For determination of particle size distribution of fine, coarse and all-in-aggregates by sieving.
REFERENCE STANDARD
IS : 2720 (Part 4) – 1985 – Method of test for soil (Part 4-Grain size analysis)
EQUIPMENT & APPARATUS:
Balance
Sieves
Sieve shaker
PREPARATION SAMPLE
After receiving the soil sample it is dried in air or in oven (maintained at a temperature of 600C). If clods are there in soil sample then it is broken with the help of wooden mallet.
PROCEDURE
The sample is dried to constant mass in the oven at a temperature of 1100±50C and all the sieves which are to be used in the analysis are cleaned.
The oven dry sample is weighed and sieved successively on the appropriate sieves starting with largest. Each sieve is shaken for a period of not less than 2 minutes.
On completion of sieving the material retained on each sieve is weighed.
CALCULATION
The percent retained (%), Cumulative retained (%) & percent finer (%) is calculated.
Percent retained on each sieve = Weight of retained sample in each sieve / Total weight of sample
The cumulative percent retained is calculated by adding percent retained on each sieve as a cumulative procedure.
The percent finer is calculated by subtracting the cumulative percent retained from 100 percent.
REPORT
The result of the sieve analysis is reported graphically on a semi log graph, taking sieve sizes on log scale and % finer in arithmetic scale. The observation is maintained in observation sheet.
SAFETY & PRECAUTIONS:
Clean the sieves set so that no soil particles were struck in them
While weighing put the sieve with soil sample on the balance in a concentric position.
Check the electric connection of the sieve shaker before conducting the test.
In geotechnical engineering, hydrometer analysis is primarily used to know the grain size distribution of a fine grained soil. In this post I will share with you the following things.
What is a hydrometer?
Why hydrometer analysis is done?
What is the theory behind hydrometer analysis of soil?
How to perform hydrometer analysis of soil?
WHAT IS HYDROMETER?
Fig-1 Hydrometer
A hydrometer is an instrument which is used to measure the relative density of a liquid. Hydrometer is made of glass and primarily consists of two parts;
A cylindrical stem with graduation marks
A bulb at bottom weighted with mercury
The lower the density of the liquid the more the hydrometer will sink. Consider water and petrol for example. The density of petrol is lower than that of water, therefore the depth of immersion of a hydrometer will more in case of petrol than water.
WHY HYDROMETER IS USED FOR GRAIN SIZE ANALYSIS OF FINE GRAINED SOIL?
In case of fine grained soil, sieve analysis test does not give reliable test result. This because a fine grained soil consist of different sizes of particles starting from 0.075 mm to 0.0002 mm. and it is not practicable to design sieve having so smaller screen size. Also there is a chance of lost of sample during sieving. Therefore hydrometer analysis is done for grain size analysis of fine grained soils.
WHAT IS THE THEORY BEHIND HYDROMETER ANALYSIS TEST OF SOIL?
Hydrometer analysis is based on Stokes law. According to this law, the velocity at which grains settles out of suspension, all other factors being equal, is dependent upon the shape, weight and size of the grain.
In case of soil, it is assumed that the soil particles are spherical and have the same specific gravity. Therefore we can say that in a soil water suspension the coarser particles will settle more quicjly than the finer ones.
If V is the terminal velocity of sinking of a spherical particle, it is given by;
V = 1/18 [(Gs-Gw)/n)]*D2
Where,
V = Terminal velocity of soil particle (cm/s)
D = Diameter of soil particle (cm)
Gs = Specific gravity of soil particle
Gw = specific gravity of water
n = viscosity of water (g-s/cm2)
HOW TO DO HYDROMETER ANALYSIS OF SOIL
EQUIPMENT
Hydrometer
Dispersion cup with mechanical stirrer with complete accessories
Two glass jar of 1 litre capacity
Deflocculating agent (sodium Hexa metaphosphate solution prepared by dissolving 33g of sodium Hexa metaphosphate and 7g of sodium carbonate in distilled water to make one litre solution)
Stop watch
Thermometer
Scale
PROCEDURE
Take about 50g in case of clayey soil and 100g in case of sandy soil and weigh it correctly to 0.1g.
In case the soil contains considerable amount of organic matter or calcium compounds, pre-treatment of the soil with Hydrogen Peroxide or Hydrochloric acid may be necessary. In case of soils containing less than 20 percent of the above substances pre-treatment shall be avoided.
To the soil thus treated, add 100 cc of sodium hexametaphosphate solution and warm it gently for 10 minutes and transfer the contents to the cup of the mechanical mixer using a jet of distilled water to wash all the traces of the soil.
Stir the soil suspension for about 15 minutes.
Transfer the suspension to the Hydrometer jar and make up the volume exactly to 1000 cc by adding distilled water.
Take another Hydrometer jar with 1000cc distilled water to store the hydrometer in between consecutive readings of the soil suspension to be recorded. Note the specific gravity readings and the temperature T0C of the water occasionally.
Mix the soil suspension roughly, by placing the palm of the right hand over the open end and holding the bottom of the har with the left hand turning the jar upside down and back. When the jar is upside down be sure no soil is tuck to the base of the graduated jar.
Immediately after shaking, place the Hydrometer jar on the table and start the stopwatch. Insert the Hydrometer into the suspension carefully and take Hydrometer readings at the total elapsed times of ¼, ½, 1 and 2 minutes.
After 2 minutes reading, remove the Hydrometer and transfer it to the distilled water jar and repeat step no-8. Normally a pair of the same readings should be obtained before proceeding further.
Take the subsequent hydrometer readings at elapsed timings of 4, 9, 16, 25, 36, 49, 60 minutes and every one hour thereafter. Each time a reading is taken remove the hydrometer from the suspension and keep it in the jar containing distilled water. Care should be taken when the Hydrometer recorded to see that the Hydrometer is at rest without any movement. As time elapses, because of the fall of the solid particles the density of the fluid suspension decreases reading, which should be checked as a guard against possible error in readings of the Hydrometer.
Continue recording operation of the Hydrometer readings until the hydrometer reads 1000 approximately.
CALIBRATION OF THE HYDROMETER
The hydrometer shall be calibrated to determine its true depth in terms of the hydrometer reading (see Fig-2) in the following steps:
Fig-2 Hydrometer calibration
Determine the volume of the hydrometer bulb, VR. This may be determined in following way:
By measuring the volume of water displaced. Fill a 1000-cc graduate with water to approximately 700 cc. Observe and record the reading of the water level. Insert the hydrometer and again observe and record the reading. The difference in these two readings equals the volume of the bulb plus the part of the stem that is submerged. The error due to inclusion of this latter quantity is so small that it may be neglected for practical purposes.
Determine the area, A, of the graduate in which the hydrometer is to be used by measuring the distance between two graduations. The area, A, is equal to the volume included between the graduations divided by the measured distance.
Measure and record the distances from the lowest calibration mark on the stem of the hydrometer to each of the other major calibration marks, R.
Measure and record the distance from the neck of the bulb to the lowest calibration mark. The distance, H1, corresponding to a reading, R, equals the sum of the two distances measured in steps (3) and (4).
Measure the distance from the neck to the tip of the bulb. Record this as h, the height of the bulb. The distance, h/2, locates the center of volume of a symmetrical bulb. If a nonsymmetrical bulb is used, the center of volume can be determined with sufficient accuracy by projecting the shape of the bulb on a sheet of paper and locating the center of gravity of this projected area.
Compute the true distances, HR, corresponding to each of the major calibration marks, R, from the formula:
HR = H1 + ½ [h – (VR/A)]
Plot the curve expressing the relation between HR and R as shown in Figure 3. The relation is essentially a straight line for hydrometers having a streamlined shape.
Fig-3 Typical hydrometer calibration chart
CALCULATIONS
If the temperature during the experiment is constant, then the the following formula can be used to calculate the diameter of the soil particles
D2 = K HR/t
Where
T = time in minutes
D = diameter of soil particle in mm
K = 30n/(G-gw)
The percentage finer N may be obtained from
N% = G*V/((G-1)*W) * (r – rw)*100
Where
V = Volume of soil suspension (1000 cc)
W = weight of dry soil taken for the test
r = Hydrometer reading in distilled water
rw = Hydrometer readings in soil suspension
G = Specific gravity of soil particles
Since V = 1000 cc, the above equation may be conveniently represented as follows:
Ct = Correction for temperature (positive if the test temperature is more than the temperature at which the hydrometer is calibrated and vice versa) (see table-1)
Cd = Correction for dispersing agent. This is determined as mentioned below
The addition of a dispersing agent to the soil suspension results in an increase in density of the liquid and necessitates a correction to the observed hydrometer reading. The correction factor, Cd, is determined by adding to a 1000-ml graduate partially filled with distilled or demineralized water the amount of dispersing agent to be used for the particular test, adding additional distilled water to the 1000-ml mark, then inserting a hydrometer and observing the reading. The correction factor, Cd is equal to the difference between this reading and the hydrometer reading in pure distilled or demineralized water.
Table-1 Temperature Correction (Ct) for Hydrometer Analysis
Water content of a soil mass is defined as the ratio of weight of water present in a given soil mass to the weight of dry soil.
i.e. water content = (weight of water in soil mass)/(weight of dry soil)
Water content is usually expressed in percentage (%).
water content of soil
HOW MUCH SAMPLE REQUIRED FOR TEST?
The minimum amount of sample required for the test is primarily dependent upon the maximum particle size present in that given soil mass. The table given below can be used as a reference for deciding minimum weight of moist soil sample required.
Maximum particle size in the soil (mm)
Minimum weight of soil sample (g)
0.425
20
2.0
50
4.75
100
10.0
500
19.0
2500
EQUIPMENTS NEEDED
Moisture cans (must be made of heat resistant material)
Oven with temp. control (1050c to 1100c)
Desiccator (used for cooling)
Balance (having a readability of 0.01 g for specimens having mass of 200 g or less and 0.1 g for specimens having mass of over 200 g.
STANDARD TEST PROCEDURE TO FOLLOW
Determine the weight (g) of the empty moisture can plus cap (W1), and also record the can number.
Place the required amount of moist soil in the can with its cap to avoid loss of moisture.
Determine the combined weight of the can and moist soil (W2).
Remove the cap from the top of the can and place it at the bottom of the can.
Put the can with moist soil sample in the oven for 24 hours.
After complete drying of soil mass, remove it from the oven and let it cool down to room temperature inside a desiccator. Determine the combined weight of the dry soil sample plus the can and its cap (W3).
CALCULATIONS INVOLVED
Calculate the weight of moisture = W2-W3
Calculate the mass of dry soil = W3-W1
Calculate the water content (%) as given below
w (%) = [(W2-W3)/(W3-W1)]*100
HOW TO REPORT TEST RESULT?
Minimum of three numbers of specimens of the same sample should be tested. The average value of the 3 observations should be reported. The water content of the soil is reported to two significant figures.
HOW TO DETERMINE WATER CONTENT OF SOIL ON SITE USING ALCOHOL METHOD? (IS-2720-PART-2)
WATER CONTENT TEST OF SOIL BY ALCOHOL METHOD
PURPOSE
It is a rapid method of determining the moisture content. Though less accurate, it is more suitable as a field test. The method shall not be used if the soil contains a large proportion of clay, gypsum, calcareous matter or organic matter.
APPARATUS
Evaporating Dish– 10 to 15 cm in diameter.
Palette Knife or Steel Spatula-having a blade 10 cm long and 2 cm wide.
Balance – of sufficient sensitivity to weigh the soil samples to an accuracy of 0.4 percent of the mass of the soil taken for the test.
Methylated Spirit
methylated spirit (alcohol) used for on site determination of water content of soil
PROCEDURE
Clean the evaporating dish, dry and weigh (W1).
Take appropriate quantity of soil specimen in the evaporating dish and weigh (W2).
Pour methylated spirit over the soil at the rate of about one millilitre for every gram of soil taken so that the soil is well covered.
Work the methylated spirit well into the soil with the palate knife and break up any large lumps of soil.
Place the evaporating dish on a surface which will not be affected by heat and ignite the methylated spirit. Stir the soil constantly with the spatula or knife taking care to see that none of the soil is lost.
After the methylated spirit has burnt away completely, allow the dish to cool and weigh it with the contents (W3).
CALCULATIONS
The percentage of water content shall be calculated as follows:
HOW TO DO UNCONFINED COMPRESSIVE STRENGTH TEST OF SOIL?
UNCONFINED COMPRESSIVE STRENGTH TEST OF SOIL
PURPOSE
The primary purpose of this test is to determine the unconfined compressive strength, which is then used to calculate the unconsolidated undrained shear strength of the clay under unconfined conditions. According to the ASTM standard, the unconfined compressive strength (qu) is defined as the compressive stress at which an unconfined cylindrical specimen of soil will fail in a simple compression test. In addition, in this test method, the unconfined compressive strength is taken as the maximum load attained per unit area, or the load per unit area at 15% axial strain, whichever occurs first during the performance of a test.
STANDARD REFERENCE
ASTM D 2166 – Standard Test Method for Unconfined Compressive Strength of Cohesive Soil
SIGNIFICANCE
For soils, the undrained shear strength (su) is necessary for the determination of the bearing capacity of foundations, dams, etc. The undrained shear strength (su) of clays is commonly determined from an unconfined compression test. The undrained shear strength (su) of a cohesive soil is equal to one-half the unconfined compressive strength (qu) when the soil is under the f = 0 condition (f = the angle of internal friction). The most critical condition for the soil usually occurs immediately after construction, which represents undrained conditions, when the undrained shear strength is basically equal to the cohesion (c). This is expressed as:
su = c = qu/2
Then, as time passes, the pore water in the soil slowly dissipates, and the intergranular stress increases, so that the drained shear strength (s), given by s = c + s‘tan ϕ’ , must be used. Where s‘= intergranular pressure acting perpendicular to the shear plane; and s‘ = (s – u), s = total pressure, and u = pore water pressure; c’ and ϕ’ are drained shear strength parameters.
EQUIPMENT
Compression device
Load and deformation dial gauges
Sample trimming equipment
Balance
Moisture can
TEST PROCEDURE
(1) Extrude the soil sample from Shelby tube sampler. Cut a soil specimen so that the ratio (L/d) is approximately between 2 and 2.5. Where L and d are the length and diameter of soil specimen, respectively.
(2) Measure the exact diameter of the top of the specimen at three locations 120° apart, and then make the same measurements on the bottom of the specimen. Average the measurements and record the average as the diameter on the data sheet.
(3) Measure the exact length of the specimen at three locations 120° apart, and then average the measurements and record the average as the length on the data sheet.
(4) Weigh the sample and record the mass on the data sheet.
(5) Carefully place the specimen in the compression device and center it on the bottom plate. Adjust the device so that the upper plate just makes contact with the specimen and set the load and deformation dials to zero.
Fig-1
(6) Apply the load so that the device produces an axial strain at a rate of 0.5% to 2.0% per minute, and then record the load and deformation dial readings on the data sheet at every 20 to 50 divisions on deformation the dial.
(7) Keep applying the load until (1) the load (load dial) decreases on the specimen significantly, (2) the load holds constant for at least four deformation dial readings, or (3) the deformation is significantly past the 15% strain that was determined in step 5.
Fig-2
(8) Draw a sketch to depict the sample failure.
(9) Remove the sample from the compression device and obtain a sample for water content determination. Determine the water content as in Experiment
ANALYSIS
(1) Convert the dial readings to the appropriate load and length units, and enter these values on the data sheet in the deformation and total load columns.
(Confirm that the conversion is done correctly, particularly proving dial gage readings conversion into load)
(2) Compute the sample cross-sectional area A0 = π*(d2)/4
(3) Calculate the deformation (ΔL) corresponding to 15% strain (e).
Strain (e) = ΔL / L0
Where L0 = Original specimen length (as measured in step 3).
(4) Computed the corrected area, A’ = A0 / (1-e)
(5) Using A’, compute the specimen stress, sc = P/A’
(Be careful with unit conversions and use constant units).
(6) Compute the water content, w%.
(7) Plot the stress versus strain. Show qu as the peak stress (or at 15% strain) of the test. Be sure that the strain is plotted on the abscissa. (See fig-3)
This test involves the measurement of the resistance to penetration of a sampling spoon under dynamic loading
The resistance is empirically correlated with some of the engineering properties of soil such as density index, consistency, angle of internal friction, bearing capacity etc.
This test is useful for general exploration of erratic soil profile for finding depth to bed rock or hard stratum and to have an approximate indication of the strength and other properties of soil, particularly the cohesionless soil, from which it is difficult to obtain undisturbed samples.
The importance of this test is that, even though empirical, the soil design of foundations in sand is mostly based on the N-value. N value serves as the basic parameter for geotechnical design in sand.
In this test, a thick wall standard split spoon sampler, 50.8 mm outer diameter and 35 mm inner diameter, is driven into the undisturbed soil at the bottom of the bore hole under the blows of a 63.5 kg drive weight with 75 cm free fall. The minimum open length of the sampler should be 60 cm. the number of blows required to drive the sampler 30 cm beyond the seating drive of 15 cm, is termed as the penetration resistance N.
RELATION BETWEEN SPT (N) VALUE WITH DIFFERENT SOIL PROPERTIES IN CASE OF COHESION LESS SOIL.
SPT VALUE (N)
COMPACTNESS
ANGLE OF FRICTION (φ)
UNIT WEIGHT (γ) in T/m3
0 – 4
Very loose
< 28
1.1 – 1.8
4 – 10
Loose
28 – 30
1.4 – 2.0
10 – 30
Medium
30 – 36
1.7 – 2.2
30 – 50
Dense
36 – 41
1.7 – 2.3
> 50
Very dense
> 41
2.0 – 2.3
RELATION BETWEEN SPT (N) VALUE WITH DIFFERENT SOIL PROPERTIES IN CASE OF COHESIVE SOIL.
HOW TO DO STANDARD PENETRATION TEST (SPT) OF SOIL ON SITE?
STANDARD PENETRATION TEST (SPT) PROCEDURE
AIM
To perform standard penetration to obtain the penetration resistance (N-value) along the depth at a given site.
EQUIPMENT & APPARATUS
Tripod (to give a clear height of about 4 m; one of the legs of the tripod should have ladder to facilitate a person to reach tripod head.)
Tripod head with hook
Pulley
Guide pipe assembly
Standard split spoon sampler
A drill rod for extending the test to deeper depths
Heavy duty post hole auger (100 mm to 150 mm diameter)
Heavy duty helical auger
Heavy duty auger extension rods
Sand bailer
Rope (about 15 m long & strong enough to lift 63.5 kg load repeatedly)
A light duty rope to operate sand bailer
Chain pulley block
Casing pipes
Casing couplings
Casing clamps
Measuring tapes
A straight edge (50 cm)
Tool box
Standard Penetration Test Setup
PROCEDURE
Identify the location of testing in the field
Erect the tripod such that the top of the tripod head is centrally located over the testing spot. This can be reasonably ensured by passing a rope over the pulley connected to the tripod head and making the free end of the rope to come down and adjusting the tripod legs such that the rope end is at the testing spot. While erecting and adjusting the tripod legs, care should be taken to see that the load is uniformly distributed over the three legs. This can be achieved by ensuring the lines joining the tips of the tripod legs on the ground forms an equilateral triangle. Further, it should be ensured that the three legs of the tripod are firmly supported on the ground (i.e. the soil below the legs should not be loose and they should not be supported on a sloping rock surface or on a small boulder which may tilt during testing.)
Advance the bore hole, at the test location, using the auger. To start with advance the bore hole for a depth of 0.5 m and clear the loose soil from the bore hole.
Clean the split spoon sampler and apply a thin film of oil to the inside face of the sampler. Connect an A-drill extension rod to the split spoon sampler.
Slip the 63.6 kg weight on to the guide pipe assembly and connect the guide pipe assembly to the other end of the A-drill rod.
The chain connected to the driving weight is tied to the rope passing over the pulley at the tripod head. The other end of the rope is pulled down manually or with help of mechanical winch. By pulling the rope down, the drive weight, guide pipe assembly, A-drill rod and the split spoon sampler will get vertically erected.
A person should hold the guide pipe assembly split spoon sampler to be vertical with the falling weight lowered to the bottom of the guide assembly.
Now place a straight edge across the bore touching the A-drill rod. Mark the straight edge level all round the A-drill rod with the help of a chalk or any other marker. From this mark, measure up along the A-drill rod and mark 15 cm, 30 cm and 45 cm above the straight edge level. Lift the driving weight to reach the top of the guide pipe assembly travel and allow it to fall freely. The fall of driving weight will transfer the impact load to the split spoon sampler, which drive the split spoon sampler into the ground. Again lift the drive weight to the top of travel and allow it to fall freely under its own weight from a height of 75 cm. as the number of blows are applied, the split spoon sampler will penetrate into the ground and the first mark (15 cm mark) on the drill rod approaches the straight edge.
Count the number of blows required for the first 15 cm, second 15 cm and the third 15 cm mark to cross down the straight edge.
The penetration of the first 15 cm is considered as the seating drive and the number of blows required for this penetration is noted but not accounted in computing penetration resistance value. The total number of blows required for the penetration of the split spoon sampler by 2nd and 3rd 15 cm is recorded as the penetration resistance or N-value.
After the completion of the split spoon sampler by 45 cm, pull out the whole assembly. Detach the split sampler from A-drill rod and open it out. Collect the soil sample from the split spoon sampler into a sampling bag. Store the sampling bag safely with an identification tag for laboratory investigation.
Advance the bore hole by another 1 m or till a change of soil strata which ever is early.
The test is repeated with advancement of bore hole till the required depth of exploration is reached or till a refusal condition is encountered. Refusal condition is said to exist if the number of blows required for the last 30 cm of penetration is more than 100.
The test will be repeated in number of bore holes covering the site depending on the building area, importance of the structure and the variation of the soil properties across the site.
The SPT values are presented either in the form of a table or in the form of bore log data.
The N-value observed during testing is not utilized directly in assessing soil properties. These values are corrected to account for
The overburden pressure
Dilatancy in saturated fine sands and silts
CORRECTION FOR OVERBURDEN PRESSURE
The penetration resistance of soil depends on the over burden pressure. At deeper depth in-situ soil will have higher overburden pressure hence its response to SPT test will be better when compared to the behavior of the same soil at shallow depth.
Bazaraa (1967 Bowels, p99) proposed the following corrections to the actual count N, based on the over burden pressure
For p0 <= 75 kPa
For p0 > 75 kPa
Where
N’ = corrected N value
N = observed N-value
P0 = over burden pressure, (kPa) = γ x D
D = depth of testing (m)
γ = unit weight of soil at the time of testing
N’ is increased from the actual blow count when p0 <=75 kPa
N’ is decreased from the actual blow count when p0 >75 kPa
CORRECTION FOR THE DILATANCY IN SATURATED FINE SANDS AND SILTS
When dynamic loads are applied on silty and fine sandy soils in saturated state the pore pressure in such soil will not be in a position to get dissipated due to low permeability. Hence, during dynamic loading (i.e. application of blows) the pore water will offer a temporary resistance to dynamic loads. This leads to higher value of N-value which is unsafe. Therefore when SPT is performed in saturated silts and fine sands and if the observed N-value is more than 15, a correction has to be applied to reduce the observed values. This correction is applied on the N-value corrected for over burden pressure (N’).
If the stratum (during testing) consists of fine sand & silt below water table, the corrected N-value (N’) has to be further corrected to get the final corrected value N”.
