Lab 7 - Unconfined and Triaxial Test PDF

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Download Lab 7 - Unconfined and Triaxial Test PDF

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Q-Test and CU Test CIVE 334 Dr. Song Due April 24th, 2018

Purpose The triaxial compression test is a sophisticated test procedure for determining the shear strength of soil, and usually involves three tests, the Unconsolidated Undrained (UU), the Consolidated Undrained (CU) and Consolidated Drained (CD) tests. Also used, is the Q-Test, which is an unconfined compression test. Each of these tests is used to determine different parameters of soil, including angle of friction, cohesion of soil and pore-water parameter.

Equipment Used

Figure 1. Triaxial Testing Chamber

Other Equipment : -

Specimen Trimmer / Wire Saw Balance Porcelain Evaporating Dish Oven Spoon

-

Rubber Membrane Vacuum Source Water

Test Procedure Q-Test 1. Obtain soil specimen for the test, trim to the proper size for the machine, measure diameter, D, and length, L of the specimen used 2. Place specimen into the apparatus, between the loading plates, adjusting the plates so that they hold the specimen in place 3. Turn machine on, and record loads until the specimen fails 4. Unload the specimen, and then remove specimen from the apparatus

UU Test 1. Place the triaxial cell with specimen inside on the platform of the compression machine 2. Make proper adjustments so that the piston of the triaxial cell rests on the top plate on top of the specimen 3. Fill chamber of triaxial cell with water. Apply hydrostatic pressure, σ3 to specimen through the chamber fluid and wait 10 minutes for specimen to stabilize 4. Check for proper contact between the piston and top plate on the specimen, and begin the test 5. Take load cell readings continuously over course of the test 6. Unload specimen, lower triaxial cell and turn off the machine. Drain the cell, remove specimen and determine moisture content.

CU Test 1. Place Triaxial cell with the saturated specimen on the compression machine platform and make adjustments so that the piston of the cell makes contact with the top platen of the specimen 2. Fill the chamber of the triaxial cell with water, and apply the hydrostatic pressure σ3 to the specimen through the fluid 3. Connect drainage lines from the specimen to calibrated burette and leave the lines open. When water level in burette becomes constant, it will indicate consolidation is complete. Record volume of water in burette 4. Connect drainage lines to the pore-pressure measuring device 5. Check contact between pistons and the top platen 6. Start the compression on the specimen, take readings with the load cell 7. At completion of the test reverse the compression machine, and lower the triaxial cell. Shut off the machine. Release the chamber pressure, and drain the water out of the cell. Determine moisture content of the specimen

Results Q-Test Data Tables Table 1. Stress on Specimen

Specimen Deformation, ΔL (in.) 0.00 0.01 0.02 0.03 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.24 0.28 0.32 0.36

Vertical Strain, ε 0 0.00333 3 0.00666 7 0.01 0.01333 3 0.02 0.02666 7 0.03333 3 0.04 0.04666 7 0.05333 3 0.06 0.06666 7 0.08 0.09333 3 0.10666 7 0.12

Load P (lb)

Correcte d Area, A (in2)

Stress, σ (lb/in2)

0.000

1.561

0.000

3.406

1.567

2.174

10.742

1.572

6.834

14.148

1.577

8.970

15.458

1.583

9.768

18.078

1.593

11.346

19.912

1.604

12.412

20.960 21.484

1.615 1.627

12.976 13.209

22.008

1.638

13.437

22.270 22.008

1.649 1.661

13.502 13.249

21.484 21.222

1.673 1.697

12.842 12.504

20.960

1.722

12.171

20.698 20.174

1.748 1.774

11.842 11.370

Table 2. Q-Test Data

Item 1. Moisture Content, w(%)

Quantity 14

2. Initial average length of specimen, Lo (in)

3

3. Initial average diameter, Do (in)

1.41

2

4. Initial area, Ao (in ):

1.56145 2

5. Unconfined compression strength, qu (lb/in )

13.5

2

6.75

6. Undrained cohesion/shear strength, cu (lb/in )

Q-Test Figures

16.000 14.000

Stress, σ (lb/in2)

12.000 10.000 8.000 6.000 4.000 2.000 0.000

0

0.02

0.04

0.06

0.08

Axial Stain, ε

Figure 1. Q-Test Stress vs. Strain Curve

0.1

0.12

Figure 2. Mohr-Coulomb Diagram for Q-Test

Sample Calculations Q-Test Strain on Specimen, ε 3∈¿=0.0033 0.1∈ ¿¿ ε=

∆L =¿ L

Corrected Area of Specimen, Ao Ac=

A o 1.56145 ¿2 =1.567 ¿2 = 1−ε 1−0.0033

Stress on Specimen, σ σ=

P 3.046 lb =2.174 lb = 2 A c 1.567 ¿2 ¿

CU Test Data Tables Table 3. Stress on Specimen for CU Test #1

Specimen Deformation (cm) 0.000 0.015 0.038 0.061 0.076 0.114 0.520 0.183 0.229 0.274 0.315 0.427 0.457 0.503 0.490 0.594 0.653 0.726 0.853

Strain, ε

Load P (N)

0.000 0.002 0.005 0.008 0.010 0.015 0.069 0.024 0.031 0.037 0.042 0.057 0.061 0.067 0.065 0.079 0.087 0.097 0.114

Corrected Area, Ao (cm2)

0.00 24.75 177.51 222.99 235.83 277.95 309.45 332.16 364.65 379.67 402.95 425.76 435.63 447.35 459.33 463.70 467.24 457.29 440.99

Deviatory Stress, Δσ (N/cm2)

8.750 8.768 8.795 8.822 8.840 8.885 9.402 8.969 9.026 9.082 9.134 9.279 9.318 9.380 9.362 9.503 9.585 9.689 9.874

Excess Pore Water Pressure, Δu (kN/m2)

Deviatory Stress, Δσ (kN/m2)

0.000 2.823 20.184 25.277 26.679 31.283 32.912 37.034 40.401 41.804 44.115 45.886 46.750 47.694 49.063 48.793 48.745 47.199 44.662

0 28.229 201.841 252.772 266.788 312.826 329.118 370.343 404.009 418.039 441.153 458.859 467.503 476.937 490.625 487.933 487.454 471.987 446.617

0.00 2.91 48.93 75.62 92.38 113.42 136.89 151.01 162.33 168.42 174.30 175.96 175.96 175.96 175.96 175.96 175.62 159.55 162.18

Table 4. Test Data CU Test #1

Beginning of Test Moist Unit weight @ beginning (kN/m3): Moisture content @ beginning (%):

18.1 32.12

Initial Length (cm), Lo

7.54

Initial Diameter (cm), Do

3.55

2

9.89798035

Specimen area (cm ), Ao 3

74.6307719

Specimen Volume (cm ), Vo After Consolidation of Saturated Specimen Cell Consolidation pressure, σ3

200 3

Net drainage during consolidation (cm ), ΔV

12.6

Volume of specimen after consolidation, Vc

62.0307719

Length of the specimen after consolidation, Lc

7.49366585

Area of specimen after consolidation, Ac

8.74996042

Table 5. Stress on Specimen for CU Test #2

Pore-Water pressure parameter, Ā 0 0.1031 0.2424 0.2992 0.3463 0.3626 0.4159 0.4078 0.4018 0.4029 0.3951 0.3835 0.3764 0.3689 0.3586 0.3606 0.3603 0.3380 0.3631

Specimen Deformation (cm)

Strain, ε

Load P (N)

Corrected Area, Ao (cm2)

Deviatory Stress, Δσ (N/cm2)

Deviatory Stress, Δσ (kN/m2)

Excess Pore Water Pressure, Δu (kN/m2)

Pore-Water pressure parameter, Ā

0.000 0.015 0.038 0.061 0.076 0.114 0.520 0.183 0.229 0.274 0.315 0.427 0.457 0.503 0.490 0.594 0.653 0.726 0.853

0.000 0.002 0.005 0.008 0.010 0.015 0.070 0.024 0.031 0.037 0.042 0.057 0.061 0.067 0.065 0.079 0.087 0.097 0.114

0.00 5.54 39.76 49.95 52.83 62.26 69.32 74.40 81.68 85.04 90.26 95.37 97.58 100.21 102.89 103.87 104.66 102.43 98.78

9.277 9.296 9.325 9.353 9.372 9.421 9.970 9.510 9.570 9.630 9.685 9.839 9.881 9.946 9.927 10.077 10.164 10.274 10.471

0.000 0.596 4.264 5.340 5.636 6.609 6.952 7.824 8.535 8.831 9.320 9.693 9.876 10.075 10.364 10.307 10.297 9.970 9.434

0 5.964 42.642 53.402 56.363 66.089 69.524 78.239 85.351 88.314 93.196 96.933 98.759 100.750 103.642 103.071 102.968 99.699 94.337

0 2.037 34.251 52.934 64.666 79.394 95.823 105.707 113.631 117.894 122.01 123.172 123.172 123.172 123.172 123.172 122.934 111.685 113.526

Table 6. Test Data CU Test #2

Beginning of Test Moist Unit weight @ beginning (kN/m3): Moisture content @ beginning (%):

18.1 32.12

Initial Length (cm), Lo

7.51

Initial Diameter (cm), Do

3.57

2

Specimen area (cm ), Ao

10.00982

Specimen Volume (cm3), Vo

75.17376

After Consolidation of Saturated Specimen Cell Consolidation pressure, σ3 (lb/in2)

50

Net drainage during consolidation (cm3), ΔV

8.1

Volume of specimen after consolidation, Vc (cm3)

67.07376

Length of the specimen after consolidation, Lc (cm)

7.481514

2

Area of specimen after consolidation, Ac (cm )

9.277207

CU Test Sample Calculations Initial Volume, Vo 2

Area ∗ Length=10.01 cm ∗7.51 cm =71.17 cm

3

0 0.3416 0.8032 0.9912 1.1473 1.2013 1.3783 1.3511 1.3313 1.3349 1.3092 1.2707 1.2472 1.2225 1.1884 1.1950 1.1939 1.1202 1.2034

Volume after consolidation, Vc V o−∆ V =75.17 cm3−8.1 cm3=67.07 cm3 Length after consolidation, Lc

( )

(

)

( )

(

)

1

Lc = Lo

1 Vc 3 67.07 cm3 3 =7.48 cm =7.51 cm Vo 75.17 cm3

Area after consolidation, Ac 2

A c= A o

2 Vc 3 67.07 cm3 3 2 =9.28 cm =10.01 cm 3 Vo 75.17 cm

Pore-water parameter, Ā kN m2 ∆u = Ā= =0.1031 ∆σ kN 28.23 2 m 2.91

Vertical Strain, ε ε=

CU Test Figures

∆ L 0.015 = =0.002 7.48 Lc

60.000 50.000

Δσ (N/cm2)

40.000 30.000 20.000 10.000 0.000 0.000

0.020

0.040

0.060

0.080

0.100

0.120

Axial Strain, ε

Figure 3. Stress vs Strain CU Test #1

12.000 10.000

Δσ (lb/in2)

8.000 6.000 4.000 2.000 0.000 0.000

0.020

0.040

0.060

0.080

Axial Strain, ε

Figure 4. Stress vs Strain CU Test #2

0.100

0.120

Φ= 45°, c’ = 0

Figure 5. Mohr-Coulomb Diagram CU Tests #1 & #2

Discussion According to the lab manual, Figures 1, represent very soft soil due to the qu of 13.5lb/in2. The pore-water parameters in Tables 3 and 5 represent clay with high sensitivity and normally consolidated clay, respectively. This observation does not make sense because a single clay specimen cannot should, in theory, not act as two different types of clay just because a different load was applied to it. Also, the value for Φ does not make sense, as clays have a cohesion factor larger than 0.

Sources of Error Determining the true values for this test are difficult given the apparent error in the data gathering. Looking at Figures 3 and 4 it can be seen that there are a few data points that throw off the graph dramatically. This could have potentially led to a different stress value at failure, leading to different numbers being used in calculations, resulting in different answers. Also, we were given this data without doing the procedure, so it difficult to say exactly where this error may have occurred or for what reason....