HKDSE Geography/E1/Solutions to Landslides

The Geotechnical Engineering Office of the Civil Engineering and Development Department is responsible for slope protection in Hong Kong.

On Natural Slopes

Blocking and Diversion of Debris

These include

• Protective barriers
• Check dams
• Boulder/debris fences
• Debris diversion

Effectiveness: These measures may not work if the barriers are too weak to hold the debris, or if the amount of slope materials that collapsed is too much for these measures to handle.

Drainage

Drainage system drains away water so as to decrease infiltration. The pore water pressure will not increase as quickly during a landslide, maintaining strength.

Effectiveness: An efficient drainage system can reduce infiltration and thus the landslide risk. However, a faulty drainage system can worsen matters, such as the leaked pipe at Kwun Lung Lau.

Land Use Planning

The GEO has records of all landslide sites. Land use planning is carried out. Areas with great landslide risk may apply appropriate stabilisation measures. Some slopes may be evacuated because the slope is very unstable and the cost of stabilisation is too high.

For example, some isolated dwellings at the foot of Queen's Hill were evacuated. (Source)

The Landslide Risk Assessment Framework evaluates the potential danger of landslides for development projects.

Land Use Restriction

• Hillside squatters are relocated and the huts are demolished. In the '80, there were 27 landslide-related deaths in squatter areas; there were only two in the '90s.
• Black spots of unauthorised cultivation are identified for clearance.
• Careful study and government approval are legally required before slope cutting.

On Man-Made Slopes

Slope Maintenance

• The government maintains government slopes
• Private land owners are required to maintain their slopes, but the government provides free advisory services for them. This is important as 1/3 of Hong Kong's man-made slopes are private.
• The Catalogue of Slopes identifies the maintenance responsibility of slopes. This aids different parties in identifying their responsibilities.

Ensuring that the appropriate persons maintain slopes avoids unnecessary deaths.

Regular Checking

• Existing man-made slopes are inspected each year to have their risks evaluated.
  • An inspector may check for: cracks, surface damage, damage to drainage channels
  • If problems are found, repair work will be carried out.

On very dangerous slopes, they have instruments monitoring water pressure and ground water level. This way, in the event of high landslide risk, the area can be evacuated on time.

Qualified geotechnical engineers carry out engineer inspections at least once every five years, and may issue warnings if slopes are found to be unstable.

Slope Strengthening

By altering the landscape:

• Loose materials like unstable/overhanging boulders are removed from slopes. Thus the additional stress is removed.
• Large trees are removed from slopes. Thus the additional stress is removed. They are replaced by smaller plants with roots that are equally capable of retaining soil.
• Afforestation is carried out with technologies like hydroseeding. Specific vegetation and grass types that are good at retaining soil (i.e. have dense, long roots) are planted. Grass can create a woven map over the slope.

By geotechnical means:

• Soil nails are a technique of slope enforcement. Nails composed of a steelbar, surrounding grouted column and a nail head are called soil nails. They are inserted into slopes are regular intervals, anchoring them to the slope. This prevents slope materials from sliding down.
• Protective retaining walls can support slope surfaces.
• Impermeable layers can prevent water from infiltrating. Chunam plaster and shotcrete are examples. Shotcrete involves spraying concrete on slopes at high velocity. It is cheap and easy to adopt.

Effectiveness note:

• Walls that are too weak may fail (e.g. Kwun Lung Lau)
• Impermeable layers like shotcrete provide some surface protection, but no strong support.
• The effectiveness of hard structures wane over time.

Others

Warnings

The Landslip Warning System is jointly operated by the GEO and the HKO. They provide instant announcements of the warnings and take precautionary measures.

Education

The public are educated about:

• Slope maintenance
• Precautionary measures during rainstorms
• What to do during landslides

It has the added benefit of creating support in the community.

This is done in several ways: Exhibitions, talks in public schools, talks for the general public, incorporating slope protection into the Dynamic Earth module of the geography syllabus, preparing education toolkits, leaflets, booklets, handbooks, constructing the Hong Kong Slope Safety website, online courses

Advisory

The GEO also has a 24-hour service for government departments to seek advice after landslides. There is also a free advisory service for private slope maintenance.