MOISTURE CONTENT TEST OF SOIL(IS-2720-PART-2-1973)

Moisture Cans

Objective

For determination of the moisture content of soil by oven drying method.

Equipment’s & Accessories

• Oven (105⁰C to 110⁰C min.)
• Metal container
• Balance (0.01 g accuracy)

Procedure

1. The number of the container is recorded, cleaned, dried and weighed.(W₁)
2. About 15-30 g of soil is placed in the container and the weight of soil with the sample is recorded.(W₂)
3. The can with the soil is placed in oven for 24hours maintained at a temperature 105⁰ to 110⁰C.
4. After drying the container is removed from the oven and allowed to cool at room temperature.
5. After cooling the soil with container is weighed.(W₃) 

Calculation

         
W₁=Mass of container, g
W₂=Mass of container and wet soil, g
W₃=Mass of container and dry soil, g

Report

The water content of the soil is reported to two significant figures.

Safety & Precautions

• Use hand gloves while removing containers from oven after switching off.
• The soil should be loosely placed in the bin, so as to allow easy evaporation of moisture
• Over heating should be avoided.
• Oven dry soil should not be kept open to the atmosphere (for cooling) as it may absorb moisture from atmosphere
• Ensure the identification number on the base and the lid of the bin is same

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 (q_u) 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 (s_u) is necessary for the determination of the bearing capacity of foundations, dams, etc. The undrained shear strength (s_u) of clays is commonly determined from an unconfined compression test. The undrained shear strength (s_u) of a cohesive soil is equal to one-half the unconfined compressive strength (q_u) 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:
s_u = c = q_u/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, s_c = P/A’
(Be careful with unit conversions and use constant units).
(6) Compute the water content, w%.
(7) Plot the stress versus strain. Show q_u as the peak stress (or at 15% strain) of the test. Be sure that the strain is plotted on the abscissa. (See fig-3)

Fig-3

(8) Calculate shear strength s_u as follows,
s_u = c (or cohesion) = q_u/2

SIEVE ANALYSIS OF SOIL(IS-2720-PART-4-1985)

Objective

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

1. 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.
2. 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.
3. 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.

LIQUID LIMIT TEST OF SOIL USING CASAGRANDE APPARATUS(IS-2720-PART-5-1985)

CASAGRANDE APPARATUS

Objective

For determination of the liquid limit of soil using casagrande apparatus.

Reference Standard

IS : 2720(Part 5)-1985- Methods of test for soils : Determination of liquid and plastic limit.

Equipment & Apparatus

• Oven
• Balance (0.01g accuracy)
• Sieve [425 micron]
• Casagrande apparatus

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. The soil passing 425 micron sieve is used in this test.

Procedure

1. About 120 gm. of air dried soil from thoroughly mixed portion of material passing 425 micron IS sieve is obtained.
2. Distilled water is mixed to the soil thus obtained in a mixing disc to form uniform paste. The paste shall have a consistency that would require 30 to 35 drops of cup to cause closer of standard groove for sufficient length.
3. A portion of the paste is placed in the cup of casagrande device and spread into portion with few strokes of spatula.
4. It is trimmed to a depth of 1 cm. at the point of maximum thickness and excess of soil is returned to the dish.
5. The soil in the cup is divided by the firm strokes of the grooving tool along the diameter through the centre line of the follower so that clean sharp groove of proper dimension is formed.
6. Then the cup is dropped by turning crank at the rate of two revolutions per second until two halves of the soil cake come in contact with each other for a length of about 12 mm. by flow only.
7. The number of blows required to cause the groove close for about 12 mm. is recorded.
8. A representative portion of soil is taken from the cup for water content determination.
9. The test is repeated with different moisture contents at least 3 times for blows between 10 and 40.

Calculation

• A ‘flow curve’ is to be plotted on a semi-logarithmic graph representing water content in arithmetic scale and the number of drops on logarithmic scale.
• The flow curve is a straight line drawn as nearly as possible through four points
• The moisture content corresponding to 25 blows as read from curve is the liquid limit of that soil

Report

The liquid limit is to be reported to the nearest whole number.

Safety & Precautions

• Soil used for liquid limit determination should not be oven dried prior to testing.
• In LL test the groove should be closed by the flow of soil and not by slippage between the soil and the cup
• After mixing the water to the soil sample , sufficient time should be given to permeate the water throughout out the soil mass
• Wet soil taken in the container for moisture content determination should not be left open in the air, the container with soil sample should either be placed in desiccators or immediately be weighed.

PLASTIC LIMIT TEST OF SOIL(IS-2720-PART-5-1985)

Plastic Limit Test

Objective

For determination of the plastic limit of soil.

Reference Standard

IS : 2720(Part 5)-1985- Methods of test for soils : Determination of liquid and plastic limit.

Equipment & Apparatus

• Oven
• Balance (0.01 g accuracy)
• Sieve [425 micron]
• Flat glass surface for rolling

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. The soil passing 425 micron sieve is used in this test.

Procedure

1. A soil sample of 20 gm. passing 425 micron IS sieve is to be taken.
2. It is to be mixed with distilled water thoroughly in the evaporating dish till the soil mass becomes plastic enough to be easily moulded with fingers.
3. It is to be allowed to season for sufficient time, to allow water to permeate throughout the soil mass.
4. 10 gms. of the above plastic mass is to be taken and is to be rolled between fingers and glass plate with just sufficient pressure to roll the mass into a thread of uniform diameter throughout its length. The rate of rolling shall be between 60 and 90 stokes per minute.
5. The rolling is to be continued till the thread becomes 3 mm. in diameter.
6. The soil is then kneaded together to a uniform mass and rolled again.
7. The process is to be continued until the thread crumbled with the diameter of 3 mm.
8. The pieces of the crumbled thread are to be collected in a air tight container for moisture content determination.

Report

The Plastic limit is to be determined for at least three portions of soil passing 425 micron IS sieve. The average of the results calculated to the nearest whole   number is to be reported as the plastic limit of the soil.

Safety & Precautions

• Soil used for plastic limit determination should not be oven dried prior to testing.
• After mixing the water to the soil sample , sufficient time should be given to permeate the water throughout out the soil mass
• Wet soil taken in the container for moisture content determination should not be left open in the air, the container with soil sample should either be placed in desiccators or immediately be weighed.

DIRECT SHEAR TEST OF SOIL(IS-2720-PART-13-1986)

Shear Box Assemblies

Objective

Determination of shear strength parameters of a silty or sandy soil at known density and moisture content.

Reference Standards

IS: 2720(Part 13)-1986- Methods of test for soils: Direct shear test.

Equipment / Apparatus

• Shear box
• Box container
• Porous stone and grid plate
• Tamper, Balance , Sieve(4.75 mm)
• Loading frame, Proving ring, Dial gauge.

Preparation sample

One kg of air dry sample passing through 4.75mm IS sieve is required for this test.

Procedure

1. Shear box dimensions is measured, the box is set up by fixing its upper part to the lower part with clamping screws, and then a porous stone is placed at the base.
2. For undrained tests, a serrated grid plate is placed on the porous stone with the serrations at right angle to the direction of shear. For drained tests, a perforated grid is used over the porous stone.
3. An initial amount of soil is weighed in a pan. The soil is placed into the shear box in three layers and for each layer is compacted with a tamper. The upper grid plate, porous stone and loading pad is placed in sequence on the soil specimen.
4. The pan is weighed again and the mass of soil used is computed.
5. The box is placed inside its container and is mounted on the loading frame. Upper half of the box is brought in contact with the horizontal proving ring assembly. The container is filled with water if soil is to be saturated.
6. The clamping screws is removed from the box, and set vertical displacement gauge and proving ring gauge to zero.
7. The vertical normal stress is set to a predetermined value. For drained tests, the soil is allowed to consolidate fully under this normal load. (Avoid this step for undrained tests.)
8. The motor is started with a selected speed and shear load is applied at a constant rate of strain. Readings of the gauges are taken until the horizontal shear load peaks and then falls, or the horizontal displacement reaches 20% of the specimen length.
9. The moisture content of the specimen is determined after the test. The test is repeated on identical specimens under different normal stress values.

Calculation

• The density of the soil specimen is calculated from the mass of soil and the volume of the shear box.
• The dial readings are converted to the appropriate displacement and load units by multiplying with respective least counts.
• Shear strains are calculated by dividing horizontal displacements with the specimen length, and shear stresses are obtained by dividing horizontal shear forces with the shear area.
• The shear stress versus horizontal displacement is plotted. The maximum value of shear stress is read if failure has occurred, otherwise read the shear stress at 20% shear strain. The maximum shear stress versus the corresponding normal stress is plotted for each test, the cohesion and the angle of shearing resistance of the soil is determined from the graph.

Safety & Precautions

• Before starting the test, the upper half of the box should be brought in contact of the proving ring assembly.
• Before subjecting the specimen to shear, the fixing pins should be taken out.
• The rate of strain should be constant throughout the test.

DETERMINATION OF FIELD DENSITY OF SOIL BY CORE CUTTER METHOD (IS-27270-PART-29)

Aim

To determine the field density of soil by core cutter method

Reference

IS-2720-Part-29-Determination of dry density of soil in place by the core-cutter method

Apparatus

1. Cylindrical core cutter
2. Steel rammer
3. Steel dolly
4. Balance
5. Steel rule
6. Spade or pickaxe
7. Straight edge
8. Knife

Core cutter, Dolly & Rammer

Procedure

1. Measure the height (h) and internal diameter (d) of the core cutter and apply grease to the inside of the core cutter
2. Weigh the empty core cutter (W₁)
3. Clean and level the place where density is to be determined.
4. Drive the core cutter, with a steel dolly on its top, into the soil to its full depth with the help of a steel rammer.
5. Excavate the soil around the cutter with a crow bar and gently lift the cutter without disturbing the soil in it.
6. Trim the top and bottom surfaces of the sample and clean the outside surface of the cutter.
7. Weigh the core cutter with soil (W₂)
8. Remove the soil from the core cutter, using a sample ejector and take representative soil sample from it to determine the moisture content.

Precautions

1. Core cutter method of determining the field density of soil is only suitable for fine grained soil (Silts and clay). This is because collection of undisturbed soil sample from a coarse grained soil is difficult and hence the field properties, including unit weight, cannot be maintained in a core sample
2. Core cutter should be driven into the ground till the steel dolly penetrates into the ground half way only so as to avoid compaction of the soil in the core.
3. Before lifting the core cutter, soil around the cutter should be removed to minimize the disturbances.

Observations and Calculations

Calculate wet unit weight (γ_wet) of the soil using the following relationship
γ_wet = (W₂-W₁)/V
Where,
W₁ = Empty weight of core cutter
W₂ = Weight of core cutter + soil
V = Volume of core cutter (πd²h/4)
D = Inner diameter of core cutter
H = Height of core cutter

CBR TEST OF SOIL-10+ MOST IMPORTANT NOTES TO REMEMBER

1. California Bearing Ratio or CBR test is the ratio of the force per unit area required to penetrate a soil mass with a standard circular piston of 50 mm dia, at the rate of 1.25 mm/min to that of force required to penetrate sample of compacted stone having CBR of 100%.
   
   

   CBR test Equipments
2. The standard load corresponding to 2.5 mm and 5 mm penetration of the plunger into the standard sample is reported to be 1370 kg and 2055 kg respectively.
3. CBR test may be conducted in the laboratory either on remoulded or undisturbed soil specimens. CBR test can also be conducted in the field.
4. CBR test is done both on the soaked and unsoaked samples. Soaking of specimen simulate the worst field condition that a subgrade soil can be subjected to, similar to monsoon and post-monsoon conditions.
5. Both during soaking and penetration test, the specimen is covered with equal surcharge weights to simulate the effect of overlying pavement or the particular layer under construction. Each surcharge slotted weight, 147 mm in dia with a central hole of 53 mm in dia and weighing 2.5 kg is considered approximately equivalent to 6.5 cm of construction.
6. The initial portion of the load-penetration curve of CBR test is generally convex upward. But sometimes the initial portion becomes concave upward. This is due to one or more of the following reasons.

o   Top layer of soaked soil is too soft or slushy after soaking in water
o   The top surface of the soil specimen is not even
o   The penetration plunger of the loading machine is not vertical resulting in the bottom surface of plunger not being horizontal and not fully in contact with top surface of the specimen.

7. The initial concavity in the curve indicates that during the initial application of load, the plunger penetrated at a more rapid rate and later further penetration values are consistent with respect to the load applied.
8. Generally the CBR value at 2.5 mm penetration is higher and this value is adopted as the CBR value of the soil sample. However if higher CBR value is obtained at 5.0 mm penetration, the CBR test is to be repeated to verify the result. If CBR value at 5.0 mm penetration is higher in the repeat test also, this higher value is adopted as the CBR value of the soil sample.
9. Presence of coarse grained particles would result in poor reproducibility of result. Therefore material passing 20 mm sieve is only used in the test.
10. CBR test is an empirical test method and cannot be related accurately with any fundamental or physical property of the soil or pavement material tested.
11. Indian Road Congress (IRC) has standardized the guidelines for the design of flexible pavements based on CBR test (vide IRC-37) and this method is being followed for the design of flexible pavements for all the categories of roads in India.
12. As per IRC guidelines whenever possible the remoulded specimens for CBR test should be prepared by static compaction otherwise by dynamic compaction so as to achieve the desired dry density.
13. Often it is required to determine the CBR value of a soil compacted at some other desired percentage of MDD, other than the MDD value as per heavy (modified) or light (standard) compaction. For example as per MORTH, the soil subgrades of highways are to be compacted in the field to 97% of MDD by heavy compaction obtained in the laboratory. In this case compaction test may be carried out by applying different number of blows per layer (say 25, 40 & 55 blows/layer) and a graph is plotted with number of blows/layer vs the dry density achieved. From this graph it is possible to determine the number of blows required to obtain any desired value or percentage of dry density by interpolation and then to conduct CBR test on specimen compacted accordingly.
14. According to IRC:37-2001, if the maximum variation in CBR values between the three specimens tested in the laboratory exceed the permissible variation in CBR values for different ranges (as given in the table below), the CBR test should be repeated on additional three specimens and the average value of six specimens is to be adopted as the CBR value.

CBR value (%)	Maximum permissible variation in CBR values between 3 individual test value (±, %)
< 5	1
5 – 10	2
11 – 30	3
31 or above	5

WHAT ARE THE ATTERBERG LIMITS OF SOIL?

Atterberg Limits of Soil

The term Atterberg limits is named as per the Swedish agriculturist Albert Atterberg. When water is added into a soil mass, it changes its state from solid to liquid. He divided the entire range from solid to liquid into four stages:

1. The solid state
2. The semi-solid state
3. The plastic state
4. The liquid state

He set arbitrary limits, known as Atterberg limits or consistency limits, for these division in terms of water content. Thus the Atterberg limits are the water content at which the soil mass passes from one state to the next state. These limits are presented as percentage of moisture present inside the soil. The Atterberg limits which are commonly used for engineering purposes are:

1. Liquid limit
2. Plastic limit
3. Shrinkage limit

What is Liquid Limit of Soil?

Liquid limit is the water content corresponding to the arbitrary limit between liquid and plastic state of consistency of a soil. It is defined as the minimum water content at which the soil is still in the liquid state, but has a small shearing strength against flowing.

What is Plastic Limit of Soil?

Plastic limit is the water content corresponding to an arbitrary limit between the plastic and semi-solid states of consistency of a soil. It is defined as the minimum water content at which a soil will just begin to crumble when rolled into a thread approximately 3 mm in diameter.

What is Shrinkage Limit of Soil?

Shrinkage limit is defined as the maximum water content at which a reduction in water content will not cause a decrease in the volume of a soil mass. It is lowest water content at which a soil can still be completely saturated

Triaxial Shear Test-UU (IS-2720-Part-11)

Aim

To determine shear strength parameters of the given soil sample by conducting unconsolidated undrained (UU) triaxial shear test.

Theory And Application

Triaxial testing is a test used to measure the shear parameters of a given soil. The test is performed on a cylindrical soil/rock samples. This test is considered to be the most conveniently available conditions to suit the field situations.

Apparatus Required

• Triaxial testing machine complete with triaxial cell
• Water pressure unit with hand pump
• Provong ring
• Dial gauge
• Rubber membranes
• Membrane stretcher
• Sample trimming apparatus
• Bins for moisture content determinations
• Balance and box of weights
• Drying oven

TRIAXIAL APPARATUS

Procedure

1. Trim the soil specimen (prepared from the sampling tube of an undisturbed sample tube using universal extractor frame or from a compacted soil specimen as per standard proctors method, at optimum moisture content or any other moisture content to suite the field situations). Using the trimming apparatus if necessary the trimmed specimen should be 76.2 mm long and 38.1 mm in diameter. The diameter and the length are measured at not less than 3 places and the average values are used for computation. Note the weight of the specimen (W1).
2. The specimen is then enclosed in a 38.1 mm diameter and about 100 mm long rubber membrane, using the membrane stretcher. Spreading back the ends of the membrane over the ends of the stretcher and applying suction between the stretcher and the rubber membranes does by inhalation. The membrane and stretcher are then easily slide over the specimen, the suction is released and membrane is unrolled from the ends of the stretcher.
3. Use non-porous stones on either side of the specimen as neither any pressure is to be measured nor any drainage of air or water is allowed.
4. Remove the porous cylinder from its base removing the bottom fly nuts.
5. The pedestal at the centre of the base of the cylinder on which the specimen is to be placed is cleaned and a 38.1 mm diameter rubber O-ring is rolled over to its bottom. The specimen along with the non-porous plate on either side is centrally placed over the pedestal and the bottom edge of the machine covering the specimen is sealed against the pedestal by rolling back the O-ring over the membrane.
6. The cap is placed over the top plate of the specimen and the top of the rubber membrane is sealed against the cap by carefully rolling over it another O-ring. This arrangement of rubber O-ring forms the effective seal between the specimen with the membrane and the water under pressure. The specimen is checked for its verticality and co-axiality with the cylinder chamber.
7. The chamber along with the loading plunger is carefully placed over its base without disturbing the soil specimen and taking care to see that the plunger rests on the cap of the specimen centrally. The loading frame is then adjusted so that it just touches the plunger top by naked eye. The chamber is then rotated if necessary such that the dial gauge, recording compression, rests centrally over the top of the screw which can be locked at any level and which is attached to the top of the cylinder chamber carrying the specimen. The cylinder is then attached to the base plate tightly by means of tightening the nuts.
8. The valve to drain out the chamber and the valve to drain out the air and water from the sample are closed and the air lock nut at the top of the cylinder is kept open to facilitate the exit of air as water enters the chamber through another valve which connects the chamber to the water storage cylinder subjected to a pressure by a hand pump or by any other means.
9. The water storage cylinder is filled with water completely and its top is then closed by means of a valve. Necessary pressure is built up in the cylinder by working the hand pump and the pressure communicated to the cylinder where the specimen is placed, by opening the connecting valve. The cylindrical chamber is allowed to be filled up completely which is indicated by the emergence of water through the air lock nut at the top of the chamber. Then the airlock nut is closed to develop necessary confining pressure by using the hand pump (or by any other means) and the same is maintained constant.
10. If necessary, bring the loading plunger down until it is in contact with the specimen top cap by means of hand operated loading device. This is indicated by a spurt in the reading of the proving ring dial gauge.
11. For this position, adjust the deformation dial gauge reading to zero.
12. Record the initial reading of the proving ring and compression dial gauge.
13. The vertical load is applied to the specimen by starting the motor at the loading frame. The change in the proving ring dial gauge gives the measure of the applied load. The deformation dial gauge gives the deformation in the soil specimen, which can be used to compute strain in the soil.
14. Take readings of proving ring dial gauge at 0.5, 1.0, 1.5, 2.0% (or any other smaller values) of strain and for every 1.0% strain thereafter up to failure or 20% strain whichever is earlier.
15. Throughout the test, make sure that the chamber, containing pressure is kept constant at the desirable value as indicated by the pressure gauge on the water cylinder. If necessary, the pressure can be made good for any possible losses by working the hand pump.
16. After specimen has failed or 20% strain is recorded, as the case may be (a) stop application of load (b) disconnect the chamber from water storage cylinder by closing the linger valve (c) open the air lock knob a little and (d) open the valve to drain out the water in the cylinder. After a few seconds open the airlock nut completely to facilitate quick draining out of water, by entry of air at top of the cylinder.
17. After the water is completely drained out, take out the cylinder from loading frame carefully, loosen the nuts and remove the Lucite cylinder from ts base, without disturbing the sample.
18. Note the space of the failed specimen, angle of shear plane if any and dimensions of the specimen.
19. Wipe the rubber membrane dry and find its weight W2 that should be same as W1.
20. Remove the membrane from the specimen and take a representative specimen preferably from the sheared zone.
21. Repeat the test with three specimens of the same soil sample subjected to three different lateral pressures (confining) of 0.5, 1.0 and 1.5 kg/cm² (5, 10 and 15 psi or 50, 100 and 150 kpa)

Calculations

1. Axial strain=ΔL/L=change in length/initial length. This is expressed as a % for convenience.
2. The stress intensity applied vertically is obtained by dividing the load, P by the cross-sectional area of the specimen. At any time when axial strain is e. area = A₀/(1-e) where A0 is the initial cross-sectional area of the sample. The intensity of stress = P/A. as the sample is enclosed in a rubber membrane and is sealed at either end, its volume is constant as no air or water can escape. So as the length decreases due to compression, area should increase which is assumed to be uniform. Therefore, A=A₀/(1-e)

Results

1. A graph is drawn between the deviator stress and strain. The deviator stress is the difference between the stresses in axial and radial direction i.e. (σ₁-σ₃) and is equal to the vertical stress P/A. σ₃ is the lateral confining pressure at any time, which is constant for a test. From the plot, determine the second result at half the ultimate stress, which can be taken as modulus of elasticity.
2. The mohr’s circle of stress to define the state of stress at failure is drawn for each sample. The circle has for its centre point (σ₁+σ₃)/2 and the radius equal to (σ₁-σ₃)/2. An envelope, which approximates to a straight line, is drawn touching the circle. The intercept made on Y-axis and the slope of the envelope gives the values of strength parameters of the soil C and φ respectively.

CBR TEST OF SOIL (IS-2720-PART-16)

Objective

Determination of CBR of soil either in undisturbed or Remoulded condition

Reference Standards

IS: 2720(Part 16)-1973- Methods of test for soils: Laboratory determination of CBR.

Equipments / Apparatus

• Compression machine
• Proving ring, Dial gauge, Timer
• Sampling tube
• Split mould
• Vernier caliper, Balance

CBR test of soil

Preparation sample

The test may be performed
(a) On undisturbed soil specimen
(b) On remoulded soil specimen
(a) On undisturbed specimen
Undisturbed specimen is obtained by fitting to the mould, the steel cutting edge of 150 mm internal diameter and pushing the mould as gentky as possible into the ground. When the mould is sufficiently full of soil, it shall be removed by under digging. The top and bottom surfaces are then trimmed flat so as to give the required length of specimen.
(b) On remoulded Specimens

The dry density for remoulding should be either the field density or if the subgrade is to be compacted, at the maximum dry density value obtained from the Proctor Compaction test. If it is proposed to carry out the CBR test on an unsoaked specimen, the moisture content for remoulding should be the same as the equilibrium moisture content which the soil is likely to reach subsequent to the construction of the road. If it is proposed to carry out the CBR test on a soaked specimen, the moisture content for remoulding should be at the optimum and soaked under water for 96 hours.
Soil Sample – The material used in the remoulded specimen should all pass through a 19 mm IS sieve. Allowance for larger material may be made by replacing it by an equal amount of material which passes a 19 mm sieve but is retained on a 4.75 mm IS sieve. This procedure is not satisfactory if the size of the soil particles is predominantly greater than 19 mm. The specimen may be compacted statically or dynamically.
I. Compaction by Static Method
The mass of the wet soil at the required moisture content to give the desired density when occupying the standard specimen volume in the mould is calculated. A batch of soil is thoroughly mixed with water to give the required water content. The correct mass of the moist soil is placed in the mould and compaction obtained by pressing in displacer disc, a filter paper being placed between the disc & soil.
II. Compaction by Dynamic Method
For dynamic compaction , a representative sample of soil weighing approximately 4.5 kg or more for fine grained soils and 5.5 kg or more for granular soil shall be taken and mixed thoroughly with water. If the soil is to be compacted to the maximum dry density at the optimum water content determined in accordance with light compaction or heavy compaction, the exact mass of soil required is to be taken and the necessary quantity of water added so that the water content of soil sample is equal to the determined optimum water content. The mould with extension collar attached is clamped to the base plate. The spacer disc is inserted over the base plate and a disc of coarse filter paper placed on the top of the spacer disc. The soil water mixture is compacted into the mould in accordance with the methods specified in light compaction test or heavy compaction test.

Procedure

1. The mould containing the specimen with the base plate in position but the top face exposed is placed on the lower plate of the testing machine.
2. Surcharge weights, sufficient to produce an intensity of loading equal to the weight of the base material and pavement is placed on the specimen.
3. To prevent upheaval of soil into the hole of the surcharge weights, 2.5 kg annular weight is placed on the soil surface prior to seating the penetration plunger after which the remainder of the surcharge weight is placed.
4. The plunger is to be seated under a load of 4 kg so that full contact is established between the surface of the specimen and the plunger.
5. The stress and strain gauges are then set to zero. Load is applied to the penetration plunger so that the penetration is approximately 1.25 mm per minute.
6. Readings of the load are taken at penetrations of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10.0 and 12.5 mm.
7. The plunger is then raised and the mould detached from the loading equipment.

Calculation

Load-Penetration curve:
The load penetration curve is plotted taking penetration value on x-axis and Load values on Y-axis. Corresponding to the penetration value at which the CBR is desired, corrected load value is taken from the load-penetration curve and the CBR calculated as follows
California bearing ratio = (P_T/P_S)x100
Where
P_T = Corrected unit (or total) test load corresponding to the chosen penetration curve, and
P_S = Unit(or total) standard load for the same depth of penetration as for P_S taken from standard code.

Report

The CBR values are usually calculated for penetration of 2.5 mm and 5 mm. The CBR value is reported correct to the first decimal place.

Safety & Precautions

• Clean the sieves with the help of a brush, after sieving
• 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.

MODIFIED PROCTOR COMPACTION TEST OR HEAVY COMPACTION TEST (IS-2720-PART-8)

Objective

To determine moisture content and dry density relationship using heavy compaction or modified compaction method as per IS-2720-Part-8.

Apparatus

• Metal mould (volume = 1000 cm3)
• Balance (capacity = 10 kg, least count = 1g)
• Oven (105 to 1100C)
• Sieve (19 mm)
• Metal rammer (weight = 4.9 kg)

Compaction Mould and Rammer

Procedure

1. Dry the soil sample by exposing it to air or sun light.
2. Sieve the air dried soil through 19 mm sieve.
3. Add suitable amount of water with the soil and mix it thoroughly. For sandy and gravelly soil add 3% to 5% of water. For cohesive soil the amount of water to be added should be 12% to 16% below the plastic limit.
4. Weigh the mould with base plate attached to the nearest 1g. Record this weight as ‘W₁’.
5. Attach the extension collar with the mould.
6. Compact the moist soil into the mould in five layers of approximately equal mass, each layer being given 25 blows, with the help of 4.9 kg rammer, dropped from a height of 450 mm above the soil. The blows must be distributed uniformly over the surface of each layer.
7. After completion of the compaction operation, remove the extension collar and level carefully the top of the mould by means of straightedge.
8. Weigh the mould with the compacted soil to the nearest 1 g. Record this weight as ‘W₂’.
9. After weighing remove the compacted soil from the mould and place it on the mixing tray. Determine the water content of a representative sample of the specimen. Record the moisture content as ‘M’.
10. Broken up the remainder of the specimen and repeat step 5 to step 9 by adding suitable increment of water to the soil. For sandy and gravelly soils the increment in general is 1% to 2%. For cohesive soils the increment in general is 2% to 4%.
11. The total no. of determinations made shall be at least five, and the moisture contents should be such that the optimum moisture content, at which the maximum dry density occurs, is within that range.

Calculation

Bulk density, γ_b in g/cm³ of each compacted specimen is calculated from the following equation.
γ_b = (W₂-W₁)/V_m
where,
W₁ = Weight in g of mould + base plate
W₂ = Weight in g of mould + base plate + soil
V_m = Volume of mould i.e. 1000 cm³.
Dry density, γ_d in g/cm³ of each compacted specimen is calculated from the following equation.
γ_d = 100 γ_b/(100+M)
Where,
γ_b = Bulkdensity of soil in g/cm³.
M = Moisture content of soil

Graph

The dry densities, γ_d , obtained in a series of determinations is plotted against the corresponding moisture content ‘M’. A smooth curve is then drawn through the resulting points and the position of the maximum on this curve is determined, which is called maximum dry density (M.D.D). And the corresponding moisture content is called optimum moisture content (O.M.C.).

Compaction Curve

HOW TO DETERMINE SPECIFIC GRAVITY OF SOIL?

Objective

For determination of specific gravity of soil solids by pycnometer method.

Equipment & Apparatus

• Pycnometer
• Sieve(4.75 mm)
• Vacuum pump
• Oven
• Weighing balance
• Glass rod

Preparation sample

After receiving the soil sample it is dried in oven at a temperature of 105 to 115⁰C for a period of 16 to 24 hours.

Procedure

1. Dry the pycnometer and weigh it with its cap(W₁)
2. Take about 200 g to 300 g of oven dried soil passing through 4.75mm sieve into the pycnometer and weigh again(W₂)
3. Add water to cover the soil and screw on the cap.
4. Shake the pycnometer well and connect it to the vacuum pump to remove entrapped air for about 10 to 20 minutes.
5. After the air has been removed, fill the pycnometer with water and weigh it (W₃).
6. Clean the pycnometer by washing thoroughly.
7. Fill the cleaned pycnometer completely with water up to its top with cap screw on.
8. Weigh the pycnometer after drying it on the outside thoroughly (W₄). 

Calculation

The Specific gravity of soil solids (Gs) is calculated using the following equation.

 Where
W₁=Empty weight of pycnometer
W₂=Weight of pycnometer + oven dry soil
W₃=Weight of pycnometer + oven dry soil + water
W₄=Weight of pycnometer + water full

Report

The result of the specific gravity test is reported to the nearest two digits after decimal.

Safety & Precautions

• Soil grains whose specific gravity is to be determined should be completely dry.
• If on drying soil lumps are formed, they should be broken to its original size.
• Inaccuracies in weighing and failure to completely eliminate the entrapped air are the main sources of error. Both should be avoided.

Reference Standard

IS : 2720 (Part 4) – 1985 – Method of test for soil (Part 4-Grain size analysis)

DETERMINATION OF FIELD DENSITY OF SOIL BY SAND REPLACEMENT METHOD (IS-2720-PART-28)

Aim

To determine the field density of soil at a given location by sand replacement method

Reference

IS-2720-Part-28-Determination of dry density of soils in place, by the sand replacement method

Apparatus

1. Sand pouring cylinder
2. Calibrating can
3. Metal tray with a central hole
4. Dry sand (passing through 600 micron sieve)
5. Balance
6. Moisture content bins
7. Glass plate
8. Metal tray
9. Scraper tool

Theory and Application

Determination of field density of cohesion less soil is not possible by core cutter method, because it is not possible to obtain a core sample. In such situation, the sand replacement method is employed to determine the unit weight. In sand replacement method, a small cylindrical pit is excavated and the weight of the soil excavated from the pit is measured. Sand whose density is known is filled into the pit. By measuring the weight of sand required to fill the pit and knowing its density the volume of pit is calculated. Knowing the weight of soil excavated from the pit and the volume of pit, the density of soil is calculated. Therefore, in this experiment there are two stages, namely

1. Calibration of sand density
2. Measurement of soil density

Sand Replacement Method

Procedure

Stage-1 (Calibration of Sand Density)

1. Measure the internal dimensions (diameter, d and height, h) of the calibrating can and compute its internal volume, V_c = πd²h/4.
2. Fill the sand pouring cylinder (SPC) with sand with 1 cm top clearance (to avoid any spillover during operation) and find its weight (W₁)
3. Place the SPC on a glass plate, open the slit above the cone by operating the valve and allow the sand to run down. The sand will freely run down till it fills the conical portion. When there is no further downward movement of sand in the SPC, close the slit. Measure the weight of the sand required to fill the cone. Let it be W₂.
4. Place back this W₂ amount of sand into the SPC, so that its weight becomes equal to W₁ (As mentioned in point-2). Place the SPC concentrically on top of the calibrating can. Open the slit to allow the sand to run down until the sand flow stops by itself. This operation will fill the calibrating can and the conical portion of the SPC. Now close the slit and find the weight of the SPC with the remaining sand (W₃)

Stage-2 (Measurement of Soil Density)

1. Clean and level the ground surface where the field density is to be determined
2. Place the tray with a central hole over the portion of the soil to be tested.
3. Excavate a pit into the ground, through the hole in the plate, approximately 12 cm deep (same as the height of the calibrating can). The hole in the tray will guide the diameter of the pit to be made in the ground.
4. Collect the excavated soil into the tray and weigh the soil (W)
5. Determine the moisture content of the excavated soil.
6. Place the SPC, with sand having the latest weight of W₁, over the pit so that the base of the cylinder covers the pit concentrically.
7. Open the slit of the SPC and allow the sand to run into the pit freely, till there is no downward movement of sand level in the SPC and then close the slit.
8. Find the weight of the SPC with the remaining sand (W₄).

Precautions

• If for any reason it is necessary to excavate the pit to a depth other than 12 cm, the standard calibrating can should be replaced by one with an internal height same as the depth of pit to be made in the ground.
• Care should be taken in excavating the pit, so that it is not enlarged by levering, as this will result in lower density being recorded.
• No loose material should be left in the pit.
• There should be no vibrations during this test.
• It should not be forgotten to remove the tray, before placing the SPC over the pit.

Observations and Calculations

Enter all the data as per the table given below and calculate accordingly.

Sl no	Data (Calibration of Unit Weight of Sand)	Trial-1
1	Volume of the calibrating container, V (cm3)
2	Weight of SPC + sand, W1 (g)
3	Weight of sand required to fill the conical portion on a flat surface, W2 (g)
4	Weight of SPC + sand (after filling calibrating can), W3 (g)
5	Weight of sand required to fill the calibrating container, Wc = (W1-W2 –W3) (g)
6	Unit weight of sand, γsand = (Wc)/V   (g/cm3)

Sl. no	Data (Determination of Density of Soil)	Trial-1
1	Weight of the excavated from the pit (W)  (g)
2	Weight of sand + SPC, before pouring, W1 (g)
3	Weight of SPC after filling the hole & conical portion, W4  (g)
4	Weight of sand in the pit Wp = (W1-W4-W2)  (g)
5	Volume of sand required to fill the pit Vp=Wp/γsand  (cm3)
6	Wet unit weight of the soil γwet=W/Vp  (g/cm3)
7	Dry unit weight of the soil γdry=γwet/(1+m)  (g/cm3) (where ‘m’ is the moisture content of soil)