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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Soil Exploration

What is soil exploration?

The process of sampling, field and laboratory investigation to obtain the physical properties of the subsoil is known as soil exploration.

Why do soil exploration?

·        To determine the properties of soil and groundwater level which affect the design and safety of the structure.

·        To determine the geological conditions of rock and subsoil

·        To select the suitable type and depth of the foundation

·        To determine the sequence, thickness, the lateral extent of the soil strata and position of bedrock

Subsurface exploration is done in the following stages:

A.  Reconnaissance:

According to this stage general investigation is done.

It helps in:

·        Deciding future programme of site investigation

·        Scope of work

·        Methods of exploration to be adopted

·        Type of sample to be taken

B.   Preliminary investigation:

It helps to determine the depth, thickness, extent and composition of each soil stratum at site

C.   Detailed investigation:

The purpose of this is to determine the engineering properties of soil in different strata

It includes:

·        Intensive boring programme

·        Sampling and testing of samples in the laboratory

·        Field tests such as vane shear test, permeability test and plate load test

 

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Capillarity Of Soil

Capillarity of soils

·        At the time of rainfall, because of gravity the rain water percolates into the ground

·        And starts stored in the form of the ground reservoir in the soil pores over impervious stratum

·        Upper surface of the zone of full saturation of the soil is known as the water table

·        Atmospheric pressure is subjected at the water table  

Definition:-

Because of gravity force, the percolating water should go on downward and the soil above the water table should be completely dry, but it is not happening because at some height above the water table the soil is completely saturated, So this phenomenon of rising water is known as capillarity of soil.

zones of soil water

1.     Zone-1 (submerged zone)

·        This exist below the ground water table

·        Soil in this zone is in submerged condition

·        Pore pressure is hydrostatic

2.     Zone-2 (capillarity zone)

·        Soil in this zone is completely saturated upto some height above the water table

3.     Zone-3 (zone of aeration)

·        exist at the some height above the capillarity zone

·        In this zone, soil is able to retain small water droplets, surrounded on all sides by air

·        This water is known as contact moisture or hygroscopic water

·        This contact moisture retained by the soil against the gravity drainage 

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Compaction Equipment

Compaction equipment and their application:-

1.  Sheep foot roller

·        suitable for fine-grain soils

·        Not suitable for use on cohesionless granular materials

2.  Tamping rollers

·       Also knowns as high speed non-vibratory rollers

·       Suitable for all soils expect sand and soft clays

3.  Vibrating rollers

·       These are used to control the effective depth by transmitting vibration.

4.  Smooth drum vibratory compactor

·       They are used on granular materials of size vary from large rocks to fine sand

·       By applying them 3 compaction force will create

a.   Pressure

b.   impact

c.    vibrations

5.  Pneumatic tyred rollers

·        suitable for coarse and fine soils

·        not suitable for very soft clay

·        They applying kneading effect  (applying deep pressure to change in different shape) to compact the soil

·        used to proof roll roadways subgrade, field bases and on earth-fill dams

6.  Manually operating compactors

I.       Vibrating compactors

·        Suitable for coarse soil

·        Used where large compactors not possible

II.  Rammer compactors

·        There are self-propelled rammers

·        used where lift thickness is small (3-4 inch)

7.  Tampers and Rammers

·        Suitable for all type soils

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Effect of compaction on properties of soil

 Properties of soil

1.   Soil Structure:-

If the soils compacted at water content less than Optimum Water Content(OMC) usually have a flocculated structure. (Because attractive forces > Repulsive forces between particles)

If the soils compacted at water content more than Optimum Water Content(OMC) usually have a dispersed structure. (Because attractive forces < Repulsive forces between particles)

2.   Permeability

Permeability of soil depends upon the size of voids.  The minimum permeability occurs at or slightly above the OMC. Beyond OMC, the permeability shows a slight increase.

3.   Compressibility

At low-stress value, a soil compacted wet side (dispersed soil) of the optimum is more compressible than the soil compacted dry side (flocculated soil) of optimum. And at high-stress value is vice versa.

4.   Swelling

 More swelling occurs on the dry side of optimum because a soil on the dry side of optimum has a higher water deficiency and a more random particle arrangement.

5.   Shrinkage

Soil compacted on the wet of optimum tends to exhibit more shrinkage upon drying than those compacted on the dry of optimum.

6.   Shear Strength

The shear strength of the compacted soils depends upon the soil type, the moulded water content, drainage conditions and the method of compaction.

   

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