LANDSLIDES is a movement of mass rock, debrisor earth down a slope due to gravity. The materials may move by falling, toppling, sliding, spreading, or flowing. When soil, rock, and other earth debris can no longer hold it together and gives way to gravity, landslides happen. Earthquake shaking and other factors can also induce landslides underwater causing tidal waves and damage to coastal areas. These landslides are called submarine landslides. Submarine landslides sometimes cause tsunamis that damage coastal areas. Volcanic landslides, also called lahars, are among the most devastating type of landslides.

GEOLOGISTS, scientists who study the physical formations of the Earth, sometimes describe landslides as one type of mass wasting. A mass wasting is any downward movement in which the Earth's surface is worn away. Other types of mass wasting include rockfalls and the flow of shore deposits called alluvium.

The Philippines is an example of a country that is prone to landslides. According to the MGB, up to 80 percent of the country's total land area is landslide prone, making the country the fourth most exposed to landslide risk, after Indonesia, India and China. More than 100,000 people are exposed in the Philippines, according to the UN International Strategy for Disaster Reduction (UNISDR).

ADDITIONAL INFORMATION :

DID YOU KNOW? The largest subaerial (on land) landslide in Earth's recorded history was connected with the 1980 eruption of Mount St. Helens volcano in Washington state, USA. That landslide had a volume of 2.8 cubic kilometers (0.67 cubic miles) of material and the landslide traveled about 22.5 kilometers (14 miles) down the North Fork Toutle River. Average landside depth was 46 meters (150 feet) with a maximum depth of 182 meters (600 feet). The landslide velocity was 112-240 kilometers per hour (70-150 miles per hour).

Some landslides move at many meters per second, while others creep along at an centimeter or two a year. The amount of water, ice, or air in the earth should also be considered. Some landslides include toxic gases from deep in the Earth expelled by volcanoes. Some landslides, called mudslides, contain a high amount of water and move very quickly. Complex landslides consist of a combination of different material or movement types.

Landslides are being mitigated significantly, with the use of geometrical steps that adjust the geometry of the hillside (in general the slope). Hydrogeological techniques for the reduction of groundwater levels or the reduction of the material's water content. Chemical and mechanical methodology in which efforts are made to increase the shear strength of the instability mass or to incorporate active external forces (anchors, rock or earth nailing) to counteract the destabilizing forces or passive forces (structural wells, towers or hardened soil).

Stability increases when ground water is prevented from rising in the landslide mass by (1) covering the landslide with an impermeable membrane, (2) directing surface water away from the landslide, (3) draining ground water away from the landslide, and (4) minimizing surface irrigation. Slope stability is also increased when a retaining structure and/or the weight of a soil/rock berm are placed at the toe of the landslide or when mass is removed from the top of the slope.

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