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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-07-052
Date: September 2007
The procedure described in this section is used for undisturbed or laboratory compacted specimens of subgrade soils. This can include specimens classified as Type 1 (152-mm [6-inch] diameter specimens) or Type 2 (71-mm [2.8-inch] diameter specimens) material.
8.1.1 Assembly of Triaxial Chamber Specimens trimmed from undisturbed samples and laboratory compacted specimens are placed in the triaxial chamber and loading apparatus in the following steps.
126.96.36.199 Place a dry porous stone on the top of the sample base of the triaxial chamber as shown in Figure 4. Paper filters should be placed between the porous stone and the sample.
188.8.131.52 Carefully place the specimen on the porous stone. Place the membrane on a membrane expander, apply vacuum to the membrane expander, then carefully place the membrane on the sample and remove the vacuum and the membrane expander. Seal the membrane to the pedestal (or bottom plate) with an O ring or other pressure seals.
184.108.40.206 Place the dry porous stone and the top platen on the specimen, fold up the membrane, and seal it to the top platen with an O ring or some other pressure seal. Paper filters should be placed between the porous stone and the sample.
After the "specimen assembly" is in-place, the top platen shall be checked to ensure that it is level. A "cross-check" level, or similar, may be used for this determination.
220.127.116.11 If the specimen has been compacted or stored inside a rubber membrane and the porous stones and sample are already attached to the rubber membrane in place, steps 18.104.22.168, 22.214.171.124, and 126.96.36.199 are omitted. Instead, the "specimen assembly" is placed on the base plate of the triaxial chamber.
188.8.131.52 Connect the specimen's bottom drainage line to the vacuum source through the medium of a bubble chamber. Apply a vacuum of 7 kPa (1 psi). If bubbles are present, check for leakage caused by poor connections, holes in the membrane, or imperfect seals at the cap and base. The existence of an airtight seal ensures that the membrane will remain firmly in contact with the specimen. Leakage through holes in the membrane can frequently be eliminated by coating the surface of the membrane with liquid rubber latex or by using a second membrane.
184.108.40.206 When leakage has been eliminated, disconnect the vacuum supply and place the chamber on the base plate, and the cover plate on the chamber. Insert the loading piston and obtain a firm connection with the load cell. Tighten the chamber tie rods firmly. The cover plate of the triaxial chamber shall be checked to ensure that it is level after tightening the tie rods. A "cross-check" level, or similar, may be used for this determination.
220.127.116.11 Slide the assembly apparatus into position under the axial loading device. Positioning of the chamber is extremely critical in eliminating all possible side forces on the piston rod. Couple the loading device to the triaxial chamber piston rod.
Bolt or firmly fasten the triaxial chamber to the bottom loading platen of the test device. For Type 1 samples, a minimum of 4 bolts or fasteners should be used, for Type 2 samples a minimum of 3 bolts should be used. After fastening the triaxial chamber to the bottom platen, the top of the chamber shall be checked to ensure that it is level.
8.1.2 Conduct the Resilient Modulus Test The following steps are required to conduct the resilient modulus test on a subgrade specimen which has been installed in the triaxial chamber and placed under the loading frame.
18.104.22.168 Open all drainage valves leading into the specimen to atmospheric pressure.
22.214.171.124 If it is not already connected, connect the air pressure supply line to the triaxial chamber and apply the specified pre-conditioning confining pressure of 41.4 kPa (6 psi) to the test specimen. A contact stress of 10 percent ± 0.7 kPa (± 0.1 psi) of the maximum applied axial stress during each sequence number shall be maintained.
126.96.36.199 Conditioning Begin the test by applying a minimum of 500 repetitions of a load equivalent to a maximum axial stress of 27.6 kPa (4 psi) and corresponding cyclic stress of 24.8 kPa (3.6 psi) using a haversine shaped load pulse consisting of a 0.1 second load followed by a 0.9 second rest period. If the sample is still decreasing in height at the end of the conditioning period, stress cycling shall be continued up to 1,000 repetitions prior to testing.The foregoing stress sequence constitutes sample conditioning, that is, the elimination of the effects of the interval between compaction and loading and the elimination of initial loading versus reloading. This conditioning also aids in minimizing the effects of initially imperfect contact between the sample cap and the test specimen.
If the total vertical permanent strain reaches 5 percent during conditioning, the conditioning process shall be terminated. For recompacted samples, a review shall be conducted of the compaction process to identify any reason(s) why the sample did not attain adequate compaction. If this review does not provide an explanation, the material shall be re-fabricated and tested a second time. If the sample again reaches 5 percent total vertical permanent strain during preconditioning, then the test shall be terminated and the appropriate item on the data sheet shall be completed. No further testing of this material is necessary.
If the sample is a thin-wall tube, sample handling procedures shall be reviewed to determine if the sample was damaged. If this review does not provide an explanation, another thin-wall tube sample shall be used for the testing. If the sample from the second thin-wall tube also reaches 5 percent total vertical permanent strain during preconditioning, then the test shall be terminated and the appropriate item on the data sheet shall be completed. No further testing of this material is necessary.
NOTE 13: The operator/technician shall conduct appropriate QC/QA comparative checks of the individual deformation output from the two vertical transducers during the conditioning phase of each Mr test in order to recognize specimen misplacement and misalignment. During the preconditioning phase, the two vertical deformation curves should be viewed to ensure that acceptable vertical deformation ratios are being measured. Desired vertical deformation ratios (Rv) are defined as Rv = Ymax/Ymin < 1.10, where Ymax equals the larger of the two vertical deformations and Ymin equals the smaller of the two vertical deformations. Unacceptable vertical deformations are obtained when Rv > 1.30. In this case, the test should be discontinued and specimen placement/alignment difficulties alleviated. Once acceptable vertical deformation values are obtained, (Rv < 1.30) then the test should be continued to completion. It is emphasized that specimen alignment is critical for proper Mr results.
188.8.131.52 Testing Specimen The testing is performed following the loading sequence shown in Table 1. Begin by decreasing the maximum axial stress to 13.8 kPa (2 psi) (Sequence No. 1, Table 1) and set the confining pressure to 41.4 kPa (6 psi).
NOTE 14: The contact stresses shown in Table 1 should be adjusted to compensate for the resultant force created by the chamber pressure (upward force) and the weight of the chamber piston rod, including the LVDT holder, (downward force). Instructions for adjusting the contact load are given in Appendix D of this procedure.
184.108.40.206 Apply 100 repetitions of the corresponding cyclic axial stress using a haversine shaped load pulse consisting of a 0.1 second load followed by a 0.9 second rest period. Record the average recovered deformations for each LVDT separately for the last five cycles on Worksheet T46.
220.127.116.11 Increase the maximum axial stress to 27.6 kPa (4 psi) (Sequence No. 3) and repeat step 18.104.22.168 at this new stress level.
22.214.171.124 Increase the cyclic stress to 6 psi (Sequence No. 3) and repeat step 126.96.36.199 at this new stress level.
188.8.131.52 Continue the test for the remaining load sequences in Table 1 (4 to 15) recording the vertical recovered deformation. If at any time the total vertical permanent strain (after preconditioning) exceeds 5 percent, stop the test and report the results on Worksheet T46.
184.108.40.206 After completion of the resilient modulus test procedure, check the total vertical permanent strain that the specimen was subjected to during the resilient modulus (after preconditioning) portion of the test procedure. If the total vertical permanent strain did not exceed 5 percent, continue with the quick shear test procedure (Section 220.127.116.11). If the total vertical permanent strain exceeds 5 percent, the test is completed. No additional testing is to be conducted on the specimen.
18.104.22.168 Apply a confining pressure of 27.6 kPa (4 psi) to the specimen. Apply a load so as to produce an axial strain at a rate of 1 percent per minute under a strain controlled loading procedure. Continue loading until either (1) the load values decrease with increasing strain, (2) 5 percent strain is reached (from the initiation of the quick shear test) or (3) the capacity of the load cell is reached. Data from the internally mounted deformation transducer in the actuator shaft and from the load cell shall be used to record specimen deformation and loads at a maximum of 3 second intervals.
NOTE 15: It has been noted that even though some samples visually bulge and appear to have failed, they do not achieve the above definition of failure at the maximum strain value (5 percent). In some cases, the stress-strain curves "level out" and the load values remain at, or near, constant and do not decrease with increasing strain. If a sample appears to fail without achieving the aforementioned criteria, a comment note should be added to the test data reporting sheet to document this occurrence.
22.214.171.124 At the completion of the triaxial shear test, reduce the confining pressure to zero and remove the sample from the triaxial chamber.
126.96.36.199 Remove the membrane from the specimen and use the entire specimen to determine moisture content in accordance with LTPP Protocol P49. Record this value on Form T46.
188.8.131.52 Plot the stress-strain curve for the specimen for the triaxial shear test procedure.