**3.2 Ground subsidence**

Subsidence is the sinking or settling of the Earth's surface. It can happen through a number of processes. Land subsidence may result from settling of local low-density soils, or the cavity of natural or man-made voids underground. Subsidence may occur gradually over many years as sagging or depressions form on the earth's surface. In rare cases, a sudden landing such as a dangerous ground hole may swallow any part of the structure in that location, or leave a dangerous, steep hole [8–10].

## **3.3 Land slide**

A landslide may be defined as the movement of a mass of rock, debris, or land down a slope. Landslides are a kind of "mass wasting", which refers to any downward movement of soil and rocks under the straight influence of gravity. The word "landslide" includes five slope movement patterns: falls, slides, slides, spreads and flow [8, 11].

Landslides have several causes. Slope movement happens when forces acting on the lower slope surpass the force of the earth materials that make up the slope. Causes contain aspects that increase the effects of slope forces and aspects that contribute to a decrease or increase in strength. Landslides can start on slopes that are already on the edge of movement due to precipitation, snowmelt, changes in water level, table erosion, changes in groundwater, earthquakes, volcanic activity, disturbance caused by human activities, or any combination of these factors. Earthquake vibration and other aspects can also activate underwater landslides. These landslides are called undersea landslides. Sub-sea landslides sometimes cause tsunamis that damage seaside areas [9, 11].

### **3.4 Slope failure**

Slope failure is the phenomenon of suddenly collapsing slope due to poor selfholding capacity of the earth under the influence of rain or earthquake. Due to the sudden collapse of the slope, many people fail to escape from it if it occurs near a residential area, resulting in a high death rate [12].

## **4. Geological masses**

The size of the land that will be affected or will affect the engineering work. All rocks and many soil masses have discontinuities and their presence in rocks or soil mass is of prime importance for all engineering work in rocks or soil.

Mass movement, also called mass wasting, is the movements of soil and rock debris down slopes in response to gravitational pull, or the rapid or gradual sinking of the Earth's surface in a mostly vertical direction. Previously, the term mass wasting referred to a variety of processes by which large masses of cortical material are transported by gravity from one place to another. More recently, the term mass movement has been replaced to include processes of mass wasting and inundation of confined areas of the Earth's surface. The group movements on the ramps and the submersible group movements are often assisted by the water and the importance of both types is the role that each plays in changing the earthly shapes [13, 14].
