[<< wikibooks] HKDSE Geography/E1/Causes of Landslides
== Strength Factors ==


=== Rainfall ===
Rainfall is the immediate and direct cause of landslides.
Water acts as a lubricant. When a large amount rainwater seeps into soil, the soil's pore water pressure increases. The friction and internal cohesion of slope materials reduce. This reduces the strength of slopes and destabilises slopes.


=== Geology ===
Granite and coarse-grained volcanic rocks are vulnerable to weathering in a hot, wet climate like Hong Kong's. Feldspar is weathered chemically into clay. Quartz is resistant to chemical weathering, but is loosened. Joints, like those in granite, allow water to infiltrate and weather the rock to a great depth. Rapid chemical weathering creates a thick soil profile on slopes.


=== Vegetation Cover ===
Vegetation cover reduces surface runoff during heavy rainstorms. Plant roots bind soil particles together. However, plants also release organic acids which worsen soil cohesion 
Frequent hill fires destroy the vegetation cover, leaving slopes unprotected.


=== Construction ===

During development, concrete is used to cover the slope. Concrete is impermeable, so infiltration decreases in that area. The landslide risk is reduced. However, the overland flow will increase. Infiltration will increase downslope. Water acts as a lubricant. When a large amount rainwater seeps into soil, the soil's pore water pressure increases. The friction and internal cohesion of slope materials reduce. This reduces the strength of slopes and destabilises the slopes.


=== Cultivation ===
Unauthorised cultivation threatens slope stability. When people cut into slopes for cultivation without applying stabilisation measures, the internal cohesion of slope materials may reduce. The cohesion is further weakened by ploughing. Moreover, the removal of vegetation induces heavy rainsplash, so soil erosion is serious. The internal cohesion of slope materials is further compromised.


=== Poor Drainage ===
Both surface and subsurface drainage can drain away water during rainstorms. Poor management of the drainage system will increase the amount of infiltration. Water acts as a lubricant. When a large amount rainwater seeps into soil, the soil's pore water pressure increases. The friction and internal cohesion of slope materials reduce. This reduces the strength of slopes and destabilises the slopes.


== Stress Factors ==


=== Slope Gradient ===
Slopes with steeper gradients are more vulnerable to landslides. This is because gravity has a greater influence on the slope, increasing the sliding force. In Hong Kong, the general topography favours landslides since a lot of slopes have gradients >26°, which means they have a high risk of landslides.


=== Cut Slopes ===
Cutting and filling slopes increases slope gradient, sometimes increasing it to 45°. This greatly increases stress.


=== Loose Materials and Large Trees ===
Unstable materials like large boulders add to the stress of the slope. Very large trees may have the same effect. They worsen the landslide risk.