Seeking Solutions to 'Slides

Ka-boom! A little after midnight on March 6, 2000, several boulders came sliding down the cliffs surrounding Waimea Bay on the island of Oahu. One, the size of school teacher's desk, crashed into the car of two women returning home. Luckily, no one was injured.

'Slides can happen in any mountainous area

Do we know when the next landslide will happen? Where will the next landslide happen? And just what is a landslide and what causes it to happen?

"Landslides," according to Dr. Horst Brandes, assistant professor of Engineering at the University of Hawaii Manoa, "are a natural part of life around mountains. And the steeper the slope, the more common the occurrence."

Landslides occur when a top layer of earth and rocks begins to split away from the main body of the mountain. "This usually happens because of intense rainfall, earthquakes, and even volcanic eruptions," said Brandes. "Sometimes, there is no clear event that triggers a landslide. Instead, it is a result of the natural breakdown or weathering, of soil and rock that occurs continuously, particularly in wet and hot climates such as Hawaii's," Brandes continued. Once a landslide event begins, the top layer of earth and rock begins its slippery slide down the mountainside, picking up momentum as it goes, much like the first drop of a roller coaster.

Unfortunately, landslides occasionally take a heavy toll on people and their possessions. For example, people's homes are being uprooted and twisted on their foundations by the slowly moving ground along the eastern slope of Honolulu's Manoa Valley. A tragic landslide occurred without warning at the popular hiking trail to Sacred Falls, killing eight people and injuring dozens. The recent Waimea landslide has kept Kamehameha Highway, the only link between Sunset Beach and Haleiwa, closed for weeks. What was normally a 10-minute commute between the two communities became an hours-long drive around the other side of the island.

The temporary road at Waimea.

How are engineers involved with landslides? Civil engineers work with geologists to study the patterns of soil movement in suspected landslide areas. They determine the factors that make certain areas more susceptible to landslides. These factors include the type and distribution of soils and rock; the steepness of the terrain; the way that water flows above and below the ground surface; and the degree of weathering and fracturing of the bedrock.

The civil engineer may then make recommendations for "mitigation" or "remediation," where future landsliding activity is suspected. For example, the hazard from future rock falls at the Waimea site will be reduced by shifting the road away from the mountainside, creating a buffer zone area where rocks may fall and not damage the passing traffic. In the case of the Manoa landslide, extensive repairs to the hillside have included securing the ground surface to the bedrock below with large structural anchors and installing extensive drainage systems that keep the water from undermining the stability of the soil.

Through the study and work of engineers, the causes of landslides are better understood. This knowledge and information helps people protect themselves and their property. "One can't really stop the forces of nature, but we can sure try and minimize its effects," said Brandes.





 

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