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What is the relationship between water and landslide?

Landslides often occur during or after long-term rainfall or a torrential rain, when the slope is with high water content. Water has double effects to the slope stability:

(1) Increase the weight of slope materials

As water can increase the weight of soil and rock materials, it is known as a factor that triggers landslide. The pore space of soil and rocks exposed on the slope is about 10% to 35% of their size. When it is dry, the pore space is occupied by air. However, after long-term rainfall, the pore space is fully occupied by water and therefore increase the weight of sediments. In other words, the power that drives the sediments to move downward is therefore increased.

(2) Reduce the strength of rock

When the water passes through the pore space of specific rocks, some of their easy-to-dissolve cementation substances (ex. calcium carbonate) can reduce the cohesion of slope materials and drive the particles to move downwards along the slope. Water can also soften the shale layer and even cause the expansion of specific clay materials, resulting in a decrease of friction between rock layers. Also, when the groundwater passes through loose slope materials that are composed of coarse and fine soil and rocks, fine particle substances can be washed away and result in the hollowing out of backend slope materials. When the water retained in rock fissures is iced, the force applied to the pore wall can further disintegrate the rock along its layer, joint or other fissure (frost wedging). These loose rock materials will then move downwards.

For more information, please go to the “Taiwan Geoscience Portal” website.