Field Identification Of Soil

COLLEGE OF

GEO-TECHNICAL ENGINEERING

ENGINEERING, PUNE.

ENROLL. NO:

2009-10

ASSIGNME NT NO. 1

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 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

>400

by thumb nail. Medium hard

Can be readily indented with

200 to 400

thumb nail. Stiff

The thumb can be pressed in

100 to 200

with light pressure. Deformable

Thumb can be pressed in with

50 to 100

light pressure. 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

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.

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

25-40%

sandy gravel 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, subangular 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, redbrown etc.

Strength and Structure:- The in-situ strength of a deposit of coarsegrained 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.

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 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 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. Nonplastic 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 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.

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.

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.

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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.