int. J. Rock Mech. Mm. Sci. & Geomech. Abstr. Vol. 26. No. 5, pp. 427-434. 1989 0148-9062/89 $3.00 +0.00 Printed in Great Britain Pergamon Press pk

I N T E R N A T I O N A L S O C I E T Y F O R R O C K M E C H A N I C S

C O M M I S S I O N O N T E S T I N G M E T H O D S

SUGGESTED METHOD FOR LARGE SCALE SAMPLING AND TRIAXIAL TESTING OF JOINTED ROCK

CONTENTS

Technical Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434

Co-ordinators O. P. Natau (F.R.G.)

Th. O. Mutschler (F.R.G.)

427

428 ISRM: LARGE SCALE SAMPLING AND TRIAXIAL TESTING SUGGESTED METHOD

INTRODUCTION

The Commission on Standardization of Laboratory and Field Tests, subsequently renamed The Commission on Testing Methods, was first established in 1967. Answers to the questionnaire circulated to all members of the International Society for Rock Mechanics (ISRM) clearly showed a general desire for standardized testing procedures. The tests were characterized and a priority for their standardization was agreed upon. It was decided that research tests, including many of the rock physics tests, were beyond the scope of standardization.

The present document has been produced through the efforts of a working group of the Commission consisting of a number of individuals. A list of contributors is given on the title page. Most of the work has been conducted by means of correspondence co-ordinated by Professor O. P. Natau with assistance of Mr Th. O. Mutschler of the Federal Republic of Germany.

The purpose of this Suggested Method is to specify the sampling and triaxial testing apparatus and procedure for large cylindrical specimens of pervasively jointed rock mass. The method of large scale triaxial testing in laboratory is well proven for many kinds of rock mass and for a wide range of geotechnical designs as underground openings, foundations and rock slopes.

Any person interested in these recommendations and wishing to suggest additions or modifi- cations should write to the Secretary General, International Society for Rock Mechanics, Lab6ratorio Nacional de Engenharia Civil, 101 Avenida do Brasil, P-1799 Lisboa Codex, Portugal.

ACKNOWLEDGEMENTS

The following persons contributed to the preparation and reviewed the drafts of this suggested method: J. A. Franklin (Canada), J. A. Hudson (U.K.).

430 ISRM: LARGE SCALE SAMPLING AND TRIAXIAL TESTING SUGGESTED METHOD

'~ Q Axial Loading ram • ~ (~ Pressure gaucJe

(~ Axial deformation measurement

( ~ _ ~ Q Radial deformation measurement

"" 0 Axial Load measurement @ Confining pressure

measurement

~ ~ A ' (~ Spacer 't T @ HydrauLic pressure

m supply

I ! I I i ,

Fig. 2. Design for a large triaxial testing apparatus.

should provide a sufficient volume of air" in the core barrel at a pressure of 100 kPa. Water flushing is permissible only for hard and water-resistant rocks.

Testing equipment 3. (a) The triaxial testing equipment essentially in-

eludes a triaxial cell, pressurizing and axial loading equipment and load and displacement measuring systems with the requirements as outlined below.

Triaxial cell and axial loading derice 4. (a) One of many possible designs 3 of a triaxial

testing apparatus is shown in Fig. 2. It consists of an integrated system of a cell and a loading device where the base of the cell provides the reaction for the axial force. (b) The cell must be strong enough to bear the combined loads of confining pressure and axial force. It should have windows to allow an optical control of the specimen during the test. (e) The device for applying confining pressure should be capable of fine regulation of the pressure to a range of I%. Due to the large volume involved, the use of water is more convenient than oil. A filled pressure vessel is connected on one side to a source of regulated air pressure and on the other to the triaxial cell. Thus, volume changes of pressure fluid are possible and can be estimated. (d) The axial loading device shall be able to apply sufficient force to reach failure at the selected confining pressure at a constant deformation rate. (e) Platens having a Rockwell hardness of not less than C30 shall be placed at both ends of specimen. The diameter of the platen shall be greater than that of the specimen. The platens must be of such

thickness that the surface of the platen in contact with the specimen does not bend by more than i mm/m at maximum load. To carry out con- solidated and drained tests, the platens must be perforatedL Their surfaces must be ground and polished to a flatness of 0.01 mm. (f) A flexible jacket 5 of suitable material shall be used to prevent the confining fluid from entering the specimen. One way of connecting this to the platens is shown in Fig. 3. The jacket must be flexible enough to accommodate specimen expan- sion during testing.

Equipment for measuring and recording loads, pressures and displacements

5. (a) The axial load shall be measured continuously with an accuracy of 2%. Because of the large forces and the dimensions of the specimens, the capacity of one load cell might not be sufficient. In such cases a tripod of load cells should be used.

F

Q LOAD PLATEN Q LAMP

RUBBER JACKET Q SPECIMEN

Fig. 3. Connection of the jacket to the platens.

ISRM: LARGE SCALE SAMPLING AND TRIAXIAL TESTING SUGGESTED METHOD 431

(b) The cell pressure shall be measured with an accuracy of I%. (c) The axial displacement shall be measured di- rectly between the platens at three points with an accuracy of not less than 0.5%. If electronic trans- ducers are used, an additional optical reading of vernier scales should be provided. (d) The radial displacement shall be measured as the change of circumference at three or more levels with an accuracy of not less than 0.5°/,. If elec- tronic transducers are used, an additional optical reading should be provided. (e) Axial load, confining pressure and displacement data can either be read intermittently or recorded continuously. A simultaneous evaluation shall in- dicate the point of failure. The radial displacement shall be included to calculate the axial stress ~.

PROCEDURE

Sampling procedure

Sampling location. 6. (a) The area to be sampled should first be examined

with regard to its geological characteristics. The sampling can be done from the ground surface, from an exploratory hole or from an exploration gallery. (b) The location of the sample and the fabric lithology and pattern of discontinuities in the surroundings shall be recorded. (c) The drilling site must be carefully prepared without blasting. The last layers should be re- moved by hand before drilling starts. (d) The area to be sampled should be prestressed to avoid exfoliation during coring and to prevent loosening of the specimen. This can be achieved by a deadweight in the form of a circular steel platen, with an additional central rockbolt, if necessary. Rockbolting should be reserved for difficult cases because, if used, the bolt must be removed before testing and the remaining hole filled with a suitable material; thus a certain disturbance cannot be avoided. The deadweight should be grouted to the rock surface to give a uniform distribution of the prestressing.

Drilling and retriet, al of the core

7. (a) An annular slot is cut into the rock mass using the calyx core barrel and regulating the thrust to give a steady return of air for effective flushing. Excessive thrust may cause cuttings to obstruct the slot and prevent further drilling. Too little thrust may cause grinding and polishing of the core bits and vibrations which disturb the specimen. If groundwater enters the drillhole the use of air flushing is not possible. (b) After drilling, the barrel is withdrawn and a steel casing with a wall thickness of 2mm is inserted into the annular space. The upper rim of the casing should have four hanger attachments to

RMMS 2b ~ F

facilitate retrieval. The seam of the casing must be tight or the casing must be seamless. (c) The annular space between the core and the casing is filled with gypsum plaster in the following manner. First, the base of the annulus is sealed with a layer of gypsum. Then the surface of the core and especially the joints are sealed by a thin layer of watery gypsum. When the seals have hardened the sample is totally embedded in gypsum. (d) The core shall be marked to show specimen number and in situ orientation. (e) After hardening of the gypsum plaster (about l h), the core is withdrawn with a crane. The sample breaks at a tension fracture at its base which often coincides with a plane of weakness. If the force of the crane proves to be insufficient, a basal fracture can be created by driving wedges into the annular slot. (f) The specimen shall be sealed with gypsum plaster at both ends to prevent drying and to protect it against mechanical damage during trans- portation. If the specimen is to be stored in the laboratory, the sealing shall be checked on arrival and, if necessary, renewed. In a cool humid room, storage of several months is possible.

Testing procedure

Preparation of the specimen. 8. (a) The specimen shall be a right circular cylinder

having a length-to-diameter ratio of at least I: 15. (b) The ends of the specimen shall be flat to 0.5 mm/m and shall not depart from the perpen- dicular to the longitudinal axis of the specimen by more than 0.01 tad or I cm per 100cm. In most cases, this requires that they are prepared using a chisel and then trowelled with plaster or cement mortar grout. The strength of grout should be similar to that of the rock mass to be tested. The grout layer should be kept as thin as possible and its thickness is to be reported. (c) The sides of the specimen shall be smooth, free from abrupt irregularities and straight to within 2% of the diameter. The steel casing and the grout between it and the rock is to be removed just before testing. Breakouts of smaller pieces of rock are to be filled with the grout material used for the ends. In cases where damage may occur during prep- aration, the surrounding gypsum plaster can be left in place. If so, this is to be reported. (d) The specimen is to be measured before testing, its height to the nearest i mm, and its circumfer- ence at three equidistant locations to the nearest I mm. The average circumference shall be used for calculating the cross-sectional area. (e) The moisture content shall be reported in accordance with Suggested Methods for Determin- ing Water Content, Porosity, Density, Absorption and Related Properties in Rock Characterization, Testing and Monitoring [5].

432 ISRM: LARGE SCALE SAMPLING AND TRIAX1AL TESTING SUGGESTED METHOD

/

START )

Choice of confining / pressure steps ¢3[n)

I Appt yinQ of ] confinincj pressure

Deviatoric I Loading

modulus of deformaLion D I")

further steps

No

No

/ No

YeS

modulus of / / deformation O z

first step

Yes

UnLoodinq- reLoadin 9 cycLe

/U modulus of / / nLoodinq E (,r<,~..o.) / /

I I

Yes

I ,Lure I reached !

Reduction of deviatoric Load to 0

modulus of / unLoadinq E [, =,D--.) /

Fig. 4. Flow chart of a triaxial test.

ISRM: LARGE SCALE SAMPLING AND TRIAXIAL TESTING SUGGESTED METHOD 433

(f) The system of discontinuities in the specimen is to be measured with the aid of a 5 × 5 cm square net, fixed or marked to its curved surface in accordance with Suggested Methods for the Quan- titative Description of Discontinuities in Rock Masses [6]. Photos of the specimen shall be taken from all sides and marked to show the orientation, the specimen number and a scale.

Assembling procedure 9. (a) Details of procedure may be modified if

required by the particular design of testing equipment employed. (b) Tool the lower endface while the sample is still in its steel casing. Rotate the specimen and place it on the lower load platen. Tool the upper endface parallel to the lower one. Remove the steel casing and if possible the surrounding gypsum. Measure and record the structural and lithologieal data. Measure height and circumference. Lower the jacket and upper platen over the specimen and connect the jacket with the lower platen. The tightness can be controlled by applying vacuum to an outlet in the platen. (c) Place the assembled system in the triaxial ceil. Specimens of weak material can be secured by creating a vacuum until the cell is filled and the confining pressure applied. (d) Install the displacement transducers on the

(o)

~+~I" t,'P"

CONFINING PRESSURE ¢3

(b)

o

R cell

i Eli.

AXIAL (;TRAIN ~1

Fig. 5. Multi-stage technique with increasing and decreasing confining pressure steps.

out when the first radial deformations occur. After the cycle, the axial load is to be increased until the axial stress-axial strain curve shows a horizontal tangent (point B, Fig. 5). (d) The axial stress is then decreased to give an isotropic state of stress a[. The next confining pressure increment or decrement a~ ~ is applied (point C or C', Fig. 5). (e) The axial load is increased keeping the con- fining pressure constant until the peak strength is reached (point D or D', Fig. 5). (f) The procedure according to (c) and (d) can be repeated a third time.

specimen and connect them to the recording system. Analysis of the specimen after testing (e) Close the triaxial cell and connect the hydraulic lines. Fill the cell with the pressure fluid and bleed the cell and the lines to remove all air. (f) Check the data acquisition system.

Loading program 10. (a) Because of the high cost of large-scale tests, a

multi-stage technique is useful. The test shall be carried out in increasing and decreasing confining pressure steps to determine upper and lower bounds for the strength parameters, and to deter- mine the deformation properties from initial load- ing paths (Fig. 4). The test should be carried out at stress levels which are relevant for the application. (b) The axial load and the confining pressure are to be increased simultaneously to the first confining pressure step a] (point A, Fig. 5) 7. Wait until the deformations remain unchanged for a period of 15 min. (Soft materials which show consolidation due to pore or joint fluid movements need a special treatment which is not the subject of this paper.) (c) The axial load is then to be increased, keeping the confining pressure constant and at a constant strain rate within the limits of 0.1-0.01%/min. To separate elastic and plastic deformations, unload- ing-reloading cycles should be included in the first confining pressure step. The first cycle should be carried out at 30% of the estimated deviatoric stress at the peak. A second cycle should be carried

I I.(a) If the specimen is not totally destroyed after testing, the following should be undertaken. (b) Photos of the specimen shall be taken from all sides, marked to show the orientation, the speci- men number and a scale. Additional details, such as the mode of failure, shall be photographed if necessary. (c) The specimen is to be dismantled step by step, noting further details of structure and lithology. In particular, the mode of failure shall be analyzed and recorded. (d) If possible, small samples should be taken from undestroyed parts of the specimen to permit index testing of the rock matrix.

CALCULATIONS

12. (a) The axial stress shall be calculated by dividing the axial load applied to the specimen by the actual cross-section area 6. (b) The strength envelopes can be approximated mathematically by linear or, if required, bilinear expressions according to at = m~a3 + b~. The pos- ition of the straight lines is fixed by the ordinate bi, the tangent of the slope m+ and the range of confining pressure to which they apply (Fig. 6) using parameters m and b. The global friction angle ~0 and the value for the "'hypothetical" or "appar-

434 ISRM: LARGE SCALE SAMPLING AND TRIAXIAL TESTING SUGGESTED METHOD

7 b2 1

CONFINING PRESSURE 0" 3

Fig. 6. Bilinear strength envelope and its description by means of two straight portions with the parameters m, and b,.

ent" cohesion c (in the sense of Coulomb's failure theory) may be calculated from:

~p, = arcsin m , - I I - sin ~Pi m~ + ! ; c i = b, 2 cos ~

(c) Provided that there are no strain-hardening effects in the tested material, a multi-stage test with increasing confining pressure steps will give a lower bound for the friction angle and an upper bound for the cohesion. A test with decreasing confining

pressure steps will give a lower bound for cohesion and an upper bound for the friction angle.

REPORTING OF RESULTS

13.(a) The report shall include at least the following items: (b) Source of specimen, including geographic location, date and method of sampling. (c) Lithologic description of the rock mass, includ- ing its structure and joint system. (d) Details of specimen preparation in the labora- tory, including the thickness of the gypsum at the surface of the specimen, also history and environ- ment of the test specimen storage. (e) Orientation of the loading axis with respect to the specimen joint system. (f) Water content at time of test. (g) Date of testing. (h) Specimen diameter and height. (i) Test duration and/or stress and displacement rates. (j) Graphs of test results. (k) Mode of failure. (1) Photos of the specimen before and after testing.

Notes

I. Sampling equipment at the University of Karls- ruhe, West Germany (60cm max dia, 160cm max height).

2. For a 60cm dia a volume of 7m~/min of com- pressed air is needed.

3. Large triaxial cell at the University of Karisruhe, West Germany: capacity: 100cm max dia, 170 cm max height, 6.4 MN max axial force, 2 MPa max confining pressure.

4. A very good method to measure the pore fluid pressure is by installing pressure gauges at the sides of the specimen. The measuring tubes are best guided through the load platens [3].

5. Jackets from 4 mm rubber are well proven. 6. The correction of the cross-sectional areas calcu-

lated from the middle of the radial displacement. If buckling occurs, the maximum radial deformation shall be used.

7. For the particular type of triaxial cell this is reached by increasing the cell pressure alone.

REFERENCES I. Karman von T. Festigkeitsversuche unter allseitigem Druck. Ztg

d. Vereins Deutscher lngenieure, Jg. 55 (19L L). 2. M/iller L. Grundsfitzliches t~ber gebirgstechnologische GroOver-

suche. Geol. u. Bauw., Jg. 27 (H. L), 3-8 (1961). 3. Wichter L. and Gudehus G. Ein Verfahren zur Entnahme und

Prfifung yon gekliifteten Grol3bohrkernen. Proc. 2. Nat. Tagung f Felsmechanik, Aachen (1976).

4. Natau O., FrShlich B. and Mutschler Th. Recent developments of the large scale triaxial test. Proc. 5th Int. Congr. of ISRM, Melbourne, pp. A65-A74 (19831.

5. ISRM Commission on Standardization of Laboratory and Field Tests. Suggested Methods for Determining Water Content, Por- osity, Density, Absorption and Related Properties in Rock Charac- terization, Testing and Monitoring, pp. 8L-89. Pergamon Press, Oxford (198L).

6. ISRM Commission on Standardization of Laboratory and FieLd Tests. Suggested Methods for the Quantitath'e Description of Discontinuities in Rock Masses, pp. 3-52. Pergamon Press, Oxford (1981).