Unit - 1

Origin of Soil

1.1 Rock Cycle and Origin of Soil:

There are 3 primary sorts of rocks: sedimentary, igneous, and metamorphic. Each of those rocks are shaped with the aid of using bodily changes—which include melting, cooling, eroding, compacting, or deforming—which are a part of the rock cycle.

Sedimentary Rocks:

• Sedimentary rocks are shaped from portions of different current rock or natural cloth. There are 3 one-of-a-kind sorts of sedimentary rocks: clastic, natural (organic), and chemical.
• Clastic sedimentary rocks, like sandstone, shape from clasts, or portions of different rock. Organic sedimentary rocks, like coal, shape from hard, organic substances like plants, shells, and bones which are compressed into rock.
• The formation of clastic and natural rocks starts off evolved with the weathering, or breaking down, of the uncovered rock into small fragments.
• Through the method of erosion, those fragments are eliminated from their supply and transported with the aid of using wind, water, ice, or organic hobby to a brand-new location. Once the sediment settles somewhere, and sufficient of it collects, the bottom layers come to be compacted so tightly that they shape strong rock.
• Chemical sedimentary rocks, like limestone, halite, and flint, shape from chemical precipitation. A chemical precipitate is a chemical compound—for instance, calcium carbonate, salt, and silica—that bureaucracy whilst the answer it's far dissolved in, normally water, evaporates and leaves the compound behind.
• This happens as water travels thru Earth’s crust, weathering the rock and dissolving a number of its minerals, transporting it elsewhere. These dissolved minerals are prompted whilst the water evaporates.

Metamorphic Rocks:

• Metamorphic rocks are rocks which have been modified from their unique shape with the aid of using big warmness or strain.
• Metamorphic rocks have classes: foliated and nonfoliated. When a rock with flat or elongated minerals is positioned beneath big strain, the minerals line up in layers, growing foliation.
• Foliation is the aligning of elongated or platy minerals, like hornblende or mica, perpendicular to the course of strain this is applied. An instance of this alteration may be visible with granite, an igneous rock.
• Granite consists of lengthy and platy minerals that aren't first of all aligned, however whilst sufficient strain is added, the ones minerals shift to all factor withinside the equal course even as getting squeezed into flat sheets. When granite undergoes this method, like at a tectonic plate boundary, it becomes gneiss (pronounced “nice”).
• Nonfoliated rocks are shaped the equal way, however they do now no longer incorporate the minerals that have a tendency to line up beneath strain and therefore do now no longer have the layered look of foliated rocks.
• Sedimentary rocks like bituminous coal, limestone, and sandstone, given sufficient warmness and strain, can grow to be nonfoliated metamorphic rocks like anthracite coal, marble, and quartzite. Nonfoliated rocks also can shape with the aid of using metamorphism, which takes place whilst magma comes in touch with the encompassing rock.

Igneous Rocks:

• Igneous rocks (derived from the Latin phrase for fire) are shaped whilst molten warm cloth cools and solidifies. Igneous rocks also can be made a pair of various ways. When they may be shaped inner of the earth, they may be referred to as intrusive, or plutonic, igneous rocks.
• If they may be shaped outdoor or on pinnacle of Earth’s crust, they may be referred to as extrusive, or volcanic, igneous rocks.
• Granite and diorite are examples of not unusual place intrusive rocks. They have a rough texture with big mineral grains, indicating that they spent heaps or tens of thousands and thousands of years cooling down withinside the earth, a time path that allowed big mineral crystals to grow.
• Alternatively, rocks like basalt and obsidian have very small grains and an exceedingly quality texture.
• This takes place due to the fact whilst magma erupts into lava, it cools greater fast than it'd if it stayed withinside the earth, giving crystals much less time to shape. Obsidian cools into volcanic glass so fast whilst ejected that the grains are not possible to look with the bare eye.
• Extrusive igneous rocks also can have a vesicular, or “holey” texture. This takes place whilst the ejected magma nonetheless has gases inner of it so whilst it cools, the fueloline bubbles are trapped and come to be giving the rock a bubbly texture. An instance of this will be pumice.

1.2 Clay Mineralogy:

There are 3 most important corporations of clay minerals:

1. Kaolinite - additionally consists of dickite and nacrite; fashioned through the decomposition of orthoclase feldspar (e.g., in granite); kaolin is the important constituent in China clay.
2. Illite - additionally consists of glauconite (an inexperienced clay sand) and are the commonest clay minerals; fashioned through the decomposition of a few micas and feldspars; main in marine clays and shales.
3. Montmorillonites - additionally consists of bentonite and vermiculite; fashioned through the alteration of mafic igneous rocks wealthy in Ca and Mg; weak linkage through cations (e.g., Na+, Ca++) outcomes in excessive swelling/shrinking potential.

Key Takeaways:

There are three types of clay minerals:

1. Kaolinite
2. Illite
3. Montmorillonites

1.3 Mechanical analysis of soil:

• Many times, laboratory tests are neither feasible nor practicable
• At such instances, if a name tag can be put to a soil, a knowledgeable soil engineer will be able to gain much data from simply identifying the oil type.
• Field tests and field identification is thus a great tool in soil engineering.
• The soils to be identified in field may be coarse grained soil or fine-grained soil.

1.Coarse Grained Soil:

• On the field coarse grained soils are identified by visual inspection and must be described on the bases of grading, grain-shape, colour, in-situ strength, structural features and presence of fines etc.

Grading:

• By visual inspection one can judge the coarse grain soil by its grading and can classify soil as well-graded, uniformly graded or poorly graded.

Grain shape:

• Grains can be defined on the field based on their shape, usually term used to describe the grain shape are angular, sub-angular and rounded.

Colour:

• Based on the colour of soil at field, soil can be expressed as white, yellow, brown, red brown etc.

Strength and structure:

• The term compact and loose are used to determine in-situ strength of deposit of coarse-grained soil.
• The structure of coarse-grained soils recognized in the field can be described as homogeneous or stratified.

Presence of fines:

• If soil contains some organic matter; it should be indicated.
• If it has some fines, and not causing cohesion, it should also be noted.

2. Fine Grained Soil:

• Following test are carried out for identification of fine-grained soil.
• For this, the soil is sieved on site through 425-micron sieve and the fraction passing through is taken for the tests.

Dry strength test:

• The prepared sample is completely dried in sun or by air drying. Its strength is tested by breaking lump between fingers
• Resistance to breaking, termed as dry strength is a measure of the plasticity and is considerably influenced by the colloidal fraction content of the soil.
• If the dry sample can be easily powdered it is said to have low dry strength, thus indicates silt or sandy silt.
• If considerable finger pressure is required to break the lump, it has medium dry strength, thus indicates silty clays and clays of low plasticity.
• If the lump cannot be powered by fingers at all, it has a high dry strength which represents a lightly plastic clay.
• The presence of water-soluble cementing materials, such as calcium carbonate or iron oxides, may also cause high dry strength.
• Soil with high strength is treated with a little line hydrochloric acid. A strong reaction indicates that the strength may be due lo calcium carbonate as cementing agent rather than colloidal clay.

Dilatancy Test:

• Dilatancy means reaction to shaking About 5CC soil sample is taken and enough water is added to nearly saturate it.
• The pat of soil is placed in the open palm of the hand and shaken horizontally be striking vigorously against the other hand several times.
• The pat is then squeezed between the fingers.
• The appearance and disappearance of water with shaking and squeezing is called a positive reaction. This reaction is called quick if water appears and disappears slowly and no reaction if water condition does not appear to change.
• The type of reaction is observed and recorded. Inorganic sits show a quick reaction whereas clays show no reaction or slow reaction

Toughness Test:

• The soil sample used in dilatancy test is dried by working and moulding till it reaches the consistency of putty.
• The time required to dry the sample is indicative of its plasticity. Further, the moisture content is reduced by rolling and re-tolling the soil into a thread of 3 mm diameter till it reaches the plastic limit.
• The resistance to moulding at plastic limit is called the toughness. After the thread crumbles, the pieces are lumped together and the slight kneading action is continued until the lump also crumbles.
• If the lump can still be moulded slightly drier than plastic limit and if high pressure is required to roll the thread between the palms of the hand, the soil is said to have high toughness.
• Medium toughness is indicated by medium thread and a lump formed of the threads slightly below the plastic limit will crumble.
• While low toughness is indicated by a weak thread that breaks easily and cannot be dumped together when drier than plastic limit.
• Non-plastic soils cannot be rolled into 3 mm diameter threads at any water content.

Organic silt or clay:

• If the amount of organic matter is small it is difficult to identify.
• Organic soil has a distinctive odour when fresh and wet.
• By heating the wet soil sample, odour can be made more noticeable.
• Fibrous organic soils, such as muck or peat, are dark brown to black in colour with distinctive odour.

Other identification tests:

• High plasticity is indicated on the field, if we cut the dry o: slightly moist lump of soil with a knife blade, it produces a shiny soil surface. A dull surface indicates clay or silt of low plasticity.
• Silt sticks to fingers will wash away easily and brush off, if dry, whereas wet clay sticks to the fingers, gives a greasy feel and does not was easily.
• In soil suspension of water of about 10 cm depth.

1. Clay-size particles will remain in suspension for several hours or even days.
2. Within 5 to 60 minutes silt will settle.
3. Whereas sand will settle easily with is half a minute.

Key Takeaways:

Field tests and field identification is thus a great tool in soil engineering. The soils to be identified in field may be coarse grained soil or fine-grained soil.

1.4 Grain size distribution curve:

• The effects of mechanical evaluation (sieve and hydrometer analyses) are typically provided with the aid of using semi-logarithmic plots referred to as particle-length distribution curves.
• The particle diameters are plotted in log scale, and the corresponding percentage finer in mathematics scale. The particle-length distribution curve for soil A is the mixture of the sieve evaluation effects provided in Table and the effects of the hydrometer evaluation for the finer fraction.
• When the effects of sieve evaluation and hydrometer evaluation are combined, a discontinuity typically happens withinside the variety wherein they overlap. This is due to the fact soil debris are typically abnormal in shape.
• Sieve evaluation offers the intermediate size of a particle; hydrometer evaluation offers the diameter of a sphere that might settle on the equal price because the soil particle.

1.5 Particle shape and size:

The classification of soils according to the predominant or effective size is called the particle size classification.

The various terms used in this classification are as follows:

• Clay: less than 2
• Silt :2 to 75 μ
• Sand: 75 to 4.75 mm
• Gravel: 4.75 mm to 80 mm
• Pebbles: 80 mm to 300 mm
• Boulders: More than 300 mm

1.6 Weight Volume relationships:

• The soil mass in general is a three-phase system composed of soil, liquid and gaseous matter. The solid phase comprises of mineral, organic matter or both.
• The mineral portion consists of particles of different size and shapes.
• The organic fraction is the plant and animal residue which may be present in various stages of decomposition.
• The solids enclose the open spaces termed as voids. The liquid phase is generally water and fills partly or wholly the voids.
• The gaseous phase, usually air, occupies the voids not filled by water. The relative volume wise and weight wise proportions of the three phases in a soil mass are important factors influencing its physical properties.
• It is therefore necessary to study them. Though the three phases precut cannot be separated as shown in below Fig. For better understanding they are shown as occupying separate spaces.
• The diagrammatic representation of the different phase present in a soil mass is called the phase diagram.
• Fig. Shows a three-phase system of natural soil which is partially saturated soil.
• When it is fully saturated i.e., all the voids are filled with water and thus gaseous phase is absent and the soil becomes a two-phase system in fig.

Fig 1: Natural soil

Fig 2: Partially saturated soil

Fig 3: Dry condition

Fig 4: Saturated condition

• Also, if the soil becomes totally dry and consists of only solid and gaseous phases. In this case also it is a two-phase system.

Ww=Weight of water

Ws =weight of soils

V =total volume of soil mass

Va =volume of air

Vw =volume of water

Vv =volume of voids

Vs =volume of solids

• For Partially Saturated Condition,

Total weight, W=Ws+Ww+Wa

Wa=0

∴W=Ws+Ww

Total volume, V= Vs+Vv= Vs+Va+Vw (3 phase system)

• For dry condition,

W=Ws

V=Vs+Va (2 phase)

• For fully saturated soil,

W=Ws+Ww

V=Vs+Vw (2 phase)

1.7 Specific gravity, Unit Weight, Void Ratio, Moisture content and relationships:

A soil mass consists of solid particles forming a porous structure. The voids in the soil mass are filled with air and water. The three constituents are blended together to form a complex material.

Fig: 5

A diagrammatic representation of different phases in a soil mass is called the Phase Diagram or the Block Diagram, for the purpose of making calculations convenient. In this all the 3 phases – solid, liquid and air – of the soil mass, are segregated and placed in 3 different layers.

Fig: 6

Usually, a 3-phase diagram is used, as shown in the figure below. As per convention, we write the volume on the left side and mass or weight on the right side of the diagram. Total volume is designated as V and it is equal to the sum of Volume of air (Va), Volume of water (Vw) and Volume of solids (Vs). Volume of voids (Vv) is the sum of volumes of air and water. Similarly, total mass is designated as M. Since the Mass of air (Ma) is very small, it is neglected. Therefore, total mass is the sum of Mass of water (Mw) and Mass of solids (Ms). Same convention is used for weights except it is designated by the letter W instead of letter M.

Fig: 7

However, sometimes, a 2-phase diagram may also be considered, as shown below. The convention as discussed above is applicable here also.

i)                   When the soil is absolutely dry, the water phase may disappear

Ii)                 When the soil is fully saturated, the air phase may disappear

Fig: 8

1. Water Content

w =  100

w ≥ 0 always

      w = water content (%)

      Ww = weight of water

      Ws = weight of solids

2.  Void Ratio

e =

e > 0 always

      e = void ratio

      Vv = volume of voids

      Vs = volume of solids

      Void ratio of fine-grained soils > Void ratio of coarse-grained soils

3.  Porosity

n =  100

0 < n < 100 always

      n = porosity (%)

      Vv = volume of voids

      V = total volume of soil

4.  Degree of Saturation

S =  100

0 ≤ S ≤ 100 always (S=0 for perfectly dry soil and S=100 for fully saturated soil)

      S = degree of saturation (%)

      Vw = volume of water

      Vv = volume of voids

5.  Air Content & Percentage Air Voids

ac =  100 = 1 - S

0 ≤ ac ≤ 100

      ac = air content (%)

      Va = volume of air

      Vv = volume of voids

na =  100

      na = air voids (%)

      Va = volume of air

      V = total volume of soil mass

6.  Unit Weight

a)     Bulk Unit Weight: total weight per unit volume

Ƴ =

      Ƴ = bulk unit weight

      W = total weight of soil mass = Wa + Ww

      V = total volume of soil mass = Va + Vw + Vs

b)    Dry Unit Weight: weight of soil solids per unit volume

Ƴd =

      Ƴd = bulk unit weight

      Ws = total weight of soil solids

      V = total volume of soil mass = Va + Vw + Vs

c)     Saturated Unit Weight: weight of fully saturated soil sample per unit volume

Ƴsat =

      Ƴsat = saturated unit weight

      Wsat = total weight of saturated soil mass

      V = total volume of soil mass = Va + Vw + Vs

d)    Submerged Unit Weight: weight of fully saturated soil sample per unit volume

Ƴsub =

      Ƴsub = submerged unit weight of solids = Ƴsat – Ƴwater ≈ Ƴsat/2

      Wsat = total weight of saturated soil mass

      V = total volume of soil mass = Va + Vw + Vs

7.  Specific Gravity & Apparent/ Mass Specific Gravity

Specific gravity of soil solids (G) is the ratio of the unit weight of a given volume of solids to the unit weight of an equivalent volume of water at 4ᵒC.

G =

Furthermore, apparent specific gravity (Gm) is the ratio of total weight of a given mass of soil to the weight of an equivalent volume of water.

Gm =

Important Relationships:

1. Relation between e and n

n = and e =

2.  Relation between e, w, G and S

S  e = w  G

3.  Bulk unit weight in terms of G, w, e and Ƴw

Ƴ = =

4.  Saturated unit weight in terms of G, e and Ƴw

If soil is fully saturated, S = 1

Ƴsat =

5.  Dry unit weight in terms of G, e and Ƴw

If soil is dry, S = 0

Ƴd =

6.Submerged unit weight in terms of G, e and Ƴw

Ƴsub = Ƴsat – Ƴwater

Ƴsub = - Ƴw

Ƴsub =

7.Relation between Ƴ, Ƴd and w

Ƴd =

1.8 Relative Density:

• Relative density is the degree of compactness of cohesionless soil.
• Relative density or density index is the ratio of the distinction among the void ratios of a cohesionless soil in its loosest kingdom and current herbal kingdom to the distinction among its void ratio with inside the loosest and densest states.
• Determination of relative density is useful in comparing compaction kingdom of coarse-grained soils and additionally assessing the secure bearing ability in case of sandy soils.

Relative Density = (emax - e)/ (emax - emin)

Where,

Emax = void ratio of coarse-grained soil (cohesionless) in its loosest kingdom.

Emin = void ratio of coarse-grained soil (cohesionless) in its densest kingdom.

e = void ratio of coarse-grained soil (cohesionless) in its herbal current kingdom withinside the field.

1.9 Consistency of Soil:

Consistency of soil can be determined in following ways:

Determination of Liquid Limit:

Following are the two Methods used for the Determination of Liquid Limit in the Laboratory

1. By Casagrande's method
2. By Cone penetrometer

By Casagrande's method (IS 2720 part 5-1985):

• In the laboratory, standard apparatus knows as Casagrande’s apparatus, along with the standard tools is used to determine the liquid limit.
• It consists of a brass’ cup mounted on a hard rubber base. The cup can be raised and made to fall on the hard rubber base.
• The distance of the fall can be regulated by screws.
• The fall is made 1 cm before the start of the test.
• There are two types of grooving tools as shown in the Fig. 1.22.1, to be used for different types of soils. For more sandy soils, ASTM tool is used and for clay soils, Casagrande tool or spatula is used.
• About 120 grams of air-dried soil passing through IS sieve of 425 micron is taken and mixed with water such that the soil attains a putty like consistency.
• A portion of the paste is placed in the cup and is levelled so as to have a maximum depth of 1 cm
• A groove is cut in the soil placed in the cup and is levelled so as to have a maximum depth of 1 cm.
• A groove is cut in the soil placed in the cup by using appropriate grooving tool.
• In cutting the groove, the tool is drawn through the sample along the symmetrical axis of the cop, holding the tool perpendicular to the cup
• The handle is rotated at the rate of 2 revolutions per second and the number of blows required to close the groove for a distance of 1.25 cm is noted.
• The groove should close by flow and not by slippage of soil.
• Then about 10 gm of soil near the groove is taken to determine its water content. By altering the water content of the soil and repeating the procedure 4 to 5 readings of water content in the range to 10 to 40 blows are obtained.
• A graph is then plotted between number of blows on a logarithmic scale water content on a natural scale as shown in Fig.

Fig 9: Liquid limit apparatus

Fig 10: Casagrande tool

Fig 11: ASTM tool

• It will be seen that the semi-logarithmic plot is a straight line whose equation is

W₁-W₂ = If

Where w1xw2, are the water contents corresponding to blows N1 and N2. If is the slope of line and it is called the flow index. The line is called the flow curve.

• The liquid limit is then determined by reading the water content corresponding to 25 blows, on the flow curve.

Determination of Plastic Limit: (IS-2720 part 5-1985)

• In order to determine the plastic limit about 15 gm of air-dried soil sample passing through IS sieve 425 u is taken and is mixed with a sufficient quantity of water which would enable the soil mass to become plastic enough to be easily shaped into a ball.
• A portion of the ball is taken and rolled on a glass plate with the palm of the hand into a thread of uniform diameter. When a diameter of 3 mm is reached, the soil is remoulded into a ball. The process of making the thread and remoulding is continued till the sample at a diameter of 3 mm just start crumbling.
• Some of the crumbled pieces are then taken for water content determination by oven drying method.
• The test is repeated twice more with fresh samples and the average of the three values is taken as the plastic limit.
• A steel rod of 3 nun diameter kept by the side of the glass plate helps in comparing the thread diameter easily.

Fig 12: Plastic limit determination

Fig 13: Plastic limit determination

Determination of Shrinkage Limit: IS-2720, Part 6-1972 Reaffirmed 1975

• About 30 gms of soil sample passing 425-micron sieve is taken in an evaporating disk.
• The soil is mixed with sufficient quantity of water to bring the soil to a consistency that it may flow.
• The soil mixture is placed in the shrinkage dish in three equal quantities so as to fill the dish. The excess soil is removed and the dish is weighed with the soil.
• The soil pat is allowed to dry till the colour changes from dark to light.
• The dish is placed in the oven at 110°C till its weight become constant.
• The shrinkage dish is weighed with dry sample and the volume of dry pat of soil is measured displacement of mercury.
• The shrinkage limit is calculated as shown in Fig.
• Fig. Shows the arrangement for shrinkage limit test.
• Fig. Shows the phase diagrams.
• Fig represents the soil sample in plastic state which fill the container of know volume V1 and weighs w1. As the sample is gradually dried, the water content at a certain stage becomes equal to the shrinkage limits as in Fig.

Fig 14: Shrinkage limit apparatus

Fig 15: Shrinkage limit apparatus

• At this point the volume is decreased to V₁, the corresponding perfectly saturated state.
• Beyond shrinkage limit, sample continues to dry and its weight continues to reduce. But its volume remains V, as now the decrease in weight is due to loss of water from the voids.
• When there is no further loss in weight, the sample is completely dry and its weight is w, while volume is V2, as weight of air is negligible.
• The shrinkage limit is the water content at situation of Fig.
• ∴Ws =×100
• Thus, by measuring the starting weight and volume and the dry weight and volume, shrinkage limit ws can be calculated. Here is the 1imit weight of water.

1.10 Atterberg limits:

• The Atterberg limits are a primary degree of the important water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit.
• Depending on its water content material, a soil may also seem in one in every of 4 states: solid, semi-solid, plastic and liquid. In every nation, the consistency and conduct of a soil is exclusive and therefore so are its engineering properties. Thus, the boundary among every nation may be described primarily based totally on an extrade withinside the soil's conduct. The Atterberg limits may be used to differentiate among silt and clay, and to differentiate among exclusive forms of silts and clays. The water content material at which the soils extrade from one nation to the alternative are referred to as consistency limits or Atterberg's limit.
• These limits had been created through Albert Atterberg, a Swedish chemist and agronomist in 1911.They had been later subtle through Arthur Casagrande, an Austrian-born American geotechnical engineer and near collaborator of Karl Terzaghi (each pioneer of soil mechanics).
• Distinctions in soil are utilized in assessing the soils which are to have systems constructed on them. Soils whilst moist preserve water, and a few enlarge in extent (smectite clay). The quantity of growth is associated with the cap potential of the soil to absorb water and its structural make-up (the form of minerals present: clay, silt, or sand). These checks are specifically used on clayey or silty soils considering the fact that those are the soils that enlarge and decrease whilst the moisture content material varies. Clays and silts engage with water and therefore extrade sizes and feature various shear strengths. Thus, those checks are used broadly withinside the initial ranges of designing any shape to make certain that the soil can have the proper quantity of shear energy and now no longer an excessive amount of extrade in extent because it expands and shrinks with exclusive moisture contents.

1.11 Liquidity index and Consistency index:

Liquidity index:

• The liquidity index (LI) is used for scaling the herbal water content material of a soil pattern to the limits.
• It may be calculated as a ratio of distinction among herbal water content material, plastic restrict, and liquid restrict:

LI=(W-PL)/(LL-PL)

Wherein W is the herbal water content material.

Consistency index:

• The consistency index (Ic) suggests the consistency (firmness) of a soil.
• It is calculated as CI = (LL-W)/(LL-PL), wherein W is the present water content material.
• Soil on the liquid restrict may have a consistency index of 0, even as soil on the plastic restrict may have a consistency index of one and if W > LL, Ic is negative. That approach soil is withinside the liquid state.
• Moreover, sum of Liquidity index and Consistency index same to 1 (one)

1.12 Activity:

• In any unique clay stratum the ratio of the plasticity index to the clay fraction content material is about constant, and can be described as the “activity” of the clay.
• Values of hobby are given for lots clays and additionally for the greater not unusual place minerals.
• It is proven that hobby is associated with the mineralogy and geological records of clays, and to the share in their shear electricity contributed through real cohesion.
• Field records is provided which suggests that the problems of taking excellent undisturbed samples in deep beds of touchy clay are restrained to the one’s clays with a hobby of much less than 0.75.

1.13 Soil Structure:

• Soil structure is the geometrical arrangement of soil particles with each other.
• Properties of soil such as permeability compressibility, shear strength etc. is greatly affected by soil structure.
• The following types of soil structure are generally recognized

1. Single grained structure
2. Honeycomb structure
3. Flocculent structure
4. Dispersed structure
5. Composite structure

1. Single grained structure:

• When each panicle of soil scales out of suspension separately and independently then such soil structure is known as single grained structure.
• The gravitational forces acting on the particles of soil cases the settlement.
• The specific surface of all particles is comparatively less, thus the surface forces are too small to be considered.
• These surface forces are neglected for all practical purpose in case of course grained soils (Diameter> 0.02 m).
• Such soil particle has high void ratio when the deposits in a loose state and low void ration in a dense state.

Fig 16: Single grained structure

2. Honeycomb structure:

• Honeycomb structure is seen in the soil particles having diameter between 0.0002-0.02 mm, usually observed in silts.
• In case of such soils, both gravitational forces and surface forces plays an important role.
• The soil particles settle due to gravitational forces and the surface forces at the contact area, are large enough compared to the submerged weighs to prevent the grains from rolling down immediately.
• The grains which are in contact are weld together until miniature arches are formed, with relatively large void space which is formed as honeycomb structure.
• In such type of structure comparatively large amount of water is enclosed within voids.
• Such type of structure has high voids ratio.

Fig 17: Honeycomb structure

3. Flocculent structure:

• Flocculent structure is formed when there is an edge contact between the clay platelets.
• The attractive electrical forces between adjacent soil particles at the time of deposition, is the basic reason for the formation of such type of structure.
• As the concentration of dissolved minerals is the water increases, the tendency of flocculation also increases.
• Flocculent structures are usually observed in clays with fine particles.

Fig 18: Flocculent structure

4. Dispersed structure:

• Dispersed structure is also seen in clays with fine particles.
• The repulsion electrical forces between adjacent soil particles at the time of deposition are responsible to form dispersed structure.

Fig 19: Dispersed structure

5. Composite Soil Structure:

• In case of composite soil following two structures can be found,

1. Coarse grain skeleton
2. Cohesive matrix

1.Coarse grain skeleton:

• When the void space between the single particles is filled with the clay particles; it is called as coarse grain skeleton.
• The bulky particles form a continuous relatively incompressible frame-work.

2.Cohesive matrix:

• When the clay content is more as compared with coarse particles; cohesive matrix is formed.
• Such type of soil formation is relatively more compressible.

Key Takeaways:

The following types of soil structure are generally recognized

• Single grained structure
• Honeycomb structure
• Flocculent structure
• Dispersed structure
• Composite structure

1.14 Engineering classification of Soil:

Soil can be extensively labeled as follows:

1. Classification primarily based totally on grain length
2. Textural type
3. AASHTO type
4. Unified soil type

Grain length type machine:

• Grain length type structures had been primarily based totally on grain length. In this machine the phrases clay, silt, sand and gravel are used to signify most effective particle length and now no longer to suggest nature of soil type.
• There are numerous type structures fin use, however typically used structures are proven here.

Textural Classification:

• The type of soil completely primarily based totally on particle length and their percent distribution is referred to as textural type machine.
• This machine specially names the soil relying on the share of sand, silt and clay. The triangular charts are used to categories soil through this machine.

AASHTO type:

• AASHTO type is in any other case referred to as PRA type machine. It became in the beginning advanced in 1920 through the U.S. Bureau of Public Roads for the type of soil for dual carriageway subgrade use.
• This machine is advanced primarily based totally on particle length and plasticity traits of soil mass.
• After a few revisions, this machine became followed through the AASHTO in 1945. In this machine the soils are divided into seven essential corporations. Some of the essential corporations similarly divided into subgroups.
• A soil is classed through intending from left to proper at the type chart to discover first the organization into which the soil check information will fill.
• Soil having best fractions are similarly labeled primarily based totally on their organization index. The organization index is described through the subsequent equation.

Group index = (F – 35) [0.2 + 0.005 (LL – 40)] + 0.01(F – 15) (PI – 10)

F – Percentage passing 0.075mm length

LL – Liquid limit

PI – Plasticity index

Unified soil type:

• Unified soil type machine became in the beginning advanced through Casagrande (1948) and became referred to as airfield type machine.
• It became followed with a few amendments through the U.S. Bureau of Reclamation and the U.S. Corps of Engineers.
• This machine is primarily based totally on each grain length and plasticity traits of soil.
• The equal machine with minor amendment became followed through ISI for well-known engineering purpose (IS1498 – 1970). IS machine divides soil into 3 essential corporations, coarse grained, best grained and natural soils and different miscellaneous soil materials.
• Coarse grained soils are people with greater than 50% of the cloth large than 0.075mm length.
• Coarse grained soils are similarly labeled into gravels (G) and sands (S). The gravels and sands are similarly divided into 4 classes in keeping with gradation, silt or clay content.
• Fine grained soils are the ones for which greater than 50% of soil finer than 0. Half mm sieve length. They are divided into 3 sub-divisions as silt (M), clay, and natural salts and clays (O). Primarily based totally on their plasticity nature they may be delivered with L, M and H image to signify low plastic, medium plastic and excessive plastic respectively.

1.15 Types of Soil Classification-IS:

• It is based on USCS with the modification that fine-grained soils are sun-divided into 3 groups of low, medium and high compressibility, instead of 2 groups as in the case of USCS.

1. Low compressibility (L): LL < 35
2. Medium compressibility (I): 35 < LL < 50
3. High compressibility (H): LL > 50

• The soil is classified into 18 groups: 8 groups of coarse-grained, 9 groups of fine-grained and 1 group of Peat.
• The basic soil components as per ISSCS are as shown in the table below.

1.16 USCS (Unified Soil Classification System):

• USCS was first developed by A. Casagrande for the purpose of air-field construction during WW-II. However, it was later modified by Bureau of Reclamation and Corps of Engineers of USA to be applicable to dams, foundations etc.
• It is the most popular system. It classifies coarse-grained soils on the basis of their grain size distribution and the fine-grained particles on the basis of their plasticity characteristics. The symbols used for various soil types in USCS are shown in the table below.

The soil is first classified into 3 categories:

1. Coarse-grained soils: These are soils having 50% or more being retained on 0.075 mm sieve. They are further classified into 8 groups.

• These are termed as gravel (G) if 50% or more of coarse fraction is retained on 4.75 mm sieve otherwise it is called sand (S).
• Furthermore, if this soil contains less than 5% fines, which is well-graded (W), it is termed as GW or SW. However, if it is poorly graded (P), the soil is designated GP or SP.
• If the soil contains more than 12% fines, it may be designated as GM, GC, SM or SC where M stands for silt and C stands for clay.
• If fines are in the range of 5 to 12%, they may be termed as GW-GM, SP-SM etc.

Fig: 20

2.     Fine-grained soils: If 50% or more passes through 0.075 mm sieve, it is termed as fine-grained soil. They are further classified into 6 groups.

• These are further sub-divided into two types:

a)     Soils of low compressibility (L) if liquid limit ≤ 50% and are designated the symbols ML, CL, OL. (O stands for organic)

b)    Soils of high compressibility (H) if liquid limit > 50% and are designated the symbols MH, CH, OH.

• The exact type of soil is determined from the plasticity chart, in which the A-line separates the clays from the silts. The equation for A-line is as follows:

IP = 0.73 (wL – 20)

• If the value of IP ranged from 4 to 7, then dual symbols are used.
• When the soil is oven-dried and its liquid limit decreases by more than 30%, it is classified as organic (OL, OH). Otherwise, it is inorganic (ML, MH).

3.     Highly organic soils: It is identified by visual inspection. It is also known as Peat (Pt).

1.17 HRB:

• HRB stands for Highway Research Board.
• The device of type is primarily based totally on each the particle length distribution in addition to the plasticity traits of the soil. This device is broadly speaking used for pavement construction.
• In this device, soils are divided into 7 number one corporations, specific as A-1, A-2…..A-7. Group A-1 is split into sub-corporations and organization A-2 is split into 4 sub-corporations.

1.18 ASTM:

• ASTM's geotechnical engineering requirements are instrumental in specifying, testing, and investigating the physical/mechanical houses and function behaviors of floor and subsurface earth substances which can be applicable to a production project.
• Such earth substances consist of soil, size stones, slate, soil-cement mixtures, floor water, bituminous geomembranes, slurry, rock mass, aquifers, and geotextiles.
• These geotechnical engineering requirements permit engineering companies and production agencies to study the elastic characteristics, flow, and erosion conduct of the stated earth substances to make sure protection and save you unexpected risks associated with the erection of civil structures.

Numericals:

Q. The Void ratio and Specific gravity of sample of clay are 0.73 and 2.7 respectively. If the voids are 92% saturated find the bulk density, dry density and the water content?

Given:

e =0.73

G=2.7

S=92%

Find:

1)Water content(W)

W=

∴W =24.87%

2)Bulk density ( and dry density (

∴

Q. A Soil sample in its natural state has, when fully saturated, a water content of 32.5%. Determine the void ratio, dry density and saturated unit weights. Assume G=2.69.

Given:

w =32.5%

G=2.69

Find: Void ratio(e), Dry unit weights (()

We know,

For fully saturated S=1

e =w.G=

∴e =0.874

Dry unit weight

For fully saturated soil S=1

∴e=

Q. A saturated clay has a water content of 40% and bulk specific gravity is 1.84. Determine the void ratio, and specific gravity of particles.

Given:

G=1.84

w=40%

e=

e=0.736

We know, n=

n=

n=0.424

∴G=0.78

Q. A saturated soil sample weighting 1.78 gm has a volume of 96cc.If G=2.67 determine the void ratio, water content and unit weight of the soil?

Given:

W=178gm

V=96cc

G=2.67

Find: e=?,

18.19=

1+e=1.45

∴e= 1.45-1 =0.45

Q. The grading curve of a soil gives the effective size as 0.16mm,Find Cu and Cc. Classify the soil.

Does not meet the requirement of well graded sand, given soil is poorly graded sand.

References:

1. Principles of Geotechnical Engineering by Braja M. Das, Cengage learning
2. Soil Mechanics and Foundation Engineering by K.R. Arora, Standard Publishers
3. Soil mechanics and Foundation Engineering by B.N.D. Narsingarao, Wiley India Pvt. Ltd.
4. Basic and applied soil mechanics, by Gopal Ranjan, A.S.R Rao, New age International publishers