Rocks + Weathering: Case Studies Flashcards
1
Q
Key facts about Hong Kong
A
- Hong Kong is one of the most densely populated areas in the world - 1075 km^2 with a population of 7 million
- most areas of Hong Kong have naturally steep terrain - upper slopes steeper than 30°
- most of the population is concentrated in less steep areas on both sides of Victoria harbour
2
Q
Causes of landslides in Hong Kong
A
- a combination of high rainfall (wet season May to sept), steep slopes and dense human developments on the islands
- the geology of Hong Kong is constructed mainly from 3 rock types: sedimentary rocks, granites, and volcanic rocks
- the sedimentary rock forms the lowlands
- the granites + volcanic rocks are situated on higher grounds + are prone to failure -> both are seriously weathered, although granite rocks tend to be weathered more deeply
- volcanic rocks are more resilient + less prone to weathering + therefore less prone to slope failure
- vegetation is believed to have increased the problem in the 1966 landslides - trees held back many smaller landslides + allowed larger ones (washouts) to occur
- other forms of landslides involve debris valances and rockslides
3
Q
Examples of landslides
A
- 1947-1997, more than 470 people died as a result of landslides
- June 1996 - rainstorms triggered massive landslides
4
Q
Cause of June 1996 landslide
A
- rainfall had been high for the first 10 days in June - over 300mm had fallen, compared with 130mm in a normal year
- on 11 + 12 June over 400mm fell - nearly 1/3 of this happened in one hour
- 15 June - the area had received over 1650mm of rainfall
- over 700 landslides were recorded in Hong Kong that month
5
Q
Managing landslides in Hong Kong: GEO
A
- the slope safety system is managed by the Geotechnical Engineering Office (GEO) - has staff establishment of over 700
- the GEO maintains its slope safety through investigating + researching the causes of significant + serious landslides to improve the slope safety systems
- the GEO is continuously updating + maintaining the Catalogue of Slopes which contains info of some 57,000 sizeable man-made slopes in Hong Kong
6
Q
Managing landslides: Govt.
A
- govt. intervention to ensure that private owners of slopes take responsibility for slope safety - if a slope owner doesn’t comply with the regulation, prosecution will lead to a HK$50,000 fine + imprisonment for up to one year
- this has had successful results - risks from landslides has been reduced by 50% since 1977
- however, as a result of continued pop. growth, developers increasingly building up the slopes - the risk of landslides increases
7
Q
Managing slopes: drainage
A
- vital to remove excess water from slopes - surface drainage systems + protective covers are two methods used to protect slopes
- surface drains are very vulnerable to blockage - without proper drain maintenance, landslides are more common than on slopes without drains - due to confusion over responsibility many are not maintained in Hong Kong
- man made slopes are used in Hong Kong for slope stabilisation - these contain drains to intercept + direct water away from slopes
-the slope is usually protected from infiltration + the erosive effect of water by impermeable hard covers
8
Q
Managing slopes: greening techniques
A
3 main types used in Hong Kong:
- mulching system -> provides a protective cover that makes it impossible for natural vegetation to grow on the slope; a natural vegetation cover is able to grow through the mat, securing it in place
- long-rooting grass - fast + cost-effective -> it’s applied by drilling planter holes into a hard cider. The drilled hole is filled with soil mix and fertiliser + the long rooting grass is planted within
- fibre reinforced soil system -> constructed by mixing polyester fibre into sandy soils = capable of resisting tension
9
Q
Adv of mulching system?
A
- higher adhesive capacity on steep slopes
- higher resistance to rain erosion
- higher water-retaining capacity
- long-lasting fertilisers
- adaptable to rough surfaces
10
Q
Adv of planting long rooting grass?
A
- reduces moisture from soil, decreasing the risk of loss of cohesion
- natural + environmentally friendly
- cost-effective
- fast + easy installation
- can be applied on steep slopes
- low maintenance
11
Q
Adv of fibre reinforced soil system?
A
- self-sustained vegetation system with low maintenance
- fibre strengthens soil particles to prevent erosion
- visual improvements of slope with various plant species
- restoration of natural habitats on the slope
12
Q
What was the Abbotsford landslide
A
- Dunedin, New Zealand
- 8 august 1979 - 7 hectare section of Abbotsford started down hill at a rate of 3 metres a minute
- displacement of 50 metres took place in about 30 mins - leaving a small rift 30 metres deep at the head of the slope
13
Q
Main cause of Abbotsford landslide
A
- the landslide was a block of sandstone resting on a bed of weaker clay -> a permeable hard rock resting on an impermeable rock
- the slope was dipping at an angle of 7 degrees
- leaking water main + heavy rainfall led to excess water being collected in the impermeable clay -> reducing its strength + cohesion, causing the sandstone to slip along the boundary of the two rocks
14
Q
Other contributing factors to the landslide?
A
- deforestation in area - reduced evapotranspiration + less binding of the soil by plant roots
- urbanisation - modified the slope by cutting + infilling, altering surface drainage
- quarrying of material at the toe of the slope - removed support from base
15
Q
Impacts of Abbotsford landslide
A
- houses + 17 people were on board of the landslide - no one killed
- 69 homes destroyed or damaged
- over 200 people displaced
- total cost from the destruction of homes, infrastructure + relief operation = over 7 million
- depressed house prices in surrounding area + expense of prolonged public enquiry
- psychological trauma
