1

CE 341: Soils

Triaxial Test Lab

Dr. Gerber

David Gelder12/11/2010

ContentsIntroduction.................................................................................................................................................2

Test Procedure............................................................................................................................................2

Discussion of Results...................................................................................................................................3

Conclusion...................................................................................................................................................5

APPENDIX....................................................................................................................................................6

2

IntroductionThe purpose of this lab is to determine the cohesion intercept, the friction angle, total stress, and effective stress of a soil using an isotropically consolidated, undrained triaxial test with pore water pressure.

Test ProcedureThe Unconfined Compression Lab was performed by the entire lab section: David Gelder, Michael Murray, Ashley Frost, Jen Tovar, Seishi Yamagata, Reed Crosby, Keelan Jensen and Ben Ford in CB 265—the Soils Lab—on Dec. 8, 2010. Tests were performed in general accordance with ASTM D 2850 – Standard Test Method for Unconsolidated Undrained Triaxial Compression Test on Cohesive Soils, ASTM D 4767 – Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils, ASTM WK3821 – New Test Method for Consolidated Drained Triaxial Compression Test for Soils (This standard is still in development), and modified for student laboratory experience.

Discussion of ResultsDrainage conditions during shearing will affect the strength parameters of soil significantly. If the sample is drained and slow shearing takes place, pore pressures will not develop and the test is called a “drained test.” However, if the sample is not allowed to drain and/or shearing occurs quickly, pore pressure is developed in the specimen and the test is called an “undrained test” (see p. 95 of the Lab Manual). In soil mechanics, effective stress decreases as pore pressure increases. In the CU-bar triaxial test discussed in this report, draining did not occur during shearing, and therefore pore pressures increased and the effective stress decreased relative to the total stress. In other words, the strength parameter of the samples decreased. The fact that the drainage valve was closed during loading makes this an undrained soil experiment as opposed to drained.

If A-bar had been negative, it would have been because the sample was heavily over-consolidated. The result as far as Mohr’s circle is concern is that the angle of the failure envelope would have been zero because no strength gain would have occurred.

Total Stress takes into account the stress based on the load which is being applied to the specimen. Effective stress takes into account total stress minus the pore pressure. The pore pressure in all three samples is increasing as the load increases, but at a slower rate. Therefore the deviator stress value for the effective stress circles is less than that for the total stress circles. Both are important in engineering analysis because it gives us a clear picture of what is taking place. With just the effective strength circles, we would not be able to assess the values or relationships between effective stress, total stress, and pore pressure.

3

0% 5% 10% 15% 20% 25%0

1000

2000

3000

4000

5000

6000

7000

(19.8, 5791)

(20.0, 3356)

(20.0, 2239)

Sample 1Sample 2Sample 3

Vertical Strain, εv

Dev

iato

r St

ress

, Δσd

(psf

)

Figure 1: Deviator Stress vs. Strain

0% 5% 10% 15% 20% 25%0

500

1000

1500

2000

2500

3000

3500

4000

(19.8, 2848)

(20.0, 1489)

(20.0, 572)

Sample 1

Sample 2

Sample 3

Vertical Strain, εv

Pore

Wat

er P

ress

ure,

Δu

(psf

)

Figure 2: Pore Pressure vs. Strain

4

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 120000

1000

2000

3000

4000

Normal Stress, σ (psf)

Shea

r Str

ess,

τ (p

sf)

φ' = 30.3°, c' = 140 psf

φcu = 18.9°, cu = 230 psf

Figure 3: Mohr’s Circle for Total and Effective Stress

In Figure 3 the failure envelopes are observed. The failure envelopes were defined as the best fit line between the total stress and effective stress circles, respectively. I decided that the samples had failed when they reached the failure envelope. Another way that I determined that the sample had failed was by observing that the shearing stress began to decrease as the vertical strain increased. That means that structurally the sample was no longer able to sustain an increasing load. It is observed that the more consolidated samples were also stronger (the diameters of the circles increased). That means that with less void space the sample sustained a higher shearing stress before it failed. There is an error in this figure. The smallest circle representing the effective stress is shifted too much to the right. I looked at the data and calculations for a while and couldn’t decide why it was wrong, but ideally it should also be tangent to the failure envelope.

5

There are some important implications. The first is that the Triaxial Test is a very accurate test. It can be used for CD, CU, and UU tests. The Unconfined Shear test is not a very good test and should only be used if the project involves a relatively low load and if the budget is limited. Another implication is that strength increases as the initial confining strength increases. That means that lower in the ground the soil will be stronger because it is more confined laterally and vertically by surrounding soil. That most likely explains why soil under a footing would shear in an upside-down semi-circular shape—because the failure plane is a function of soil depth.

There are some sources of error. The readings of the initial confining pressure are read by eyesight and therefore are only accurate to within 1 psi. The temperature in the room is different from the temperature of the in situ soil. It is important to note that error was constrained by taking three points instead of two. That made the drawing of the best-fit-lines on Figure 3 easier. Still, this was a human process and the variation of c will be about 50 psf based on the limited accuracy of the graph.

ConclusionThe friction angle and the cohesion intercept were determined and are located on Figure 3. This was a Consolidated-Undrained (CU) Triaxial Test. It is a somewhat expensive test (~$150/sample), but it is also a very reliable test. Both total and effective stress strength parameters were determined from the test. The total and effective strength grew as the initial confining pressure increased. The failure envelope was defined as the best-fit-line tangent to all three samples (Figure 3).

6

APPENDIX

7

Table 1: Sample 1 DataSample 1

v Ac Ac d u   3 1 ' 1 ' 3 p p' q

(%) (in2) (ft2) (psf) (psf) A-bar (psf) (psf) (psf) (psf) (psf) (psf) (psf)0 6.069871 0.042152 0 0 -- 1440 1440 1440 1440 1440 1440 0

0.000495 6.072877 0.042173 151.5196 54.72 0.361141 1440 1591.52 1536.8 1385.28 1515.76 1461.04 75.759810.001007 6.075989 0.042194 271.3632 106.56 0.392684 1441 1712.363 1605.803 1334.44 1576.682 1470.122 135.68160.001502 6.079 0.042215 416.2 178.56 0.429024 1442 1858.2 1679.64 1263.44 1650.1 1471.54 208.10.001997 6.082014 0.042236 515.908 241.92 0.468921 1443 1958.908 1716.988 1201.08 1700.954 1459.034 257.9540.002509 6.085136 0.042258 595.3918 295.2 0.495808 1444 2039.392 1744.192 1148.8 1741.696 1446.496 297.69590.003003 6.088156 0.042279 658.2485 345.6 0.52503 1445 2103.249 1757.649 1099.4 1774.124 1428.524 329.12430.003498 6.09118 0.0423 707.0945 387.36 0.547819 1446 2153.095 1765.735 1058.64 1799.547 1412.187 353.54730.003993 6.094206 0.042321 758.2546 426.24 0.562133 1447 2205.255 1779.015 1020.76 1826.127 1399.887 379.12730.004505 6.09734 0.042343 795.6518 459.36 0.577338 1448 2243.652 1784.292 988.64 1845.826 1386.466 397.8259

0.005 6.100373 0.042364 832.7884 491.04 0.589634 1449 2281.788 1790.748 957.96 1865.394 1374.354 416.39420.005495 6.103409 0.042385 867.0565 519.84 0.599546 1450 2317.056 1797.216 930.16 1883.528 1363.688 433.52820.006007 6.106552 0.042407 897.9732 545.76 0.607769 1451 2348.973 1803.213 905.24 1899.987 1354.227 448.98660.006502 6.109594 0.042428 925.8095 568.8 0.614381 1452 2377.809 1809.009 883.2 1914.905 1346.105 462.90470.006997 6.112639 0.042449 953.6176 590.4 0.619116 1453 2406.618 1816.218 862.6 1929.809 1339.409 476.80880.007509 6.115792 0.042471 975.0234 610.56 0.6262 1454 2429.023 1818.463 843.44 1941.512 1330.952 487.51170.008003 6.118843 0.042492 1001.601 629.28 0.628274 1455 2456.601 1827.321 825.72 1955.801 1326.521 500.80060.008498 6.121897 0.042513 1021.095 646.56 0.633202 1456 2477.095 1830.535 809.44 1966.548 1319.988 510.54760.008993 6.124954 0.042534 1042.45 662.4 0.635426 1457 2499.45 1837.05 794.6 1978.225 1315.825 521.22510.009505 6.12812 0.042556 1060.71 678.24 0.639421 1458 2518.71 1840.47 779.76 1988.355 1310.115 530.3552

Table 2: Sample 2 DataSample 2

v Ac Ac d u   3 1 ' 1 ' 3 p p' q

(%) (in2) (ft2) (psf) (psf) A-bar (psf) (psf) (psf) (psf) (psf) (psf) (psf)

0 6.157522 0.042761 0 0 --288

0 2880 2880 2880 2880 2880 0

0.000498 6.160591 0.042782 268.5716 105.12 0.391404288

0 3148.572 3043.452 2774.88 3014.286 2909.166 134.2858

0.000997 6.163664 0.042803 547.1551 228.96 0.418455288

0 3427.155 3198.195 2651.04 3153.578 2924.618 273.5775

0.001495 6.16674 0.042825 733.6907 336.96 0.459267288

0 3613.691 3276.731 2543.04 3246.845 2909.885 366.8454

0.001993 6.169819 0.042846 874.7615 437.76 0.500434288

0 3754.762 3317.002 2442.24 3317.381 2879.621 437.38080.002509 6.173007 0.042868 1002.144 531.36 0.530223 288 3882.144 3350.784 2348.64 3381.072 2849.712 501.0718

8

0

0.003007 6.176092 0.04289 1106.097 614.88 0.5559288

0 3986.097 3371.217 2265.12 3433.049 2818.169 553.0487

0.003505 6.179181 0.042911 1188.507 689.76 0.580358288

0 4068.507 3378.747 2190.24 3474.254 2784.494 594.2536

0.004003 6.182272 0.042932 1268.505 761.76 0.600518288

0 4148.505 3386.745 2118.24 3514.252 2752.492 634.2523

0.004502 6.185366 0.042954 1335.617 826.56 0.61886288

0 4215.617 3389.057 2053.44 3547.808 2721.248 667.8085Sample 2 (cont’d)

v Ac Ac d u   3 1 ' 1 ' 3 p p' q(%) (in2) (ft2) (psf) (psf) A-bar (psf) (psf) (psf) (psf) (psf) (psf) (psf)

0.005498 6.191565 0.042997 1463.359 947.52 0.647497288

0 4343.359 3395.839 1932.48 3611.679 2664.159 731.6794

0.005997 6.194668 0.043019 1516.323 997.92 0.658118288

0 4396.323 3398.403 1882.08 3638.162 2640.242 758.1617

0.006495 6.197775 0.04304 1565.517 1044 0.666873288

0 4445.517 3401.517 1836 3662.758 2618.758 782.7583

0.006993 6.200885 0.043062 1612.3381088.6

4 0.675194288

0 4492.338 3403.698 1791.36 3686.169 2597.529 806.1688

0.007509 6.204106 0.043084 1655.368 1130.4 0.682869288

0 4535.368 3404.968 1749.6 3707.684 2577.284 827.6842

0.008007 6.207222 0.043106 1695.3671169.2

8 0.689691288

0 4575.367 3406.087 1710.72 3727.684 2558.404 847.6835

0.008505 6.210342 0.043127 1731.3831205.2

8 0.696137288

0 4611.383 3406.103 1674.72 3745.692 2540.412 865.6915

0.009003 6.213464 0.043149 1767.3621236.9

6 0.699891288

0 4647.362 3410.402 1643.04 3763.681 2526.721 883.681

0.009502 6.21659 0.043171 1796.3551270.0

8 0.707032288

0 4676.355 3406.275 1609.92 3778.177 2508.097 898.1773

Table 3: Sample 3 DataSample 3

v Ac Ac d u   3 1 ' 1 ' 3 p p' q

(%) (in2) (ft2) (psf) (psf) A-bar (psf) (psf) (psf) (psf) (psf) (psf) (psf)0 6.201582 0.043067 0 0 -- 5425.92 5425.92 5425.92 5425.92 5425.92 5425.92 0

0.0005 6.204685 0.043088 523.1144 138.24 0.264263 5425.92 5949.034 5810.794 5287.68 5687.477 5549.237 261.5572

0.001 6.20779 0.04311 996.9925 313.92 0.314867 5425.92 6422.912 6108.992 5112 5924.416 5610.496 498.49620.001

5 6.210899 0.043131 1354.934 491.04 0.362409 5425.92 6780.854 6289.814 4934.88 6103.387 5612.347 677.46720.002 6.21401 0.043153 1632.105 662.4 0.405856 5425.92 7058.025 6395.625 4763.52 6241.973 5579.573 816.05270.002 6.217125 0.043174 1864.991 829.44 0.444742 5425.92 7290.911 6461.471 4596.48 6358.415 5528.975 932.4953

9

50.003 6.220243 0.043196 2056.897 984.96 0.478857 5425.92 7482.817 6497.857 4440.96 6454.369 5469.409 1028.4490.003

5 6.223364 0.043218 2225.009 1133.28 0.509337 5425.92 7650.929 6517.649 4292.64 6538.424 5405.144 1112.5040.004 6.226488 0.04324 2362.886 1258.56 0.532637 5425.92 7788.806 6530.246 4167.36 6607.363 5348.803 1181.4430.004

5 6.229616 0.043261 2496 1383.84 0.554423 5425.92 7921.92 6538.08 4042.08 6673.92 5290.08 12480.005 6.232746 0.043283 2609.11 1497.6 0.573989 5425.92 8035.03 6537.43 3928.32 6730.475 5232.875 1304.5550.005

5 6.23588 0.043305 2717.948 1608.48 0.591799 5425.92 8143.868 6535.388 3817.44 6784.894 5176.414 1358.9740.006 6.239017 0.043327 2814.905 1709.28 0.607225 5425.92 8240.825 6531.545 3716.64 6833.373 5124.093 1407.4530.006

5 6.242156 0.043348 2911.071 1804.32 0.619813 5425.92 8336.991 6532.671 3621.6 6881.455 5077.135 1455.5350.007 6.2453 0.04337 2990.998 1893.6 0.6331 5425.92 8416.918 6523.318 3532.32 6921.419 5027.819 1495.4990.007

5 6.248446 0.043392 3066.926 1977.12 0.644659 5425.92 8492.846 6515.726 3448.8 6959.383 4982.263 1533.4630.008 6.251595 0.043414 3142.084 2054.88 0.653986 5425.92 8568.004 6513.124 3371.04 6996.962 4942.082 1571.0420.008

5 6.254748 0.043436 3207.957 2125.44 0.662553 5425.92 8633.877 6508.437 3300.48 7029.898 4904.458 1603.9780.009 6.257904 0.043458 3265.017 2194.56 0.672144 5425.92 8690.937 6496.377 3231.36 7058.428 4863.868 1632.5080.009

5 6.261063 0.04348 3327.077 2257.92 0.67865 5425.92 8752.997 6495.077 3168 7089.459 4831.539 1663.539

10

1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,0000

500

1000

1500

2000

2500

3000

3500

Stress Path, p' (psf)

Stre

ss P

ath,

q (p

sf)

Figure 4: Stress Paths

11