1、Standard Method of Test for Direct Shear Test of Soils under Consolidated Drained Conditions AASHTO Designation: T 236-08 (2013)1ASTM Designation: D3080-72 (2003) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-1a
2、T 236-1 AASHTO Standard Method of Test for Direct Shear Test of Soils under Consolidated Drained Conditions AASHTO Designation: T 236-08 (2013)1ASTM Designation: D3080-72 (2003) 1. SCOPE 1.1. This method describes procedures for determining the consolidated drained shear strength of a soil material
3、in direct shear. The test may be conducted in either a single shear or in a double shear, as shown in Figure 1. The direct shear test is well suited to a consolidated drained test because the drainage paths through the test specimen are short, thereby allowing excess pore pressures to be dissipated
4、fairly rapidly. The test can be made on all soil material,2and on undisturbed or remolded samples. Figure 1Test Specimens in (a) Single and (b) Double Shear 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable la
5、w.TS-1a T 236-2 AASHTO 1.2. The test results are applicable to field situations where complete consolidation has occurred under the existing overburden and failure is reached slowly so that excess pore pressures are dissipated. The test is also useful in determining the shearing resistance along rec
6、ognizable weak planes within the soil material. Note 1If failure is forced to occur on or near a horizontal plane at the middle of the specimen, it may not necessarily occur along the weakest plane, thereby overestimating shear strength parameters. Only when weak plane(s) are recognizable within the
7、 soil mass or interfaces between dissimilar materials are being tested, and the plane or interface at question is placed within the limits of the forced failure zone, can the shear resistance along these planes or interfaces be evaluated. The usefulness of direct shear test results was discussed in
8、the Symposium on Direct Shear Testing of Soils; the proceedings appear in ASTM Special Technical Publication 131. 1.3. The test is not suited to the development of exact stressstrain relationships nor for evaluating any other associated quantities such as moduli within the test specimen because of t
9、he non-uniform distribution of shearing stresses and displacements. The slow rate of displacement provides for dissipation of excess pore pressures, but it also permits plastic flow of soft cohesive soils. Care should be taken that the testing conditions represent those being investigated. 1.4. The
10、values stated in SI units are to be regarded as the standard. 2. SUMMARY OF METHOD 2.1. The method consists of (a) placing the test specimen in the direct shear device, (b) applying a predetermined normal stress, (c) providing for drainage or wetting of the test specimen, (d) consolidating the speci
11、men under the normal stress, (e) unlocking the frames that hold the test specimen, and (f) applying a shearing force to shear the specimen (Figures 1 and 2). Generally three or more specimens are tested, each under a different normal stress to determine the effects upon shear resistance and displace
12、ment. The range in normal stresses should be appropriate for the soil conditions being investigated. Figure 2Typical Direct Shear Box for Single Shear 3. APPARATUS 3.1. Shear DeviceA device (square or circular) to hold the specimen securely between two porous stones in such a way that torque cannot
13、be applied to the specimen. The shear device shall provide means for applying a normal stress to the faces of the specimen, for measuring change in thickness of the specimen, for permitting drainage of water through the porous stones, and for submerging the specimen in water. The device shall be cap
14、able of applying a shearing force to shear the specimen along a predetermined shear plane (single shear) or shear planes (double shear) parallel to the faces of the specimen. The frames that hold the specimen shall be sufficiently rigid to 2015 by the American Association of State Highway and Transp
15、ortation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 236-3 AASHTO prevent their distortion during shearing. The various parts of the shear device shall be made of material not subject to corrosion by substances within the soil or soil moisture. 3.2. Porous Sto
16、nesThe porous stones shall consist of silicon carbide, aluminum oxide, or metal that is not susceptible to corrosion by soil substances or soil moisture. The proper grade of stone depends upon the soil being tested. The stone should be coarse enough to develop adequate interlock with the specimen an
17、d fine enough to prevent excessive intrusion of the soil into the pores. Exact criteria for this have not been established. For normal soil testing, medium grade stones with a permeability of about 0.5 to 1 mm/s are appropriate. 3.3. Loading Devices: 3.3.1. Device for applying the normal force shall
18、 be capable of applying the specified force quickly, without exceeding it, and capable of maintaining it with an accuracy of 1 percent for the duration of the test. 3.3.2. Device for applying the shear force. The capabilities will depend upon whether a controlled-displacement test or controlled-stre
19、ss test is used. The former is generally preferred because the ultimate stress as well as the maximum stress can be determined. Controlled displacement equipment shall be capable of shearing the specimen at a uniform rate of displacement, with less than 5 percent deviation, and should permit adjustm
20、ent of the rate of displacement over a relatively wide range. The rate depends upon the consolidation characteristics of the soils.3The rate is usually maintained with a motor and gear box arrangement and the shear force is determined by a force indicating device such as a proving ring. Controlled-s
21、tress equipment, if used, should be capable of applying the shear force in increments to the specimen in the same manner and to the same degree of accuracy as that described under Section 3.3.1. 3.4. Calibration DiskThe calibration disk should be a copper, aluminum, or hard steel disk of approximate
22、ly the same height as the test specimen and at least 1 mm (0.04 in.) but no more than 5 mm (0.20 in.) smaller in diameter than the ring. 3.5. Moisture RoomFor storing samples as well as for preparing samples where moisture loss during preparation does not exceed 0.5 percent. 3.6. Trimmer or Cutting
23、RingFor trimming oversized samples to the inside dimensions of the shear box with a minimum of disturbance. An exterior jig may be needed to maintain the succession of ringstwo or threein axial alignment. 3.7. BalanceSensitive to 0.1 g or to 0.1 percent of the specimen mass. 3.8. Displacement Indica
24、torsTo measure change in thickness of the test specimen, with a sensitivity to 0.002 mm (0.0001 in.), and to measure horizontal displacement with a sensitivity of 0.02 mm (0.001 in.). 3.9. Drying OvenThat can be maintained at 110 5C (230 9F). 3.10. ContainersSuitable containers made of material resi
25、stant to corrosion and not subject to change in mass or disintegration on repeated heating and cooling. Containers shall have close-fitting lids to prevent loss of moisture from samples before initial mass determination and to prevent absorption of moisture from the atmosphere following drying and b
26、efore final mass determination. One container is needed for each moisture content determination. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 236-4 AASHTO 3.11. Equipment for Remolding or Comp
27、acting Specimens. 3.12. Miscellaneous EquipmentIncluding a timing device with a second hand, distilled or demineralized water, spatulas, knives, straightedge, wire saws, etc., used in preparing the sample. 4. CALIBRATION 4.1. The calibration step is necessary in order to determine the deformation of
28、 the testing device when subjected to consolidation load. For each consolidation step, the corresponding device deformation will be subtracted from the observed deformation so that only deformations due to sample consolidation will be reported at the end of each consolidation step. Calibration of th
29、e testing device must be performed when the device is originally placed in service as well as whenever any of its components are changed. An annual calibration, at a minimum, should be performed. 4.2. Assemble the single shear direct shear device with the calibration disk used in place of the test s
30、pecimen. The calibration disk should be a copper or hard steel disk of approximately the same height as the test specimen and at least 1 mm (0.04 in.) but no more than 5 mm (0.20 in.) smaller in diameter than the ring. Note 2The double shear device will require two calibration disks. 4.3. Position a
31、nd adjust the normal displacement indicator so it can be used to measure either consolidation or swell from the “calibration disk” reading. Record the zero or “no load” reading. 4.4. Apply increments of normal force up to the equipment limitations and record both the applied normal force and the nor
32、mal displacement indicator reading for future reference in determining the thickness of the test specimen and compression within the test apparatus itself. Remove the applied normal force in reverse sequence of the applied force and again record the normal displacement indicator readings and normal
33、force. Calculate the average of the two recorded deformation values corresponding to each value of applied normal loading sequences (loading and unloading). Plot the average deformation of the device as a function of the applied load. As indicated in Section 4.1, this information will be used in det
34、ermining the thickness of the test specimen and its compression within the testing device at the end of each consolidation loading step. 4.5. Remove the calibration disk. Note 3Other equally accurate methods for calibrating the apparatus are acceptable. 5. SAMPLE PREPARATION 5.1. If an undisturbed s
35、ample is used, it should be large enough to provide a minimum of three similar specimens. Prepare the specimens so that moisture loss is negligible. Trim oversized specimens to the inside diameter of the direct shear device and to the length of the trimmer. Extreme care shall be taken in preparing u
36、ndisturbed specimens of sensitive soils to prevent disturbance of their natural structure. Determine the initial mass of the specimen for subsequent use in determining initial moisture content. 5.2. If specimens of compacted soil are used, they shall be compacted to the moisture content and unit wei
37、ght as prescribed in the test request. They may be compacted directly in the shear device, in a mold of equal dimensions and extruded into the shear device, or in a larger mold and trimmed as in Section 5.1. 2015 by the American Association of State Highway and Transportation Officials.All rights re
38、served. Duplication is a violation of applicable law.TS-1a T 236-5 AASHTO 5.3. Minimum specimen diameter for circular specimens, or width of rectangular (square) specimens, shall be approximately 50 mm (2.00 in.). For undisturbed samples, the diameter of the sampling tube shall be greater than the s
39、pecimen diameter to reduce disturbance in the specimen and prevent lateral displacement (Note 4). Note 4The diameter of undisturbed test specimens cut from tube samples should be at least 6 mm (0.25 in.) less than the diameter of the sampling tube to minimize disturbance caused by sampling. 5.4. Min
40、imum specimen thickness shall be approximately 13 mm (0.5 in.), but not less than six times the maximum grain diameter. 5.5. Minimum specimen diameter-to-thickness ratio shall be 2:1. For square or rectangular specimens, the minimum width-to-thickness ratio shall also be 2:1. 6. PROCEDURE 6.1. Assem
41、ble the shear box with the frames aligned and locked in position. A light coating of grease between the frames may be used to ensure watertightness during consolidation and reduce friction during shear. TFE-fluorocarbon spacers or TFE-fluorocarbon-coated surfaces may also be used to reduce friction
42、during shear. Carefully insert the test specimen. Connect the loading devices. Position and/or activate the displacement indicators for measuring shear deformation and changes in specimen thickness. Determine the initial thickness of the specimen. Note 5The decision to dampen the porous stones befor
43、e insertion of the specimen and before application of the normal force depends upon the problem under study. For undisturbed samples from below the water table, the porous stones are usually dampened. For swelling soils, wetting should probably follow application of the normal force to prevent swell
44、 not representative of field conditions. 6.2. Consolidate each test specimen under the appropriate normal force. As soon as possible after applying the initial normal force, fill the water reservoir to a point above the top of the specimen. Maintain this water level during the consolidation and subs
45、equent shear phases so that the specimen is at all times effectively submerged. Allow the specimen to drain and consolidate under the desired normal force or increments thereof prior to shearing. During the consolidation process, record the normal displacement readings before each increment of norma
46、l force is applied and at appropriate times.4Plot the normal displacement readings against elapsed time. Allow each increment of normal force to remain until primary consolidation is complete. The final increment should equal the previous normal force developed and should produce the specified norma
47、l stress. Note 6The normal force used for each of the three or more specimens will depend upon the information required. Application of the normal force in one increment may be appropriate for relatively firm soils. For relatively soft soils, however, several increments may be necessary to prevent d
48、amage to the specimen. The initial increment will depend upon the strength and sensitivity of the soil. This force should not be so large as to squeeze the soil out of the device. 6.3. Shear the SpecimenAfter consolidation is complete, unlock the frames and separate them slightly (0.64 mm 0.025 in.)
49、 so the specimen can be sheared. Apply the shearing force and shear the specimen slowly to ensure complete dissipation of excess pore pressure. The following guide for total elapsed time to failure (tf) may be useful in determining rate of loading: tf= 50 t50 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 236-6 AASHTO where: t50 =time required for the specimen to achieve 50 percent consolidation under the normal force. Note 7If the normal displace
