field identification of soil

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2009- 10 GEO-TECHNICAL ENGINEERING ENROLL. NO: ASSIGNMENT NO. 1 FIELD IDENTIFICATION OF

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Page 1: Field Identification of Soil

2009-10

GEO-TECHNICAL ENGINEERING

ENROLL. NO:

ASSIGNMENT NO. 1 FIELD IDENTIFICATION OF SOIL

Page 2: Field Identification of Soil

Importance 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  color,  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. Color is often helpful in distinguishing between soil types, and with

experience, one may find it useful in identifying the particular soil type. Color

may also indicate the presence of certain chemicals. Color often varies with

moisture content of a soil. For this reason, the moisture content at the time of

color identification should be included. Some of the more familiar color

properties are listed  below. Generally, colors 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 gray, including almost black, contain

organic  colloidal  matter. In contrast, clean, bright looking shades of gray, olive

green, brown, red, yellow, and white are associated with inorganic  soils.  Gray-

blue  or  gray-  and  yellow-mottled  colors frequently  result  from  poor

Page 3: Field Identification of Soil

 drainage. Red, yellow, and yellowish brown result from the presence  of  iron

oxides. White to pink may indicate considerable silica, calcium carbonate, or

aluminum 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:-

Consistency Field Identification Unconfined 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

Page 4: Field Identification of Soil

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

trimming them to the desired size.

4) Mixing bowl:- Use  a  small  bowl  with  a rubber-faced  pestle  to  pulverize  the

fine-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.

Page 5: Field Identification of Soil

Coarse Grained Soil:-Coarse grained soils are easily identified by visual

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. 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:-

Main content Descriptive term Subsidiary constituent

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 Upto 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 percent of gravel. Very fine unifom 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

inspection is 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

Page 6: Field Identification of Soil

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-

brown etc.

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 honmogeneous.

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 not

individually 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.

Page 7: Field Identification of Soil

Specimens to be analysed

Sample analysis:-

Specimen 1:- It shows gravels with varying sizes. It has a sub-angular

structure. The clean bright shade of gravels indicate its association with

inorganic soils. Also it indicates it has less moisture content as compared to

specimen 2.

Specimen 2:- It shows gravels of uniform size. They may have more moisture

content as compared to specimen 1 due to their dark appearance. The presence

of red, yellow and yellowish-brown gravels show presence of compounds like

iron oxides and small amounts of silicates. They basically have sub-angular or

rounded structures.

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

vigorously in the palm of one hand which may also be struck several

times with the other hand, 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

Page 8: Field Identification of Soil

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

broken or crushed with fingers, the breaking strength is an indication of

the relative amounts 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 plastic limit is called the toughness. The water content of a wet

soil sample is gradually 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 dry the pat is an indication of its

plasticity. After the treads crumble, the pieces are lumped together and

Page 9: Field Identification of Soil

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

when cut 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 not wash 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

combination with 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-gray, or black colour, presence of organic

matter may be suspected. Organic soils usually have a distinctive

organic odour, specially 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

Page 10: Field Identification of Soil

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:-

1. Compaction test:-

It determines the stability of soil

after densification.

2. Percolation test:-Vertical and lateral movement of water through the soil by gravity is percolation. It is necessary on sites related with water like dams, purification houses, etc.

Page 11: Field Identification of Soil

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

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

Page 12: Field Identification of Soil

Advantages of field identification:-

It is very economical. It can be carried out in short duration of time. No pre-setting is required

Disadvantages of field identification:-

It is just an approximation. You cannot completely rely on it. 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.

Page 13: Field Identification of Soil

It aids us to determine approximate compressive strength of soil and thus helps us in constructive measures.