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# The Shear Strength Of The Soil Under A Constant Cell Pressure (Lab Report Sample)

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This experiment examined the behavior of soil under loading. The test was to determine the shear strength of the soil under constant cell pressure. Four Samples were tested under unconsolidated undrained test. Each condition was recorded. The results showed that there is a correlation in the experiment findings and theory. However, the third result is contrary to theory expectation. This is because the sample was contaminated. This led to an irregular result that was not in tandem with the first and the second results. These results bear significant effects on the determination of soil shear strength and resistance.

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Abstract
This experiment examined the behavior of soil under loading. The test was to determine the shear strength of the soil under constant cell pressure. Four Samples were tested under unconsolidated undrained test. Each condition was recorded. The results showed that there is a correlation in the experiment findings and theory. However, the third result is contrary to theory expectation. This is because the sample was contaminated. This led to an irregular result that was not in tandem with the first and the second results. These results bear significant effects on the determination of soil shear strength and resistance.
Introduction
This experiment examined the behavior of soil under loading. The aim of this test was to determine the shear strength, as well as the shearing resistance of cohesive soil under constant confining pressure with zero change in moisture content, using unconsolidated undrained test. The four samples were compressed at a constant strain-controlled rate, and not allowed to drain. Each condition was recorded. The test is useful in the determination of the total strength parameters for soils subjected to various conditions. The triaxial test was used since it is more reliable and can measure both drained and undrained shear strength.
Apparatus
1. A triaxial cell, the size of the specimen to be tested, and suitable for the highest cell pressure. It should have an end with a similar diameter as the samples’.
2. A cell pressure system for maintaining, measuring and applying the appropriate pressure of water in the cell.
3. A back pressure system for measuring, maintaining and applying the appropriate pressure within the sample as well as measuring water in and out of the specimen.
4. Axial loading machine for the application of axial compression to the sample at different speeds.
5. Triaxial compression chamber
6. Rubber membrane to suit the size of the sample
7. Miscellaneous apparatus
Experiment procedure
1. Measure initial mass, length, and diameter of the specimen. Measure the rubber membrane, thickness.
2. Put the sample in a rubber membrane and Set it in a triaxial chamber.
3. Raise the cell pressure to the desired value. This can be 70 kPa for the first case followed by 140 kPa in the second instance.
4. Shear the specimen at an interval of 1%/min.
5. Record О”L, and Пѓd at 10 seconds interval.
6. Continue until the deviator stress exhibits a final figure or 20% axial strain.
7. Upon completion of the experiment, release the cell pressure to 0. Then vent the pressure, bring down the cell by lowering the platen. Drain the cell; clean the assembly and the porous stone.
8. Sketch the failure mode.
9. Re-measure the weight of the soil sample's weight and put it in the oven.
10. Repeat the procedure for the second and third specimen at 140 kPa and 210 kPa cell pressure respectively.
11. Calculate axial strain. Оµa
12. Calculate the sample's vertical load.
13. Calculate corrected area of the specimen (Ac)
14. Plot Пѓd against axial strain for the three tests.
15. Plot Пѓd vs. Оµa for the three tests on a single sheet.
16. Plot Mohr circle using Пѓ1 and Пѓ3 at failure. These should produce a similar Пѓd value.
17. Find the final moisture content of the sample.
Results and calculations
MOISTURE CONTENT
container no
1
2
3
4
mass of wet soil + container
g
39.6
27.4
28.6
51.2
mass of dry soil + containerg3524.525.445.2mass of container(m1)g5.45.35.45.4mass of moisture(m1-m3)g4.62.93.26mass of dry soil (m3-m1)g29.619.2239.8moisture=(m2-m3)/(m3-m1)%15.415.11615.08average moisture content% 15.4
Sample 1
16.2/76 x100=21.32%
E=21.32%
Sample 2
16.83/76 x100=22.18%
E=22.18%
Sample 3
20.57/76 x100=27.07%
E=27.07%
Sample 4
14.68/76 x100=19.32%
E=19.32%
B.cell pressure
QUOTE /4
QUOTE /4
=1134.11mm2
Volume=AREA X HEIGHT
Volume =A0XL0=A(L0-X) OR A =V0/(L0-X) OR A=A0/(1-E)
Where
A0=initial area
X-Displacement
L0-initial length
V0-initial volume
=1134.11X76
=86192.36mm3
Sample 1
Deviator stress= P/A
454/1134.11=0.40N/MM2
P/A=Q1-Q3
0.40=Q1-0.013
Q1=0.413N/MM2
Sample2
Deviator stress= P/A
Where
P-load
A-area
=543/1134.11=0.48N/MM2
P/A=Q1-Q3
Where
Q1= MAJOR PRINCIPLE STRESS
Q3-CELL PRESSURE
0.48=Q1-0.15
Q1=0.63N/MM2
Sample 3
Deviator stress=P/A
=667/1134.11=0.59N/MM2
P/A=Q1-Q3
0.59=Q1-0.034
Q1=0.624N/MM2
Sample 4
Deviator stress=P/A
=1059/1134.11
=0.933N/MM2
P/A=Q1-Q3
0.93=Q1-0.032
Q1=0.96N/MM2
A. Failure strain as per the final value for the four samples
Mohr table
sampledeviator stress(Q1-Q3)cell pressure(Q3)major principle stress(Q1)Q11315100415 4152372.8200572.8 572.83429.6400829.6 829.64755.18001555.11555.1
Mohr circle
From the graph cu=100
Angle Йё =26°
Discussion
Besides reading and calibration issues, the test procedures were executed appropriately. The first and second tests' results were in good correlation, therefore, satisfactory and in the right magnitude. The third test was not equally successful as its specimen had an improper seal which made the sample partially saturated during the experiment. This consequently led to a steeper friction angle and a higher shear stress at failure. This is normal due to surface tension and water effects and therefore, could not be avoided.
The second error may be attributable to improper calculation of either the first or the second sample's relative density. The shear stress at failure is expected to increase progressively since the tests were carried out on progressively denser specimens. If the third sample had not been contaminated, its angle of failure would be near the first and second sample's values.
The results of this experiment give an estimation of how the soil represented by the samples would behave in the field. Nevertheless, in an ideal lab situation results are bound to differ slightly.
Conclusion
The triaxial test is of significance in the understanding of soil behavior. It is crucial in the measurement of stiffness and strength, determination, and monitoring of the internal reaction of a particular medium. It also allows monitoring of pore pressures and observation of the volume changes during the experiment. In-Depth understanding of material behavior and adequate assessment of its qualities allows the Engineer to enhance designs and to keep the risk of failures at the minimum.
UU triaxial test provides the shear strength of soil at various confining stresses. Shear strength is vital in every geotechnical analysis and design. The value of shear strength as regards to a particular soil is pivotal in the acquisition of solutions to problems concerning the stability of that soil's mass in the determination of the load that soil can bear.
APPENDIX – I
SAFE WORKING PROCEDURE
TRIAXIAL TEST
A.PRIOR TO STARTING WORK
1. Avoid loose cloth...

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