Mass movement Flashcards
The Hong Kong government started a slope-retro fitting project known as the…
landslip preventative measures programme in 1977.
What was the LPM’s aim?
The LPM scheme aimed to reduce risks from landslides by upgrading man-made slopes to modern safety standards.
The LPM Program has an annual expenditure of about HK$870 million to upgrade substandard government slopes and undertake safety-screening of old private slopes.
Man-made slopes are the main type of…
slope stabilisation method in Hong Kong.
Explain how the drainage system in man-made slopes reduce chance of mass movement?
The man-made slopes have open concrete U-shaped channels constructed at the crest and toe of each slope. These drains intercept and direct water away running on the slopes. This reduces the amount of water that infiltrates into the soil so the soil is less likely to become saturated. Therefore, the likelihood of cohesive bonds between soil particles being broken (due to saturation) is also reduced.
Furthermore, the shear stress from the weight of water on the slope is reduced.
How does the nature of the material of man-made slopes reduce mass movement.
The slope body of man-made slopes is usually protected from infiltration and erosive effects of water by an impermeable hard cover which is made out of chunam plaster, a mixture of soil, lime and cement, and shotcrete.
What is an evaluation point for drainage systems in man-made slopes?
Hong Kong’s tropical climate is inviting to fast growing natural vegetation. Vegetation can take rooting easily to soil debris or decomposed vegetation. This can lead to drains and channels quickly being overgrown with vegetation ultimately blocking the drain. Therefore, drainage systems must be properly maintained in order to discharge rainwater and surface water safely and effectively from the slopes.
However, often in Hong Kong, due to confusions of the responsibility of maintenance, these tasks are rarely carried out (e.g. many private slopes are simply unattended for decades after their construction, with no maintenance being carried out until failure occurs).
Explain how the Mulching System helps to reduce mass movement.
(Greening technique)
The Mulching system provides a protective cover that vegetation can grow through, to vegetation hold the protective cover in place. The roots of vegetation increases the internal strength of the soil. Also, the cover also absorbs excessive rainwater, which is the primary cause of landslides in Hong Kong.
What is an evaluative point for the Mulching system?
The Mulching system is not easily applicable on uneven surfaces. Therefore, it could be argued that mulching is likely to be in ineffective at reducing mass movement as Hong Kong’s undeveloped terrain is hilly to mountainous, with very little flat land. Furthermore, using planting methods are not strong enough to stabilise or protect hills from sliding.
Explain how a Fibre Reinforced Soil System reduces mass movement.
The Fibre Reinforced Soil system has a minimum of 3.3kg of fibre threads per one cubic meter of sandy materials. This mixture is directly applied on the slope and it increases the amount of tensile stress a slope can take, so the increasing the internal strength of the slope.
How has the Geotechnical Engineering Organisation (GEO) attempted to reduce mass movement?
The GEO is continuously updating, maintaining and disclosing the Catalogue of Slopes, which contains information of some 57,000 sizeable man-made slopes in Hong Kong. All of the slopes are mapped on the Slope Information System (SIS) for the public to obtain information about a particular slope and their location. This allows the public to report poorly maintained slopes and their state if they seem to be at risk of collapsing.
What is an evaluative point for the SIS?
The effectiveness is indicated by the declining casualty rate due to landslides.
When was the GEO established?
The GEO was established in 1977.
Overall, using statistics, have attempts of reducing mass movement being successful?
Yes,
According to risk assessment calculations, the overall landslide risk arising from old sub-standard man-made slopes to the whole community of Hong Kong has been reduced to about 50% of the risks that existed in 1977. However, reducing the risks landslides would be more successful, the government should make sure that the real estate industry takes precautionary measures before building on slopes or hilly areas.
Using examples, explain the causes of shallow landslides.
Shallow landsides are caused by erosion or shallow processes such as cleft water pressures behind loose rock blocks. This is the most common type of failure in Hong Kong and probably the most significant in terms of risk. They typically occur during or very shortly after severe rainstorms.
-74 incidents were reported between 1978 and 1995. There have been 9 reported fatalities since 1926.
Despite being caused by rainfall, the risk of the failures can be increased by human activity. For example, according to the GEO, the Shum Wam failure of 1995 killed 2 people and was considered to be triggered by flow down roads at the head of the landslide exacerbated by blocked drains.
Using an example, explain how rapid urban development can cause landslides.
Urbanisation expands due to the lack of space in highly populated regions. This leads to construction on harsher geographical surroundings, such as hills and mountains. Frequently, the slopes are unstable due to increase shear stress from human activities. Therefore, the risk of landslides becomes greater. For example, during the last three decades landslides have occurred with increasing frequency in the Highlands of Sri Lanka, due to the increasing population people have been migrating upslope for the construction of houses on unstable lands.
