“St. Francis’ Canossian College has the threat of experiencing mass wasting in the future. Do you agree?

What is mass wasting?What is landslide?

• Mass wasting is the geomorphic process by which soil, regolith, and rock move down slope under the force of gravity.

• Landslide is a kind of fast movement by rocks and soils of all sizes collapse through either sliding or slumping along a curved surface.

The landslide in 2000.

Did SFCC experienced landslides before?

• occurred on 24 August 2000 • involving unregistered fill on the hillside

adjacent to No. 2B Bowen Road on Hong Kong Island.

• Debris with an estimated volume of 120 m3, • transported downstream for a long distance

beyond the landslide site (up to about 250 m on plan) to reach Hong Kong Park below Kennedy Road.

• To reach also our school!!!!!!!!

Physical FactorsThe potential shear threats of slopes near our school

Steep gradientLarger

gravitational force

force>resistance

stressSteep slope

The distribution of rainfall

Day

Jan Feb

Mar

Apr

May

Jun Jul Aug

Sep

Oct

Nov

Dec

Total

70.3  

27.6

40.9

547.7

 208.3

 443.3

304.0

 600.7

152.6

204.1

96.8

56.0 

The rainfall intensity is the highest in August in 2000, therefore it is one of the reasons of the occurrence of landslide.

The rainfall distribution in 2000

Human factorLack of maintenance of the

drainage system• According to the Civic Engineering and Development department, there has been no maintenance work done on the slopes on Kennedy Road other than the two below.

Large populationdensely populated (as there are many residential buildings)More pressure to the slope systemStress

There are many students in our school.

Large Population

Different type of land use nearby

Uphill and downhill sides: Roads, residential buildings, school…etc.

Many land use = many human activities

much pressure to the slope

StressTransport land use

Residential land use

The rainfall intensity of Wanchai is not high comparing to that in the eastern part of Kowloon

It is not usual for Wanchai to have prolonged heavy rainfall.

light rainfall would not add weight to slopes or create lubrication effect

Strength

Potential shear strength – Physical Factor

Distribution of rainfall

Human FactorSubsurface drainage system

Surface and subsurface drainage system were built on the slope

can drain water from the slope surface or soil

can effectively reduce the rising rate of pore water pressure in the soil after rainstorm.

less pressure to the slope

strength

Measures

• Done by the Civil Engineering and Development Department

• Aim: increase the strength and reduce the stress of the soil on slopes

1. Covering slopes with impermeable layers

Protect slope surface with masonry walls (a kind of impermeable layer of covering)

water cannot percolate into rocks and both physical and chemical weathering could not take place. maintain the stability of slope

There would not be regolith

Taken on Kennedy Road

2. Afforestation

Planting vegetation covergrass can help to bind the soil with their long and dense roots, producing a “woven mat” covering the slope

less likely to be washed away

3. Removing large trees

since densely vegetated area added pressure and weight to the slope

The government removed large tress from steep slopes

4. Put up warning signs

People would learn to avoid slopes on rainy days

Minimize destruction of landslide

Fewer casualties

Does SFCC has the threat of experiencing mass wasting in

the future?• The chance is low.• Potential strength> potential stress• The management of slope is updated

and comprehensive.• But mass wasting is still possible, if

given that a sudden stress is caused to the slopes.

Physical FactorRainfall intensity

• The nature is dynamic.• In summer, there might be chances

of prolonged heavy rain to occur as the summer monsoon brought moisture from the south (sea) to inland.

creates lubrication effecttrigger landslide

Human FactorLack of maintenance

Lack of maintenance surface and subsurface drainage system

blockedwater trap inside the permeable layersSubsurface water increases weight of the

soil masswater acts as lubricant, reducing frictional

resistance and increasing mobility of flow.Raises water table, increases pore water

pressure, more stress

• Also, there are several reasons that might trigger landslide.

Also, there are several reasons that might trigger landslide.

Our school

Ｗｈｅｒｅ　ａｒｅ　ｔｈｅ　ｈｏｌｅｓ？

Reflection

• 1. Although the government has carried out several measures to deal with the landslide problem on Kennedy road, no one could promise the safety of our slopes.

• Our earth is dynamic, hazards could occur any time.

• So in summer, watch out!• Also, to be a responsible citizen, don’t throw

rubbish into the drainage system, or you might be the one who trigger landslides!