1、Standard Method of Test for Triaxial Compressive Strength of Undrained Rock Core Specimens without Pore Pressure Measurements AASHTO Designation: T 226-90 (2013) ASTM Designation: D2664-86 American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Was
2、hington, D.C. 20001 TS-1a T 226-1 AASHTO Standard Method of Test for Triaxial Compressive Strength of Undrained Rock Core Specimens without Pore Pressure Measurements AASHTO Designation: T 226-90 (2013) ASTM Designation: D2664-86 1. SCOPE 1.1. This test method covers the determination of the strengt
3、h of cylindrical rock specimens in an undrained state under triaxial compression loading. The test provides data useful in determining the strength and elastic properties of rock, namely: shear strengths at various lateral pressures, angle of internal friction (angle of shearing resistance), cohesio
4、n intercept, and Youngs modulus. It should be observed that this method makes no provision for pore pressure measurements. Thus the strength values determined are in terms of total stress that is not corrected for pore pressures. 1.2. The values stated in inch-pound units are to be regarded as the s
5、tandard. 1.3. This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety concerns associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determi
6、ne the applicability of regulatory limitations prior to use. 2. REFERENCED DOCUMENTS 2.1. ASTM Standards: D4543, Standard Practices for Preparing Rock Core as Cylindrical Test Specimens and Verifying Conformance to Dimensional and Shape Tolerances E4, Standard Practices for Force Verification of Tes
7、ting Machines E122, Standard Practice for Calculating Sample Size to Estimate, with Specified Precision, the Average for a Characteristic of a Lot or Process E691, Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method 3. SIGNIFICANCE AND USE 3.1. Rock
8、is known to behave as a function of the confining pressure. The triaxial compression test is commonly used to stimulate the stress conditions under which most underground rock masses exist. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplicatio
9、n is a violation of applicable law.TS-1a T 226-2 AASHTO 4. APPARATUS 4.1. Loading DeviceA suitable device for applying axial load to the specimen. It shall be of sufficient capacity to apply load at a rate conforming to the requirements set forth in Section 7.2. It shall be verified at suitable time
10、 intervals in accordance with the procedures given in ASTM E4, Verification of Testing Machines, and comply with the requirements prescribed therein. 4.2. Pressure-Maintaining DeviceA hydraulic pump, pressure intensifier,1or other system of sufficient capacity to maintain constant the desired latera
11、l pressure, 3. 4.3. Triaxial Compression Chamber2An apparatus in which the test specimen may be enclosed in an impermeable flexible membrane; placed between two hardened platens, one of which shall be spherically seated; subjected to a constant lateral fluid pressure; and then loaded axially to fail
12、ure. The platens shall be made of tool steel hardened to a minimum of Rockwell 58 HRC, the bearing faces of which shall not depart from plane surfaces by more than 0.0005 in. (0.013 mm) when the platens are new and which shall be maintained within a permissible variation of 0.001 in. (0.025 mm). In
13、addition to the platens and membrane, the apparatus shall consist of a high-pressure cylinder with overflow valve, a base, suitable entry ports for filling the cylinder with hydraulic fluid and applying the lateral pressure, and hoses, gauges, and valves as needed. 4.4. Deformation Measuring DeviceH
14、igh-grade dial micrometers or other measuring devices graduated to read to 0.0001 in. (0.0025 mm); and accurate within 0.0001 in. (0.0025 mm); in any 0.0010 in. (0.025 mm) range, and within 0.0002 (0.005 mm) in any 0.0100 in. (0.25 mm) range shall be provided for measuring axial deformation due to l
15、oading. These may consist of micrometer screws, dial micrometers, or linear variable differential transformers securely attached to the high-pressure cylinder. 4.4.1. Electrical resistance strain gauges applied directly to the rock specimen in the axial direction may also be used. In addition, the u
16、se of circumferentially applied strain gauges will permit the observation of data necessary in the calculation of Poissons ratio. In this case two axial (vertical) gauges should be mounted on opposite sides of the specimen at mid-height and two circumferential (horizontal) gauges similarly located a
17、round the circumference, but in the direction perpendicular to the axial gauges. 4.5. Flexible Membrane3A flexible membrane of suitable material to exclude the confining fluid from the specimen, and that shall not significantly extrude into abrupt surface pores. It should be sufficiently long to ext
18、end well onto the platens and when slightly stretched be of the same diameter as the rock specimen. 5. SAMPLING 5.1. The specimen shall be selected from the cores to represent a true average of the type of rock under consideration. This can be achieved by visual observations of mineral constituents,
19、 grain sizes and shape, and partings and defects such as pores and fissures. 6. TEST SPECIMENS 6.1. PreparationThe test specimens shall be prepared in accordance with ASTM D4543. 6.2. Moisture condition of the specimen at time of test can have a significant effect upon the indicated strength of the
20、rock. Good practice generally dictates that laboratory tests be made upon specimens representative of field conditions. Thus, it follows that the field moisture condition of the specimen should be preserved until time of test. On the other hand, there may be reasons for 2015 by the American Associat
21、ion of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 226-3 AASHTO testing specimens at other moisture contents including zero. In any case, the moisture content of the test specimens should be tailored to the problem at hand and
22、reported in accordance with Section 9.1.6. 7. PROCEDURE 7.1. Place the lower platen on the base. Wipe clean the bearing faces of the upper and lower platens and of the test specimen, and place the test specimen on the lower platen. Place the upper platen on the specimen and align properly. Fit the f
23、lexible membrane over the specimen and platens and install rubber or neoprene O-rings to seal the specimen from the confining fluid. Place the cylinder over the specimen, ensuring proper seal with the base, and connect hydraulic pressure lines. Position the deformation measuring device and fill the
24、chamber with hydraulic fluid. Apply a slight axial load, approximately 25 lbf (111 N), to the triaxial compression chamber by means of the loading device in order to properly seat the bearing parts of the apparatus. Take an initial reading on the deformation device. Slowly raise the lateral fluid pr
25、essure to the predetermined test level and at the same time apply sufficient axial load to prevent the deformation measuring device from deviating from the initial reading. When the predetermined test level of fluid pressure is reached, note and record the axial load registered by the loading device
26、. Consider this load to be the zero or starting load for the test. 7.2. Apply axial load continuously and without shock until the load becomes constant, or reduces, or a predetermined amount of strain is achieved. Apply the load in such a manner as to produce a strain rate as constant as feasible th
27、roughout the test. Do not permit the strain rate at any given time to deviate by more than 10 percent from that selected. The strain rate selected should be that which will produce failure of a similar test specimen in unconfined compression, in a test time of between 2 and 15 min. The selected stra
28、in rate for a given rock type shall be adhered to for all tests in a given series or investigation (Note 1). Maintain constant the predetermined confining pressure throughout the test and observe and record readings of deformation as required. Note 1Results of tests by other investigators have shown
29、 that strain rates within this range will provide strength values that are reasonably free from rapid loading effects and reproducible within acceptable tolerances. Note 2If the specimen diameter is not the same as the piston diameter through the chamber, a correction must be applied to the measured
30、 load to account for differences in area between the specimen and the loading piston where it passes through the seals into the chamber. 7.3. To make sure that no testing fluid has penetrated into the specimen, the specimen membrane shall be carefully checked for fissures or punctures at the complet
31、ion of each triaxial test. If in question, weigh the specimen before and after the test. 8. CALCULATIONS 8.1. The following calculations and graphical plots shall be made: 8.1.1. A stress difference versus axial strain curve shall be constructed (Note 3). Stress difference is defined as the maximum
32、principal axial stress, 1, minus the lateral pressure, 3. The value of the lateral pressure, 3, shall be indicated on the curve. Note 3Because total deformation is recorded during the test, suitable calibration for apparatus deformation must be made. This may be accomplished by inserting into the ap
33、paratus a steel cylinder having known elastic properties and observing differences in deformation between the assembly and steel cylinder throughout the loading range. The apparatus deformation is then subtracted from the total deformation at each increment of load in order to arrive at specimen def
34、ormation, from which the axial strain of the specimen is computed. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 226-4 AASHTO 8.1.2. Mohr stress circles shall be constructed on an arithmetic pl
35、ot with shear stresses as ordinates and normal stresses as abscissas. At least three triaxial compression tests, each at a different confining pressure, shall be made on the same material to define the envelope to the Mohr stress circles. Note 4Because of the heterogeneous nature of rock and the sca
36、tter in results often encountered, it is considered good practice to make at least three tests of essentially identical specimens at each confining pressure or single tests at nine different confining pressures covering the range investigated. Individual stress circles shall be plotted and considere
37、d in drawing the envelope. 8.1.3. A “Best-Fit” smooth curve (the Mohr envelope) shall then be drawn approximately tangent to the Mohr circles as in Figure 1. The figure shall also include a brief note indicating whether a pronounced failure plane was or was not developed during the test and the incl
38、ination of this plane with reference to the plane of major principal stress. Note 5If the envelope is a straight line, the angle the line makes with the horizontal shall be reported as the angle of interval friction (or the slope of the line as tan depending upon preference), and the intercept of th
39、is line at the vertical axis reported as the cohesion intercept, C. If the envelope is not a straight line, values of (or tan ) should be determined by constructing a tangent to the Mohr circle for each confining stress at the point of contact with the envelope, and the corresponding cohesion interc
40、ept noted. Figure 1Typical Mohr Stress Circles 9. REPORT 9.1. In addition to the plots discussed in Section 8, Calculations, the report should include the following: 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of appl
41、icable law.TS-1a T 226-5 AASHTO 9.1.1. Sources of the specimen including project name and location, and if known, storage environment. The location is frequently specified in terms of the borehole number and depth of specimen from collar of hole. 9.1.2. Physical description of the specimen including
42、 rock type; location and orientation of apparent weakness planes, bedding planes, and schistosity; large inclusions of inhomogeneities, if any. 9.1.3. Dates of sampling and testing. 9.1.4. Specimen diameter and length, conformance with dimensional requirements. 9.1.5. Rate of loading or deformation
43、or strain rate. 9.1.6. General indication of moisture condition of the specimen at time of test such as: as-received, saturated, laboratory air-dry, or oven-dry. It is recommended that the moisture condition be more precisely determined when possible and reported as either water content or degree of
44、 saturation. 9.1.7. Type and location of failure. A sketch of the fractured specimen is recommended. Note 6If it is a ductile failure and 1 3is still increasing when the test is terminated, the maximum strain at which 1 3is obtained shall be clearly stated. 10. PRECISION AND BIAS 10.1. An interlabor
45、atory study was conducted in which six laboratories each tested five specimens of three different rocks, three confining pressures, and four replications. The specimens were prepared by a single laboratory from a common set of samples and randomly distributed to the testing laboratories for testing.
46、 The study was carried out in accordance with ASTM E691. Details of the study are given in ISR Research Report “Interlaboratory Testing Program for Rock Properties (ITP/RP) Round Two,” 1994. Tables 1 through 3 give the repeatability (within a laboratory) and reproducibility (between laboratories) fo
47、r the method at confining pressure of 10, 25, and 40 MPa. 10.1.1. The probability is approximately 95 percent that two test results obtained in the same laboratory on the same material will not differ by more than the repeatability limit. Likewise, the probability is approximately 95 percent that tw
48、o test results obtained in different laboratories on the same material will not differ by more than the reproducibility limit. Table 1Compressive Strength (MPa) at 10 MPa Confining Pressure Berea Sandstone Tennessee Marble Barre Granite Average value 127 173 282 Repeatability 5.29 32.2 13.5 Reproduc
49、ibility 22.5 38.3 25.7 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 226-6 AASHTO Table 2Compressive Strength (MPa) at 25 MPa Confining Pressure Berea Sandstone Tennessee Marble Barre Granite Average value 179 206 366 Repeatability 8.69 43.3 22.5 Reproducibility 34.7 51.8 31.0 Table 3Compressive Strength (MPa) at 40 MPa Confining Pressure Berea Sandstone Tennessee Marble Barre Granite Average value 215 237 N/A Repeatability 7.95 42.4 N/
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