Mass Movements: When the Ground Gives Way

Hello! Welcome to your study notes on Mass Movements. Ever wondered why landslides happen after a heavy typhoon? Or why some hillsides are covered in concrete? This chapter will answer those questions! We'll explore why rock and soil sometimes decide to slide, flow, or fall downhill, and what we can do about it. It's a really important topic, especially for us living in a hilly city like Hong Kong. Let's get started!


What Exactly is Mass Movement?

In simple terms, mass movement (also known as mass wasting) is the downhill movement of soil, rock, and other loose materials on a slope, mainly due to the force of gravity.

Analogy Time! Imagine you're building a tower with toy blocks. If you stack them straight up, they're stable. But if you start making the tower lean, eventually, gravity will win and the blocks will tumble down. Mass movement is like that, but for a whole hillside!

Gravity is the ultimate driver of all mass movements. It's constantly pulling everything towards the Earth's centre. On a slope, this pull has two parts: one part that holds the material onto the slope, and another part that pulls it downwards along the slope.

Key Takeaway

Mass movement is the downslope slide of earth materials, and gravity is the number one cause. It's a natural part of how landscapes are shaped through erosion and other external processes.


The Science of a Slope: A Balancing Act

Don't worry if this seems tricky at first! To understand why a slope fails, we need to think about two competing forces. It's like a tug-of-war on the hillside.

1. Shear Stress: This is the force trying to pull the material downhill. It's caused by gravity. A steeper slope or heavier material means higher shear stress.

2. Shear Strength: This is the force trying to hold the material in place. It's the internal resistance of the slope, coming from things like friction between particles and the binding power of plant roots.

A mass movement happens when the 'downhill' force wins the tug-of-war.

$$ \text{Mass Movement occurs when } \textbf{Shear Stress} > \textbf{Shear Strength} $$

Quick Review Box

Stable Slope: Shear Strength is GREATER than Shear Stress. (The slope holds itself together).
Unstable Slope (Failure!): Shear Stress is GREATER than Shear Strength. (The slope gives way).


Factors Influencing Mass Movement

So, what can tip the balance and cause Shear Stress to overcome Shear Strength? These factors can be natural or caused by humans.

Natural Factors (Nature's Triggers)

• Water: This is a HUGE factor. Too much water from heavy rain is very dangerous because it:
    - Adds weight to the soil, increasing shear stress.
    - Lubricates soil particles, allowing them to slide past each other easily.
    - Reduces friction between particles by filling the spaces between them, which weakens the slope's shear strength.

• Slope Gradient (Steepness): The steeper the slope, the greater the pull of gravity (shear stress). This is why you see more landslides in mountainous areas.

• Geology (Rock Type): Weak, weathered, or fractured rocks have very low shear strength and are much more likely to fail. In Hong Kong, the heavily weathered granite is a major culprit.

• Lack of Vegetation: Plant and tree roots act like a natural net, binding soil particles together and increasing shear strength. Where vegetation is removed, the slope is left exposed and weak.

• Triggering Events: An earthquake or a volcanic eruption can shake the ground and trigger a massive landslide in seconds. In Hong Kong, the main trigger is intense rainfall from typhoons.

Human Factors (Our Impact)

Human activities can dramatically increase the risk of mass movement, often by changing the natural balance of a slope.

• Deforestation: Removing trees for farming, construction, or logging destroys the root network that holds the soil together. This is a very common cause of human-induced landslides.

• Slope Modification (Undercutting): When we cut into the base of a slope to build roads or buildings, we remove the slope's support, making it unstable. It's like kicking out the legs from under a table!

• Loading a Slope: Constructing heavy buildings or dumping waste on the top of a slope adds extra weight, which increases the shear stress and can lead to failure.

• Poor Drainage: Leaking water mains, broken sewers, or blocked drains can saturate the ground with water, weakening it in the same way heavy rain does. This is a significant issue in urban areas.

Key Takeaway

Both natural and human factors can cause mass movements. The most dangerous combination is often steep slopes + heavy rain + human interference. Water is the biggest single trigger for making a stable slope unstable.


Types of Mass Movement (From Slow to SUPER Fast)

Mass movements are often classified by their speed.

Slow Movement: Soil Creep

This is the slowest type of mass movement, happening at just a few millimetres per year! It's the gradual downhill movement of soil. You can't see it happening, but you can see its effects over time, like tilted trees, fences, and telephone poles.

Fast Movements: Landslides

Landslide is a general term for the rapid downslope movement of a mass of rock or soil. This is the type we are most concerned about because it is a major geological hazard that can cause widespread destruction and loss of life.

Examples of fast movements include rockfalls (rocks falling from a cliff) and mudflows (a river of mud and debris), which are all types of landslides.

Did you know?

The deadliest landslide in recent history was in 1970 in Peru. An earthquake on Mount Huascarán triggered the collapse of a glacier and rock face, creating a debris avalanche that travelled at over 300 km/h and buried two entire towns, killing over 20,000 people.


A Closer Look: Landslides in Hong Kong

Hong Kong is a world leader in managing landslide risk, but why is the risk so high here in the first place?

The "Recipe" for Landslides in Hong Kong

Think of it as a combination of four key ingredients:

1. Steep Slopes: Hong Kong is very hilly, with many natural and man-made slopes.

2. Intense Rainfall: We get heavy summer rains and typhoons that can dump hundreds of millimetres of rain in a single day, saturating the ground.

3. Weak Geology: Much of our bedrock is granite, which weathers into a thick layer of loose, sandy soil that becomes very weak when wet.

4. Dense Urban Development: For decades, hillsides were cut into to create space for buildings and roads, creating thousands of man-made slopes that need to be managed.

Managing the Risk: How Hong Kong Fights Landslides

After several disastrous landslides in the past, Hong Kong's government developed a comprehensive slope management and prevention system. This includes:

• Strengthening Slopes ("Hard" Engineering):
    - Retaining Walls: Concrete walls at the bottom of a slope to hold it back.
    - Soil Nails: Long steel bars are drilled into a slope to pin the loose soil to the stable rock underneath.
    - Greening Techniques: Planting special vegetation to bind the soil naturally.

• Controlling Water ("Hard" Engineering):
    - Surface Drains (Channels): To catch rainwater and guide it away safely.
    - Weepholes: Small holes in retaining walls to let groundwater out, preventing pressure from building up.

• Planning and Prevention ("Soft" Management):
    - Land-use Zoning: Restricting development on slopes that are too dangerous.
    - Regular Checks & Maintenance: The government regularly inspects all slopes and orders repairs.
    - Public Warning System: The Landslip Warning issued by the Hong Kong Observatory alerts the public to high risk.

A local example often studied is the Sham Wan Landslide, which highlights the dangers of unmanaged slopes and led to improvements in Hong Kong's slope safety system.

Key Takeaway

Hong Kong's high landslide risk comes from its geography, geology, and weather. The city manages this risk through a combination of engineering solutions (strengthening and drainage) and careful planning and public warnings.