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 Natural slope
Conditions for landslides to occur in HK 1.Heavy rainfall Soil becomes saturated Heavy rainfall Ground surface Very little water Some water in the soil spaces. All soil spaces are filled with water. Soil spaces All soil spaces are filled with water. Soil particles cannot 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 HK 5.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 HK 6.Triggering action pilingexplosion 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?
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 slopes Retaining 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