Success Soil Mechanics

8/12/2019 Success Soil Mechanics

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SUCCESS SOILMECHANICS----------------------------------------------------------------------------------------------------------------------------------------------

FIELD IDENTIFICATION OF SOILImportance of Field Identification:-Field identification of soil is of great importance for civil engineering. Sometimes the

lack of time and facilities makes laboratory soil testing impossible in military

construction. Even when laboratory tests are to follow, field identification tests

must be made during the soil exploration. Soil types need to be identified so that

duplicate samples for laboratory testing are held to a minimum. Each test may be

performed with a minimum of time and equipment. However, the classification

derived from these tests should be considered an approximation. The number of

tests used depends on the type of soil and the experience of the individual using

them. Experience is the greatest asset in field identification; learning the

technique from an experienced technician is the best method of acquiring the

skill. If assistance is not available, you can gain experience by getting the feel

of the soil during laboratory testing.

Visual Examination:-1. Colour of soil:-

Visual examination should establish the colour, grain size, grain shapes (of the

coarse-grained portion), some idea of the gradation, and some properties of the

undisturbed soil. It helps in Unified classification of soil. Colour is often helpful indistinguishing between soil types, and with experience, one may find it useful in

identifying the particular soil type. Colour may also indicate the presence of

certain chemicals. Colour often varies with moisture content of a soil. For this

reason, the moisture content at the time of colour identification should be

included. Some of the more familiar colour properties are listed below. Generally,

colours become darker as the moisture content increases and lighter as the soil

dries. Some fine-grained soils (OL, OH) with dark drab shades of brown or

grey, including almost black, contain organic colloidal matter. In contrast, clean,

bright looking shades of grey, olive green, brown, red, yellow, and white areassociated with inorganic soils. Grey-blue or grey and yellow-mottled colours

frequently result from poor drainage. Red, yellow, and yellowish brown result from

the presence of iron oxides. White to pink may indicate considerable silica,

calcium carbonate, or aluminium compounds.

2. Compressive strength:-Knowing the consistency index (consistency) of the soil we can have the value of

compressive strength by field determination. However, this value is an

approximate value and can be used as a guideline. It can be tabulated as follows:-


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Consistency Field IdentificationUnconfined Compressive

strength (kN/m2)

Hard Can be indented with difficulty by thumb nail. >400

Medium hard Can be readily indented with thumb nail. 200 to 400

Stiff The thumb can be pressed in with light pressure. 100 to 200

Deformable Thumb can be pressed in with light pressure. 50 to 100

Soft Thumb can be pressed easily. 25 to 50

Very soft The fist can be pressed easily. < 25

Instrumentation:-The instruments and tools used while soil testing are:1. Pioneer tools:- Use a pick and shovel or a set of entrenching tools for

collecting samples. A hand auger is useful if samples are desired from depths

of more than a few feet below the surface.2. Stirrer:- The spoon issued as part of the mess equipment serves in mixing

materials with water to the desired consistency. It also can aid in collecting

samples.

3. Knife:- Use a combat knife or pocket knife for collecting samples and trimmingthem to the desired size.

4. Mixing bowl:- Use a small bowl with a rubber-faced pestle to pulverize thefine-grained portion of the soil. Both may be improvised. You could use a canteen

cup and wood pestle.

5. Paper:-Several sheets of heavy paper are needed for rolling samples.6. Pan and heating element:- Use a pan and heating element to dry samples.7. Scales: -Use balances or scales to weigh samples of soil.8. Soil testing kit:- Consists of pH papers, pH meters, test tubes, chemical

capsules and various other accessories.

Coarse Grained Soil:-Coarse grained soils are easily identified byvisual inspection on the basis of particle size. Rounded to angular, bulky, hard rock

fragments of average diameter more than 20 cm are described as boulders.

Stones of sizes between 20 cm and 6 cm are termed as cobbles. Soil fraction

between the sizes of 60 mm and 2 mm is known as the gravel fraction. Soil with

particle size visible to the naked eye but less than 2 mm is classified as sand.


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Sand is further divided into coarse, medium and fine fractions for which sieve

analysis is usually required.

Mixtures of gravel and sand are given in dual designations in the following table:

Maincontent

Descriptive termSubsidiaryconstituent

Gravel

Gravel and san About 50%

Gravel with some sand or sandy gravel 25-40%

Gravel with little sand 10-25%

Gravel with a trace of sand Up to 1%

If sand is the main constituent, gravel and sand will interchange in the above

descriptive terms, for example, sand with some gravel or gravelly sand will represent

sand having 25% to 40% of gravel. Very fine uniform sand is difficult to be

distinguished visually from silt. However, when dry, it does not hold together (no

cohesion) and feel gritty in contrast to the very slight cohesion and smooth feel of

the dried silt.

The description of coarse grained soils should give grading, grain-shape, colour, in-situ

strength and structural features, and presence of fines if any.

Grading:- The grading or particle size distribution as judged by visual inspectionis expression by the terms well-graded, poorly graded or uniformly graded.

Grain size:- The terms used to describe the grain shape are angular, sub-angular and rounded. Angular particles have sharp edges and relatively plane sides

with unpolished surfaces. Sub-angular particles are similar to angular but have rounded

edges. Rounded particles have smoothly curve sides and no edges.

Colour:- The colour of the soil is expressed as brown, white, yellow, red- brownetc.

Strength and Structure:- The in-situ strength of a deposit of coarse-grained soil is expressed as compact or loose. If a pick is required for excavation,

it is compact and if it can be done with a spade, it is loose.The arrangement and state of aggregation of soil particles in a soil mass is known

as soil structure. The structure of coarse grained which may be recognized in the

field is described as homogeneous. If it is composed of arranged in strata or

layers, e.g. Stratified alluvium, it is said to have stratified structure.

Pressure of Fines:- If the soil contains some fines (particle size notindividually visible to the naked eye), but not sufficient to cause cohesion. This

should be noted. Presence of any organic matter, if any, should also be indicated.


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Fine Grained Soil:-Fine grained are identified by performing the following simple tests on the minus

425 micron IS sieve size particles. In the field, use of the sieve is not intended. The

coarser particles that interfere with the tests may be simply removed by hand.

1.Dilatancy or Shaking test:- When a wet pat of soil is shaken vigorouslyin the palm of one hand which may also be struck several times with the otherhand, the surface may become glossy and show free water. If the pat of the

soil is then squeezed between fingers, the free water disappears and the

surface becomes dull i.e. Dilates. With silt and sands, a rapid or good reaction

of appearance of water during shaking and of its disappearance during squeezing

will be exhibited, but this phenomenon will not be noticeable with clays.

Take a small sample of your soil (approximately 1 cubic inch) and mix it with

about 1 tablespoon of water. Form the sample into a cube or ball and place it

in the palm of your hand face up. With you other hand, hit the side of your

hand that is holding the soil sample. This will impart a vibration to the soil

sample. If the vibrations causes the water to come to the surface of the soil

(soil surface starts to shine), the soil has a permeability indicative of a silty

soil and likely has a low plasticity. If the water appears quickly and the cube

of soil starts to slump down, the soil is likely a silt. If water appears slowly

and the cube generally retains its shape, it is likely a low plastic clay . The

water does not appear and the ball holds its shape very well, you likely have a

highly plastic clay

2.Dry Strength Test:- If a small piece of dry fine grained soil is brokenor crushed with fingers, the breaking strength is an indication of the relativeamounts of silt or clay. If the soil can be powdered easily with the fingers, it

is said to have slight dry strength and indicates silt or sandy silt. If

considerable strength is required, but the soil can still be broken into small

pieces without great difficulty, it is said to have medium dry strength, and is

indicative of silty clays and clays of low plasticity. When the pat of dry soil

cannot be broken with fingers, it has high dry strength and represents a highly

plastic clay. The presence of high strength, 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 dilute hydrochloric acid. A

strong reaction indicates that the strength may be due to calcium carbonate as

cementing agent, rather than colloidal.

If you can find a dry sample of soil, try to break it. Soil with very high dry

strength is typically highly plastic. A soil that is very easy to crumble is likely a

low plastic silt. A soil that takes some effort but is still easy to break is

likely a low to moderately plastic clay.

3.Toughness test:- The consistency or the resistance to moulding at the plasticlimit is called the toughness. The water content of a wet soil sample isgradually reduced by working and moulding until it reaches the plastic limit

when soil threads should crumble at about 3 mm diameter. The time required to


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dry the pat is an indication of its plasticity. After the treads crumble, the

pieces are lumped together and a slight kneading action continued until the lump

also crumbles. If the lump can still be moulded slightly drier than the plastic

limit and the threads can still be rolled with considerable pressure, the soil is of high

toughness which represents clays of high plasticity or fat clays. Medium

toughness represents soils of medium plasticity whose threads are medium tough

and a lump formed of the threads slightly below the plastic limit crumbles.Weak threads that break easily cannot be lumped together, when drier than

the plastic limit indicate slight toughness representing soils of low plasticity.

The number of times the procedures can be repeated is an indication of the

plasticity index of the soil. Non- plastic soils cannot be rolled into 3 mm

diameter threads at any water content.

4.Other identification tests:- If a dry or slightly moist lump of soil whencut or rubbed with considerable pressure with a knife blade produces a shiny

surface, high plasticity is indicated. A dull surface indicates silt or clay of low

plasticity. Wet clay sticks to the fingers, gives a greasy feel and does notwash off readily, whereas, silt will wash away easily or brush off, if dry. In a

soil suspension of water of about 10 cm depth, sand will settle within half a

minute, most of the silt in about 5 to 60 minutes, whereas, clay-size particles

will remain in suspension for at least several hours or may even remain for

several days.

5.Organic silt or clay:- Finely divided organic matter present in combinationwith mineral soil materials is not easily recognized, particularly if the amount of

organic matter is small. However, if the soil has a dark-brown, dark-grey, or

black colour, presence of organic matter may be suspected. Organic soils usually

have a distinctive organic odour, especially when fresh and wet. Sometimes the

organic odour can be made more noticeable by heating the wet sample. Highly

organic clays have a very weak and strong feel at the plastic limit. Fibrous

organic soils, such as peat or muck, are usually dark brown to black in colour,

have organic matter in various stages of decomposition and possess a

characteristic odour.

Other Tests Conducted:-

Some more tests are conducted on the site using various handy tools and

apparatus. These tests are conducted only when we require detailed study of soil

properties. They are:-

Compaction test: -It determines the stability of soil after densification

Percolation test:-Vertical and lateral movement of water through the soil bygravity is percolation. It is necessary on sites related with water like dams,

purification houses, etc.

Erosion test: -Useful in the area of flowing waters.

Radiation test: - This actually is not feasible to do on site. It is used forcalculating water content in the soil using radio isotopes.


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Advantages Of Field Identification:-1. It is very economical.2. It can be carried out in short duration of time.3. No pre-setting is required

Disadvantages Of Field Identification:-1. It is just an approximation.2. You cannot completely rely on it.3. Experienced people are required.

Conclusion:-It is a must do procedure.

It helps in Unified soil classification which makes it more easy to understand the

physical properties and thus derive its chemical properties.

It aids us to determine approximate compressive strength of soil and thus helps

us in constructive measures.

UNIFIED SOIL CLASSIFICATION SYSTEMThe Unified Soil Classification System (USCS) is a soil classification system used in engineering

and geology to describe the texture and grain size of a soil. The classification system can be

applied to most unconsolidated materials, and is represented by a two-letter symbol. Each letter

is described below (with the exception of Pt):

First And/Or Second Letters Second Letter

Letter Definition Letter Definition

G Gravel P Poorly Graded (Uniform Particle Sizes)

S Sand W Well-Graded (Diversified Particle Sizes)

M Silt H High Plasticity

C Clay L Low Plasticity

O Organic

If the soil has 512% by weight of fines passing a #200 sieve (5% < P#200 < 12%), both grainsize distribution and plasticity have a significant effect on the engineering properties of the soil,

and dual notation may be used for the group symbol. For example, GW-GM corresponds to "well-

graded gravel with silt."

If the soil has more than 15% by weight retained on a #4 sieve (R#4 > 15%), there is a

significant amount of gravel, and the suffix "with gravel" may be added to the group name, but the

group symbol does not change. For example, SP-SM could refer to "poorly graded SAND with

silt" or "poorly graded SAND with silt and gravel."


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AASHTO Soil Classification System (from AASHTO M 145 or ASTM D3282)

General ClassificationGranular Materials (35% or less passing the

0.075 mm sieve)

Silt-Clay Materials (>35%

passing the 0.075 mm sieve)

Group Classification

A-1

A-3

A-2

A-4 A-5 A-6

A-7

A-1-a A-1-bA-2-

4

A-2-

5

A-2-

6

A-2-

7

A-7-5

A-7-6

Sieve Analysis,%

passing

2.00 mm (No. 10) 50max

0.425 (No. 40)30

max

50

max

51

min

0.075 (No. 200) 15 max25

max

10

max

35

max

35

max

35

max

35

max

36

min

36

min

36

min36 min

Characteristics of

fraction passing

0.425 mm (No. 40)

Liquid Limit 40

max

41

min

40

max

41

min

40

max

41

min

40

max41 min

Plasticity Index 6 max N.P.10

max

10

max

11

min

11

min

10

max

10

max

11

min

11 min1

Usual types of

significant constituent

materials

stone

fragments,

gravel and sand

fine

sand

silty or clayey gravel and

sandsilty soils clayey soils

General rating as a

subgradeexcellent to good fair to poor

Plasticity index of A-7-5 subgroup is equal to or less than the LL - 30. Plasticity index of A-7-6 subgroup is greater than LL - 30
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IS CODE SOIL CLASSIFICATION SYSTEMTERMINOLOGY:-1.Clay An aggregate of microscopic and sub-microscopic particles derived from the

chemical decomposition and disintegration of rock constituents. It is plastic within a

moderate to wide range of water content.

2.Silt A fine-grained soil with little or no plasticity. If shaken in the palm of thehand, a part of saturated inorganic silt expels enough water to make its surface appearglossy. If the pat is pressed or squeezed between the fingers, its surface again

becomes dull.

3.Sand and Gravel Cohesionless aggregates of angular, subangular, sub-rounded,rounded, flaky or flat fragments of more or less unaltered rocks or minerals.

According to this system, gravel is a fraction of the soil material between 75 mm and the

4.75-mm IS Sieve size and sand is the material between the 4.75-mm IS Sieve size and the

75-micron IS Sieve size.

CLASSIFICATION:- Soils shall be broadly divided into three divisions as givenbelow:

1.Coarse-Grained Soils In these soils, more than half the total material byweight is larger than 75-micron IS Sieve size. The coarse-grained soils shall be divided

into two subdivisions, namely:

a) Gravels In these soils, more than half the coarse fraction (+ 75 micron) is largerthan 4.75-mm IS Sieve size. This subdivision includes gravels and gravelly soils.b) Sands In these soils, more than half the coarse fraction (+ 75 micron) is smallerthan 4.75-mm IS Sieve size. This subdivision includes sands and sandy soils.

2.Fine-Grained Soils In these soils, more than half of the material by weight issmaller than 75-micron IS Sieve size. The fine-grained soils shall be further divided

into three subdivisions on the basis of the following arbitrarily selected values of liquid

limit:

a) Silts and clays of low compressibility having a liquid limit less than 35(represented by symbol L )

b) Silts and clays of medium compressibility having a liquid limit greater than 35and less than 50 (represented by symbol I)

c) Silts and clays of high compressibility having a liquid limit greater than 50(represented by symbol H )

[NOTE In this system the fine-grained soils are not divided according to particle

size but according to plasticity and compressibility. The term compressibility here

shall imply volume change, shrinkage during dry periods and swelling during wet

periods, as well as, consolidation under load. Soil particles finer than 2-micron may;

however, be designated as clay-size particles and the particles between 75-micron and

2-micron as silt-size particles.]3. Highly Organic Soils and Other Miscellaneous Soil Materials These soils contain

large percentages of fibrous organic matter, such as peat, and particles of decomposed


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vegetation. In addition, certain soils containing shells, concretions, cinders, and other

non-soil materials in sufficient quantities are also grouped in this division.

Groups: The coarse-grained soils shall be further divided into eight basic soilgroups. The fine-grained soils shall be further divided into nine basic soil groups (see

Table 2 of IS 1498-1970).

Highly organic soils and other miscellaneous soil materials shall be placed in one group.The groups shall be designated by symbols.

[NOTE These groups are broad, based on basic properties of soil; therefore,

supplemental detailed word descriptions are required to point out peculiarity of a

particular soil and differentiate it from others in the same group.]

The basic soil components are given in Table 1 of IS 1498-1970.

The various subdivisions, groups and group symbols are given in Table 2 of IS 1498-1970.

The laboratory classification criteria for classifying the fine-grained soils are given in

the PLASTICITY CHARTshown in Fig. The A line has the following linear equation

between the liquid limit and the plasticity index:Ip = 0.73 ( WL20 ) where

Ip = plasticity index, and

WL= liquid limit.

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