 12-hour period yesterday — and more heavy showers are on the way.

"It's Mother Nature that's making its own stream at this point ... we've got to wait for the water to recede before we can do anything more," Ishikawa said.

Ishikawa said the mudslide, fell outside the townbound lane of the Pali tunnels, adding that no other mudslide had ever fallen in that area before.

The culprit had been the tremendous rains that hit the area in a short amount of time, he said. Until the rains subside and the hillside becomes more stable, there is virtually nothing anyone could do, he said.

"The rain has not let up. And we'll continue to have mud and water heading down onto the road," he said. "We'll just have to monitor it tonight."

BLAME THE RAIN

Maureen Ballard, national weather service meteorologist, said by yesterday afternoon the area in the general vicinity of the Pali had received nearly 14 inches of rain in the previous 48 hours — the most of any location on O'ahu. Of that amount, more than 8.13 inches had fallen between 2 a.m. and 2 p.m.

Stephen Martel, University of Hawai'i professor of geology and geophysics, said the recent earthquake probably contributed little to either the Pali mudslide or one that occurred on Kapa'a Quarry Road near Kalaniana'ole Highway later yesterday afternoon.

Martel put the blame on the showers.

"If we keep getting heavy amounts of rain, we could be in for more slides," Martel said. "It's pretty much that simple.

"The potential for rock falls goes up when it rains. The rainfall can trigger landslides, it can trigger rock falls and it can trigger landslides that turn into rock falls. In some cases it can trigger rock falls that evolve into things called debris flows and avalanches — so there are a lot of different slope failures. But the odds on all of these go up when you have heavy rains."

What can residents do? Be vigilant, Martel advised.

"If people live at the base of a cliff and they see unusual amounts of water coming into their yard, or they hear unusual loud noises up slope that might be because of falling rocks, they might want to get up and get out of the house."

Martel said O'ahu's geological infrastructure — with its steep terrain and warm, moist climate with vegetation that breaks and fractures rocks — makes the island susceptible to mudslides and rock falls. While the island hasn't changed that much geologically over eons, humans have ramped up the risk by developing homes near the base of slopes.

"These are natural processes, and until we move in and invade the area where they occur, they won't affect us. But we're progressively encroaching on steeper and steeper slopes."

ROCK-FALL RISK

Windward Community College professor of geology Floyd McCoy agreed with Martel that human development into the islands' mountainous regions was flirting with danger.

"How do people think these slopes developed?" asked McCoy. "By falling rocks over long geological periods of time — 10 or 12 life spans or more. So, if you move highways or housing developments into the middle of these areas, it's only natural that it going to be risky."

Landslides have plagued the Castle Junction area of Windward O'ahu for decades, and efforts to mitigate the problem culminated in a 19-month, $7.8 million effort to scrape an eroding, nearly vertical cliff into a gently sloped, landscaped hillside.

The work on the hillside along Kalaniana'ole Highway ended in December 2004, after crews removed 250,000 cubic yards of dirt, the equivalent of 18,000 truck loads.

In addition to the Windward-side work, the state in March 2005 announced a $4 million rock-fall mitigation project to protect homes in Nu'uanu and motorists in Pacific Palisades.

The plan is to remove boulders and install protective fencing and netting in two O'ahu areas considered at greatest risk for rock falls.

Reach Will Hoover at whoover@honoluluadvertiser.com.