EXPERIMENT 9 , Test Name: UNCONFINED COMPRESSION (UC) TEST , soil and foundation
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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 condi tions. 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.
Equipment
1. Compression device,
2. Load and deformation dial gauges,
3. Sample trimming equipment,
4. Balance,
5. Moisture can.
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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. Calculate the deformation (Z) corresponding to 15% strain (E).
Test Name: UNCONFINED COMPRESSION TEST
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Where L0 = Original specimen length (as measured in step 3).
6. 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.
7. 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.
8. 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.
9. Draw a sketch to depict the sample failure.
10.Remove the sample from the compression device and obtain a sample for water content determination. Determine the water content as in Experiment 1.
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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
3. Compute the strain,
4. Computed the corrected area
5. Using A’, compute the specimen stress,
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(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 example data.
8. Draw Mohr’s circle using qu from the last step and show the undrained shear strength, su = c (or cohesion) = qu/2. See the example data.
EXAMPLE DATA
Date Tested: MAY 22, 2022
Tested By: HM Class, Group A
Project Name: HM Lab
Sample Number: A, B
Visual Classification: Brown silty clay, medium plasticity, moist CL.
Sample data: Diameter (d) = 7.29 cm
Length (L0) = 14.78 cm Mass = 1221.4 g
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