HKDSE Geography/E1/Causes of Landslides

Rainfall
Rainfall is the immediate and direct cause of landslides. Persistent rainfall exceeding 70mm/hr has a high chance leading to landslide

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.

The Landslide Potential Index measures the capability of a rainstorm to produce landslides. It is devised by the CEDD and tells the relative severity of the rainstorm in relation to the rainstorm in the past 20 years. The highest index recorded is the 2008 one, with the Cafeteria Old Beach landslide. The rainstorm causing the Kwun Lung Lau Landslide had an LPI of 10. (Source)

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. (Refer to spheroidal weathering)

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.

There are around 57000 slopes of significant size in Hong Kong, of which a third of them are private property. (Source)

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.

After a persistent 48-hour rain, a drainage pipe leaked. The water table rose rapidly. Water acted as a lubricant. When a large amount rainwater seeped into soil, the soil's pore water pressure increased. The friction and internal cohesion of slope materials reduced. Eventually, the weak protective masonry wall failed. The slope collapsed and killed five pedestrians.

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.