Section Two

Collapsing slopes

What is a landslide?

soil layer

rock layer

wet soil

Landslide takes place when large masses of loose rocks and soil move downslope rapidly under gravity.

Types of landslide

Rapid movement of soil mass

Types of landslide

Rock fall

What are slopes?

Man-made slope Man-made slope

Natural slope

Conditions for landslides to occur in HK

1. Heavy rainfall

Soil becomes saturatedHeavy rainfall

Ground surface

Very little water

Some water in the soil spaces.

All soil spaces arefilled with water.

Soil spaces

All soil spaces are filledwith water. Soil particlescannot hold together.

Ground surface

After heavy rain, all soil spaces are filled with water. Soil becomes saturated.

Soil particles cannot hold together.

Conditions for landslides to occur in HK

2. Steep slopes

Hong Kong is hilly with many steep slopes. The steeper the slope, the greater is the chance of landslide.

Conditions for landslides to occur in HK

3. Slope cutting and slope filling

Conditions for landslides to occur in HK

3. Slope cutting and slope filling• After people have cut a slope, the soil on the upper part of the cutting is no longer supported from below.•When the soil becomes saturated, the slope becomes unstable.

Conditions for landslides to occur in HK

4. Thick soil layer on slopes

Soil layer is thick that more soil will slide down slope.

In Hong Kong, the layer of soil is thick. This is related to the hot and wet climate as well

as the main type of rock in Hong Kong which is called granite.

High temperatures and high rainfall are favourable for weathering.

Granite has lots of joints that water can seep into the joints easily. That favours the breaking down of rocks’ minerals.

Conditions for landslides to occur in HK5. Removal of vegetation•Without trees, there are no roots to bind the soil. There is also no organic matter to stick the soil together.

Conditions for landslides to occur in HK6. Triggering action

piling explosion

earthquake

passing of heavy vehicles

A landslide may cause by a sudden shock.

Summary of the causes of landslide

thick soil layer on slope

heavy rainfall

removal of vegetation

slope cutting and slope filling

buildings increase weight on slope

Rock layer Granite is easily weathered to form thick soil

steep slope

How can landslides affect us?

How can landslides affect us?

1. Loss of life

How can landslides affect us?

2. Property damage

How can landslides affect us?

3. Disruption of traffic

How can we prevent landslides?1. Careful design of projects on slopes

Reduce slope angles as much as possible

How can we prevent landslides?2. Stabilising slopesRetaining wall and drainage channels are built to stabilize the slopes.

Preserve natural vegetation.

How can we prevent landslides?

3. Monitoring and maintaining slopes

The slope owners are responsible for the maintenance and safety of their slopes.

How can we prevent landslides?

4. Good farming practices

Contour ploughing

Building low walls