soil studies

 

                       INTRODUCTION

 

Importance of soil studies in Civil Engineering

     

                   Soil is an unconsolidated material that has resulted from the disintegration of rocks. The soil voids are in turn filled with a gas or liquid or both. These components, viz., solid particle, gas and liquid, ply a significant role in the fundamental behavior of soil. The engineering properties, such as permeability, consolidation and shear strength of soil deposit are governed by the mode of formation, stress history, groundwater conditions and physicochemical characteristics of the parent material

Geological origin of soils with special reference to soil profiles in India

 

      Approximate depth of 20km of the earth crust mineral and rocks are originated from soil.

Rocks types are grouped into three major classes

i.                    Igneous

ii.                  Sedimentary

iii.                Metamorphic

 

 Residual and transported soil

      The soil obtained from their formation place is called residual soil.

      The soil obtained from Fare away their formation place is called transported soil.

 

 Alluvial deposits (transported soil)

            Soils that are carried and deposits by rivers are called alluvial soil. Alluvial deposits also occur at some places in the peninsular India. The thickness of alluvial deposits is more than 100m presence of alternating layers of sand, silt and clay deposits.

 

Lake deposits

            Soils carried by rivers, while entering a lack, deposit all the coarse particles because of a sudden decrease in velocity. Such coarse soil deposits are called lack deposits soil.

 

 Local soil found in J&K

 

 Dunes and loess

            Dunes are formed due to the accumulation of such wind deposits sands. Dunes are rather common occurrence in the desert areas .Sands from dunes may be used to a limited extent for construction purpose

            Wind blown silt and clay deposited with some cementing minerals in a loose, stable condition are classified as loess deposits soil.

Glacial deposits

            Soil of heterogeneous nature transported and deposited by moving glaciers are called glacier deposit soil.

 

 Black cotton soils

            The primary bed rock basalt or trap, quartzite and sedimentary rocks are reported. This is residual soil and consists of minerals montmorillonite and illite. The soil surface is hard during summer and become slushy during the rainy season.

2. Physical Properties of Soils

 

2.1 Constituents of soil and representation by a phase diagram

            A soil mass is combination of solid particles and voids. Soil mass consists of solid particle, water and air.

            Top, medium and bottom layer are called phases.

            Soil sample consists of three phase when both air and water are present in voids such as in partially saturated soils. Soil sample consist of two phases when the voids are filled with air only or with water only such as dry soil and fully saturated soil.

 

2.2 Definitions of

Void ratio

            The void ratio ‘e’ is normally expressed as a decimal.

                        Vv (volume of voids) / Vs (volume of the solids)

                       

                                    Vv = Vw (volume of water) + Va (volume of air0

                       

 Porosity ‘n’

                        Vv(volume of voids ) /  V(total volume of soil sample)

 

 Water content ‘w’

                        w   =   Mw(mass of  soil) / Ms(mass of solids)

                                   

 Degree of saturation ‘S’

                        S  =  Vw(volume of water) / Vv(volume of voids)

 

Specific gravity ’G’

                        G =  Ps(density of solids) / Po (density of water at 4 degree C )

 

 Unit weight ‘Y’

                        Y   =   W (weight of solids) / M(mass of solids)

 

 

 

Bulk density ‘pt’/bulk unit weight

                        Pt   = M (mass of solids) / V (volume of solids )

 

Dry unit weight ‘yd’

                        Ws (weight of solid0 / V(volume of solids)

 

Saturated unit weight

                        Ysat  =   W / V

 

 Submerged unit weight of soil grains

                        Yb  =  Y (G – 1) / 1+e

 

 

 3. Classification and Identification of Soils

 

Engineering properties of soil,

 

Particle size

 

 Shapes

            Shape play an important role in case of much fined grained soils.

i.                    Angular

ii.                  Sub angular

iii.                Rounded

iv.                Sub rounded

v.                  Well rounded

vi.                Flaky

 

particle size classification of soils

 

3.2 Gradation and its influence on engineering properties

            Particle size analysis or gradation of soil is also known as Mechanical Analysis

1.      Sieve analysis

2.      Sedimentation analysis (Net mechanical analysis).this method of sedimentation analysis is used for fine grained soil having particle of size less than 75 microns.

            Particle sizes greater than 75 micron, known as coarse grained soil.

 

3.3 Relative density and its use in describing cohesion less soils

            The relative density is an important index property of cohesion less soils. Relative density is term as ‘the relative compactness i.e. denseness or looseness of a natural soil deposit.’ It is also known as density index or degree of density.

 

3.4 Behavior of cohesive soils with change in water content,

Atterberg’s limit - definitions, use and practical significance

            A Swedish agriculture engineering atterbeg describe in 1911 that a fine grained soil can exist in four states namely; liquid, plastic, semi-solid and solid.

             The water contents at which the soil changes from one state to the other are known as consistency limits or atterberg limit.

            Fine grained soil may be mixed with water to form a plastic paste which can be molded into any desired shape. The addition of water reduces the cohesion but the soil can still be molded into different forms.

            Further addition of water reduces the cohesion until the material no longer retains its shape under its own weight but flow as liquid.

            Enough water may be added until the soil grains are dispersed in a suspension.

            If water is evaporated or removed from such a soil suspension the soil pass through different stages of consistency as mention below

1.      Liquid stage                      e.g. : Thick juice

2.      Plastic stage                      e.g. : Soft butter

3.      Semi- solid stage               e.g. : Cheese

4.      Solid stage                        e.g. : Chocolate (hard only)

 

  

The important atterberg limit or consistency limits which are most useful for engineering purpose are:

i.                    Liquid limit

ii.                  Plastic limit

iii.                Shrinkage limit

 

 

 

3.5 Field identification tests for soils

            Soil can be broadly grouped as

            Coarse grained soil (non-cohesive)

                                    Which soils are consisting of mineral fragments which are easily identified by the eye on the basic of grain size. The major materials are gravel and sand.

 

            Fine grained soil (cohesive) 

                                    Fine grained soils are silts and clay they are categorized as inorganic soil and organic soil 

i.                    inorganic soils

a.       dry strength test

b.      plasticity

ii.                  organic soil

 

3.6 Soil classification system as per BIS 1498; basis, symbols, major divisions

and sub divisions, groups, plasticity chart; procedure for classification of a

given soil

4. Flow of Water Through Soils

4.1 Concept of permeability and its importance

            The property of soil which permits the flow of water through the connecting voids is called permeability.

1.      To calculate the rate of settlement of building and other structure.

2.      To analysis seepage through dams.

3.      To study losses from irrigation canals.

4.      To determine yield of wells.

 

4.2 Darcy's law, coefficient of permeability, seepage velocity and factors

affecting permeability

            considering one dimensional flow in a saturated medium obeying laminar flow and soil is fully saturated Darcy (1856) demonstrated experimentally that the flow velocity is proportional to the hydraulic gradient

                                                v ∞ i

                                                v= ki

where,

                        v = velocity of flow (mm/s or m/s)

                        k= coefficient of permeability (mm/s or m/s)

                        i= hydraulic gradient = h/L

                                    h= difference head pressure

                                    L= length of specimen

Rate of flow , q(m3/s) is given as

                        Q= kiA

                                    A= cross sectional area

 

4.3 Comparison of permeability of different soils as per BIS

 

4.4 Measurement of permeability in the laboratory

                        There are two laboratory experiments for the determination of the coefficient of permeability, viz.; the constant head and falling or variable head permeameter.

 

Constant head permeameter

            This test is preferred for coarse grained soils. The soil specimens placed at an appropriate density in the peermeameater. A steady vertical flow of water under a constant total head is maintained. After saturation, of soil sample, a certain quantity of water passing through the soil for a given time is collected and ‘q’ is calculated.

                        Q =Akh/L

                                    Or

                        k = qL/Ah

Falling head permeameter

            For fine grained soil, such as silt and clay this is generally used. After saturate of soil sample

 

5. Effective Stress

 

1 Stresses in subsoil

            It is defined as force per unit area.

            The total stress or unit pressure is the total load per unit area and this pressure is due to following ;

i.                    self weight of soil

ii.                  superimposed load on the soil

 

5.2 Definition and meaning of total stress, effective stress and neutral stress

 

Total stress

            It is the load acting on the soil mass per unit areas. It is the sum of effective stresses and neutral stresses.

            It is represented by ᵨ (sigma)

                        Sigma = superimposed load including self weight (P) / cross sectional area (A)

Where,

                        P = Ysat * h * A

                        ᵨ = Ysat * h * A / A

so,

                        ᵨ =  Ysat * h

            Total stress = intergranular stress + pore water pressure

 

Effective stress

            The stresses transmitted from particle to particle of soil mass through their point of contact are termed as effective stress.

            The effective stress -ᵨ at any point in the soil mass is equal to the total stress minus the pore water pressure.

           

                        -ᵨ = ᵨ  -  u

Where,

                        ᵨ   =    total stress

                         u =    pore water pressure or neutral stress

 

neutral stress

                        The stress induced due to filling of pore water in the voids of the soil mass which tries to separate out the soil grain is termed stress as neutral stress. Sometime it is also known as pore water pressure. It is denoted by ‘u’

                        u = Yw * h

            Yw = unit weight of water

            h    = height of saturated soil mass

it is not effective in increasing the shear strength of the soil mass.

 

            u  =  ᵨ  -  ᵨ-

 

6. Deformation of Soils

6.1 Meaning, conditions/situations of occurrence with emphasis on practical

significance of:

a) Consolidation and settlement

            the change in volume of soil due to expulsion of pure water under an applied load is termed consolidation.

 

Settlement

            It is a process by which the soil particle are artificially rearranged and packed together by mechanical mean in order to decreases the voids of the soil.

            It is a vertical downward displacement of the structure due to decrees in the volume of soil mass on which it is built.

i.                    Uniform settlement

ii.                  Differential settlement

iii.                Tilt

 

b) Creep

            slow and gradual lateral advancement of the soil is termed as creep.

            Creep has effective on pipe line, bench mark on road and railway, retaining and breast walls, which are on or just at the foot of a slope.  

 

c) Plastic flow

            the term plastic flow indicates continuous deformation at a continuous state of stress.

 

d) Heaving

            it is the tendency of the soil to move up.

 

e) Lateral movement

           

f) Freeze and thaw of soil

 

6.2 Definition and practical significance of

compression index,

 

 coefficient of consolidation,

 

degree of consolidation.

 

6.3 Meaning and rate of settlement and their effects

total settlement,

 

 uniform settlement and

 

differential settlement;

 

 

 

6.4 Settlement due to

construction operations and

i.                    Subsidence of the area surrounding excavation

ii.                  Lateral moment of the soil at sides of the excavated trenches

iii.                Heaving of soil at the bottom of the excavated space

 

 lowering of water table

            due to lowering water table is the increase of effective pressure on soil.

                        Effective stress = total stress – pore pressure

 

 

6.5 Tolerable settlement for different structures as per BIS

7. Shear Strength Characteristics of Soils

7.1. shear strength

Concept and Significance

 

7.2 Factors contributing to shear strength of

cohesive and

 

cohesion less soils,

 

Coulomb's law

 

7.3 Examples of shear failure in soils

8. Compaction

 

8.1 compaction

Definition and necessity

            Compaction of soil mass improves the engineering properties of soil. The properties of the soil which are important for constructions.

i.                    High shear strength

ii.                  Low permeability

iii.                Increase density of soil

iv.                Stable and long span structure

v.                  Reduce future settlement

 

8.2 Laboratory compaction test (standard and modified proctor test as per BIS)

 

definition and importance of

Optimum water content,

            The OMC corresponding to the maximum dry unit weight or maximum dry density is known as OMC. The OMC is achieved at higher OMC for cohesive soils (fine) compare to non cohesive soil (coarse, sandy)

 

Maximum dry density;

            The density of the soil corresponding to maximum compaction is known as MD. Maximum dry density is achieved at optimum water content. Highly plastic soil (organic) have low MDD as compare to cohesive less or sandy soil.

 

 Moisture dry density relationship for typical soils with different comp active efforts

 

8.3. Compaction control; Density control, measurement of field density by core

cutter method and sand replacement method, moisture control,

 

 Proctor's needle and its use, thickness control,

 It is a rapid  and accurate method roe fine grained soil

9. Soil Exploration

 

9.1 soil exploration

Purpose

1.      To determined the basic properties of soil which is affected the design and safety of structure

2.      To determined the condition of ground water

3.      To analyses the causes of failure of existing work.

4.      To determined the extent and properties of the material to be used for construction.

 

and necessity

9.2 Reconnaissance,

methods of soil exploration,

 

Trial pits,

 

borings (auger, wash, rotary, percussion to be briefly dealt)

 

9.3 Sampling; undisturbed, disturbed and representative samples; selection of

type of sample; thin wall and piston samples; area ratio, recovery ratio of

samples and their significance, number and quantity of samples, resetting,

sealing and preservation of samples.

 

9.4 Presentation of soil investigation results

10. Bearing Capacity of soil

 

10.1 Concept of bearing capacity

            Load carrying capacity of soil or rock as its bearing capacity.

 

10.2 Definition and significance of

Ultimate bearing capacity ‘qu’

the ultimate bearing capacity is defined as the minimum gross pressure intensity at base of foundation at which the soil fails in shear.   

   

   Net ultimate bearing capacity ‘qnu’

            The minimum net  pressure intensity at the base of foundation that causes shear failure.

                        Net ultimate bearing capacity = ultimate bearing capacity – overweight at footing

                                    qnu = qu – Y df

                                                Y  = unit weight of soil

                                                Df = depth of foundation

Net safe bearing capacity and  

             net safe bearing capacity is the ultimate bearing capacity divided by factor of safety.

 

allowable bearing pressure

            Allowable bearing pressure is the net loading intensity at which neither the soil fail in shear nor there excessive settlement.

 

10.3 Guidelines of BIS (IS 6403) for estimation of bearing capacity of soil

 

10.4 Factors affecting bearing capacity

·         Type of soil

·         Physical properties of soil

·         Position of water table

·         Type of foundation

·         Amount of settlement

 

10.5 Concept of vertical stress distribution in soils due to foundation loads,

pressure bulb

                             

 

 

10.6 Applications of SPT, unconfined compression test and direct shear test in

estimation of bearing capacity

 

10.7 Plate load test (no procedure details) and its limitations

            It is a field test

            Performed on uniform sandy

            Determine ultimate bearing capacity of soil

            The rigid plate is loaded with gradually increasing load and the settlements are measured for each increment of load. The ultimate bearing capacity is taken as the load at which the plate starts shrinking at a rapid rate.

Procedure

·         Excavated a pit of size not less than 5 time of plate size.

·         The bottom of pit is kept equal to the depth of foundation.

·         30 cm for sandy soil and bigger for clayey soil.

·         The ground is leveled and plate is placed in a center

·         A firstly load  700 kg /m3 is applied

·         The test should be conducted until the settlement about 25 mm has occurred.

  

10.8 Improvement of bearing capacity by sand drain method, compaction, use

of geo-synthetics. 

11. Foundation Engineering

 

Concept of

shallow and deep foundation;

            A foundation is said to be shallow if the depth of foundation is equal to or less than width of foundation

            On the other hand if the depth of foundation is more than width of foundation it is known as deep foundation.

 

types of shallow foundations and their suitability

 Isolated,

Combined,

Strip,

 Mat,.

 

Factors affecting the depth of shallow foundations,

 

 deep foundations,

            when the soil at or near the ground level is unable to bear load of structure, deep foundation are required to transfer the load to deep strata.

i.                    Pile foundation

ii.                  Pier foundation

iii.                Well or caisson foundation 

 

 Type of piles and their suitability;

            A long slender structure member relatively small diameter made up of timber, concrete, or steel or composite material driven or installed into the weak ground to support the structure through suitable mean.

 

 Pile classification on the basis of material,

 

Pile group and

 

Pile cap.