Rocks and weathering (case studies)

Question 1

HK: How is the climate and landscape?

Annual precipitation, topography

Answer 1

• The climate is very wet due to its geographical location and being an island
• 94 inches of annual rain
• It is quite hilly with high relief in some areas

Question 2

HK: Factors increasing landslide vulnerability

(physical)

Answer 2

• Wetness, increases chance of flows
• Steep gradient, increases chance of rockfall
• HK is particularly vulnerable due to hilly topographical setting

Question 3

HK: How much of the land in the city is mountainous?

Answer 3

60%

Question 4

HK: How many landslides occur every year in the city?

Answer 4

Average of 300

Question 5

HK: How many people died in landslided between 1947 and 1997?

Answer 5

Over 470 people

Question 6

HK: When was one of the largest landslides?

Answer 6

In 1966

Question 7

HK: What were the effects of the catastrophic 1966 landslides?

Answer 7

• Killed 64
• Made 2500 homeless
• 8000 people had to be evacuated

Question 8

HK: What caused the 1966 landslides?

Answer 8

• Over 300mm of rain falling in the first 10 days of June
• Followed by over 400mm on the 11th and 12th, an incredibly large amount for such a short period of time

Question 9

HK: How much rainfall had fallen and how many landslides has been reported in HK by June 15th?

Answer 9

• 1650mm of rain had fallen
• 700 landslides had been reported

Question 10

HK: How did vegetation contribute to the landslides?

Answer 10

High levels of vegetation stopped smaller landslides from occuring, so material built up and large washouts were caused by the high precipitation levels

Question 11

HK: What is the population density?

Answer 11

17,311 people per square mile

Question 12

HK: How steep are upper slopes?

Answer 12

Up to 30 degrees

Question 13

HK: What about land use in HK makes it particularly vulnerable to mass movement?

Answer 13

There is near constant bulding work, and vibration and agitation from construction can decrease slope stability

Question 14

HK: Solutions to manage slopes: when was the slope safety system implemented, who implemented it and how many staff members does it have?

Answer 14

• Implemented in 1977
• By the Geotechnical engineering Office
• Staff of over 700

Question 15

HK: How much has slope risk reduced by due to the slope safety system?

Answer 15

50% reduction in slope risk since 1977

Question 16

HK: What does the slope safety system do?

Answer 16

• Reduce risk by monitoring at risk slopes and implementing strategies such as the drainage networks and greening techniques
• Raise public awareness of how to manage slope risk

Question 17

HK: Drainage networks: what are they and what is their limitation?

Answer 17

• Collect surface runoff (overland flow) and move it off slope areas as heavy rainfall is a significant risk factor for landslides in HK
• However, they must be properly maintained and the issue of which landowner (or the government) has the responsibility to maintain them means they are frequently blocked and cannot serve their purpose
• Cost 3.8 Billion Dollars

Question 18

HK: Greening techniques: definition

Answer 18

Greening techniques are a soft engineering method to reduce the risk of mass movements by using natural vegetation

Question 19

HK: Mulching system and advantages

Answer 19

• Provides a protective cover that makes it possible for vegetation to grow through the mat, securing it in place.
• Higher adhesive capacity on steep slopes
• High resistance to rain erosion
• Higher water retaining capacity
• Long lasting fertilisers
• Adaptable to rough surfaces

Question 20

HK: Planting long-rooting grass and its advantages

Answer 20

• Holes are drilled into a hard cover which are filled with soil mix and fertilisers and long rooted grass is planted in them
• Natural and environmentally friendly
• Fast and easy installation
• Low maintenance
• Applicable to steep slopes

Question 21

HK: Fibre reinforced soil system

Answer 21

• Constructed by mixing polyester fibre into sandy soils to allow them to resist tension
• Self sustained vegetation system with low maintenance
• Visual improvements to the slops with planting of plant species
• Restoration of natural habitats on the slope