Collapsing Slopes

Question 1

What are landslides?

Answer 1

Landslides, which is a mass movement, are the collapse of slopes. They occur when a mass of soils and rocks of all sizes moves down a slope rapidly due to gravity.

Question 2

Landslide situation in Hong Kong

Answer 2

In nature terrain, which covers about 60% of the total land area, about 300 landslides occur each year.
Some landslides also occur on man-made slopes, and these slopes can be found in built-up areas

Question 3

How do landslides affect us?

Answer 3

Landslides that occur in built-up areas cause injuries and deaths. They may bury buildings, and damage infrastructure such as water pipes, gas pipes and power lines. The supply of these service will be cut off. Landslides often block roads and stop traffic. This greatly disturbed and disrupted people’s daily lives.

• cause injuries and death
• bury buildings
• damage infrastructures and cutting off the supply of these services (water pipes, gas pipes, power lines)
• block roads and stop traffic

-> disrupted people’s daily life

Question 4

What is slope stability controlled by?

Answer 4

It is controlled by the balance of shear stress and shear strength.
/
It is controlled by the balance of driving force and resisting force.

Question 5

What is shear stress?

Answer 5

Shear stress drive movements. It pulls the slope material downhill under the force of gravity. It is mainly controlled by the slope gradient and the weight of the slope materials. The larger the slope gradient, the greater the shear stress. The heavier the slope materials is, the greater the tendency for them to fall down.

Question 6

What is shear strength?

Answer 6

Shear strength resists the movement down the slope. It prevents the slope materials from sliding down. It is affected by the cohesion of slope materials and the force of friction.
The cohesion is the force that cause the soil grains to stick together.
Friction is the resistance to motion between soil grains.

Question 7

What is shear stress and shear strength?

Answer 7

Shear stress drives movement. It pulls the slope materials downhill under the force of gravity. It is mainly controlled by the slope gradient and the weight of the slope materials. The larger the slope gradient, the greater the shear stress. The heavier the slope materials, the greater the tendency for them to fall down.

Shear strength resists the movement down the slope. It prevents the slope materials from sliding down. It is mainly affected by the cohesion of the slope materials and the force of friction. Cohesion is the force that cause the soil grains to stick together while friction is the resistance to motion between soil grains.

Question 8

What is the connection between shear stress and shear strength?

Answer 8

When shear strength is greater than shear stress, the slope remains stable. In contrast, when the shear stress is greater than the shear strength, the slope becomes unstable and may collapse

Question 9

What are the causes of landslides in Hong Kong? (Nature factors)

Question 10

What is cohesion?

Answer 10

The FORCE that CAUES SOIL GRAINS to stick together

Question 11

What is friction?

Answer 11

The RESISTANCE TO MOTION between soil grains

Question 12

Why does slope collapse from the shear stress perspective? How does shear strength help prevent slope collapse?

Answer 12

Gravity increases the shear stress to the slope and makes it easier to collapse. But on the other hand, cohesion of weathered materials and friction along the surface increases the shear strength that holds the weathering materials together and prevents them from falling downslope.

Question 13

In the physical factors / natural factors, how does the nature of rocks affect shear stress and shear strength?

Answer 13

Nature of rocks affects shear strength. Rocks that are more resistant to weathering have greater cohesion. Rocks like granites are hard when fresh, but they can be easily weathered under wet and hot climate. (As a result, since HK is made up of granites, it is often weathered) Weathered rocks produce a thick layer of loose materials that reduces cohesion. The shear strength of these loose materials are then small, leading a landslide which can happen very easily.

Question 14

In the physical factors / natural factors, how does the slope gradient affect shear stress and shear strength?

Answer 14

Slope gradient affects the shear stress. Slope materials are more affected by gravity as the gradient of a slope is steep. The larger the slope gradient, the stronger the force pulling the slope materials downhill. (Risks of landslides is particularly high when the slope gradient is stepper than 30) (about 60% of natural terrain in HK has slope angles over 20)

Question 15

In the physical factors / natural factors, how does the weight of the slope affect shear stress and shear strength?

Answer 15

The weight of the slope affects the shear stress. The heavier is the slope, the greater the shear stress as it is more affected by gravity. On the other hand, the lighter the slope, the more stable the slope is

Question 16

In the physical factors / natural factors, how does the vegetation cover affect shear stress and shear strength?

Answer 16

The vegetation cover affects the shear strength. Dense vegetation increases both cohesion for slope materials and friction of the slope surface. Both cohesion and friction drops where there is sparse / little vegetation cover as there are no roots to hold the soil together.

Question 17

In the physical factors / natural factors, how does the rainfall affect shear stress and shear strength?

Answer 17

Rainfall affects shear stress and shear strength respectively.
During heavy rainfall, the weight of a slope increases when it is saturated with water. The larger amount of rainwater adds much weight to the slope. Therefore, slope materials are more likely to fall down.
In addition, a large amount of rainwater seeps into the soil or rock on slopes. Too much rainwater acts as a lubricant that reduces (the force of) friction. It also reduces the cohesion of the slope materials. Only suitable amount of rainwater can increase the cohesion of slope materials. Therefore, the shear strength decreases.