Landslides are one of the most destructive natural disasters seen across the world — especially in hilly regions like Sri Lanka, India, Nepal, Japan, and many mountainous countries. Every year, hundreds of homes, roads, bridges, and farms are damaged due to slope failures. But what actually causes a landslide?

From a civil engineering perspective, the answer is a combination of soil behaviour, groundwater movement, slope geometry, weather, and human activity.

In this article, we break it down in the simplest way possible so that anyone can understand — even without engineering background. Let’s dive in! 🚀

🧱 1. Weak Soil or Rock Layers (Poor Geological Conditions)

One of the biggest reasons slopes fail is the type of soil or rock beneath them. Every soil type has different strength properties — like cohesion, friction angle, and shear strength.

👉 Weak soils include:

🟤 Soft clay
🌫️ Loose sand
🪨 Highly weathered rock
🌋 Residual soils formed from decayed rock

When these soils absorb water, they lose strength — just like wet sand collapsing under your feet. In civil engineering terms, we call this loss of shear strength, and it is one of the major triggers of landslides.

Why it happens:

Long-term weathering breaks rock into weak particles
Areas with old landslides have already disturbed soil
Poorly compacted fill soil (from construction) settles unevenly

Simple example:

Try building a sandcastle with overly wet or overly dry sand — it collapses. The same thing happens on hillsides.

🌧️ 2. Heavy Rainfall & Water Pressure Build-Up

Rainwater is the number one cause of most landslides in tropical countries.
When soil becomes saturated, water fills the gaps between particles, increasing pore water pressure.

⚙️ What does this mean in civil engineering?

Increasing water pressure = decreasing soil strength.

The slope becomes heavier, weaker, and can no longer support itself.

Rain causes landslides through:

Infiltration of water into the soil
Rising groundwater table
Softening clay layers
Increasing weight of soil
Washing away the slope (erosion)

Easy explanation:

Think of a sponge. When dry, it's stiff. When filled with water, it becomes soft and collapses easily.

That’s exactly what happens to a hillside terrace or natural slope during intense rainfall.

🚜 3. Human Activities & Construction Without Proper Engineering

Humans often accelerate natural disasters through unplanned development.
Civil engineers call this anthropogenic causes.

Common mistakes include:

Why it is dangerous:

Cutting a hill removes the original support.
Filling a valley adds extra load on soft soil.
Altering natural drainage sends water into unstable areas.

Civil engineers design slopes using factor of safety (FOS) to ensure stability.
When construction happens without engineering expertise, slopes fail easily.

🪵🌲 4. Deforestation & Loss of Vegetation Cover

Trees and plants are nature’s own retaining structures! 🌱

Their roots:

Bind soil together
Reduce erosion
Absorb large amounts of rainwater
Anchor soil to bedrock

When forests are cleared:

Soil loses reinforcement
Rainwater flows directly into the slope
Surface erosion increases
Water infiltration increases
The slope becomes unstable

Simple comparison:

Grass is like the “glue” for soil.
Removing it makes the soil free to slide.

This is why areas with landslides often show:

Recent land clearing
Tea plantation expansions
Illegal timber cutting
Shifting cultivation

🌀 5. Earthquakes & Vibrations

Earthquakes shake the ground and reduce soil strength instantly.

In engineering terms:

Earthquake waves = increased shear stress
Loose soil (especially sand) = loss of friction
Steep slopes = rapid movement

Even small vibrations from heavy trucks or blasting can trigger a landslide in weak soil.

Critical risk zones:

Mountain roads
Quarry sites
Slopes with loose fill material

Earthquake-induced landslides are common in Japan, China, and Nepal — but Sri Lanka has experienced some during minor tremors too.

💧 6. Poor Drainage (Surface & Subsurface)

Even if the soil is strong, water can destroy it if it has nowhere to go.

Poor drainage causes:

Water accumulation
Increased pore water pressure
Erosion at slope toes
Softening of soil layers

Civil engineering solutions include:

Weep holes
Drainage paths
Berm drains
Subsoil drains (French drains)
Proper road shoulder drains

But when these systems clog or do not exist, slopes fail naturally.

Simple Example:

If a flower pot has no hole at the bottom, the soil becomes soggy.
Slopes behave exactly like that.

🛤️ 7. Natural Erosion Over Time

Rivers, streams, and rainwater slowly erode the base (toe) of a slope.

When the toe weakens, the upper soil loses support and collapses.

Causes:

Riverbank erosion
Wave action near coastal slopes
Continuous rainfall runoff
Poorly managed water channels

As civil engineers, we call this toe failure — a major reason for roadside landslides.

🧊 🌡️ 8. Weathering, Climate Change & Long-Term Soil Degradation

Climate plays a huge role in slope stability.
High temperatures, alternating wet and dry seasons, and freezing in some countries all weaken rock.

Over time:

Rocks break into smaller particles
Clay minerals form
Cracks appear
Soil structure becomes loose

Climate change intensifies rainfall patterns, causing:

Sudden heavy downpours
Extended wet seasons
Unpredictable storms

This increases landslide frequency worldwide.

🛠️ 9. Poorly Designed Retaining Structures

Not all retaining walls are engineered correctly.

Common problems:

No weep holes
No filter material
Poor backfilling
Insufficient foundation depth
No reinforcement

When water builds up behind a poorly designed wall, the pressure becomes too high, causing the wall to tilt or collapse — triggering a landslide above or below it.

A retaining wall without drainage = a disaster waiting to happen.

🛣️ 10. Road Cutting, Tunneling & Infrastructure Works

Roads built along hillsides require careful slope stabilization.

Mistakes include:

Cutting slopes at nearly vertical angles
Not protecting slopes with shotcrete
No gabion walls or geotextiles
Daylight cuts exposing weak soil
Vibrations from heavy machines

Similarly, tunneling or blasting disturbs deep soil layers, creating fractures that later become sliding surfaces.

🧪 11. Failure of Fill Slopes & Embankments

Construction projects often require filling to raise land levels.

But fill material must be:

Properly compacted
Layered correctly
Tested for density
Free of organic matter

Improperly compacted soil settles unevenly, absorbs water quickly, and becomes heavy — causing deep-seated failures.

This is a common problem in:

Highway embankments
Housing schemes
Agricultural terraces

🏞️ 12. Natural Slope Geometry – Too Steep, Too High, Too Weak

Even without rainfall or human activity, some slopes are naturally unstable.

Factors include:

Height of the slope
Angle of inclination
Depth of weathering
Type of rock
Pre-existing cracks

Civil engineers measure stability using:

Slope angle (θ)
Cohesion (C)
Friction angle (ϕ)
Unit weight (γ)
Factor of safety (FOS)

If the FOS is below 1.5 (recommended), the slope is unsafe.

🌍 Summary: What Really Causes Landslides?

✔️ Natural Causes

Heavy rainfall
Weak soil
Earthquakes
Weathering
Natural erosion

✔️ Human Causes

Deforestation
Unplanned construction
Bad road cutting
Poor drainage
Excessive load on slopes

✔️ Engineering Causes

Wrong retaining wall design
Improper fill compaction
Poor slope stabilization
Disturbance of slope toe

A landslide usually occurs due to a combination of several causes, not just one.

🛑 How We Can Prevent Landslides (Simple Methods)

🌱 1. Plant more trees & avoid clearing hillsides

Roots strengthen soil naturally.

🚧 2. Build proper retaining walls & provide drainage

Drainage is the key to slope stability.

🛣️ 3. Avoid steep slope cutting

Always follow recommended safe angles.

🌧️ 4. Maintain road drains & water paths

Keep water flowing away from the slope.

🧱 5. Follow engineering guidelines for fill construction

Compaction is critical.

🧭 6. Get geotechnical investigations before building

Soil tests reduce disaster risks.

🌍 What Causes Landslides? – A Civil Engineer’s Perspective (Explained Simply!) ⛰️💧🚧