Hello. This is Kkoogongnam.

Recently, not only Korea, but also India and other countries have been experiencing heavy rainfall.

I hope everyone is safe.

While watching the news, I thought about why landslides occur when it rains on civil engineering perspective.

A landslide is a phenomenon in which soil moves down a slope, such as a mountain or hillside, due to the decreased stability of the soil during heavy rainfall. This phenomenon is affected by various factors such as the characteristics of the ground, rainfall, and the shape of the slope.

**1. Soil Saturation and Soil Bearing Capacity Reduction (Stress reduction)**

When it rains, the soil becomes saturated, which means it fills with water. Soil is a composition of earth particles that create inter-particle stresses (bearing capacity), which are made difficult by the placement of water molecules between them. This results in a significant decrease in bearing capacity and makes the soil less stable.

(See slope/effective stress/shear stress analysis below)

**2. Reduced friction (stress reduction)**

On landslide slopes, soil tends to flow downward by gravity. When the friction force between the soil particles decreases, the soil slides and becomes more prone to landslides. When it rains, the presence of water between the soil particles reduces the frictional forces.

(See Slope/Effective Stress/Shear Stress Analysis below)

**3. Influence of groundwater (effective stress change)**

When it rains, the level of groundwater rises. This can infiltrate into the slope, disrupting the stress balance of the soil, increasing the pressure on the soil and causing landslides. It’s the same principle of saturated soil losing its bearing capacity that we talked about earlier.

(See Slope/Effective Stress/Shear Stress Analysis below)

**4. Soil fluidity**

When rain infiltrates the soil, it fills the gaps between the soil particles. This fluidity reduces the strength and viscosity of the soil and causes the soil to behave more like a liquid.

**5. Analysis on Geotechnical perspective: Slope/Effective stress/Shear stress**

Let’s take a look at the simple “infinite slope” analysis with an illustration.

Let’s consider the behavior of soil of volume L,H, denoted by W, on an infinite slope on both sides of the β angle.

(1) W = the weight of that volume of soil. (kN/m)

The weight of the soil should be calculated as the unit weight of the soil with water, right?

(2) T = the force to slide to the left due to the weight of the soil (kN/m)

You can think N and T divided force caused by β angle slope.

Here, remember that the force to slide = affected by **r_sat!**

(3) N = force acting perpendicular to the collapsing surface (kN/m)

(4) “Effective stress” of the soil acting on the face to collapse (kN/m2)

- L/cosβ = Length of the bottom of the slope

Stress = force per area, so it makes sense to divide by the length.

And since we need to subtract the water pressure to get the “effective stress”, we subtract that corresponding to the height H, current groundwater level.

(5) “Shear stress” of the soil (kN/m2), the force to be resisted

The “shear stress” of the soil is used to resist, which can be found as the “effective stress” at that point.

(6) Factor of Safety (Fs) = Stress to resist / Stress to collapse

This is calculated by dividing the length of the slope L/cosβ by the force to collapse T=W sinβ, right?

It seems like a complicated equation, but if you look at it one by one, you can check it.

① The higher Fs, the greater the resisting force, so the slope is “safe”.

② The larger β, the larger sinβ and smaller cosβ, so the “factor of safety” decreases.

– You can understand that a large β means the slope is steeper and more unstable.

③ If it does not rain and only the dry unit weight is considered, the “resisting force” becomes larger and the “safety factor can be increased” because the rw, water pressure, is ignored.

④ If it rains a lot and “upward infiltration” occurs, the water pressure “rwH” becomes larger and the “resisting force” becomes smaller, and the “safety factor” can be lowered.

(in the same way that frictional forces are reduced and effective stresses are reduced).

Due to these factors, heavy rainfall increases the probability of landslides due to soil movement on slopes or in mountainous areas. Therefore, on geotechnical perspective, it is important to identify landslide risk areas in advance and take appropriate safety measures to minimize the loss of life and property.