Slope failures, types and causes of slope failures

Introduction

Slope failure is generally the failure of sloped surfaces due to physical factors that may result in loss of properties and even life.

Why do we need to understand slope failures?

You must have heard about plain and non-plain areas. So, if we talk about it the plain areas are those where the earth or lands do not show any elevation or slight elevations that can be neglected. Non-plain areas are those where the lands have sufficiently larger elevations that need to be considered for any activity.

Now, if any land has elevations then it forms slope. For example, consider mountains they generally have slopes though it may be narrow or steep but if any kind of construction or any other activity, if happens there, then, these slopes need to be taken into account and how are they going to affect the outputs of the work going on.

But here we will not talk about the difficulty faced for routes and inaccessibility of the area rather we will talk about the physical difficulties of failure of slopes.

The failure of slopes is generally unexpected and result in lots of damages to life and property.

Due to these damages, we need to prevent the failure of such slopes that may result in above losses, and that’s why it is necessary to understand the failure of slopes in order to devise the methods, processes and actions that need to be taken to prevent the failure of slopes.

Slope

The slope is generally the action of any exposed ground surface (or anything in general) for being at an angle of β from horizontal.

Slopes are provided in the construction of highways and railways in order to provide super elevations

Slopes may be provided during an embankment

It is provided in the construction of dams and similarly many other constructions and other daily life work that also makes it very necessary to understand its failure.

Types of slopes

Generally, there are two types of slopes

Infinite and finite slopes

Infinite slopes

Infinite slope basically represents the constant sloped surface with an infinite extent. Since an infinite extent is practically not possible so, the sloped surface with a very large extent is considered to be an infinite slope. Generally, the mountain slopes come into this category.

Finite Slopes

It represents the constant sloped surfaces with a finite extent. Inclined surfaces of earth dams, embankments, and excavations are some examples of this category.

As described earlier, the two types of slopes that are Infinite and Finite slopes on the basis of the extent of the sloping surface, but there are two more, on the basis of formation and that is natural and man-made slopes.

Natural and man-made slopes

Natural slopes are those that are formed due to the geological features of the earth whereas man-made slopes are constructed for some specific purposes by man or get formed due to the cutting and filling etc.

Triggering mechanism and causes of slope failures

Many physical phenomenons are there that results in the slope failures. But despite these failures, there some phenomenon that acts as triggering mechanisms that further result in the slope failure.

These triggering mechanisms are:

·        Intense Rain-Fall

·        Water-Level Change

·        Seepage Water Flow

·        Volcanic Eruption

·        Earthquake

·        Human Activity

 Causes of slope failures:

Erosion: The sloped surfaces get eroded due to wind or flowing water over time. This erosion of the surfaces results in the formation of steep slopes, increasing the base angel β, and hence, increasing the tangential component of the driving force. Consider the below examples.

In the above example, 

Steady Seepage: Consider the water flowing downstream the slope. This water may get into the surface of the slope resulting in the seepage. Now the downward seepage forces due to gravity make the slope susceptible to instability. The seepage water results in pore water pressure which finally results in the decrease of the shear strength.

Rainfall: A long period of rainfall generally results in the saturation of soil and hence, weakens the underlying soil layers and finally results in the failure. Also, the long periods of rainfall result in the erosion of soil that has been discussed earlier.

Earthquakes: The earthquakes induce the dynamic shear forces which result in the sudden buildup of pore water pressure that reduces the available shear strength and results in the slope failure.

External loading: The additional loadings over the top of the slopes may result in shear failure and hence into the failure of the slopes.

Construction activities at the toe of the slope: Construction activities at the toe of the slope may result in the improper normal reaction from the ground and result in the instability of the slope.