ASTM D4623-96
(Test Method)Standard Test Method for Determination of In Situ Stress in Rock Mass by Overcoring Method-USBM Borehole Deformation Gage
Standard Test Method for Determination of In Situ Stress in Rock Mass by Overcoring Method-USBM Borehole Deformation Gage
SCOPE
1.1 This test method covers the determination of the ambient local stresses in a rock mass and the equipment required to perform in situ stress tests using a three-component borehole deformation gage (BDG). The test procedure and method of data reduction are described, including the theoretical basis and assumptions involved in the calculations. A section is included on troubleshooting equipment malfunctions.
Note 1—The gage used in this test method is commonly referred to as a USBM gage (U.S. Bureau of Mines three-component borehole deformation gage).
1.2 The values stated in inch-pound units are to be regarded as the standard. The values given in parentheses are provided for information only.
1.3 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 4623 – 96
Standard Test Method for
Determination of In Situ Stress in Rock Mass by Overcoring
Method—USBM Borehole Deformation Gage
This standard is issued under the fixed designation D 4623; 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 * 3.1.1 deformation—displacement change in dimension of
the borehole due to changes in stress.
1.1 This test method covers the determination of the ambi-
3.1.2 in situ stress—the stress levels and orientations exist-
ent local stresses in a rock mass and the equipment required to
ing in the rock mass before excavation.
perform in situ stress tests using a three-component borehole
deformation gage (BDG). The test procedure and method of
4. Summary of Test Method
data reduction are described, including the theoretical basis and
4.1 The overcore test measures the diametral deformation of
assumptions involved in the calculations. A section is included
a small-diameter borehole as it is removed from the surround-
on troubleshooting equipment malfunctions.
ing stress field by coaxially coring a larger diameter hole.
NOTE 1—The gage used in this test method is commonly referred to as
Deformation is measured across three diameters of the small
a USBM gage (U.S. Bureau of Mines three-component borehole defor-
hole, spaced 60° apart, using a deformation gage developed by
mation gage).
the U.S. Bureau of Mines. With knowledge of the rock
1.2 The values stated in inch-pound units are to be regarded
deformation moduli, the measured borehole deformation can
as the standard. The values given in parentheses are provided
be related to the change in stress in a plane perpendicular to the
for information only.
borehole. This change in stress is assumed to be numerically
1.3 This standard does not purport to address all of the
equal, although opposite in sense to the stresses existing in the
safety problems, if any, associated with its use. It is the
parent rock mass. Deformation measurements from three
responsibility of the user of this standard to establish appro-
nonparallel boreholes, together with rock deformation moduli,
priate safety and health practices and determine the applica-
allow calculation of an estimate of the complete three-
bility of regulatory limitations prior to use.
dimensional state of stress in the rock mass.
2. Referenced Documents
5. Significance and Use
2.1 ASTM Standards:
5.1 Either virgin stresses or the stresses as influenced by an
D 3148 Test Method for Elastic Moduli of Intact, Rock
excavation may be determined. This test method is written
Core Specimens in Uniaxial Compression
assuming testing will be done from an underground opening;
D 4394 Test Method for Determining the In Situ Modulus
however, the same principles may be applied to testing in a
of Deformation of Rock Mass Using the Rigid Plate
rock outcrop at the surface.
Loading Method
5.2 This test method is generally performed at depths within
D 4395 Test Method for Determining the In Situ Modulus
50 ft (15 m) of the working face because of drilling difficulties
of Deformation of Rock Mass Using the Flexible Plate
at greater depths. Some deeper testing has been done, but
Loading Method
should be considered developmental. It is also useful for
obtaining stress characteristics of existing concrete and rock
3. Terminology
structures for safety and modification investigations.
3.1 Definitions of Terms Specific to This Standard:
5.3 This test method is difficult in rock with fracture
spacings of less than 5 in. (130 mm). A large number of tests
may be required in order to obtain data.
This test method is under the jurisdiction of ASTM Committee D-18 on Soil
and Rock and is the direct responsibility of Subcommittee D18.12 on Rock
5.4 The rock tested is assumed to be homogeneous and
Mechanics.
linearly elastic. The moduli of deformation and Poisson’s ratio
Current edition aproved July 10, 1996. Published November 1996. Originally
of the rock are required for data reduction. The preferred
published as D 4546 – 90. Last previous edition D 4546 – 90.
Considerable information presented in this test method was taken from Bureau method for determining modulus of deformation values in-
of Mines Information Circular No. 8618, and Hooker, V.E., and Bickel, D.L.,
volves biaxially testing the recovered overcores, as described
“Overcoring Equipment and Techniques Used in Rock Stress Determination,”
in Section 8. If this is not possible, values may be determined
Denver Mining Research Center, Denver, CO, 1974.
from uniaxial testing of smaller cores in accordance with Test
Annual Book of ASTM Standards, Vol 04.08.
*A Summary of Changes section appears at the end of this standard.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 4623
FIG. 1 Special Pliers, the Bureau of Mines’ Three-Component Borehole Gage, a Piston, Disassembled Piston and Washer, and a
Transducer with Nut
Method D 3148. However, this generally decreases the accu- borehole. Required accessories are special pliers, 0.005 and
racy of the stress determination in all but the most homoge- 0.015 in. (0.127 and 0.381 mm) thick, brass piston washers,
neous rock. Results may be used from other in situ tests, such and silicone grease.
as Test Method D 4394 and Test Method D 4395. 6.1.2 Strain Readout Indicators—Three strain indicators
5.5 The physical conditions present in three separate drill normally are used to read the deformations. (Alternatively, one
holes are assumed to prevail at one point in space to allow the indicator with a switch and balance unit may be used or one
three-dimensional stress field to be estimated. This assumption indicator may be used in conjunction with a manual wire
is difficult to verify, as rock material properties and the local changing to obtain readings from the three axes.) These units
stress field can vary significantly over short distances. Confi- need a full range digital readout limit of 40 000 indicator units.
dence in this assumption increases with careful selection of the Indicators need to be capable of measuring to an accuracy of
−6
−5 −6
test site. 65 3 10 in. (13 3 10 mm) with a resolution of 1 3 10 in.
−6
5.6 Local geologic features with mechanical properties (25 3 10 mm). A calibration factor must be obtained for
different from those of the surrounding rock can influence each axis to relate indicator units to microinches deflection.
significantly the local stress field. In general, these features, if The calibration factor for each axis will change proportionally
known to be present, should be avoided when selecting a test with the gage factor used. Normally, a gage factor of 0.40 gives
site location. It is often important, however, to measure the a good balance between range and sensitivity. Fig. 2 shows a
stress level on each side of a large fault. All boreholes at a typical strain indicator, calibration jig, and a switching unit.
single test station should be in the same formation. Newer data acquisition systems and microcomputer may be
5.7 Since most overcoring is performed to measure undis- substituted for the indicators.
turbed stress levels, the boreholes should be drilled from a 6.1.3 Cable—A shielded eight-wire conductor cable trans-
portion of the test opening at least three excavation diameters mits the strain measurements from the gage to the strain
from any free surface. The smallest opening that will accom- indicators. The length of cable required is the depth to the test
modate the drilling equipment is recommended; openings from position from the surface plus about 30 ft (10 m) to reach the
8 to 12 ft (2.4 to 3.6 m) in diameter have been found strain indicators. A spare cable or an entire spare gage and
satisfactory. cable should be considered if many tests are planned.
5.8 A minimum of three nonparallel boreholes is required to 6.1.4 Orientation and Placement Tools— The orientation
determine the complete stress tensor. The optimum angle each and placement tools consist of:
hole makes with the other two (trihedral arrangement) is 90°. 6.1.4.1 Placement tool or “J slot tool” as shown in Fig. 3.
However, angles of 45° provide satisfactory results for deter- 6.1.4.2 Placement rod extensions as shown in Fig. 3.
mining all three principal stresses. Boreholes inclined upward 6.1.4.3 Orientation tool or “T handle,” also shown in Fig. 3.
are generally easier to work in than holes inclined downward, 6.1.4.4 A scribing tool, for making an orientation mark on
particularly in fractured rock. the core for later biaxial testing, is optional. It consists of a
bullet-shaped stainless steel head attached to a 3-ft (1-m) rod
6. Apparatus
extension. Projecting perpendicularly from the stainless steel
head is a diamond stud. The stud is adjusted outward until a
6.1 Instrumentation:
snug fit is achieved in the EX hole, so that a line is scratched
6.1.1 Borehole Deformation Gage—The USBM borehole
deformation gage is shown in Fig. 1 (in fractured rock, the
reverse-case modification of the gage is recommended). The
More details of the gage are described in: Hooker, V.E., Aggson, J.R., and
gage is designed to measure diametral deformations during
Bickel, D.L., Improvements in the Three-Component Borehole Deformation Gage
overcoring along three diameters, 60° apart in a plane perpen-
and Overcoring Techniques, Report of Investigation 7894, U.S. Bureau of Mines,
dicular to the walls of an EX (1 ⁄2-in. (38-mm) diameter) Washington, DC, 1984.
D 4623
FIG. 2 The Calibration Device (Left Side) and a Switching Unit (Right Side)
FIG. 3 Placement and Retrieval Tool
along the borehole wall as the scribing tool is pushed inward. deformation modulus of the retrieved rock core. A schematic of
6.1.4.5 Pajari alignment device for inserting into the hole to the apparatus is shown in Fig. 4.
determine the inclination. It consists of a floating compass and
6.2 Drilling Equipment—A detailed description of the drill-
an automatic locking device which locks the compass in
ing apparatus is included in Annex A1.
position before retrieving it.
6.3 Miscellaneous Equipment—This field operation re-
6.1.5 Calibration Jig—A calibration jig (Fig. 2) is used to
quires a good set of assorted hand tools which should include
calibrate the BDG before and after testing.
a soldering iron, solder and flux, heat gun, pliers, pipe
6.1.6 Biaxial Chamber—A biaxial chamber with hand hy-
wrenches, adjustable wrenches, end wrenches, screwdrivers,
draulic pump and pressure gage is used to determine the
D 4623
FIG. 4 Schematic: Biaxial Test Apparatus
allen wrenches, a hammer, electrical tape, a yardstick, carpen- 7.1.14 Back out each micrometer 0.0040 in. (0.102 mm) a
ter’s rule, chalk, stopwatch, and a thermometer. total of 0.0080 in. (0.203 mm).
7.1.15 Balance and record.
7. Calibration and Standardization
7.1.16 Continue this procedure with the same increments
7.1 Gage Calibation—Calibrate the BDG prior to beginning
until the initial point on the micrometer is reached. This zero
and end of the test program, or more frequently if conditions displacement will be the zero displacement reading for the
require. Also recalibrate the BDG if it has undergone severe
second run.
vibration (especially to the signal cable), or if there are any 7.1.17 Repeat the operations described in 7.1.10-7.1.16.
other reasons that exist to suspect that the gage performance
7.1.18 Loosen the wing nuts, and rotate the gage to align the
has changed. The recommended calibration procedure is as piston of the U axis with the micrometer holes.
follows:
7.1.19 Retighten the wing nuts.
7.1.1 Grease all gage pistons with a light coat of silicone
7.1.20 Repeat the operations described in 7.1.6-7.1.17.
grease and install them in the gage.
7.1.21 Loosen wing nuts, and align pistons of U axis with
7.1.2 Place the gage in the calibration jig as shown in Fig. 2,
micrometer holes. Repeat the calibration procedure followed
with the pistons of the U axis visible through the micrometer
for the U and U axis.
1 2
holes of the jig. Tighten the wing nuts.
7.1.22 Determine the calibration factor for each axis as
7.1.3 Install the two micrometer heads, and lightly tighten
follows:
the set screws.
7.1.22.1 Subtract the zero displacement strain indicator
7.1.4 Set the strain indicators on “Full Bridge,” and then
readings (last reading of each run) from the indicator reading
center the balance knob and set the gage factor to correspond
for each deflection to establish the differences.
to the respective anticipated in-situ range and sensitivity
7.1.22.2 Subtract the difference in indicator units at 0.0080-
requirements. A lower gage factor results in higher sensitivity.
in. (0.203-mm) deflection from the difference in indicator units
The gage factor used should be the same for calibration, in-situ
at 0.0320-in. (0.813-mm) deflection.
testing, and modulus tests.
7.1.22.3 Divide the difference in deflection 0.0240 in.
7.1.5 Wire the gage to the indicators as shown in Fig. 5 or (0.610 mm) by the corresponding difference in indicator units
to a switching and balance unit and one indicator.
just calculated to obtain the calibration factor for that axis.
7.1.6 Balance the indicator using the “Balance” knob (if 7.1.22.4 Repeat for the second cycle and take the mean as
using three indicators).
the calibration factor.
7.1.7 Turn one micrometer in until the needle of the 7.1.22.5 See Appendix X1 for an example of the calibration
indicator just starts to move. The micrometer is now in contact
for one axis, calibrated at a gage factor of 0.40.
with the piston. Repeat with the other micrometer.
8. Procedure
7.1.8 Rebalance the indicator.
7.1.9 Record this no load indicator reading for the U axis. 8.1 The procedure for obtaining data to determine in-situ
7.1.10 Turn in each micrometer 0.0160 in. (0.406 mm), or a stresses can be divided into two testing phases: (a) strain relief
total of 0.0320 in. (0.813 mm) displacement. measurements in-situ, and (b) determination of Young’s modu-
7.1.11 Balance the indicator and record the reading and the lus of the rock by recompression in a biaxial chamber.
deflection. 8.1.1 General—Holes of two sizes are drilled for the
7.1.12 Wait 2 min to check the combined creep of the two overcore test: an EX-size (1.5-in. (38-mm) diameter) hole for
transducers. Creep should not exceed 20 μin./in. (20 μmm/mm) the deformation gage and a large-diameter overcore
...
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