Why Landslides happen ?

Why landslides happen?

Because of failure of slopes, So to understand this let's discuss Stability Of Slopes.

·       Landslides are the downward and outward movements of slope materials because of exhaustion of required shear strength (strength of the material against the structural failure)

·       Slope materials are composed of natural rock, soil, artificial fills or combination of all

·       There are 2 causes of slope failure as external or internal.

1.    External slope failure:

·       When increasing in stress at unaltered shearing resistance of the material

·        Because of

Ø gravity force

Ø Earthquake force

Ø  Seepage force

Ø excavation near the slope

Ø  Erosion due to flowing water

Ø Sudden drawdown

2.    Internal slope failure:

·       Lead to slide without any change in a surface condition which involves unaltered shearing stress in the slope materials

·       Such as

Ø Excess pore water pressure

Ø leaching of salt

Ø softening

Ø breakage in cementation bond

 

Note:  The landslides happen because of external or internal failure of slopes

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Methods Of Soil Exploration

 

Methods of exploration:

There are 4 methods of exploration

1.    Open excavation

2.    Borings

3.    Geophysical method

4.    soundings and penetration tests

 

1. Open excavation:

·       This is suitable for shallow depth up to 3m

·       For this, test pits, test trenches and open borings are used

2. Borings:

·       these are used for depths > 3m or when difficult groundwater conditions are met.

1.    Auger boring:

§  This method is suitable identification of various types of soils and also for getting information about the depth of the groundwater table.

§  Hand-operated augers are used upto depth of 6m in soft soil whereas power-driven augers are used for greater depth in harder strata

2.    Wash boring:

§  This method is used below the water table and is suitable for all types of soil except hard rock.

3.    Percussion drilling:

§  It is used when the hole is to be made is very hard or soft rock because in both cases, auger or wash borings can not be used i.e. bouldery and gravelly strata

4.    Rotary drilling:

§  This method is used for rocks, clays and sands

3. Geophysical method:

·       This method consists of identification of underground material by measuring the change in the character of the surface material due to the presence of different types of sub-surface materials.

·       This method is very useful to explore large areas much more rapidly and economically than borings methods

·       The various geophysical methods of subsurface exploration are:

§  Seismic refraction method

§  Electrical resistively method

4.Sounding and penetration tests:

·       These methods are used to measurement of the resistance to penetration of a driving rod, sampling spoon, a cone etc. under dynamic or static loading

·       The most commonly use soundings tests are

o  Standard penetration test

o  cone penetration test

These methods of soil exploration are useful for cohesionless soils.

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