Geokon 4850 User manual

Instruction Manual
Model 4850
N.A.T.M. Style
V.W. Concrete Stress cells
No part of this instruction manual may be reproduced, by any means, without the written consent of Geokon®.
The information contained herein is believed to be accurate and reliable. However, Geokon®assumes no responsibility for errors,
omissions or misinterpretation. The information herein is subject to change without notification.
Copyright © 1993-2019 by Geokon®
(Doc Rev I, 05/1/19)

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. INTRODUCTION ...................................................................................................................................................1
1.1 THEORY OF OPERATION.......................................................................................................................................1
1.2 STRESS CELL DESIGN AND CONSTRUCTION.........................................................................................................2
2. INSTALLATION ....................................................................................................................................................3
2.1 PRELIMINARY TESTS............................................................................................................................................3
2.2 STRESS CELL INSTALLATION ...............................................................................................................................3
2.2.1 Installing the Model 4850-1........................................................................................................................3
2.2.2 Installing the Model 4850-2........................................................................................................................5
2.3 INITIAL READINGS ...............................................................................................................................................5
2.4 RE-PRESSURIZING THE CELL ...............................................................................................................................6
2.4.1 Standard Re-Pressurization Technique.......................................................................................................6
2.4.2 Remote Re-Pressurization Technique .........................................................................................................7
2.5 CABLE INSTALLATION .........................................................................................................................................8
2.6 ELECTRICAL NOISE..............................................................................................................................................8
3. TAKING READINGS.............................................................................................................................................9
3.1 GK-404 READOUT BOX.......................................................................................................................................9
3.1.1 Operating the GK-404 ................................................................................................................................9
3.2 GK-405 READOUT BOX.....................................................................................................................................10
3.2.1 Connecting Sensors...................................................................................................................................10
3.2.2 Operating the GK-405 ..............................................................................................................................10
3.3 GK-403 READOUT BOX (OBSOLETE MODEL)....................................................................................................11
3.3.1 Connecting Sensors...................................................................................................................................11
3.3.2 Operating the GK-403 ..............................................................................................................................11
3.4 MEASURING TEMPERATURES.............................................................................................................................11
4. DATA REDUCTION ............................................................................................................................................12
4.1 PRESSURE CALCULATION ..................................................................................................................................12
4.2 TEMPERATURE CORRECTION .............................................................................................................................12
4.3 BAROMETRIC CORRECTION ...............................................................................................................................12
5. TROUBLESHOOTING........................................................................................................................................13
6. APPENDIX A. SPECIFICATIONS.....................................................................................................................14
A.1 STRESS CELLS...................................................................................................................................................14
A.2 THERMISTOR (SEE APPENDIX BALSO) ..............................................................................................................14
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION.....................................................................15
APPENDIX C. TEMPERATURE EFFECT ON EARTH PRESSURE AND CONCRETE STRESS CELLS 16
C.1 FORMULAS ........................................................................................................................................................16
C.2 EXAMPLES ........................................................................................................................................................19
APPENDIX D. TYPICAL CALIBRATION REPORT..........................................................................................21

FIGURES
FIGURE 1-GROUND REACTION CURVE ......................................................................................................................... 1
FIGURE 2-MODEL 4850 CONCRETE STRESS CELL ........................................................................................................ 2
FIGURE 3-MODEL 4850-1 INSTALLATION .................................................................................................................... 4
FIGURE 4-MODEL 4850-1 INSTALLATION DETAIL ....................................................................................................... 4
FIGURE 5-MODEL 4850-2 INSTALLATION DETAIL ....................................................................................................... 5
FIGURE 6-CELL RE-PRESSURIZATION GRAPH .............................................................................................................. 6
FIGURE 7-MODEL 4850 WITH REMOTE PINCHING APPARATUS .................................................................................... 7
FIGURE 8-LEMO CONNECTOR TO GK-404 ................................................................................................................... 9
FIGURE 9-LIVE READINGS –RAW READINGS..............................................................................................................10
FIGURE 10 -RADIUS (R) AND THICKNESS (D) ..............................................................................................................16
FIGURE 11 -TYPICAL CALIBRATION REPORT ...............................................................................................................21
TABLES
TABLE 1-SPECIFICATIONS ...........................................................................................................................................14
TABLE 2-THERMISTOR RESISTANCE VERSUS TEMPERATURE ......................................................................................15
TABLE 3-TYPICAL VALUES OF VARIOUS CELL PARAMETERS .....................................................................................18
EQUATIONS
EQUATION 1-CONVERT DIGITS TO PRESSURE .............................................................................................................12
EQUATION 2-TEMPERATURE CORRECTION FOR TRANSDUCER ONLY..........................................................................12
EQUATION 3-RESISTANCE TO TEMPERATURE .............................................................................................................15
EQUATION 4-EXPANSION OF LIQUID FOR A TEMPERATURE RISE OF 1°C....................................................................16
EQUATION 5-COMPRESSION OF LIQUID.......................................................................................................................16
EQUATION 6-EXPANSION OF LIQUID ...........................................................................................................................16
EQUATION 7-DEFORMATION AT THE CENTER .............................................................................................................17
EQUATION 8-DEFORMATION AT THE EDGE .................................................................................................................17
EQUATION 9-DIFFERENCE IN DEFORMATION ..............................................................................................................17
EQUATION 10 -AVERAGE TOTAL EXPANSION OF THE CELL.........................................................................................17
EQUATION 11 -COMBINED EQUATIONS........................................................................................................................17
EQUATION 12 -TOTAL EMBEDMENT ............................................................................................................................18
EQUATION 13 -TOTAL EMBEDMENT FOR CONTACT PRESSURE CELLS .........................................................................18

1. INTRODUCTION
1.1 Theory of Operation
The “New Austrian Tunneling Method”, or N.A.T.M., calls for the support of a tunnel by the
rapid application of shotcrete to the freshly exposed ground. The theory behind this method of
support, particularly useful in weaker ground, is that if the inherent strength of the ground can be
preserved, it will be almost self-supporting and will require much less artificial support in the
form of concrete or steel. To preserve the inherent cohesion of the ground it is necessary to
prevent it from breaking up in the first place and, hence, the need for a rapidly applied layer of
shotcrete.
Support Deformation
Ground Characteristic
In-Situ Stress Level
Radial
Support
Pressure
Failure
A
B
C
D
(A,B,C,D)
Figure 1 - Ground Reaction Curve
The above figure graphically shows the ground reaction curve, i.e., the amount of support
required versus the amount of inherent support and ground deformation. Thus, to prevent any
support deformation (or tunnel closure) at all, would require a support pressure exerted on the
tunnel walls equal to the original in-situ ground stress.
A strong lining with characteristics of curve A would allow only a small amount of ground
deformation, but might, because it is too strong, be uneconomical. A thinner lining which would
allow more deformation would have characteristics of curves B or C. However, a lining which is
too thin, with characteristics shown by curve D, would allow too much deformation of the rock
allowing it to weaken and ultimately fail.
The task of the N.A.T.M. stress cells is to provide a measure of the support pressure which, when
coupled with a measurement of tunnel closure, using a tape extensometer, will allow an
assessment to be made of the adequacy of the shotcrete lining, indicating the need for perhaps
more or less shotcrete to maintain stability. It is this ability to monitor the performance of the
shotcrete lining that can lead to significant reductions in tunnel support costs.

2
1.2 Stress Cell Design and Construction
The basic cell is comprised of two stainless steel rectangular plates welded together around their
periphery, leaving a thin space between the plates which is then filled with de-aired hydraulic
oil*. This fluid filled space is connected via a pressure tube to a vibrating wire pressure sensor.
Pressure applied normal to the plate is balanced by a corresponding build-up of internal fluid
pressure which is measured by the sensor.
Lugs are provided at the corners of the rectangular plates to facilitate holding the cells in place
while the shotcrete is applied.
One further refinement is required; this is the pinch tube, which is filled with mercury or de-aired
hydraulic oil and is connected at one end to the fluid filled space between the plates and the other
end is capped. The purpose of this pinch tube is to inflate the cell when the concrete around it has
fully cured and has cooled off to the ambient temperature. During concrete curing, temperatures
very often rise and will cause the cell to expand in the still green concrete. On cooling, the cell
contracts leaving a space between it and the surrounding concrete which, if allowed to remain,
would prevent the transmission of pressures from the concrete to the cell.
Pinch Tube Stress Cell Transducer Housing Instrument Cable
(4 conductor, 22 AWG)
Mounting Lug
Seal Screw
(4 places)
Side View
Top View
Figure 2 - Model 4850 Concrete Stress Cell
The vibrating wire sensor is a standard Geokon Model 4500H transducer inside an all welded
housing. The sensor is hermetically sealed and is connected via waterproof connectors to an
electrical cable leading to the readout location. The sensor housing also incorporates a thermistor
which permits measurement of temperature at the cell location.
* Most other commercially available concrete stress cells are filled with mercury in order to
achieve a sufficient cell stiffness. However, the filling procedures and the construction details of
the Geokon cell are such that mercury is not required.

3
2. INSTALLATION
2.1 Preliminary Tests
It is always wise, before installation commences, to check the cells for proper functioning. Each
cell is supplied with a calibration report which shows the relationship between readout digits and
pressure and shows the initial no load zero reading. The cell electrical leads (usually the red and
black leads) are connected to a readout box and the zero reading given on the sheet is now
compared to a current zero reading. (See Section 3 for readout instructions.) The two readings
should not differ by more than ≈50 digits after due regard to corrections made for different
temperatures, barometric pressures and height above sea level and actual cell position (whether
standing up or laying down).
By pressing on the cell, it should be possible to change the readout digits, causing them to fall as
the pressure is increased.
Checks of electrical continuity can also be made using an ohmmeter. Resistance between the
gauge leads should be approximately 180 ohms, ±10 ohms. Remember to add cable resistance
when checking (22 AWG stranded copper leads are approximately 14.7Ω/1000' or 48.5Ω/km,
multiply by two for both directions). Resistance between the green and white conductors varies
with the temperature. Compare the measured resistance with the values given in Table 2 of
Appendix B. Resistance between any conductor and the shield should exceed 20 megohm.
2.2 Stress Cell Installation
Cells are positioned on the wall of the tunnel in two ways, one way to measure tangential stresses
and the other to measure radial.
2.2.1 Installing the Model 4850-1
The Model 4850-1 is designed to measure tangential stresses in the lining. Figure 3 shows
one method of installation using short pieces of steel rebar grouted inside short boreholes
and protruding into the area where the lining will be placed.
The pressure cells are tied to these rebars using soft iron wire connected to the lugs at the
corners of the cell. The cable is fixed firmly to other pieces of rebar or to the reinforcing
mesh, if one is used, and is strung out to the readout location which typically is made of a
metal junction box with removable hinged cover. The cable is terminated inside this box.
Sufficient cable is coiled inside the box to allow it to be pulled out and connected to a
portable readout box.
Note: It is very important that the concrete makes intimate contact with the
pressure cell. Therefore, the concrete should be sprayed on, first from below and
then, after removing any rebound material, from above. The person spraying the
concrete should receive special instructions so that no open shadow zones are
created next to the cell.

4
Tunnel Cross Section
Shotcrete Lining
Model 4850-1
Rebar Dowel
Pinch Tube
Junction Box Junction Box
Instrument Cable
Figure 3 - Model 4850-1 Installation
The pinch tube is bent so that it will protrude from the lining after it has been placed. Or it
can be wrapped in foam, plastic, etc. so that it can be dug out and retrieved after
shotcreting.
Tie WireShotcrete Lining
Rebar Dowels
Pinch TubeModel 4850-1 Instrument Cable Junction Box
(grouted into boreholes)
Figure 4 - Model 4850-1 Installation Detail