Indian Standard

METHOD FOR DETERMINATION OF

STRENGTH OF ROCK MATERIALS IN
TRIAXIAL COMPRESSION

UDC 624*121*439*4

@ BIS 1991

BUREAU OF INDIAN STANDARDS

MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
NEW DELHI 110002

March 1991 Price Group 2

Rock Mechanics Sectional Committee, CED 48

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the
Rock Mechanics Sectional Committee had been approved by the Civil Engineering Division Council.

Natural rock formations are in triaxially stressed state and the strength characteristic of rock under such
a state is important in calculating the bearing capacity of foundation of rock and is also useful in slope
stability analysis. This standard is based on the suggested method of International Society of Rock
Mechanics.

In reporting the rerults of a test or analyais made in accordance with this standard, if the final value,
observed or calculated, is to be rounded off, it shall be done in accordance with IS 2 : 1960 ‘Rules for
rounding off numerical values ( revised )’ .
 Indian Standard
METHODFORDETERMINATIONOF
STkENGTHOFROCKMATERIALSIN
TRIAXIALCOMPRESSION
1 SCOPE D/3. Surfaces of the platens shall be
ground and these shall not depart from
1.1 This standard covers the mothod for determi- plane surfaces by more than +0*005 mm.
nation of strength of cylindrical rock specimens
subjected. to triaxial compression. This test The top platen which shall incorporate a
provides Individual points on the failure (peak spherical seat. The centre of curvature of
strength) envelope from several tests. From this the spherical seat should coincide with the
the value of the internal friction angle 4 and the centre of the top surface of the specimen.
apparent cohesion C may be obtained. The spherical seat shall be lightly lubricat-
ed with mineral oil. The specimen, the
1.2 This standard does not cover (a) the multiple platens and the spherical seat shall be
failure state test, and (b) the continuous failure accurately central with respect to one
state test wherein the failure envelope is pro- another.
duced with a single test employing a stepwise or
continuous procedure. A flexible membrane of suitable material
( like neoprene or butyl rubber or any
2 REFERENCES other ) , to prevent the hydraulic fluid
The following Indian Standards are necessary from entering the specimen. The men&-
adjuncts to this standard: rane shall not penetrate significantly into
the surface pores and it shall be sufficiently
IS No. Title long to extend well on to the platens. When
slightly stretched, it shall be of the same
1586 : 1968 Methods for Rockwell hardness test
diameter as the specimen. When strain
( B and C scales )
gauged specimens are tested, a moulded
9179 : 1979 Method for preparation of rock silicon rubber membrane is satisfactory.
specimen for laboratory testing
3 APPARATUS
3.1 The apparatus consists mainly of three parts
( Fig. 1 ) : a triaxial cell, a loading device and
a device for generating confining pressure.

3.2 A Triaxial Cell

This comprises:
4 A triaxial cell to apply confining pressure
to the specimen. The design of a simple
triaxial cell made of suitable material cap-
able of withstanding design pressure is
schematically shown in Fig. 2. The body
of the cell shall have an air bleeder valve,
suitable entry port for filling the cylinder
with hydraulic fluid, outlets for pore prc- 1 HP
ssure measurement and/or drainage and
electrical outlets for strain gauge instru-
mentation. P =3 Testing machine

Platens having a Rockwell Hardness of MC = Control unit for applying andcontrolling axial load
b)
not less than HRC 30 ( see IS 1586 : C = Triaxial cell
1968 ) for placing at both ends of the HP = Equipnient for generating andcontrollingconfining
specimen, the diameter of the platens shall pressure
be between D and 1.02 D, where D is the FIG 1 BLOCK DIAQRAM SHOWINGTEST
diameter of the specimen. The thickness ARRANGEMENTFOR DETERMININGTHE TRI AXIAL
of the platens shall be at least 15 mm or COMPRESSIVESTRENGTH

1
 4.3 Shape and Dimension of the Specimen

4.3.1 Test specimen shall be a right circular

cylinder.

4.3.2 The length to diameter ratio of the test

specimen shall preferably between 2 to 3.
4.3.3 The diameter of the specimen shall not be
less than ten times the largest mineral grain in
the rock and preferably not less than NX size
(approximately 54 mm) .

4.3.4 Ends of the specimen shall be flat to 0*02mm

and shall not depart from perpendicularity to the
longitudinal axis of the specimen by more than
O*OOl radians ( about 3.5 minutes ) or 0.05 mm
in .50 mm.

4.3.5 The cylindrical surface shall be smooth

and ‘free from abrupt irregularities and straight to
1 Loading piston 4b Bleeder hole within 0.3 mm over the full length of the speci-
Seals 5 Flexible membrane men, The dimensions of the specimen shall not
3. Platens with spherical 6 O-ring clamps vary by more than 0.2 mm over the length of
seating L Load applied by the the speci,men.
4a Hydraulic connection loading device
FIB. TYPICAL DESIGN FOR TRIAXIAL CHAMBER
2 4.3.6 The diameter of the test specimen shall be
measured to the nearest 0’1 mm by averaging
.
two diameters measured at right angles to each
3.3 A Loading Device for Applying and
other at about the upper height, the mid height
Controlling Axial Load
and the lower height of the specimen. It shall
a) A suitable loading machine shall be used not vary by more than 0.3 mm, over the length
for applying, controlling and measuring of the specimen. The average diameter shall he
the axial load on the rock specimen. It used for calculating the cross sectional area. The
shall be of sufficient capacity and capable height of the specimen shall be determined to
of applying load at a rate conforming to the nearest 1 *O mm.
the requirements set out in’5.6, It shall be
4.3.7 The core shall be given a specimen number
verified and calibrated at suitable time
and marked with in-situ orientation.
intervals depending upon the workload.
The two loading faces of the machine 5 TEST PROCEDURE
shall be parallel to each other. The
spherical seat of the loading machine, if 5.1 The cell shall be assembled with the speci-
any, shall also satisfy the requirements men aligned between steel platens and surround-
laid down under 3.2 ( c ) above. ed by the flexible membrane. The specimen,
the platens and the spherical seat shall be
3.4 Device for Generating and Measuring accurately aligned so that each is coaxial with
the Confining Pressure the others.
This includes the following: 5.2 The spherical seat shall be lightly lubricated
with mineral oil.
a) A hydraulic pump or pressure intensifier
or some other system of sufficient capacity 5.3 The cell shall be filled with hydraulic oil,
and capable of maintaining constant con- allowing the air to escape through an air bleeder
fining pressure within 2 percent of the valve. The air bleeder valve shall than be
desired value. closed.
b) A pressure indicating device ( pressure
5.4 The cell shall be placed into the axial load-
gauge or pressure transducer ) which shall
ing device.
be accurate to &2 percent.
5.5 The axial load and the confining pressure
4 PREPARATION OF THE TEST should be increased simultaneously and in such
SPECIMEN . a way that axial stress and confining pressure be
approximately equal, until the predetermined
4.1 The specimen shall be prepared according to
test level for the confining pressure is reached.
IS 9179 : 1979.
Subsequently the confining pressure shall be
4.2 Tbe moisture content of the specimen shall maintained within 2 percent of the prescribed
be as close to the field condition as possible. value.

a
 IS 13047: 1991

.%6 The axial load on the test specimen shall 6.4 Using parameters m and 6, the angle of
-then be increased continuously and without shock internal friction 4 and a value for the apparent
to produce an approximately constant rate of cohesion G ( in the sense (of coulomb’s failure
load or deformation; such that failure will occur theory ) may be calculated using the formulae:
within 5 to 15 minutes of initiation of loading if
carried to failure. Alternatively the stress rate . m-l
shall be within the limits of 0.5 to I.0 MPa/s.
4=
.
arc
‘In___
m -I- 1
The axial failure load shall be measured within
1 percent accuracy. c=b l---sin 4
2 cos $4
5.7 The maximum axial load and the corres-
ponding confining pressure on the specimen shall 7 REPORT
be recorded.
5.8 The axial displacement shall be measured to 7.1 The report shall give a plot of axial strength
an accuracy not less than 0.5 percent. versus confining pressure as discussed in 6.2
( Fig. 3 ), along with the values of apparent cohe-
5.9 The number of specimens to be tested, as well sion c and angle of internal friction 4 citing the
as, the number of different confining pressure range of confining pressures in which they are
values should be determined from practical consi- valid.
derations. However, at least five specimens per
rock sample are preferred.
7.2 The report of test shall also include the
6 CALCULATIONS following information:
6.1 The compressive strength of the specimen 4 Lithologic description of rock.
shall be calculated by dividing the maximum b) Orientation of the axis of loading with
axial load, applied to the specimen during the respect to anisotropy, bedding planes,
test, by the original cross sectional area of the foliations, etc.
specimen.
Cl Source of sample, location, depth, orien-
6.2 The confining pressures and the correspond- tation and date of sampling.
ing strength values for different specimen as
worked out in 6.1, are plotted with the confining 4 Storage history and environment.
pressures as abscissa and strength as ordinates e) Moisture content of sample and room
( see Fig. 3 ) . temperature at the time of the test.
6.3 A strength envelope is obtained by fitting a f) Duration of the test and stress rate.
mean curve to the above points. From practical d Date of testing and type of machine used.
considerations it may be advisable to fit a straight Number of specimens tested.
line to only the most relevant part of the curve h)
or to fit several straight lines to different parts of 3 Mode of failure of each specimen ( with
the curve. Each straight line is characterised by sketch showing failure pattern ).
calculating its gradient m ( tangent of the incli- k) Other physical properties such as specific
nation) its intercept b on the vertical (axial stress) gravity, absorption, permeability and
axis ( ,cee Fig. 3 ) . In each case the range of porosity, citing the Indian Standard
confining pressure in which the respective straight available for the determination of the
line is valid shall be shown on the abscissa. above properties.

300

/
l1
_- *
t _,lGrc----__ tan m
200
.I
t AA

03 6 IO 20 30 LO 50 60
CONFINING PRESSURE Mpa

F1o.3 STRENGTH ENVELOPE

3
m) Should it be necessary to test specimens height, specimen diameter, confining:
of shape other than cylindrical, suitable pressure and the corresponding axiak
mention of these facts shall be made in strength to three significant figures.
the report.
n) A table giving specimen number, specimen p) Any other observation.

4
 .

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I I
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.
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Revision of Indian Standards

Indian Standards are reviewed periodically and revised, when necessary and amendments, if any, are
issued from time to time. Users of Indian Standards should ascertain that they are in possession of the
latest amendments or edition. Comments on this Indian Standard may be sent to BIS giving the
following reference:

Dot : No. CED 48 (4633)

Amendments Issued Since Publication

Amend No. Date of Issue Text Affected

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