TESTING OF SOIL IN CIVIL ENGINEERING - QUALITY CONTROL

1.0   FIELD CLASSIFICATION OF SOILS

1.0   GENERAL:

Types of Soil tests for building construction works depend on properties of soil. Design of the foundation is based on soil test report of construction site. 

Soil is a very important part of the design and building of any structure. Since the soil is in direct contact with the structure, it acts as a way to transfer loads. Any analysis of forces acting on the structure must consider how stresses are spread through the soil since the structure’s stability depends on the soil’s properties. At the feasibility stage, it is important to conduct a geotechnical study of the site before the design begins. It is a key design input that helps to understand the subsoil on which the structure will stand. 

Unlike construction material like Cement, Steel, Bricks and timber, soil does not possess a constant parameter. The type of soil, its density, moisture content and so its strength and deformation characteristics vary so frequently that we cannot rely over few test results. It is thus imperative to have adequate study of soil properties in field as well as through laboratory testing.

The extent of study depends on time, importance of structure and cost of structure. Factor of safety considered also depends on factors and also on the extent of investigations. Lower the safety factor if details investigation is done or structure is of less importance or of low cost and vice-versa. Advancement in science and technology and needs of time helped as to design the structure with lower safety factor.

Experienced civil engineers are all are quite aware of the soil phenomenon that takes place during excavation and foundation work due to presence of subsoil water nearby. It is a distinct behavior of soil at different juncture of time.

The field method is used primarily in the field to classify and describe soil.

Visual observations are employed in place of precise laboratory tests to define the basic parameters.

For understanding the fundamentals of soil mechanics and soil behavior close familiarity with the common soil types is very necessary. This does not require a knowledge of locally used nomenclature like murrum, yellow earth, yellow soil or brown soil etc.

3.0   CLASSIFICATIONS AND PROPERTIES:

Soil can be classified by different ways such as:

i)      Preliminary classification by soil type.

iii)    Geological classification

iii)    By its structure

iv)    Atterberg limits

v)     By Grain size analysis

vi)    IS classifications

3.1   Along with the classification, the following properties are also required to be obtained by laboratory testing.

i)      Soil type or classification of soil based on particle size distribution and Atterbergs limits (plasticity index)

ii)     Compactness Natural (for field or in-situ) dry density or Maximum dry density (or placement) to be attained for fill material.

iii)    Moisture content - Natural moisture content or placement moisture i.e. optimum moisture content.

iv)    Shear parameters

v)     Compaction

vi)    Permeability

vii)   Swelling and Shrinkage

3.2   For determining these properties sampling is necessary which should be truly representative and the mode of sampling will depend on the purpose of soil testing.

4.0   TYPES OF SOILS:

i)     Principal types of soils:-

The following list of soil types includes the names commonly used by experienced foreman and practical engineers for field classification.

(a)   Sand and Gravel:

The cohesion less aggregates of rounded, sub angular or angular fragments of more or less unaltered rocks or minerals. Particles with a size upto 4.75mm (3/16") are referred to as sand, and those with a size from 4.75 to 150 & 200mm as gravels. Fragments with a diameter more than 200 mm are known as boulders. Hardpan is a soil that offers an exceptionally great resistance to the penetration of drilling tools. Most hardpans are extremely dense well graded somewhat cohesive aggregate of mineral particles.

(b)   Inorganic silt:

It is a fine grained soil with little or no plasticity. The least plastic is a powder form of quartz called rock flour. Some are even plastic silts. Because of its smooth texture it is often mistaken as clay, but it distinguishes from clay. When shaken in the palm of hand, a pat of saturated inorganic silt expels enough water to make its surface appearance glassy. If the pat is bent between the fingers, its surface again become dull. This procedure is known as the shaking or dilatancy test. After the pat is dried it is brittle and dust can be detached by rubbing it with the finger.

(c)   Organic Silt:

It is a fine grained more or less plastic soil with an admixture of finely divided particles of organic matter.  Sheels and visible fragments of partly decayed vegetable matter may also be present. The soil ranges in colour from light to very dark grey and it is likely to contain considerable quantity of H2S CO2 and various other gaseous products of the decay of organic matter which gives it a characteristics odour. The compressibility of organic silt is very high.

(d)   Clay:

It is an aggregate of microscopic and submicroscopic particles derived from the chemical decomposition or rock constituents. It is plastic within a moderate to wide range of water content.

3.     Dry specimen are very hard, and no powder can be detached by rubbing the surface of a dried pats with the fingers. It has a way to soapy appearance and great toughness. At higher water content they are conspicuously sticky.

(e)    Organic clay:

It is a clay that owes some of its significant physical properties to the presence of finely divided organic matter. When saturated organic clay is very compressible but when dry its strength is very high. It is usually dark grey or black in colour with soapy feel.

(f)    Peat:

It is a somewhat fibrous aggregate of microscopic and microscopic fragments of decayed vegetable matter. Its colour ranges between light brown and black. Peat is so compressible that it is almost always unsuitable for supporting foundations.

If a soil is made up of combination two different soil types, the predominant ingredients expressed as a noun, and the less prominent ingredient as a modifying adjective. For example, siltys and indicates a soil which is predominantly sand but contains a small amount of silt. A sandy clay is a soil which exhibits the properties of a clay but contains as appreciable amount of sand. The aggregate properties of sand and gravel are described qualitatively by the terms:

Loose, medium and dense, where as those of clay are described by hard-stiff medium and soft.

The above descriptions should be based on field tests data namely knowing the resistance to penetration, in-situ density, etc. The record of different colored strata encountered in adjacent borings reduces the risk of errors in correlating the boring logs. The colour difference of same soil indicates its location above or below water table. Terms such as multitude marbled-spotted are used when different colours occur in the same stratum of soil.

Dark or drab colours are usually associated with organic soils.

For unstratified and or layered stratified soils some of the typical terms for its recognition are used like:

Till: lnstratified glacial deposits of clay-slit sand gravel and boulders.

Tuff: A fine grained water or wind laid aggregate of very small mineral or rock. Fragments ejected from volcanoes during explosions.

Loess: A uniform cohesive wind blow sediment light brown in colour. Size in narrow limit of 0.01 to 0.05 mm.

Vared Clay: Consists of alternating layers of medium gray inorganic silt and darker silty clay.  The thickness of layer rarely exceeds 12mm.

Bentonite: It is a clay with high content of montrnorillonite formed (mostly) by chemical alternation of Volcanic ash.  It is when in contact with a water, dried bentonite swells more than other dried clays so also it shrinks more or drying.

5.0   CLASSIFICATION: Soils are classified by symbols of two letters, utilizing data on Grain size analysis and Atterberg's Limits.  The first letter stands for cohesionness of soil e.g.

G     Gravel

S      Sand

M     Silt

C      Clay

O      Organic soils

(a)    For coarse grained soils the second letter of classification symbol stands for either

1.      Gradation

e.g.  W         Well graded 

        P          Poorly graded or

2.     Another type of soil mixed with it

 e.g. M         Silt

        C         Clay

(b)   For the grained soils, the second letter of symbol for classification stands for compressibility.

H     High compressibility

I       Intermediate compressibility

L      Low   compressibility

This soil may be designed as

SW  Well graded sands

GP   Poorly graded gravels

SC   Sand - Clay mixture

MI   Silt of intermediate compressibility

OL   Organic soil of low compressibility etc.

5.1   It has been determined that the soil is coarse grained, it is further identified by estimating and recording the percentage of

(a)   Gravel sized particle, size range from 75 mm to 4.75 mm is sieve size (or approximately 5 mm size)

(b)   Sand size particle, size range from 4.75 mm to 75 micron is sieve size and

(c)   Silt and clay particles, size range smaller from 75 micron IS sieve. The fraction of soil smaller than 75 micron, IS Sieve that is the clay and silt fraction is referred to as fines.

5.2   Gravelly Soils:

If the percentage of gravel is greater than that of Sand, the soil is a gravel. Gravels are further identified as being clean (containing  little or no fines, that is, less than 5 percent) or dirty (containing appreciable fines, that is, more than 12 percent) depending upon the percentage of particles not  visible to the unaided eye. Gravels containing 5 to 12 percent fines are given boundary classification.  If the soil is obviously clean, classification shall be either

(a) Well graded Gravel (GW) if there is good representation of all particles sizes or

(b) Poorly Graded Gravel (GP), if there is an excess or absence of intermediate particles sizes. A well graded soil has a reasonably large spread between the largest and the finest particles and has no marked deficiency in any size. lf the soil obviously is dirty, the classification will be either.

(c) Softy Gravel (GM), if the fines have little or no plasticity or

(d) Clayey Gravel (GC), if the fines are of low to medium or high plasticity.

5.3   Sandy Soil:

If the percentage of sand is greater than gravel, the soil is a sand. The same procedure is applied as for gravels except that the word sand replaces gravel and the symbols S replaces G. The group classification for the clean sands will be either

(a) Well-graded Sand (SW) or

(b) Poorly-graded Sand (SP) and the dirty sands shall be classified as ¬

(c) Silty Sand (SM), if the fines have little or no plasticity or

(d) Clayey Sand (SC), if the fines are of low to medium or high plasticity.

6.0    FIELD IDENTIFICATION

It should be carried out during exploration & while collecting samples. These tests will give idea of the type of soil to the user.

6.1     FIELD TESTS FOR IDENTIFICATION:

6.1.1 Simple field tests for identification

(a)     Shaking or dilatancy test:

Dilatancy (Reaction to shaking): Take a small representative sample in the form of soil pat of the size of about 5 cubic centimeters and add enough water to nearly saturate it. Place the pat in one open palm of one hand and shake horizontally, striking vigorously against the other hand several times. Squeeze the pat between the fingers. The appearance and disappearance of the rd water with shaking and squeezing is referred to a reaction. This reaction is called quick if water appears and disappears rapidly; slow, if water appears and disappears slowly, and no reaction, if the water condition does not appear to change. Observe and report type of reaction as descriptive information:

Fine clean sands react quickly and distinctly.

Very fine grained cohesionless soils like silt and rock flour, react somewhat slowly than fine sands.

Plastic clays do not react to this.               .·

Cohesionless soils when moist do not stain the hands when rubbed.

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 (b)     Surface tests:

Cohesive soils can be identified by rubbing a small ball or remoulded moist soil with a clean knife blade or finger nail. A small sample of the soil at above the plastic limit is kneaded into a ball about 25mm in dia (I") and rubbed with a clean knife blade or finger nail. If the knife blade leaves a shiny surface, the soil is highly plastic.

(c)     Dry Strength: (Crushing resistance):

Completely dry the prepared soil pat. Then measure its resistance to crushing and powdering between fingers. This resistance, called dry strength is a measure of the plasticity of soil and is influence largely by the colloidal fraction content. The dry strength is designated as low, it the dry pat can be easily powdered; medium if considerable finger pressure is required and high, it cannot be powdered at all. Observed and record the dry strength as descriptive information.

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(d)    Toughness: (Consistency near plastic limit)

Dry the pat used in the dilatancy test by working and moulding until it has the consistency of putty. The time required to dry the pat is the indication of plasticity.

Roll the pat on a smooth surface or between the palms into a thread of about 3 mm in diameter. Fold and reroll the thread repeatedly to 3 mm in diameter so that its moisture content is gradually reduced until the 3 mm thread just crumbles. The moisture content at this time is called the plastic limit and the resistance to moulding at the plastic limit is called the toughness. After the thread crumbles, lump the pieces together and continue slight knealing action until the lump crumbles If the lump can still be moulded slightly drier than the plastic limit and if high pressure is required to role the thread between the palms of the hand, the soil is described as having high toughness Medium toughness is lumped together when drier than the  plastic limit Highly organic clays have very weak and spongy feed as the plastic limit Non-plastic soils cannot be rolled into thread of 3 mm in diameter at any moisture content Observe and record the toughness as descriptive information.

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(e)     Organic Content and colour:

Fresh wet organic soils usually have a distinctive odour of decomposed organic matter. This odour can be made more noticeable by heating the wet sample. Another indication of the organic matter is the distinctive dark colour. In tropical soils, the dark colour may be or may not be due to organic matter. When not due to organic matter, it is associated with poor drainage. Dry organic clay develop an earthy odour upon moistening, which is distinctive from that of decomposed organic matter.

(9)    Acid Test:

Acid test using dilute Hydrochloric Acid (HCL is primarily a test for the presence of Calcium Carbonate. For soils with high dry strength, a strong reaction indicates that the strength may be due to CaCO3 agent rather than colloidal clay. If this is so, it should be recorded in the result sheet.

(g)     Shine Test:

This test help in determining the presence of clay soil by cutting a lump of dry or slightly moist soil with a knife, the surface of soil mass is shining indicates the highly plastic clay while a dull surface indicates silt or clay of low plasticity.

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6.1.2   Field Moisture Content:

i)       Alcohol method:

This is fruited for field test. It is less accurate than oven drying method.

Apparatus:

1.         Evaporating Dish

2.         Steel spatula

3.         Balance

4.         Methylated spirit or petrol

5.         100 ml or kerosene measuring cylinder.

The soil specimen taken shall be representative of soil mass. The quantity depends of the maximum size of particles.

Testing Procedure:

The evaporating dish is cleaned, dried and weighed (Wl). The required quantity of soil is taken in the dish and weighed (W2). Absolute 100 ml. of methylated spirit is then poured over the soil and is ignited. The soil is then stirred constantly with a spatula care being taken not to lose any soil. After the whole spirit has burnt away the dish is allowed to cool and is then weighed. This process is repeated until you get a constant weight (W3) generally 3 to 4 repetitions required depending upon the clay percentages in the soil.

Calculation:

i)                      The percentage of water content

W = (W₂-W₃)/(W₃-W₁)  x 100

Where,

W         =        Water content percent

W₂       =        Weight of dish with wet soil

W₃        =        Weight of dish with dry soil

W₁       =        Weight of dish

ii)        Stove Method:

Drying of soil content be done on a stove using frying pan or sand bath and heating the soil to its constant weight (W₃ as above)

ii)                   Oven method:

This method is more suitable for laboratory testing. Thermostatically controlled even for temperature range 105° to 110°C is used for drying the wet soil.

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6.1.3   Field dry density tests:

Field dry density tests are necessary to know in situ density of soil. These tests can be carried out by digging pits and collecting material at the desired depth.  In case boring is being done samples received in samples can be used for determine the FDD. Material received in split spoon sampler of the standard penetration test also can be used for determining FDD.

Tests involves, i) taking a sample, ii) weighing it care should be taken to avoid atmospheric effect, iii) drying the samples and iv) re-weighing it. Different methods are described below:

There are in general three main methods of finding out field density. In all cases the procedure involved is to determine the weight and moisture content of soil removed from a cylindrical cavity whose volume is then measured.

(a)   Core Cutter Method:

This method is suitable only for fine grained soil. It is less accurate than sand replacement method and hence recommended when speed is essential or when the soil is well compacted.

Apparatus:

1.       Cylindrical steel core cutter of 10mm internal dia and 127.4 mm long. With a wall thickness of 3mm.

2.       Steel Dolly of the same internal dia and 25mm height with a wall thickness of 7.5 mm fitted with a lip to be fitted on top of the core-cutter.

3.       Suitable steel hammer.

Wet density    =     (Wt. of wet soil)/ (Volume of soil)

Dry density    =      (Wt.  density  x  100) / (100  + M.C.) 

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(b)       Sand Replacement method

This is suitable for fine and medium grained soils. Apparatus:

1.         Sand pouring cylinder

2.         Calibrating cane

3.         Balance - accurate to 1 g.

4.         Tray or container for calibrating excavated soil.

5.         Metal tray – 30cms square 4cm depth with a 10 cm hole in the center.

6.         Sand-clean closely graded natural sand between 600 and 300 micron IS sieve.

(i)       Bulk density of sand S = Wg/V

(ii)      Measurement of soil density

a)        Weight of sand in the hole Wb = W₁ – W₄ – W₂

b)        Wet density:  b = (Ww x s)/Wb,

c)        Dry density:  d = (100 x rb)/(100 + W)

The method can also be used for coarse grained soils by using large size pouring cylinder (200mm dia cylinder).

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(c)       Ring and water replacement method

The method is suited for coarse grained soils containing gravels cobbles boulder and rock.

Apparatus:

1.        Density ring and steel spikes diameter of the ring shall be 3 to 4 times the size of the largest particles. Inner surface approximately 10 to 20 high.

2.         Plastic film.

(a)       0.1 mm thick 2 to 4 mm square (for small rings)

(b)       0.2 mm thick 4 to 8 mm square (for large rings)

3.        Pointer Gauge assembly and supports. Horizontal bar with supports on or outside the ring with an adjustable vertical point and lock-out.

4.        Balance for weighting up to 20 kg.

5.        IS Sieves.

6.        Measuring Cylinders.

Testing procedure:

The ground is levelled at the site of the test. The ring is then placed on the levelled ground and is fixed to the surface to present any movement during the test.

The pointer gauge assembly is then set up so that the pointer can be removed and returned to a fixed position below the top of the ring. The pointer gauge bar is then removed to a safe position.

The plastics film is then spread over the test surface on the ring. The pointer gauge bar is then replaced. The plastics film ring assembly is filled with water to see that the film makes full constant with the test surface and the inside surface of the ring. The measured volume of water used is the initial reading 'Vi' for the test. The pointer gauge bar and the plastics film with water are then removed. A cylindrical cavity within the ring is then excavated using the digging tools. When the desired depth is reached all the excavated material is carefully collected in a container and weighed to the nearest 0.10 kg. A representative sample from the excavated material is then taken for the determination of moisture content.

The plastics film is then spread properly into the cavity thus formed and the pointer gauge bar is replaced. The cavity covered with the film is then filled with water to the precise level of the pointer as set for the initial volume measurement. The measured volume of water used is the final reading 'Vt' for the test.

Calculations:

i)         Volume of the cavity V = Vt – Vi where,

Vt        =      Final volume reading

Vi        =      Initial reading

V        =      Volume of the contents.

ii)        Wet density   =     Ww/V

 Where Ww = weight of the material from the cavity.

iii)      Obtain a moisture content specimen representative of the excavated material.

Dry density = Wet density / (1+(w/100))

Where:

w: moisture content of excavated materials

There are some more methods also for the determination of field dry density. But they are cumber some and less accurate and hence generally not used.

(IS 2720-33 (1971): Methods of test for soils, Part 33, This part deals with the determination of dry density of soil in-place by the water replacement method using a ring.)

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7.0    LABORATORY TESTING:

The various, properties tests of soil are described in the subsequent notes. Please, refer to it.

FOUNDATION SOIL:

The performance of soil in actual practice is greatly influence by the natural environmental condition and condition imposed by construction of structure. Soils does not have simple, well define, uniform or constant physical properties. It is varying very widely and hence uncertainty involves the soil upon which the structure is to be place, its properties are required to be determined very reliably. The samples are collected as disturbed and undisturbed sample and are tested for the required properties in Laboratory. The field officers have to inform the laboratory very precisely for various details of structure including levels etc. and test to be performed. Field officers also should approach laboratory well in advance of their requirement of test result because the lab testing is itself time consuming job for each of the test differently.

DESCRIPTION OF FOUNDATION SOILS

The following information shall be recorded to define the in place condition of soils which are to be utilized as foundation for Hydraulic or other structures:

1)         Natural moisture content (as dry) moist wet and saturated.

2)         Perviousness a drainage properties in the natural condition.

3)         Structure (as stratified uniform un-cemented lensed and attitude that is strike & dip.

4)         Type and degree of cementation;

5)         Degree of compactness (as loose a dense) and

6)         Consistency.

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