Instruction Manual
Model 4300 series
Vibrating Wire Stressmeter (EX, BX, NX)
The information contained herein is believed to be accurate and reliable. However, Geokon
No part of this instruction manual may be reproduced, by any means, without the written consent of Geokon®.
omissions or misinterpretation. The information herein is subject to change without notification.
Copyright © 1984-2019 by Geokon
(Doc Rev I, 1/09/2019)
®
®
assumes no responsibility for errors,
Warranty Statement
Geokon warrants its products to be free of defects in materials and workmanship, under normal
use and service for a period of 13 months from date of purchase. If the unit should malfunction,
it must be returned to the factory for evaluation, freight prepaid. Upon examination by Geokon,
if the unit is found to be defective, it will be repaired or replaced at no charge. However, the
WARRANTY is VOID if the unit shows evidence of having been tampered with or shows
evidence of being damaged as a result of excessive corrosion or current, heat, moisture or
vibration, improper specification, misapplication, misuse or other operating conditions outside of
Geokon's control. Components which wear or which are damaged by misuse are not warranted.
This includes fuses and batteries.
Geokon manufactures scientific instruments whose misuse is potentially dangerous. The
instruments are intended to be installed and used only by qualified personnel. There are no
warranties except as stated herein. There are no other warranties, expressed or implied, including
but not limited to the implied warranties of merchantability and of fitness for a particular
purpose. Geokon is not responsible for any damages or losses caused to other equipment,
whether direct, indirect, incidental, special or consequential which the purchaser may experience
as a result of the installation or use of the product. The buyer's sole remedy for any breach of this
agreement by Geokon or any breach of any warranty by Geokon shall not exceed the purchase
price paid by the purchaser to Geokon for the unit or units, or equipment directly affected by
such breach. Under no circumstances will Geokon reimburse the claimant for loss incurred in
removing and/or reinstalling equipment.
Every precaution for accuracy has been taken in the preparation of manuals and/or software,
however, Geokon neither assumes responsibility for any omissions or errors that may appear nor
assumes liability for any damages or losses that result from the use of the products in accordance
with the information contained in the manual or software.
TABLE of CONTENTS
1. THEORY OF OPERATION .................................................................................................................................. 1
2. INSTALLATION .................................................................................................................................................... 3
2.1 BOREHOLE REQUIREMENTS ................................................................................................................................. 3
2.2 PRELIMINARY CHECKS ........................................................................................................................................ 3
2.3 ATTACHING THE WEDGE/PLATEN ASSEMBLY ..................................................................................................... 4
2.4 SETTING THE STRESSMETER (RECOVERABLE TYPE) ............................................................................................ 4
2.5 RECOVERING THE STRESSMETER ......................................................................................................................... 5
2.6 SPLICING AND JUNCTION BOXES.......................................................................................................................... 6
3. TAKING READINGS ............................................................................................................................................. 7
3.1 GK-404 READOUT BOX ....................................................................................................................................... 7
3.2 GK-405 READOUT BOX ....................................................................................................................................... 8
3.2.1 Connecting Sensors with 10-pin Bulkhead Connectors Attached ............................................................... 8
3.2.2 Sensors with Bare Leads ............................................................................................................................. 8
3.2.3 Operating the GK-405 ................................................................................................................................ 8
3.3 GK-403 READOUT BOX (OBSOLE TE MODEL) ...................................................................................................... 9
3.3.1 Connecting Sensors with 10-pin Bulkhead Connectors Attached ............................................................... 9
3.3.2 Connecting Sensors with Bare Leads .......................................................................................................... 9
3.3.3 Operating the GK-403 ................................................................................................................................ 9
3.4 MEASURING TEMPERATURES ............................................................................................................................... 9
4. DATA REDUCTION ............................................................................................................................................ 10
4.1 CHANGE IN STRESS CALCULATION .................................................................................................................... 10
4.2 ENVIRONMENTAL FACTORS ............................................................................................................................... 10
4.3 GAUGE SENSITIVITY FACTORS .......................................................................................................................... 11
4.4 CORRECTIONS FOR TEMPERATURE CHANGES .................................................................................................... 12
5. TROUBLE SHOOTING ....................................................................................................................................... 13
APPENDIX A. SPECIFICATIONS ......................................................................................................................... 15
A.1 MODEL 4300 STRESSMETER ............................................................................................................................. 15
A.2 THERMISTOR (SEE APPENDIX B ALSO) .............................................................................................................. 15
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION ..................................................................... 16
APPENDIX C. BIAXIAL STRESS CHANGES ..................................................................................................... 17
FIGURES
IGURE 1 - VIBRATING WIRE STRESSMETER.................................................................................................................. 1
F
IGURE 2 - VIBRATING WIRE STRESSMETER INSTALLATION TOOL ASSEMBLY ............................................................. 4
F
IGURE 3 - LEMO CONNECTOR TO GK-404 ................................................................................................................... 7
F
IGURE 4 - LIVE READINGS – RAW READINGS............................................................................................................... 8
F
IGURE 5 - MODEL 4300EX SENSITIVITY FACTOR VS. ROCK MODULUS ......................................................................11
F
IGURE 6 - MODEL 4300BX SENSITIVITY FACTOR VS. ROCK MODULUS ......................................................................11
F
IGURE 7 - MODEL 4300NX SENSITIVITY FACTOR VS. ROCK MODULUS .....................................................................12
F
TABLES
ABLE 1 - SAMPLE CALCULATIONS ..............................................................................................................................10
T
ABLE 2 - SAMPLE RESISTANCE ...................................................................................................................................14
T
ABLE 3 - RESISTANCE WORK SHEET ...........................................................................................................................14
T
ABLE 4 - SPECIFICATIONS ...........................................................................................................................................15
T
ABLE 5 - THERMISTOR RESISTANCE VERSUS TEMPERATURE ......................................................................................16
T
EQUATIONS
QUATION 1 - CHANGE IN STRESS ................................................................................................................................10
E
QUATION 2 - TEMPERATURE CORRECTION .................................................................................................................12
E
QUATION 3 - RESISTANCE TO TEMPERATURE .............................................................................................................16
E
QUATION 4 - PLANE STRESS .......................................................................................................................................17
E
QUATION 5 - STRESS IN ANY DIRECTION.....................................................................................................................17
E
QUATION 6 - SECONDARY PRINCIPLE STRESSES AND ANGLE .....................................................................................17
E
1. THEORY OF OPERATION
Geokon vibrating wire stressmeters are designed primarily for long-term measurements of stress
changes in rock, by utilizing a vibrating wire transducer to measure the deformation of a thickwalled steel ring preloaded into a borehole by a wedge and platen assembly as shown in Figure
1.
1
Figure 1 - Vibrating Wire Stressmeter
In use, changing rock stresses impose changing loads on the gauge body causing the body to
deflect, and this deflection is noted as a change in tension and resonant frequency of vibration of
the vibrating wire element. The square of the vibration frequency is directly proportional to the
change in diameter of the gauge and, by calibration, to the change in stress in the rock.
The actual calibration of the gauge depends upon many factors including the host rock elastic
constants, the prestress applied during installation, the orientation of the stressmeter with respect
to the principal rock stress direction and platen contact area. Thus, the accuracy of the gauge
reading is largely indeterminate and the indicated stress magnitude can only be approximate.
A coil and magnet assembly located close to the wire is used to both excite the wire and sense
the resultant frequency of vibration. When the gauge is connected, a pulse of varying frequency
is applied to the coil and magnet assembly, and this causes the wire to vibrate at its resonant
frequency. The wire continues to vibrate, and a signal, at the gauge frequency, is induced in the
pickup coil and transmitted to the readout box where it is conditioned and displayed.
In theory, where the effective modulus of the stressmeter (approximately 28 GPa (4 x 106 PSI))
is more than two times the modulus of the host rock, conversion of the readings to changes in
stress does not require and accurate knowledge of the rock modulus, and this is the reason for
using the term stressmeter for this device. However, in most rocks, and especially in harder
rocks, the modulus must be known to improve the accuracy of the stress measurements, and
calibration curves provided herein give sensitivity factors for materials of different moduli. It
should be noted that as the rock modulus changes by a factor of 10, the gauge factor changes
only by a factor of two.
2
The stressmeter is a uniaxial device. To completely evaluate stress changes in a given plane,
three stressmeters, installed at 0°, 45°, and 90° orientations, are required.
The gauge wire in the Model 4300 Series stressmeters runs perpendicular to the direction in
which the gauge body is loaded in an effort to minimize the effects of point loading, off center
loading, etc. This gives the gauge a very high range and, since as the load increases the wire gets
tighter, the wire never goes slack.
Gauge installation is accomplished by driving a wedge between the gauge body and the platen,
which contacts the borehole walls. Preloading to desired levels is accomplished by further
driving of the wedge with the setting tool. In soft rocks, a soft rock platen and soft rock shoe are
used to increase the area of contact.
The gauge is constructed of corrosion resistant materials and should have an indefinite lifetime
under even the most severe conditions.
3
2. INSTALLATION
2.1 Borehole Requirements
Stressmeters are designed to be used in smooth-walled diamond drill holes. Stressmeters can be
installed in percussively drilled holes and drag bit drilled holes, provided that care is taken to get the
proper hole diameter with a smooth wall. If the walls are rough, the gauge response (calibration) can
be radically affected.
After drilling, the hole should be thoroughly cleaned by washing out with water or blowing out
with compressed air. The borehole diameter should then be checked with the GO-NO-GO gauges
supplied with the installation tool. If the borehole diameter is correct, the installation can
proceed.
Standard hole configurations for each of the models is as follows:
Model 4300EX Stressmeters are designed for use in EX diamond drill holes 38 mm (1.5”), and
the hole can range in diameter from 37 mm (1.45”) to 39 mm (1.55”) when using the standard
wedge and platen assembly.
Model 4300BX Stressmeters are designed for use in BX diamond drill holes 60 mm (2.36”), and
the hole can range in diameter from 59 mm (2.30”) to 61 mm (2.40”) when using the standard
wedge and platen assembly.
Model 4300NX Stressmeters are designed for use in NX diamond drill holes 76 mm (2.98”), and
the hole can range in diameter from 75 mm (2.95”) to 77.5 mm (3.05”) when using the standard
wedge and platen assembly. Oversize platens are available for oversize boreholes (consult
factory).
2.2 Preliminary Checks
Before installing the gauges in the field, perform a preliminary check by completing the
following
1) Connect the gauge to a readout box. (See Section 3 Readout instructions.)
2) Take a reading. Zero readings at the site should coincide with the factory readings within a
few digits after corrections for temperature are made. (See Section 4.4 for information on
temperature correction.)
3) A check of electrical continuity can be made using an ohmmeter. The resistance between the
two lead wires (usually red and black) should be around 180 ohms for Models 4300BX and
4300NX, 90 ohms for Model 4300EX. Remember to add the cable resistance at
approximately 14.7Ω/1000' (48.5Ω/km) at 20 °C. Multiply this factor by two to account for
both directions.
4) Using an ohmmeter check the resistance between the two thermistor wires (usually white and
green). Using Table 5 in Appendix B, convert the resistance to temperature. Compare the
result to the current ambient temperature.
4
2.3 Attaching the Wedge/Platen Assembly
The wedge/platen assemblies are shipped separately. They are held together by a nylon screw
and nut. Remove the nut and then use the nylon screw to attach the wedge/platen assembly to the
Stressmeter Body. Orient the wedge so that the narrow end is facing in the same direction as the
cable, (see Figure 2 in the next section). Tighten the nylon screw into the threaded hole in the
body. Do not overtighten the screw as it may break; it is made of nylon so that it can be
sheared easily later in the installation.
(Note: The BX size stressmeter and the NX size stress meter both use the same wedge. However,
there are two holes in the wedge. The one nearest the tip is for the BX size, the one farthest from
the tip is for the NX size).
2.4 Setting the Stressmeter (Recoverable Type)
Mount the stressmeter on the setting tool by pushing the nylon threaded pieces into the matching
holes in the setting tool head. Feed the gauge leads through the slot in the setting head. (See
Figure 2 below.)
Connect the first section of 1/4” rod to the yoke attached to the thin end of the wedge. Note that
the first section of rod has one end with a ¼-20 left handed thread on it. This will connect to the
left-handed thread in the yoke.
Attach the first section of the 3/4” positioning rod to the back of the setting tool head. Push the
stressmeter into the hole using the positioning rod. The buttons on the setting rod connectors
indicated the orientation of the wedge/ platen assembly, e.g., for taking measurements in a
vertical direction, keep the buttons to the top of the rod.
As the 3/4” positioning rod is pushed into the hole, add new sections of both 3/4” and 1/4” rod
until the desired depth has been reached. It is advisable to wear gloves during this procedure
to protect the thumb while depressing the buttons on the 3/4 inch rods.
Slide the slide-hammer over the last section of 1/4” rod and then thread the anvil block onto the
outer end of the 1/4” rod. Connect the readout box to the lead wires and take initial readings.
(See Section 3 for readout instructions.)
Figure 2 - Vibrating Wire Stressmeter Installation Tool Assembly
Holding the positioning rod firmly at its correct depth and orientation, slide the slide-hammer
back up the 1/4” rod, then side it quickly back to the anvil striking it a sharp firm blow. This will
shear the rivet holding the wedge to the platen and will pull the wedge into the platen thereby
expanding it against the wall of the borehole.
After the first blow, take another reading on the readout box and observe the change in reading.
The recommended preloads are as follows: For the EX size a reading change of 2000 digits on
channel F, for the BX size a reading change of 400 digits on channel B, for the NX size a reading
change of 200 digits on channel B.
Use as many blows of the hammer as is necessary to achieve this reading. Stop hammering
when successive blows produce little or no change of reading. Continued hammering can
break the wedge! When the target reading has been achieved, or if successive hammer blows
produce little or no change, disconnect the 1/4” rod from the wedge yoke by turning clockwise.
Remove the 1/4” rod from the hole, and then disengage the setting tool from the stressmeter by
pulling on it.
For multiple installations of gauges in a single hole, route the lead wires from deeper gauges
through the recess in the side of the setting tool head. Maintain tension on these wires as
subsequent gauges are pushed into the hole.
If necessary, after setting the gauges and obtaining the final readings, push the leads back into
the borehole and seal the borehole using an expandable rockbolt anchor or a short bolt. This will
discourage vandalism if this is a problem.
Note that the stressmeter initial readings will probably diminish slightly over the first day or two
as the instrument beds firmly into place.
5
2.5 Recovering the Stressmeter
After tests, the stressmeter can be removed from the borehole by using the setting tool.
Only the larger setting rods are required, along with the setting tool head, which is used to strike
the outer tip of the wedge. This will drive the wedge out from under the platen and allow the
stressmeter to be pulled from the hole using the electrical cable. Make sure that the setting head
is oriented so that the flat part of the front face lies opposite the wedge. The entire stressmeter
can sometimes be recovered in this way i.e., the wedge, platen, and stressmeter body.
In order to reuse the stressmeter it will require a new nylon screw. (A few spare nylon screws are
included in each shipment). However, there is a good chance that the wedge and platen may
dislodge in the borehole and be lost; therefore, it is advised to order spares of these also.
(Note: The BX size stressmeter and the NX size stress meter both use the same wedge. However,
there are two holes in the wedge. The one nearest the tip is for the BX size; the one farthest from
the tip is for the NX size).
6
2.6 Splicing and Junction Boxes
Because the vibrating wire output signal is a frequency rather than a current or voltage,
variations in cable resistance have little effect on gauge readings. Therefore, splicing of cables
has no effect, and in some cases may in fact be beneficial. For example, if multiple stressmeters
are installed in a borehole, and the distance from the borehole to the terminal box or datalogger is
great, a splice (or junction box) could be made to connect the individual cables to a single multiconductor cable. This multi-conductor cable would then be run to the readout station. For these
types of installations, it is recommended that the stressmeter be supplied with enough cable to
reach the installation depth, plus extra cable to pass through drilling equipment (rods, casing,
etc.).
Cable used for making splices should be a high quality twisted pair type, with 100% shielding
and an integral shield drain wire. When splicing, it is very important that the shield drain
wires be spliced together. Splice kits recommended by Geokon incorporate casts that are placed
around the splice and then filled with epoxy to waterproof the connections. When properly made,
this type of splice is equal or superior to the cable in strength and electrical properties. Contact
Geokon for splicing materials and additional cable splicing instructions.
Junction boxes and terminal boxes are available from Geokon for all types of applications. In
addition, portable readouts and dataloggers are also available. Contact Geokon for specific
application information.
3. TAKING READINGS
3.1 GK-404 Readout Box
The Model GK-404 Vibrating Wire Readout is a portable, low-power, handheld unit that is
capable of running for more than 20 hours continuously on two AA batteries. It is designed for
the readout of all Geokon vibrating wire gauges and transducers, and is capable of displaying the
reading in either digits, frequency (Hz), period (µs), or microstrain (µε). The GK-404 also
displays the temperature of the transducer (embedded thermistor) with a resolution of 0.1 °C.
Before use, attach the flying leads to the GK-404 by aligning the red circle on the silver “Lemo”
connector of the flying leads with the red line on the top of the GK-404 (Figure 3). Insert the
Lemo connector into the GK-404 until it locks into place.
7
Figure 3 - Lemo Connector to GK-404
Connect each of the clips on the leads to the matching colors of the sensor conductors, with blue
representing the shield (bare).
To turn the GK-404 on, press the “ON/OFF” button on the front panel of the unit. The initial
startup screen will be displayed.
After approximately one second, the GK-404 will start taking readings and display them based
on the settings of the POS and MODE buttons.
The unit display (from left to right) is as follows:
• The current Position: Set by the POS button, displayed as a letter A through F.
• The current Reading: Set by the MODE button, displayed as a numeric value followed by the
unit of measure.
• Temperature reading of the attached gauge in degrees Celsius.
Use the POS button to select position B for BX and NX models or position F for EX models.
Use the MODE button to select Dg (digits). (Other functions can be selected as described in the
GK-404 Manual.)
The GK-404 will continue to take measurements and display readings until the unit is turned off,
either manually, or if enabled, by the Auto-Off timer. If no reading displays or the reading is
unstable, consult Section 5 for troubleshooting suggestions.
For further information, consult the GK-404 manual.
8
3.2 GK-405 Readout Box
The GK-405 Vibrating Wire Readout is made up of two components: The Readout Unit,
consisting of a Windows Mobile handheld PC running the GK-405 Vibrating Wire Readout
Application; and the GK-405 Remote Module, which is housed in a weatherproof enclosure and
connects via a cable to the vibrating wire gauge to be measured. The two components
communicate wirelessly using Bluetooth®, a reliable digital communications protocol. The
Readout Unit can operate from the cradle of the Remote Module, or, if more convenient, can be
removed and operated up to 20 meters from the Remote Module.
3.2.1 Connecting Sensors with 10-pin Bulkhead Connectors Attached
Align the grooves on the sensor connector (male), with the appropriate connector on the
readout (female connector labeled senor or load cell). Push the connector into place, and
then twist the outer ring of the male connector until it locks into place.
3.2.2 Sensors with Bare Leads
Attach the GK-403-2 flying leads to the bare leads of a Geokon vibrating wire sensor by
connecting each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
3.2.3 Operating the GK-405
Press the button labeled “POWER ON (BLUETOOTH)”. A blue light will begin
blinking, signifying that the Remote Module is waiting to connect to the handheld unit.
Launch the GK-405 VWRA program by tapping on “Start” from the handheld PC’s main
window, then “Programs” then the GK-405 VWRA icon. After a few seconds, the blue
light on the Remote Module should stop flashing and remain lit. The Live Readings
Window will be displayed on the handheld PC. Choose display mode B for BX and NX
models or display mode F for EX models. Figure 4 shows a typical vibrating wire output
in digits and thermistor output in degrees Celsius on display mode B.
If no reading displays or the reading is unstable, see Section 5 for troubleshooting
suggestions. For further information, consult the GK-405 Instruction Manual.
Figure 4 - Live Readings – Raw Readings
3.3 GK-403 Readout Box (Obsolete Model)
The GK-403 can store gauge readings and apply calibration factors to convert readings to
engineering units. The following instructions explain taking gauge measurements using Mode
"B". Consult the GK-403 Instruction Manual for additional information.
3.3.1 Connecting Sensors with 10-pin Bulkhead Connectors Attached
Align the grooves on the sensor connector (male), with the appropriate connector on the
readout (female connector labeled senor or load cell). Push the connector into place, and
then twist the outer ring of the male connector until it locks into place.
3.3.2 Connecting Sensors with Bare Leads
Attach the GK-403-2 flying leads to the bare leads of a Geokon vibrating wire sensor by
connecting each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
3.3.3 Operating the GK-403
1) Turn the display selector to position B for BX and NX models or position F for EX
models.
2) Turn the unit on.
3) The readout will display the vibrating wire output in digits. The last digit may change
one or two digits while reading.
4) The thermistor reading will be displayed above the gauge reading in degrees
centigrade.
5) Press the "Store" button to record the value displayed.
If the no reading displays or the reading is unstable, see Section 5 for troubleshooting
suggestions.
The unit will automatically turn off after approximately two minutes to conserve power.
9
3.4 Measuring Temperatures
All vibrating wire stressmeters are equipped with a thermistor, which gives a varying resistance
output as the temperature changes. The white and green leads of the instrument cable are
normally connected to the internal thermistor.
Readout boxes will read the thermistor and display the temperature in degrees C.
To read temperatures using an ohmmeter: Connect an ohmmeter to the green and white
thermistor leads coming from the stressmeter. (Since the resistance changes with temperature are
large, the effect of cable resistance is usually insignificant. For long cables a correction can be
applied, equal to approximately 14.7 Ω for every 1000 ft., or 48.5Ω per km at 20 °C. Multiply
these factors by two to account for both directions.) Look up the temperature for the measured
resistance in Appendix B, Table 5.
10
Model
EX
BX
NX
Readout initial
display (R0)=
Subsequent
display (R1)=
Values entered into
Equation 1
σ =
(12,000 – 10,000) 0.50
σ =
(5,000 – 4,000) 2.5
σ =
(3,000 – 2,500) 6.0
Change in Stress =
σ = 1,000 psi
σ = 2500 psi
σ = 3,000 psi
4. DATA REDUCTION
4.1 Change in Stress Calculation
To obtain the change in stress at any given time the following equation applies:
σ = (R1 – R0) G
Equation 1 - Change in Stress
Where;
σ = Stress change, in psi.
R0 = Initial reading after the gauge has been set in place.
R1 = Reading at subsequent stress.
G = Sensitivity factor taken from Section 4.3.
Table 1 gives example calculations for the various models.
10,000 4,000 2,500
12,000 5,000 3,000
Table 1 - Sample Calculations
If using the GK-403 Readout, note that the GK-403 excites the gauge, measures the period of
255 cycles (or less) of gauge vibration, using a 6.144 MHz quartz oscillator, and displays the
period to a resolution of 0.1 microseconds in position “A”. Positions “F” and “B” are used for
stressmeters, the processor converts the period readings to units of frequency squared, which is
proportional to wire strain, gauge deflection and applied stress. A reading of 10,000 on channel
“F” corresponds to a period of 316.2 microseconds on channel “A”.
4.2 Environmental Factors
Since the purpose of the stressmeter installation is to monitor site conditions, factors that may
affect these conditions should always be observed and recorded. Seemingly minor effects may
have a real influence on the behavior of the structure being monitored and may give an early
indication of potential problems. Some of these factors include, but are not limited to, blasting,
rainfall, tidal levels, excavation and fill levels and sequences, traffic, temperature and barometric
changes, changes in personnel, nearby construction activities, seasonal changes, etc.
4.3 Gauge Sensitivity Factors
The graphs in Figure 5, Figure 6, and Figure 7 are used to determine the stress sensitivity or
gauge factor for rocks of different moduli. Sensitivity factors are based on experimental data
conducted on rock samples and can only serve as a guide. For more accurate determinations of
stress sensitivity, calibrations must be performed in samples of the rock being monitored.
11
Figure 5 - Model 4300EX Sensitivity Factor vs. Rock Modulus
Figure 6 - Model 4300BX Sensitivity Factor vs. Rock Modulus
12
Figure 7 - Model 4300NX Sensitivity Factor vs. Rock Modulus
4.4 Corrections for Temperature Changes
The materials used in the construction of the stressmeter are affected by changes in ambient
temperature. Since these gauges are normally installed underground in constant temperature
environments, corrections are not normally applied. However, if maximum accuracy is desired,
or the temperature changes are extreme, a correction may be applied.
The temperature correction factor for the gauge reading on a readout box is two digits/°C,
indicating an apparent decrease in rock stress for a temperature rise. Stress correction for
temperature is given by the equation:
σT = (R1 – R0) G + (T1 – T0) 2G
Equation 2 - Temperature Correction
Where;
σT = The stress change corrected for temperature.
R0 = Initial reading after the gauge has been set in place.
R1 = Reading at subsequent stress.
T0 = Initial temperature °C
T1 = Subsequent temperature °C
G = Sensitivity factor taken from Section 4.3.
It should be noted that this temperature correction factor is for a gauge in a free field with no
restraints. In a field condition where the gauge is firmly placed in a borehole the gauge
temperature sensitivity is also dependent on the gauge/rock interactions, and these relationships
are very complex and beyond the scope of this manual. Calibration would be required for
accurate determination of the thermal characteristics of the gauge.
13
5. TROUBLE SHOOTING
Maintenance and troubleshooting of stressmeters is confined to periodic checks of cable
connections and maintenance of terminals. The setting rods should be kept clean and the button
mechanisms kept lightly oiled.
Once installed, the gauges are usually inaccessible and remedial action is limited. Should
difficulties arise, consult the following list of problems and possible solutions. Return any faulty
gauges to the factory. Gauges should not be opened in the field. For additional troubleshooting
and support, contact Geokon.
Symptom: Thermistor resistance is too high:
There may be an open circuit. Check all connections, terminals, and plugs. If a cut is located
in the cable, splice according to instructions in Section 2.6.
Symptom: Thermistor resistance is too low:
There may be a short. Check all connections, terminals, and plugs. If a short is located in the
cable, splice according to instructions in Section 2.6.
Water may have penetrated the interior of the Stressmeter. There is no remedial action.
Symptom: Instrument Readings are Unstable:
Is the readout box position set correctly? If using a datalogger to record readings
automatically, are the swept frequency excitation settings correct?
Is there a source of electrical noise nearby? Likely candidates are generators, motors, arc
welding equipment, high voltage lines, etc. If possible, move the instrument cable away from
power lines and electrical equipment or install electronic filtering.
Make sure the shield drain wire is connected to ground. Connect the shield drain wire to the
readout using the blue clip. (Green for the GK-401.)
Does the readout work with another gauge? If not, it may have a low battery or possibly be
malfunctioning.
Symptom: Instrument Fails to Read:
Is the cable cut or crushed? Check the resistance of the cable by connecting an ohmmeter to
the gauge leads. Table 2 shows the expected resistance for the various wire combinations;
Table 3 is provided for the user to record the observed values. Cable resistance is
approximately 14.7 Ω per 1000' of 22 AWG wire. (Multiply this factor by two to account for
both directions.)
If the resistance is very high or infinite (megohms), the cable is probably broken or cut. If the
resistance is very low (<20Ω), the gauge conductors may be shorted. If a cut or a short is
located in the cable, splice according to the instructions in Section 2.6.
Does the readout or datalogger work with another gauge? If not, it may have a low battery or
possibly be malfunctioning.
14
Red
Black
White
Green
Shield
BX and NX≅ 180Ω
EX ≅90Ω
BX and NX ≅180Ω
EX ≅90Ω
3000Ω at
25°C
3000Ω at
25°C
Vibrating Wire Sensor Lead Grid - SAMPLE VALUES
Red
Black
White
Green
Shield
Red
Black
White
N/A
infinite infinite infinite
N/A infinite infinite infinite
infinite infinite N/A
infinite infinite
N/A infinite
infinite
infinite infinite infinite infinite N/A
Table 2 - Sample Resistance
Vibrating Wire Sensor Lead Grid - SENSOR NAME/## :
Red Black White Green Shield
Green
Shield
Table 3 - Resistance Work Sheet
APPENDIX A. SPECIFICATIONS
Model
EX
BX
NX
Resolution KPa (psi)
2 - 7 (0.25 - 1)
10 - 30 (1.5 - 4)
35 - 140 (5 - 20)
Accuracy2
±20 %
Operating Temperature3
–20 to +80 °C
Thermal Zero Shift %
0.02% F.S./°C
0.04% F.S./°C
0.04% F.S./°C
Resonant Frequency Range
3000 - 5000 Hz
2000 - 3500 Hz
1500 - 2500 Hz
Length mm (inches)
44 (1.75)
70 (2.75)
76 (3.0)
Outer Diameter mm (inches)
29 (1.125)
48 (1.875)
64 (2.50)
Inner Diameter mm (inches)
13 (0.5)
22 (0.875)
32 (1.25)
Weight kgm (lbs.)
0.45 (1)
0.9 (2)
1.4 (3)
Borehole Diameter mm (inches)
38 (1.485)
60 (2.36)
76 (2.98)
Gauge Material
Stainless Steel
Two conductor or four conductor, 22 gauge,
A.1 Model 4300 Stressmeter
15
Nominal Range1
Cable
Table 4 - Specifications
35 - 100 MPa (5000 - 15000 psi)
shielded, PVC jacket, 5 mm or 6 mm dia.
Notes:
1
Depends on rock modulus
2
Accuracy depends to a large extent on the roughness of the borehole walls, gauge stiffness, and
on the degree to which the platens bed into the surrounding material, thus increasing the area of
contact. It also depends on the accuracy with which the host rock elastic constants are known.
3
High temperature versions are available (−20°C to 200°C)
A.2 Thermistor (see Appendix B also)
Range: -80 to +150 °C
Accuracy: ±0.5 °C
16
Ohms
Temp
Ohms
Temp
Ohms
Temp
Ohms
Temp
Ohms
Temp
201.1K
-50
16.60K
-10
2417
+30
525.4
+70
153.2
+110
187.3K
-49
15.72K
-9
2317
31
507.8
71
149.0
111
174.5K
-48
14.90K
-8
2221
32
490.9
72
145.0
112
162.7K
-47
14.12K
-7
2130
33
474.7
73
141.1
113
151.7K
-46
13.39K
-6
2042
34
459.0
74
137.2
114
141.6K
-45
12.70K
-5
1959
35
444.0
75
133.6
115
132.2K
-44
12.05K
-4
1880
36
429.5
76
130.0
116
123.5K
-43
11.44K
-3
1805
37
415.6
77
126.5
117
115.4K
-42
10.86K
-2
1733
38
402.2
78
123.2
118
107.9K
-41
10.31K
-1
1664
39
389.3
79
119.9
119
101.0K
-40
9796 0 1598
40
376.9
80
116.8
120
94.48K
-39
9310
+1
1535
41
364.9
81
113.8
121
88.46K
-38
8851 2 1475
42
353.4
82
110.8
122
82.87K
-37
8417 3 1418
43
342.2
83
107.9
123
77.66K
-36
8006 4 1363
44
331.5
84
105.2
124
72.81K
-35
7618 5 1310
45
321.2
85
102.5
125
68.30K
-34
7252 6 1260
46
311.3
86
99.9
126
64.09K
-33
6905 7 1212
47
301.7
87
97.3
127
60.17K
-32
6576 8 1167
48
292.4
88
94.9
128
56.51K
-31
6265 9 1123
49
283.5
89
92.5
129
53.10K
-30
5971
10
1081
50
274.9
90
90.2
130
49.91K
-29
5692
11
1040
51
266.6
91
87.9
131
46.94K
-28
5427
12
1002
52
258.6
92
85.7
132
44.16K
-27
5177
13
965.0
53
250.9
93
83.6
133
41.56K
-26
4939
14
929.6
54
243.4
94
81.6
134
39.13K
-25
4714
15
895.8
55
236.2
95
79.6
135
36.86K
-24
4500
16
863.3
56
229.3
96
77.6
136
34.73K
-23
4297
17
832.2
57
222.6
97
75.8
137
32.74K
-22
4105
18
802.3
58
216.1
98
73.9
138
30.87K
-21
3922
19
773.7
59
209.8
99
72.2
139
29.13K
-20
3748
20
746.3
60
203.8
100
70.4
140
27.49K
-19
3583
21
719.9
61
197.9
101
68.8
141
25.95K
-18
3426
22
694.7
62
192.2
102
67.1
142
24.51K
-17
3277
23
670.4
63
186.8
103
65.5
143
23.16K
-16
3135
24
647.1
64
181.5
104
64.0
144
21.89K
-15
3000
25
624.7
65
176.4
105
62.5
145
20.70K
-14
2872
26
603.3
66
171.4
106
61.1
146
19.58K
-13
2750
27
582.6
67
166.7
107
59.6
147
18.52K
-12
2633
28
562.8
68
162.0
108
58.3
148
17.53K
-11
2523
29
543.7
69
157.6
109
56.8
149
Table 5 - Thermistor Resistance versus Temperature
55.6
150
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION
Thermistor Type: YSI 44005, Dale #1C3001-B3, Alpha #13A3001-B3
Resistance to Temperature Equation:
1
T=
A+B(LnR)+C(LnR)
Equation 3 - Resistance to Temperature
Where;
T = Temperature in °C.
LnR = Natural Log of Thermistor Resistance
A = 1.4051 × 10-3
B = 2.369 × 10-4
C = 1.019 × 10-7
Note: Coefficients calculated over the −50 to +150° C. span.
-273.2
3
17
APPENDIX C. BIAXIAL STRESS CHANGES
The relationship between the radial deformation of a borehole, U, and two principle stresses in
the plane of a borehole has been given by Hast (1958) and Merrill and Peterson (1961). The
equation for Plane Stress is:
U = d/Er [(σ1 + σ2) + 2 (σ1 - σ2) cos 2 θ]
Equation 4 - Plane Stress
Where;
σ1 and σ
are the principle stresses in the plane of the borehole.
2
θ is the angle measured counterclockwise from the direction of σ1.
d is the diameter of the borehole.
Er is the Young’s modulus of the rock
If it is assumes that the stress measured across the stressmeter is proportional to the radial
deformation that would have occurred in this direction if the stressmeter had not been there, then
the term d/Er can be replaced by one reflecting the relationship between the rock modulus and
the gauge modulus. Hast (1958) has shown this to be applicable for a uniaxial stressmeter.
For the measurement of stress (σR) in any direction (θ) the following equation applies:
(Note: θ is measured counterclockwise from σ1)
σR = 1/3 (σ1 + σ2) + 2/3 (σ1 - σ2) cos 2 θ
Equation 5 - Stress in any Direction
Using this relationship and three uniaxial stress change measurements at 45° to each other, the
secondary principle stresses σ1 and σ2 and the angle (θ) are given by:
σ1 = 3/2 a + ¾ b
σ
= 3/2 a – ¾ b
2
θ = 1/2 sin
Equation 6 - Secondary Principle Stresses and Angle
-1
((a – σ45)/b)
Where;
a = σ0 + σ90 /2
b = [(σ45 – a)2 +(σ0 – a)2]
1/2
To determine the θ angles, it must be determined what quadrant the angle lies in. The
inequalities to do this are as follows:
If σ45≤ a and σ0≥ 90, then 0≤ θ ≥45°
If σ45≤ a and σ0≤. 90, then 45° ≤ θ ≤ 90°
If σ45≥ a and σ0≤ 90, then 90° ≤ θ ≤ 135°
If σ45≥ a and σ0≥ 90, then 135° ≤ θ ≤ 180°
Note: θ is measured clockwise for σ0 (this is same as counterclockwise for σ1).
18
For Example:
Three gauges are set in borehole. The first is at 0° (σ0), the second at 45° (σ45) and the third at
90° (σ90), measured counterclockwise from 0. The uniaxial stress changes for each gauge are
determined by the reading change times the calibration factor. Substitute the constants into the
equations to obtain the magnitude of the changes of the two secondary principal stresses σ1
relative to 0°.
Stress Changes:
Gauge 1, σ0 = 600 psi
Gauge 2, σ45 = 800 psi
Gauge 3, σ90 = 300 psi
Calculate the values for constants, a and b:
a = σ0 + σ
b = [(σ45 – a)2 + (σ0 – a)2]
2 = 600 + 300/2 = 450
90/
1/2
= [(800-450)2 + (600-450)2]
1/2
= 380.79
σ1 = 3/2a + 3/4b = 3 x 450/2 + 3 x 380.79/4 = 960.59 psi
σ2 = 3/2a – 3/4b = 3 x 450/2 – 3 x 380.79/4 = 389.41 psi
sin 2θ = -0.92
θ = 33.40°
σ1 Direction: since σ45 > a and σ0 > σ90, then 135 < θ < 180°. Therefore, θ = 180 – 33.40 =
146.6°. This is measured clockwise from σ0.
References:
Hast, N.; THE MEASUREMENT OF ROCK PRESSURE IN MINES;
Sveriges Geologiska Undersokning, Arsbok 52, Series C, 3. 1958.
Merrill, R.H. and Peterson, J.R.; DEFORMATION OF A BORE HOLE IN ROCK;
U.S. Bureau of Mines, RI 5881.
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