Core Physical Rocks &weathering 3.4 The human impact

• Mass movement rates are altered by human activities like building, excavation, drainage, and agriculture.
• Human activities can accelerate mass movements by destabilizing slopes.
• Localized erosion is intensified by footpath trampling in recreational areas.
• Humans sometimes create mass movements by piling unstable waste soil and rock.
• Landslides result from undercutting or overloading slopes.
• Human interference with slopes typically speeds up natural processes.

Urban Slope Modification

• Urban areas experience high slope modification due to building and road construction.
• Most buildings with foundations modify the natural slope of the land.
• Large modern buildings often require material removal even on flat sites for proper foundations.
• Slope modification increases on steeper slopes.
• Cut-and-fill techniques create horizontal bases and access, leading to over-steepened slopes.
• Steep slopes, devoid of soil and vegetation, are less stable and prone to landslips during intense rainfall.

Strategies to Reduce Mass Movement

• Human activities can also reduce mass movement.

Methods of Controlling Mass Movement: Falls

• Flattening the slope is a method to control falls
• Benching the slope is a method to control falls
• Drainage is a method to control falls
• Reinforcing rock walls with grouting, cement, and anchor bolts can control falls
• Covering walls with steel mesh can control falls

Methods of Controlling Mass Movement: Slides and Flows

• Grading or benching to flatten the slope is a method to control slides and flows
• Draining surface water with ditches can control slides and flows
• Sealing surface cracks to prevent infiltration can control slides and flows
• Subsurface drainage is a method to control slides and flows
• Rock or earth buttresses at the foot of the slope can control slides and flows
• Retaining walls at the foot of the slope can control slides and flows
• Pilings through the potential slide mass can control slides and flows

Pinning & Grading

• Pinning attaches wire nets or concrete blocks to rock faces to reduce rock fall and erosion risk.
• Netting helps collect scree fragments for safe removal later, commonly used where tourism is present.
• Grading re-profiles slopes to increase stability.

Afforestation

• Afforestation involves planting new forests in upper catchments to increase interception and reduce overland flow.
• This is a long-term strategy, as young trees initially intercept small water amounts.

Landslides in Hong Kong: A Case Study

• Hong Kong has a history of landslides due to high rainfall in wet season (May-Sept), steep slopes, and dense urban development.
• Between 1947 and 1997, over 470 people died from landslides.

Hong Kong Landslide Events

• Rainstorms in June 1966 triggered landslides killing 64 people and making 2500 homeless.
• Rainfall was over 300 mm in the first 10 days of June, compared to 130 mm in a normal year.
• On June 11-12, over 400 mm fell, with nearly a third in one hour.
• By June 15, the area had received over 1650 mm of rain and over 700 landslides were recorded that month.
• Vegetation can exacerbate the problem; geographers believe trees held back smaller landslides, leading to larger "washout" events, debris avalanches, and rockslides

Hong Kong's Geography and Landslides

• With an area of 1075 km², Hong Kong is densely populated with over 7 million people (2015).
• It consists of Hong Kong Island, Kowloon Peninsula, New Territories and over 230 islands with steep terrain.
• Most of the population lives along the less steep urban areas on both sides of Victoria Harbour.
• The slopes are steeper than 30 degrees.
• Urban development triggers landslides through excavation and building works.

Hong Kong Geology

• Hong Kong's geology consists of sedimentary rocks, granites, and volcanic rocks.
• Sedimentary rocks form the lowlands.
• Granites and volcanic rocks are on higher ground and prone to failure.
• Both are weathered, with granite weathering more deeply than volcanic rocks.
• Volcanic rocks are resilient and less prone to weathering, reducing slope failure.

Landslide Management in Hong Kong

• The Hong Kong government manages landslides through the Slope Safety System, managed by the Geotechnical Engineering Office (GEO).
• The GEO has over 700 staff and maintains slope safety by investigating landslides to improve the Slope Safety Systems.
• GEO updates and discloses the Catalogue of Slopes, with information on 57,000 man-made slopes.

Government Intervention

• The Hong Kong government ensures private slope owners take responsibility for slope safety.
• Non-compliance leads to prosecution, a HK $50,000 fine, and up to one year imprisonment.
• Government intervention has reduced landslide risk by 50% since 1977.
• Continued population growth causes developers to build further up slopes, increasing landslide risk and damages.

Slope Maintenance

• Removing excess water from slopes is vital in Hong Kong due to heavy rainfall and surface runoff.
• Surface draining systems and protective covers protect slopes.
• Surface drains are vulnerable to blockage.
• Without drain maintenance, landslides are more common.
• Responsibility confusion leads to poorly maintained drains.
• Man-made slopes in Hong Kong contain drains to intercept and divert water.
• The slope is often protected from water infiltration and erosion with impermeable hard covers.

Greening Techniques

• Greening techniques use natural vegetation to reduce mass movement risk.
• The mulching system provides a protective cover for natural vegetation to grow.
• Natural vegetation secures the mat in place.
• Long-rooting grass is a fast and cost-effective system for man-made slopes.
• Planter holes are drilled into a hard cover, filled with soil mix and fertilizers, and planted with long-rooting grass.
• Fiber reinforced soil system mixes polyester fiber into sandy soils.
• This mixture resists tension.

Greening techniques summary

• Mulching system: Higher adhesive capacity on steep slopes, high resistance to rain erosion, high water-retaining capacity, long-lasting fertilizers, adaptable to rough surfaces.
• Planting long-rooting grass: Environmentally friendly, cost-effective, fast/easy to install, can be used on steep slopes with low maintenance
• Fiber reinforced soil system: Self-sustaining vegetation with low maintenance, fiber strengthens soil particles to prevent erosion, visual improvement, restoration of natural habitats