ASTM D4729-87(1997)
(Test Method)Standard Test Method for In Situ Stress and Modulus of Deformation Using the Flatjack Method
Standard Test Method for In Situ Stress and Modulus of Deformation Using the Flatjack Method
SCOPE
1.1 The flatjack test measures stress at a rock surface. The modulus of deformation and the long-term deformational properties (creep) may also be evaluated.
1.2 Limitation -The flatjack test measures stresses only at the surface of the test chamber. Undisturbed stress levels must be determined by theoretical interpretations of these data.
1.3 Assumptions and Factors Influencing the Data:
1.3.1 The stress relief is assumed to be an elastic, reversible process. In nonhomogeneous or highly fractured materials, this may not be completely true.
1.3.2 The equations assume that the rock mass is isotropic and homogeneous. Anisotropic effects may be estimated by testing in different orientations.
1.3.3 The flatjack is assumed to be 100% efficient. The design and size requirements of 5.1 were determined to satisfy this requirement to within a few percent.
1.3.4 The jack is assumed to be aligned with the principal stresses on the surface of the opening. Shear stresses are not canceled by jack pressure. Orientating the tests in three directions in each plane tested prevents the misalignment from being excessive for at least one of the tests.
1.4 The values stated in inch-pound units are to be regarded as the standard.
1.5 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D 4729 – 87 (Reapproved 1997)
Standard Test Method for
In Situ Stress and Modulus of Deformation Using the
Flatjack Method
This standard is issued under the fixed designation D 4729; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Summary of Test Method
1.1 The flatjack test measures stress at a rock surface. The 3.1 The in situ stress in the rock mass is relieved by cutting
modulus of deformation and the long-term deformational a slot into the rock perpendicular to the surface of the test adit.
properties (creep) may also be evaluated. The deformation caused by this stress relief is measured. A
1.2 Limitation—The flatjack test measures stresses only at hydraulic flatjack is placed into the slot and is pressurized until
the surface of the test chamber. Undisturbed stress levels must the above-measured displacement is canceled. This reapplied
be determined by theoretical interpretations of these data. stress is approximately equal to the stress in the rock mass at
1.3 Assumptions and Factors Influencing the Data: the test location in a direction perpendicular to the plane of the
1.3.1 The stress relief is assumed to be an elastic, reversible jack. The deformational characteristics of the rock mass are
process. In nonhomogeneous or highly fractured materials, this evaluated by incrementally loading the flatjack and measuring
may not be completely true. the deformation.
1.3.2 The equations assume that the rock mass is isotropic
4. Significance and Use
and homogeneous. Anisotropic effects may be estimated by
4.1 Tests in Orthogonal Directions— The flatjack most
testing in different orientations.
1.3.3 The flatjack is assumed to be 100 % efficient. The accurately determines the stress parallel to the long axis of the
adit, because this stress is the least affected by the presence of
design and size requirements of 5.1 were determined to satisfy
this requirement to within a few percent. the opening. (The other tangential stress is highly concen-
trated.) In addition, if the adit is in a stress field where one of
1.3.4 The jack is assumed to be aligned with the principal
stresses on the surface of the opening. Shear stresses are not the stresses is significantly larger than the others (3 or 4 times),
certain locations in the adit may be in very low compressive or
canceled by jack pressure. Orientating the tests in three
directions in each plane tested prevents the misalignment from even tensile stress. Flatjack tests in these locations can give
anomalousandmisleadingresults.Becauseofthesefactors,the
being excessive for at least one of the tests.
1.4 The values stated in inch-pound units are to be regarded test adit should have at least two, and preferably three, long (at
least 4 to 5 times the diameter), straight sections at about 90°
as the standard.
1.5 This standard does not purport to address all of the to each other. Testing should be distributed evenly in all three
sections to provide redundant data and, if results in one section
safety problems, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- are anomalous, to allow the program to produce sufficient
usable data.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
5. Interferences
2. Terminology
5.1 Personnel Prequalification:
5.1.1 Test Personnel—Allpersonnelinvolvedinperforming
2.1 Definitions:
2.1.1 cancellation pressure—the pressure in the flatjack the test, including the technicians and test supervisor, shall be
formally prequalified.
required to return the rock to its initial position.
2.1.2 skin stress—the tangential stress at the surface of an 5.1.2 Drilling Personnel—Quality drilling is important to
achievement of successful flatjack tests. The drilling personnel
opening.
2.1.3 undisturbed stress—the stress field existing in a rock should be capable of the precision drilling necessary to
successfully produce the slot and instrument holes.
mass prior to excavation of an opening.
5.2 Equipment Performance Verification— The compliance
ofallequipmentandapparatuswithperformancespecifications
This test method is under the jurisdiction of ASTM Committee D-18 on Soil
apparatus shall be verified. If no requirements are stated, the
and Rock and is the direct responsibility of Subcommittee D18.12 on Rock
manufacturer’s specifications for the equipment shall be the
Mechanics.
Current edition approved Aug. 28, 1987. Published October 1987. required level of performance. Performance verification is
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 4729 – 87 (1997)
generally done by calibrating the equipment and measurement vertically. The jacks in each group should all be placed in one
systems. Calibration and documentation shall be accomplished part of the adit within 20 ft (6.1 m) of each other along the
according to standard procedures. length of the adit.
5.3 Local Geologic Features—Local features, particularly 7.2 Surface Preparation:
faults, shear zones, etc., can influence the local stress field. 7.2.1 Rock Quality—The flatjack and deformation transduc-
Large inclusions in the rock can affect both the stress and ers should not be installed. Loose, broken, or drummy material
deformational properties. Test locations should be carefully may be detected by a dull, hollow sound when struck with a
selected so that the effects of such features are minimized or, if hammer; such material should be removed.
they are the features of interest, accounted for fully. 7.2.2 Dimensions—The prepared surface shall extend at
5.4 Influence of Excavations—Other excavations intersect- least1ft(0.30m)pasteitherendoftheflatjackslotandatleast
ing the test adit will cause complex stress concentration effects 1 ft (0.30 m) past the furthest measuring points. The transduc-
by superposition. Flatjack tests should be located at least three ers or flatjack shall be 1 ft (0.30 m) inside the prepared surface
diameters of the intersecting feature away from that feature. If at any point (see Fig. 1).
the test adit is excavated by conventional methods, then the 7.2.3 Method—Drilling to a uniform depth may be required
surfaces for testing should be further excavated by nonblasting to prepare the rock face. Residual rock between the drill holes
may be removed by moving the bit back and forth until a
techniques to remove loose material resulting from stress relief
or blasting. smooth surface is achieved. Alternatively, in hard, competent
rock, controlled blasting with very small charges may be used
to remove the residual rock. In softer material, coarse grinding,
6. Apparatus
chipping, or cutting devices may be required.
6.1 Flatjacks—Flatjacks shall be designed to operate at
7.2.4 Smoothness—Ideally, the prepared surface shall be a
pressures of several thousand pounds per square inch when
plane. The difference between the highest and lowest points on
properly installed. The jacks shall be constructed so that the
the prepared surface shall be not greater than 2 in. (50 mm).
two main plates move apart in essentially a parallel manner
7.3 Transducer Installation—Transducers shall be installed
over the range of the jack. The range shall be at least 0.25 in.
on the centerline normal to the flatjack, either at the surface or
(6 mm). The jacks should be square and no less than 2 ft (0.6
at depth.Transducers for stress determination shall be installed
m) wide.
within L/2 of the flatjack slot, where L is the width of the
6.2 Transducers:
flatjack.
6.2.1 Pressure—Electronic transducers or hydraulic gages
7.4 Slot Cutting—The slot can be formed by sawing or by
may be used to monitor flatjack pressure. The pressure trans-
drilling overlapping holes in weak or highly fractured material.
ducer shall have an accuracy of at least 620 lb/in. (60.14
Vibration should be minimized. The slot shall be no more than
MPa), including errors introduced by the readout system and a
3 in. (74 mm) wide, and extend no more than 3 in. (75 mm)
sensitivity of at least 10 lb/in. (0.069 MPa).
past the edges of the flatjack. It shall be deep enough that the
6.2.2 Deformation—Deformation transducers include dial
flatjack may be inserted 3 in. (75 mm) beyond the lowest point
gages, Whittemore-type strain gages, and electronic transduc-
on the rock fact adjacent to the slot. If drilled, care shall be
ers such as LVDT’s or linear potentiometers. The transducer
taken that the holes are straight and parallel to keep the bottom
shall have an accuracy of at least 60.0001 in. (60.0025 mm)
of the slot open to receive the jack. The slot shall be washed
and a sensitivity of at least 0.00005 in. (0.0013 mm).
clean of all dirt and cuttings, using clean water.
6.2.3 Internal Gages—Strain gages inside the flatjack shall
7.5 Relaxation Measurements—Deformation shall be mea-
be calibrated prior to installation in the jack. The effects of the
sured immediately upon completion of slot cutting and again
hydraulic oil and ambient pressure increase on the gages shall
be determined prior to testing.
6.3 Mortar—If mortar is used to cement the flatjack into the
slot, a high-early strength, nonshrink material shall be used.
The mortar may include up to 50 % clean sand by weight, with
grain size between 20- and 60-mesh. Clean, potable water shall
be used for the mortar. The cured mortar shall have a strength
greater than the stress applied by the flatjack. The modulus of
the mortar may be required to be removed from some of the
determinations of rock modulus.
6.4 SawingEquipment—Equipmentusedtosawaslotinthe
rockshouldbeofatypewherelargecenterorendholesarenot
required. These large holes can cause serious changes in the
stress field to be measured.
7. Procedure
7.1 Groups at Each Test Station—At least one group of
jacks should be tested in each adit section. Each group should
FIG. 1 Recommended Flatjack Measurement Array, Surface
have three flatjacks installed horizontally inclined 45° and Measurements
D 4729 – 87 (1997)
immediately prior to testing. If the rock undergoes strain under
constant load over a period of time, several intermediate
readings shall be taken to evaluate this effect.
7.6 Flatjack Installation—Flatjacks shall be centered in the
slot and recessed 3 in. (75 mm) from the face of the excavation
to minimize the possibility of rupture during pressurization.
The mortar, if used, should surround the jack and shall be free
from voids. The jack shall be installed to allow sufficient time
for the mortar to attain compressive strength greater than
maximum anticipated jack stress.
7.7 Flatjack Testing—The flatjack pressure shall be raised
in 100 lb/in. (0.7 MPa) increments until cancellation of all
measuring points has been achieved. Deformation shall be read
after each pressure increment. The peak pressure shall be
maintained for 15 min to check for time-dependent deforma-
tion; deformation readings shall be taken every 5 min. The
pressure shall be reduced in 100 lb/in. (0.69 MPa) decrements
to zero, with deformation read after every decrement. Zero
pressure shall be maintained for 15 min to check for time-
dependent deformation; deformation readings shall be taken
every 5 min. The cycle shall be repeated at least two more
times using equal pressure increments and decrements. The
peak jack stress of these cycles should be as high as possible
and be determined by the test engineer in the field depending
on the jack and rock strength and the cancellation pressure.
7.8 Data Recording Requirements—The data shown on Fig.
FIG. 3 Test Data Sheet
2 and Fig. 3 shall be recorded as a minimum.
Several elastic models and assumptions have been used to
8. Calculation
compensate for these factors, leading to varied and sometimes
8.1 General—The calculation of stress and modulus of
contradictory methods of data reduction. The equations pre-
deformation from flatjack data is influenced by the complex
sented here are among those more widely accepted and have
loading geometry of the test. In addition, the load applied by
been found to produce results comparable with those of other
the flatjack is not the same as the load originally acting on the
in situ methods.The analysis of data, however, is dependent on
rock. The jack expands in one direction only, so lateral and
site-specific factors such as geology and the existing stress
shear components are not restored. This is particularly signifi-
field. In the future, individualized analysis of each test by
cant when the jack is not aligned with a principal stress.
numerical techniques such as finite element methods may
prove to be the most effective approach.
8.2 Cancellation Pressure—The cancellation pressure is not
necessarily equal to the skin stress because of the factors
discussed in 8.1. Skin stress calculations fall into two major
categories: one in which deformations are measured on one
side of the flatjack slot, and one in which deformations are
measured across the slot.
8.2.1 When deformation is measured between points on one
side of the flatjack slot, the skin stress is calculated using
elastic theory and strain. Tincelin found that the strain caused
by cutting the slot was similar to the strain produced by a long
elliptical opening in an elastic plate, and the strain produced by
the flatjack was similar to that caused by uniformly loading the
edge of a semi-infinite plate. The ratio of actual stress to
cancellation pressure is shown in Table 1 for cancellation
measured at various distances from the slot, and from several
Poisson’s ratios. These factors were derived by Tincelin for a
Tincelin, M. E., “Mesure des pressions de terrains dans les mines de fer de
l’Est: Annales de l’Institut Technique de Batiment et des Travaux Publics,” serie:
Sols et Foundations,No.58,pp.972–990.TranslatedbyS.H.Britt,U.S.Geological
FIG. 2 Test Data Sheet Survey open file report No. 28927, Washington, DC, 1953.
D 4729 – 87 (1997)
TABLE 1 Ratio of Skin Stress to Cancellation Pressure for 1-m
S = rock stress normal to the jack, lbf/in. (MPa),
Square (1.09-yd Square) Flatjack
Q = rock stress parallel to the jack, lbf/in. (MPa),
Poisson’s ratio of rock
Distance C = half-length of the slot, in. (mm),
from slot
0.10 0.20 0.33 0.50
Y = distance of measuring point from center line of
jack, in. (mm),
0 0.99 0.99 0.98 0.92
A
0.1 L 0.98 0.98 0.94 0.89
Y = half-width of slot, in. (mm),
o
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