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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Compaction Of Soil

Introduction:-

Soil compaction is the process where soil particles are forced to pack more closely by reducing air voids. This can be achieved by applying some mechanical energy (dynamic or statics) on the soil.

Dynamic:- Repeated application of dead load and this is better for coarse-grained soils.

 Static:-  Steady increase of static load and this is better for fine-grained soils.

Difference between Compaction and Consolidation

 Compaction:-

                     i.            Instantaneous phenomenon

                   ii.            Soil is unsaturated

                 iii.            Reduction in the volume of air voids at given water content

Consolidation:-

                                i.            Time dependent phenomenon

                              ii.            Soil is completely saturated

                            iii.            Volume reduction is due to the expulsion of pore water from voids

                           iv.            it’s occur on account of a load placed on the soil

Advantage of compaction

                              i.            Increase in shear strength of soil

                           ii.            Improve stability and bearing capacity of soil

                        iii.            Reduction in compressibility and permeability of the soil

                         iv.            Increase load-carrying capacity of soil subgrade

                            v.            Prevention of determental settlements

                         vi.            Prevention also, undesirable volume changes through swelling and shrinkage