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Mass movement

Introduction

We can define a mass movement as a soil, mud sediments and stones movement toward down.

This movement can perform with low amount and slowly like soil creep and/or with high amount and speed like a landslide and/or stones falling from slopes and walls. On the world scale, the mass movement has had a few fatalities rather than the overall of fatalities caused from natural disasters. Nevertheles, we can not ignore a high damages at a wide level to a man-mades, and sometimes they result a structures movement and/or roads cut.

It is a necessary that a components of one inclined land first become an unstable and then sliding for a mass movement and this occurs when the separating forces overcome on a resisting forces.This equilibrium has been defined as factor of safety that is shown with F:

F= Factor of safety

F= stress/ resisting force= sum of separating forces/ some of resisting forces.

2-2- Some of the most important of mass movement at 20th century (Jone.1995):

 

Name of place

Date

Earth movement type

Fatalities and destructive effect

Jave

1919

Debris flow

5100 person killed, 140 villages damaged

Kansoo- China

1920

Loess flow

200/000 killed

California

1934

Debris flow

40 killed,4000 houses damaged

Patrachirka,peru

1962

Avalanche from ice and stone

3500 killed and casualty

Vayount, Italy

1963

Falling and sliding stone

2600 killed and casualty

Virginia

1969

Debris avalanches fall

 

Peru(Magoonmarka)

1974

Debris flow

Degradation and 451 killed

Peru (Armero)

1985

Lahar

22000 killed and casualty

One inclined surface with a factor of safety less than 1 is an instable, and therefore it is at a suitable requirements for sliding and falling, while one inclined surface with a factor of safety more than 1 is a stable. The inclined surface that it's factor of safety is equal to one, it is at a critical equilibrium requirements.

In general, 6 types of region have been identified for a landfall or a mass movement:

1-     Regions having seismicity

2-     Mountainous environments

3-     Regions that the land is detached in them (Land detach)

4-     Areas covered by thick loess sheets

5-     Regions having improper weather

6-     Regions with a fast development

The mass movement is performed at a different dimensions, and shapes that we can classify them based on the following cases:

-         Materials type: weathered soft sediments or rough and resistance stones.

-         Movement type: movement as flows, sliding or movement of deposits along with classes.

-         Speed: the movement speed can be very low or very high

Hazards risk survey caused from mass movements: The slopes stability amount has been drawn at hazard risk maps. There are two important points for surveying mass movement hazard risk:

1-     To anticipate the fall, slide and creep possibility at slopes.

2-     To determine the position, time and magnitude of event. We must know two points for anticipating a mass movement possibility.

At Grindelwald area, Swiss Alps, the Landslide hazards have been classified as follows(Kienholz 1978):

Range of hazards

Description

3

 

2

 

 

 

1

 

 

0

The houses have been damaged and people put at risk of landslide occurance.

The houses are at a little risk but area between houses have been affected by landslide and thus people put at a risk.

 

The houses are at a very low risk of landslide, but people is rarely threatened for going out from their homes A risk has not been identified.

Avoidance from mass movement:

There are a many ways for avoiding from a mass movement.

First, We must determine an areas having a movement risk given to maps relating to that area and a direct visit from land, then introduce an alternative with regards to the area having a mass movement possibility . At some of areas, the inclined landsare near to the established buildings and there is a falling and sliding possibility of the slopes components over buildings. Here we can increase the safety with making a protective wall that we must build in a piper or parts inside a protective wall in order to pass the penetrated water from upper stream.