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タイトル Japanese Geotechnical Society Standard (JGS3541-2020) Method for in-situ triaxial compression test on rocks
著者 The Japanese Geotechnical Society
出版 Japanese Geotechnical Society Standard (JGS3541-2020) Method for in-situ triaxial compression test on rocks
ページ 発行 2021/03/01 文書ID os202103010004
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  • Japanese Geotechnical Society Standard (JGS3541-2020) Method for in-situ triaxial compression test on rocks
  • 著者
  • The Japanese Geotechnical Society
  • 出版
  • Japanese Geotechnical Society Standard (JGS3541-2020) Method for in-situ triaxial compression test on rocks
  • ページ
  • 発行
  • 2021/03/01
  • 文書ID
  • os202103010004
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  • JGS 3541-2020Japanese Geotechnical Society Standard (JGS 3541-2020)Method for in-situ triaxial compression test on rocks1Scope of applicationThis standard specifies the methods of testing for obtaining the strength and deformation characteristics ofspecimens prepared in in-situ rock masses when subjected to axial compression in the confined state. Thestandard is mainly applicable to rock masses ranging from soft rocks to hard rocks.Note 1: The standard is not only applicable to homogeneous and/or continuous rock masses, but also to heterogeneousand/or discontinuous rock masses. Moreover, it is applicable to fracture zones and the like.Note 2: The standard is not only applicable to triaxial compression tests, but also to unconfined compression tests.Note 3: The standard is not only applicable to monotonic loading tests, but also to cyclic loading tests.2Reference standards and specificationsThe following standards shall form a part of the requirements of this standard by virtue of being referenced inthis standard. The latest versions (including addendums) of these reference standards shall be applicable.JIS A 0207 Technical terms for geotechnical engineeringJIS B 7507 Vernier, dial and digital calipersJIS B 7510 Precision levelsJIS B 7512 Steel tape measuresJIS B 7516 Metal rulersJGS 2134 Test method for water content of rocksJGS 2511 Method for preparation of rock specimensJGS 2533 Method for consolidated-undrained triaxial compression test on soft rocks with pore water pressuremeasurementsJGS 2561 Method for multi-stage cyclic undrained triaxial compression test on rocksJGS 2562 Method for cyclic undrained triaxial compression test to determine fatigue properties of rocksJGS 2563 Method for cyclic triaxial test to determine deformation properties of soft rocks3Terminology and definitionsThe main terminology and definitions used in this standard shall be in accordance with JIS A 0207 and/or thefollowing.3.1SpecimenSpecimen shall refer to a rock body that cuts the outcrop and the bottom of the tunnel into a columnar shape3.2Axial stress, σaAxial stress shall refer to the stress acting in the longitudinal direction of the specimen.© JGS 2021 – All rights reserved1 JGS 3541-20203.3Lateral stress, σrLateral stress shall refer to the stress acting in the radial direction of the specimen.3.4Principal stress difference, σa - σrPrincipal stress difference shall refer to the difference between the axial stress and the lateral stress.3.5Isotropic stress conditionIsotropic stress condition shall refer to the stress condition for which the axial stress and the lateral stress areidentical.3.6Compressive strength under confined condition, (σa - σr)maxCompressive strength under the confined condition shall refer to the maximum value for the principal stressdifference acting on a specimen in the confined state.3.7Cell pressure, σrCell pressure shall refer to the pressure applied within the triaxial pressure cell. The lateral stress shall be equalto the cell pressure.3.8Deformation modulus, EDeformation modulus shall refer to the secant slope and the tangent slope of the axial stress-axis strain curve.The deformation moduli determined by the secant slope and the tangent slope at 50% of the direct tensilestrength are expressed as Ets,50 and Ett,50, respectively.4Test equipment4.1Equipment to prepare specimenThe equipment for preparing the specimen is a device for cutting out the specimen, and shall be configuredfrom a coring device, a grinder, etc.Note: When the specimen is a prism, a cutting machine may be used.4.2Triaxial compression test apparatusThe triaxial compression test apparatus shall be configured from a compression device, a triaxial pressure cell,a loading plate, a loading piston, a cell pressure supply device, a load cell, displacement transducers, and apressure transducer, and shall satisfy the following conditions.Note: Fig. 1 shows the configuration of the triaxial compression test apparatus. In this configuration, since the specimen isa cylinder, preparation of the specimen is easier and the stresses in the specimen are more uniform than the case in whichthe specimen is a prism. Furthermore, as the triaxial cell is equipped with a membrane, the membrane can be easily installed,and its pressure resistance is high. The triaxial compression test apparatus shall be selected according to the site conditions,etc. Other configurations are shown in Appendix A.a)The triaxial cell in Fig. 1 has a sliding portion, and the upper side of the membrane shall move downwardwith the movement of the loading plate. The membrane shall have a thickness of 2.5 mm to 10 mm withoutconstraining the deformation of the specimen.b)There shall be sufficient load capacity with respect to the maximum cell pressure and the maximum axialcompressive force applied to the specimen.c)It shall be possible to continuously apply axial displacement or axial stress at a constant rate.d)It shall be possible to maintain the required cell pressure on a specimen for the duration of the test withinthe range of ± 4 kN/m2 at less than 200 kN/m2 and ± 2% at 200 kN/m2 or more.© JGS 2021 – All rights reserved2 JGS 3541-2020e)It shall be possible to measure the axial compression force up to the maximum axial compression force ofthe specimen within an allowable tolerance of ± 1%. In the example given in Fig. 1, the axial compressionforce can be measured by the external load cell and the internal load cell. In the measurement with anexternal load cell, the frictional force at the sliding portion of the triaxial pressure cell shall be measured,and the measured values of the axial compression force shall be corrected.f)It shall be possible to measure the cell pressure within an allowable tolerance of ± 2 kN/m2 at less than 200kN/m2 and ± 1% at 200 kN/m2 or more.g)It shall be possible to measure the axial displacement within an allowable tolerance of ± 0.1% of the heightof the specimen. Local displacement transducers shall be installed at the center height on the side of thespecimen to measure the local axial displacement. In the example given in Fig. 1, the axial displacementis also measured by an external displacement transducer.Note: When the axial displacement is only measured by an external displacement transducer on the loading plate, etc., anda local displacement transducer is not installed on the side of the specimen, it should be kept in mind that the measuredresults include the effects of the bedding errors at the top of the specimen and the deformations of the ground below thelower end of the specimen. The local displacement transducer shall be installed in the range not affected by the upper and/orlower ends of the specimen. The measurement length of the local displacement transducer is preferably about 50% to 70%of the specimen height.h)4.3a)If the displacement in the circumferential or lateral direction is measured, it shall be possible to measure itwith the same accuracy as that of the axial displacement.Other equipmentSpecimen dimension measuring equipmentMeasurement of the diameter of the specimen shall be by a vernier caliper. The caliper shall be inaccordance with JIS B 7507. Measurement of the height of the specimen shall be by a steel tape measureor a metal straight ruler. The steel tape measure shall be in accordance with JIS B 7512. The metal straightruler shall be in accordance with JIS B 7516. Measurement of the inclination of the upper end face of thespecimen shall be by a precision level. The precision level shall be in accordance with JIS B 7510.Note: If it is difficult to measure the diameter with a vernier caliper, the length of the circumference may be measured witha steel tape measure to calculate the diameter. The steel tape measure shall be in accordance with JIS B 7512.b)Specimen retrieval deviceWhen the specimen is to be retrieved after the test, the specimen can be cut at its lower end and lifted up.© JGS 2021 – All rights reserved3 JGS 3541-2020Fig. 1 Example of test equipment5Preparation of specimens5.1Selection of test locationsA suitable test area that is representative of the target rock mass shall be selected based on the geologicalobservation of the roughly excavated ground surface and the outcrop and of the drilled cores obtained aroundthe site.Note: When obtaining strength characteristics, it is desirable to conduct tests on three or more specimens with varyingisotropic stresses. Geological observation, etc. shall confirm that the characteristics of the rock masses at the chosen testlocations are the same.5.2a)Shape and dimensions of specimensThe shape of the specimen shall be a straight cylinder.Note: This standard can also be applied to specimens of straight prisms and hollow straight cylinders.b)The diameter of the specimen shall be 300 mm to 600 mm.Note 1: This standard can also be applied to specimens of straight cylinders with diameters of 100 mm to 1000 mm.Note 2: When testing rocks having coarse-grained crystals or conglomerates, the diameter of the specimen should be atleast 5.0 times the largest dimension of the constituent particles.c)The height of the specimen shall be 2.2 times the diameter. It is permissible if it is 2 to 3 times.d)The axial direction shall be vertical so that bending does not act on the specimen.© JGS 2021 – All rights reserved4 JGS 3541-20205.3Preparation of specimensa)The upper end face of the specimen shall be shaped flat with a grinder. It may be made to be flat bycovering it with mortar, etc.b)The coring device shall be installed at a prescribed position so that the drilling axis is vertical. The specimenshall be drilled to a prescribed diameter.5.4Measurement of specimensa)The inclination of the upper end face of the specimen shall be measured with a precision level. Assumingthat the central axis of the specimen is vertical, this inclination shall be confirmed to meet the requirementsspecified by JGS 2511.b)The diameter of the specimen shall be measured with a vernier caliper in two orthogonal directions nearthe top of the specimen, and the average value of these measurements shall be recorded as the initialdiameter D0 of the specimen.Note: A steel tape measure or a metal straight ruler may be used.c)The height of the specimen shall be measured at three or more positions with a steel tape or metal straightruler, and the average value of these measurements shall be recorded as the initial height H0 of thespecimen.d)If necessary, a representative sample shall be taken of the rock fragments produced when forming thespecimen, and the initial water content w0 of the specimen shall be calculated using JGS 2134 and recorded.e)The initial condition of the specimen shall be geologically observed and recorded with sketches,photographs, etc.6Assembling of test equipment6.1Installation of triaxial pressure cell and pressure supply equipmentAfter confirming the initial position of the local displacement transducers in the triaxial pressure cell, the triaxialpressure cell shall be installed on the specimen. Then, the pressure transducer and the cell pressure supplydevice shall be connected to the triaxial pressure cell, and the triaxial pressure cell shall be filled with the cellpressure medium.6.2Installation of loading deviceThe loading plate, the external displacement transducer, the piston, the load cell, and the compression deviceshall be installed on the upper end face of the specimen.Note: It should be devised so that the weight placed on the upper end face of the specimen does not act on the specimen,or cell pressure that is equivalent to the weight should be applied so as to maintain an isotropic stress state.7Test methodThis test method applies to monotonic loading for one isotropic stress.Note: This standard can also be applied to multi-step loading tests where compressive strengths for different isotropicstresses are obtained by applying isotropic stresses that change in magnitude stepwise to one specimen and performingaxial compression to the vicinity of failure.7.1a)Application of isotropic stressThe installation status of each transducer shall be checked, and the initial value shall be read as necessary.© JGS 2021 – All rights reserved5 JGS 3541-2020b)Axial and lateral stresses are applied to the specimen to achieve a prescribed isotropic stress state.Note 1: Pressure may be applied repeatedly in a stepwise manner. The number of stages is often about three to five.Note 2: If the amount of compression is estimated to be large, axial displacement ∆H (mm) shall be measured, and theconvergence of the values shall be confirmed.7.2a)Axial compression processZero values for the load cell and the axial displacement transducers shall be confirmed.Note: Zero values for the displacement transducers in a circumferential or lateral direction shall be confirmed, if necessary.b)While keeping the cell pressure constant, axial compression shall be applied continuously at a constantstrain rate. The axial strain rate shall be 0.01 to 0.1%/minute. However, if it is difficult to maintain a constantaxial strain rate, the specimen may be loaded at an axial stress rate equivalent to this axial strain rate.Note: Cyclic loading is also applicable. For the loading method, JGS 2561, JGS 2562, and JGS 2563 may be referred.c)During axial compression loading, axial compressive force P (kN) and axial displacement ∆H (mm) shall bemeasured and recorded.Note 1: The measurement interval shall be set such that it is possible to draw a smooth principal stress difference - axialstrain curve.Note 2: If necessary, circumferential or lateral displacement ∆l (mm) shall be measured and recorded.d)7.3When the axial strain rate is controlled to be constant, the compression loading shall be continued after thereading of the load cell reaches its maximum and shall be terminated when no further changes in theprincipal stress difference can be observed. If the reading of the load cell continues to increase, thecompression loading shall be terminated when the axial strain reaches 5%. When the axial stress rate iscontrolled to be constant, the compression loading shall be terminated when the axial displacementincreases rapidly.Retrieval of specimena)The compression device, the load cell, the loading piston, the loading plate, the triaxial pressure cell etc.shall be dismantled. After that, the specimen can be lifted and collected, if necessary.b)The failure condition and the deformation of the specimen shall be observed and recorded.Note: The deformation and destruction of the specimen after the compression test shall be observed and recorded from thedirection in which those conditions are the most noticeable. Also, if a failure plane is observed, it shall be observed from thedirection in which the angle of failure plane is the steepest, and it shall be recorded so that the approximate angle can beread. Moreover, the inhomogeneous nature of the specimen, the nature of the discontinuities, and the inclusions of foreignmatters, etc. shall be observed and recorded.c)If necessary, a representative sample of the rock fragments of the specimen shall be taken after the test,and the water content w of the specimen shall be calculated using JGS 2134 and recorded.8Processing of test resultsa)The axial strain εa (%) of the specimen shall be calculated by the following equation. If axial strain εa isdirectly measured, its value shall be converted into a percentage.εa = (∆H / H0) x 102where,∆H (mm): axial displacement of the specimen© JGS 2021 – All rights reserved6 JGS 3541-2020Note: When the compression of the specimen is large, due to the loading of isotropic stress, the value of H0 shall be correctedin consideration of the amount of compression by referring to JGS 2533.b)The principal stress difference (σa - σr) (MN/m2) when the axial strain is εa (%) shall be calculated by thefollowing equation.σa - σr = (P / A0) (1 - εa / 100) x 103where,P (kN): axial compressive force applied to the specimen, P = 0, for the isotropic stress stateσa (MN/m2): axial stress applied to the specimenσr (MN/m2): lateral stress applied to the specimenA0 (mm2): initial cross-sectional area of the specimenNote: When the compression of the specimen is large, due to the loading of isotropic stress, the value of A0 shall be correctedin consideration of the amount of compression by referring to JGS 2533.c)If the displacement in the circumferential or lateral direction is measured, the lateral strain εr (%) of thespecimen and Poisson’s ratio ν shall be calculated by the following equations. Also, if lateral strain εr (%) isdirectly measured, Poisson’s ratio ν shall be calculated using the same equation.εr = (∆l / πD0) x 102 = (∆d / D0) x 102ν = - (∆εr / ∆ εa)where,∆l (mm): circumferential displacement of the specimen∆d (mm): lateral displacement of the specimend)The principal stress difference - axial strain curve shall be drawn with the principal stress difference (σa σr) (MN/m2) as the vertical axis and the axial strain εa (%) as the horizontal axis.e)The maximum value for the principal stress difference shall be obtained and taken as the compressivestrength (σa - σr)max (MN/m2), and rounded off to three significant figures. Also, the strain at that time shallbe failure strain εf (%) and rounded off to three significant figures.Note: After the principal stress difference of the specimen shows the maximum value, the value which becomes constantwith the increase in axial strain is taken as the residual strength.f)Deformation modulus E (MN/m2) shall be calculated by the following equation. The secant slope Es,50(MN/m2) of the axial stress - axial strain curve at 50% of the compressive strength shall be obtained androunded off to three significant digits.Et = (∆σa,t / ∆εa,t) X 102where,∆εa,t (%): axial strain increment∆σa,t (MN/m2): axial stress increment corresponding to the axial strain incrementNote: If necessary, tangent slope Et,50 (MN/m2) shall be obtained.© JGS 2021 – All rights reserved7 JGS 3541-20209Reporting9.1Map of test siteA drawing that shows the test site and the surrounding area shall be shown.9.2a)Rock mass condition at test siteName of the test site and depth to the top surface of the specimen from the ground surfaceNote: If necessary, the groundwater level at the test site and the condition of the spring water shall be shown.b)Rock type, lithological characteristics, and conditions of discontinuities such as joints and cracksNote: For example, sandstone, granite, tuff, etc. shall be indicated.c)Rock mass classification of the test site and rock mass classification system that is applied, if a rock massclassification system is appliedd)Sketches and photographs of the rock conditions at the test site before testing9.3Items related to specimena)Shape and preparation method of the specimenb)Initial height and initial diameter of the specimenNote: If the water content is measured, the water content of the specimen in its initial state or after the test shall be reported.c)Results of observation of the specimenNote: The angles of bedding, lamination, and cracks, with respect to the axis of the specimen, and the geological featuressuch as lithological characteristics, shall be reported.9.4Items related to test methodsa)Loading method (loading device, loading pattern, etc.)b)Measurement method (measuring apparatus, arrangement of the displacement transducers, etc.)9.5Items related to test resultsa)Cell pressureb)Axial strain rate or axial stress rate during the axial compression processc)Compressive strength, (σa - σr)max (MN/m2) and failure strain, εf (%)Note: If the residual strength (σa - σr)R (MN/m2) is measured, the residual strength and the range in axial strain εR (%) overwhich it is obtained shall be reported.d)Deformation modulusThe deformation modulus obtained from the slope of the secant, Es,50, and the deformation modulus obtainedfrom the slope of the tangent, Et,50, as needed, shall be reported. If the lateral strain is measured or calculated,Poisson's ratio ν shall be reported, as needed.e)Principal stress difference – axial strain curveNote: If the circumferential displacement or lateral strain is measured, the principal stress difference – lateral strain curveand the principal stress difference – Poisson’s ratio curve shall be reported, as needed.f)Failure state of the specimen© JGS 2021 – All rights reserved8 JGS 3541-2020Photographs and sketches of the side of the specimen after loading shall be reported.g)Compressive strength - isotropic stress relationshipNote 1: If necessary, the envelope for the Mohr’s stress circles representing the compressive strengths, and the angle ofshear resistance, φ, and the vertical axis intercept, c, obtained from the envelope, shall be reported. However, if the envelopeis non-linear, the range in stress over which φ and c are obtained shall be indicated.Note 2: If necessary, the envelope for the Mohr’s stress circles representing the residual strengths, and the angle of shearresistance, φR, and the vertical axis intercept, cR, obtained from the envelope, shall be reported.9.6Other special itemsIf a method that is partially different from this standard is used, the content shall be reported.Appendix AExamples of configurations for the triaxial compression test apparatus, other than those shown in Fig. 1, aregiven below.The example shown in Fig. A1 illustrates an apparatus in which a specimen of a straight cylinder is used. Amembrane is provided on the outer peripheral surface of a triaxial cell (outer cell). Since the reaction forceagainst the cell pressure can be carried by the surrounding rock mass, this triaxial cell is advantageous for itsexcellent pressure resistance. Another triaxial cell (inner cell) is also installed in the hollow portion. The pressurein the outer cell and that in the inner cell are equal.The example shown in Fig. A2 illustrates an apparatus for which a membrane is placed on both the straightcylindrical specimen and the surrounding rock surface, and a lid-like triaxial cell is placed on the top of them.Fig. A1 Example of test equipment (Option 1)© JGS 2021 – All rights reserved9 JGS 3541-2020Fig. A2 Example of test equipment (Option 2)The example shown in Fig. A3 illustrates an apparatus for which a triaxial pressure cell is assembled aftercovering the membrane on a specimen of a straight cylinder. Attention should be paid to the friction betweenthe top plate of the triaxial cell and the loading plate. The frame for the linear guide is not always necessary.The example shown in Fig. A4 illustrates an apparatus for which a set of flat jacks is installed around a specimenof a straight square prism and the reaction force is taken by the surrounding rock mass. Although it is difficult toshape the rock mass into a straight square prism, it is conceivable to use a disk cutter or drill rows of holes andto cut flat surfaces or form smooth flat surfaces using plaster. Attention should be paid to the friction betweenthe flat jacks and the specimen.Fig. A3 Example of test equipment (Option 3)© JGS 2021 – All rights reserved10 JGS 3541-2020Fig. A4 Example of test equipment (Option 4)© JGS 2021 – All rights reserved11 Japanese Geotechnical Society Standard (JGS3541-2020)Method for in-situ triaxial compression test on rocksPublished byThe Japanese Geotechnical Society4-38-2 Sengoku, Bunkyo-ku, Tokyo 112-0011, JapanE-mail: jgs@jiban.or.jpURL: https://www.jiban.or.jp/e/C 2021 The Japanese Geotechnical Society○All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means electronic ormechanical, including photocopying, recording or any information storage and retrieval system now known or tobe invented, without written permission from the publisher.ISBN978-4-88644-125-6
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