DETERMINATION OF UNCONFINED COMPRESSIVE STRENGTH OF SOIL

DETERMINATION OF UNCONFINED COMPRESSIVE STRENGTH OF SOIL in Foundation Engineering

INTRODUCTION
TERMINOLOGY
APPARATUS
SOIL SPECIMEN & ITS TYPES
THEORY
RELEVANCE OF THE EXPERIMENT
PROCEDURE
VIDEO
OBSERVATION
DISCUSSION
REMARKS

1. DETERMINATION OF UNCONFINED COMPRESSIVE STRENGTH OF SOIL ASHISH NAYYAR(CO18212) JAPTYESH SINGH(CO18229) JOBANPREET SINGH(CO18232) NISCHAY SINGH(CO18237) RATTANBIR SINGH(CO18243) SARTHAK PUNJ(CO18248) in FOUNDATION ENGINEERING Submitted to: MOHAMMAD SAKIB PERWEZ KHAN SIR

2. INDEX INTRODUCTION TERMINOLOGY APPARATUS SOIL SPECIMEN & ITS TYPES THEORY RELEVANCE OF THE EXPERIMENT PROCEDURE VIDEO OBSERVATION DISCUSSION REMARKS

3.    INTRODUCTION                            [as per IS code 2720(Part X) ] The maximum load that can be transmitted to the subsoil by a foundation depends upon the resistance of underlying soil to the shearing deformation and compressibility. An axial load is applied at a constant rate of strain without any lateral support to the soil specimen and is increased until failure occurs.  The compressive load per unit area required to fail the soil specimen under such conditions is called the unconfined compressive strength of the soil. This test is applicable for determining strength of cohesive soils. In this presentation we shall demonstrate the unconfined compressive test as per IS code 2720 Part 10.

4. TERMINOLOGY For the purpose of this standard, the following definitions shall apply: - Unconfined Compressive Strength, q u , it is the load per unit area at which an unconfined cylindrical specimen of soil will fail in the. axial compression test. If the axial compression force per unit area has not reached a maximum value even at 20 % axial strain, q u shall be taken as the value obtained at 20% axial strain.

5. APPARATUS Unconfined Compression apparatus,  (loading frame) proving ring type . Proving capacity 1 KN Dial gauge accuracy 0.01 mm Weighing balance Oven Stopwatch Sampling Tube Split Mould (38 mm diameter, 76 mm long) Sample extractor Knife Vernier Calliper Large Mould Stopwatch Grease

6. APPARATUS USED 1.     Unconfined Compression apparatus,  to apply compressive load at constant rate of strain Can be any of the following: (provided All these loading devices shall have sufficient capacity and strain control.) Platform weighing scale equipped with a screw-jack activated yoke Hydraulic loading device; Screw jack with a proving ring; and Any other loading device. loading frame.  Also includes gear for different loading.  2.      proving ring type  with Proving capacity 1 KN For relatively weak soil with q u less than 100 KPa (1 kg/cm 2 ) load shall be measurable to 1 KPa (0.1 kg/cm 2 ). For soils with q u equal to or greater than 100 KPa (1 kg/cm 2 ) load shall be measurable to the nearest 5 KPa (0.05 kg/cm 2 ). The calibration of the proving ring shall be checked frequently, at least once a year.

7. 3.      Dial gauge  Axial deformation of the sample shall be measured with a dial gauge having a least count of 0.01 mm and travel to permit not less than 20 percent axial strain.  4       Bearing plates Instrumented by clutch with help of electric motor  5.      Weighing balance Suitable for weighing soil specimens specially. Specimens of less than 100 g shall be weighed to the nearest 0.01 g whereas specimens of 100 g or larger shall be weighed to the nearest 0.1 g. 6.      Oven Thermostatically controlled, with interior of noncorroding material capable of maintaining the temperature at 110°C + - 5°C 7. Stopwatch Timing device to indicate the elapsed testing time to the nearest second may be used for establishing the rate of strain.

8. 8.      Split mould (38 mm diameter, 76 mm long) ​ 9.      Vernier Calliper ​ Suitable to measure physical dimensions of the test specimen to the nearest O-1 mm. ​ 10.    Sampling Equipment  ​ Sampling Tube ​ Sample extractor ​ 11.    Miscellaneous Equipment ​ Specimen trimming and carving tools,  ​ remoulding apparatus,  ​ Large mould ​ Knife ​ water content cans,  ​ data sheets, etc, as required. ​ Grease or oil

9. SOIL SPECIMEN The soil specimen to be used for test shall be depend on the purpose for which it is tested and may be compacted, moulded or undisturbed.

10. SPECIMEN SIZE The specimen for test shall have a minimum diameter of 38 mm and the largest particle contained within. The test specimen shall be smaller than 1/8 th of the specimen diameter. If after completion of test on undisturbed sample, it is found that larger particles than permitted for the particular specimen size tested are present.  The height to diameter ratio shall be within 2 to 2.5.  Measurements of height and diameter shall be made with Vernier callipers or any other suitable measuring device to the nearest 0.1 mm.

11. TYPES OF SPECIMEN

12. UNDISTURBED SPECIMEN Undisturbed specimens shall be prepared from large undisturbed samples. Undisturbed sample shall be prepared from the drive sampling tube. The ejecting device shall be capable of ejecting the soil core from the sampling tube within the same direction of travel during which the sample entered the tube and with negligible disturbance of the sample. Conditions at the time of removal of the sample may dictate the direction of removal but the principle concern should be to stay the degree disturbance negligible. NOTES: Three specimens obtained by trimming and carving from undisturbed soil samples shall be tested. When the sample is ejected horizontally, a curved plate may be provided to butt against the sampling tube such that the ejected specimen slips over it freely, This will avoid bending of the specimen and facilitate bringing specimen to vertical position in many cases.

13. The specimen shall be handled carefully to stop disturbance, change in cross section or loss of water. If any type of disturbance is likely to be caused by the ejection device the sample tube shall be split lengthwise or be cut off in small sections, to facilitate removal of the specimen without disturbance.  The specimen shall be of uniform circular cross-section with ends perpendicular to the axis of the specimen. Specimen of required size could also be carved from large undisturbed specimens. Tube specimen may be tested without trimming except for squaring of ends  Where the prevention of the possible development of applicable capillary forces is required the specimens shall be sealed with rubber membranes, thin plastic coatings or with coating of grease or sprayed plastic immediately after plastic immediately after preparation and through the whole testing cycle. Representative sample cuttings taken from the tested specimen shall be used for determination of water content.

14. REMOULDED SPECIMEN The specimen could also be prepared either from a failed undisturbed specimen or from a disturbed soil sample. Just in case of failed undisturbed specimen, the fabric shall be wrapped during a thin rubber membrane and thoroughly worked with the fingers to assume complete remoulding . Care shall be taken to avoid entrapped air, to get uniform density, to remould to an equivalent void ratio as that of undisturbed specimen and to preserve the natural water content of the soil.

15. COMPACTED SPECIMEN When compacting disturbed material, it shall be done employing a mould of circular cross section. Compacted specimen could also be prepared at any predetermined water content and density. After the specimen is made, the ends shall be trimmed perpendicular to the long axis and faraway from the mould. Representative sample cuttings shall be obtained or the whole specimen shall be used for the determination of water content after the test.

16. THEORY

17. RELEVANCE OF THE EXPERIMENT To determine the shear strength of a soil triaxial shear test is conducted. Also, it’s quick and straightforward to perform. The consistency of clay is often determined using the worth of unconfined compressive strength of soil. This should be evaluated as unconfined compression test is inappropriate for dry sands or crumble clays because the materials would disintegrate without some land of lateral confinement. Shear strength of a soil the foremost important engineering property. To settle on the simplest material for the embankment, one has got to conduct strength tests on the samples selected. If it’s not evaluated then it’ll be needed to conduct the bearing capacity test within the field which isn’t always possible. Sl No. Consistency of Clay Unconfined Compressive Strength (KN/m 2 ) 1 Very Soft < = 25 2 Soft 25 – 50 3 Medium 50 – 100 4 Stiff 100 – 200 5 Very Stiff 200 – 400 6 Hard > = 400

18. PROCEDURE The soil specimen is prepared at the desired water content and density in the large mould . The sampling tube into the large mould is pushed and the sampling tube which is filled with the soil is removed. The soil sample in the sampling tube is saturated. The split mould is lightly coated with a thin layer of grease. The sample is extracted out of the sampling tube by a suitable method into the split mould , using the sample extractor and the knife. The two ends of the specimen are trimmed off in the split mould . The mould with the specimen is weighed. The specimen is removed from the split mould by splitting the mould into two parts.

19. PROCEDURE The length and diameter of the specimen is measured with Vernier callipers . The specimen is placed on the bottom plate of the compression machine. The upper plate is adjusted to make contact with the specimen. The dial gauge and the proving ring gauge is adjusted to zero. The compression load is applied to cause an axial strain at the rate of ½ to 2% per minute. The dial gauge reading is recorded and the proving ring after every 60 seconds for a strain between 6% to 12% after every 2 minutes or so beyond 12%. The test is continued until failure surfaces have clearly developed or until an axial strain of 20% is reached. The angle between the failure surface and the horizontal is measured if possible.

20. OBSERVATION Elapsed  Time (minutes) Compression Dial reading Lc=0.01mm  Strain  E=𝛿L/L  (%) Area (cm 2 ), Ac= A / (1-Ε) Proving ring dial reading 1 Div=1Kg Axial Load(P)   (Kg) Compressive Stress(P/Ac)  (Kg/cm ² ) 11.35 0.5 100 1.32 11.50 1 1 0.09 1.0 200 2.69 11.66 2 2 0.17 1.5 300 3.95 11.82 3 3 0.25 2.0 400 5.26 11.98 5 5 0.42 2.5 500 6.53 12.15 7 7 0.53 3.0 600 7.9 12.32 9 9 0.73 3.5 700 9.21 12.50 11 11 0.83 4.0 800 10.53 12.69 12 12 0.95 4.5 900 11.34 12.97 13 13 1.01 5.0 1000 13.16 13.07 14 14 1.07 5.5 1100 14.47 13.27 14 14 1.06 6.0 1200 15.79 13.43 13 13 0.96

21. Result The Unconfined Compressive Strength of Soil is 1.07 Kg/cm ². Graph

22. DISCUSSION A graph is drawn between compressive stress and strain. The maximum stress from the curve gives the value of unconfined compressive strength, qu. If no maximum value of stress is out there, the strain at 20% strain is taken as unconfined compressive strength. This test provides an immediate value of the compressive strength of soil in the remoulded condition, it is carried out within a short time to ensure that no drainage of water is permitted into or out of the specimen. In very plastic soils the axial stress does not readily reach a maximum value.

23. REMARKS The type of soil in this test depends on the purpose for which it is tested and may be compacted, remoulded or undisturbed. The specimen has minimum diameter of 38 mm. Compacted specimen could also be prepared at any predetermined water content and density. Due to lack of stopwatch the readings taken may include some error. Moreover, the specimen was already prepared so nothing can be said much about the type of soil specimen. Both the ends of the sample are shaped so that it should sit properly on the bottom plate of loading frame. Rate of loading of the sample should be constant. Readings should be taken with proper attention so as to have accurate results. Reading should be taken perpendicularly so as to remove parallax. Unconfined Compressive Strength Test is a special type of Unconsolidated Undrained (UU) test that is commonly used for clay specimen. It is special in case of triaxial compression test.

24. REPORT Unconfined compressive strength (UCS) test was carried out on the undisturbed soil sample collected from the site in tube sampler. The soil sample obtained had a diameter of 38 mm and the height of the sample was trimmed to 76 mm to attain height to diameter ratio of 2 in accordance with IS 2720 part 10. The value of unconfined compressive strength (q) of the soil sample was determined in a conventional compression testing machine at a constant strain rate of 0.6 mm/min in accordance with IS 2720 part 10. The sample was tested upto the breaking/failure load or 20% axial deformation of the sample, whichever occurred earlier. UCONFINED COMPRESSION TEST CARRIED OUT AT STATE INFORMATION COMMISSION (SECTOR-3 PANCHKULA)

25. CALCULATION Sample Calculation for sample collected from BH-1 (depth 12 m)  A = initial area of the sample = 11.3 cm²  Ac= Corrected area of the sample  =A (1-{change in length of specimen during testing/original   length of specimen})  Original length of specimen 76 mm  Final length of specimen = 60.8 mm  Strain in sample at failure = (76-60.8)/76 =20%  Ac=14.1 cm²  Load = 36.1 kg  q=Load/ Ac =2.56 kg/cm²  Undrained Shear strength (Cs) = q /2 = 1.28 kg/cm²

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