Geokon 4820 User manual

Instruction Manual
Model 4800 Series
VW Earth Pressure Cells
No part of this instruction manual may be reproduced, by any means, without the written consent of Geokon, Inc.
The information contained herein is believed to be accurate and reliable. However, Geokon, Inc. assumes no responsibility for
errors, omissions, or misinterpretation. The information herein is subject to change without notification.
Copyright © 1984-2018 by Geokon, Inc.
(REV S, 04/13/2018)

Warranty Statement
Geokon, Inc. 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, Inc. 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, Inc. or any breach of any warranty by Geokon, Inc. shall not exceed the
purchase price paid by the purchaser to Geokon, Inc. 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, Inc. 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 EARTH PRESSURE CELL DESIGN ..........................................................................................................................3
1.3 EARTH PRESSURE CELL CONSTRUCTION .............................................................................................................4
1.3.1 Model 4800 Earth Pressure Cells...............................................................................................................4
1.3.2 Model 4810 Contact ("Fat Back") Pressure Cell........................................................................................5
1.3.3 Model 4815 Hydraulic Load Cell ...............................................................................................................5
1.3.4 Model 4820 Earth Pressure "Jackout" Cell................................................................................................6
1.3.5 Model 4830 Push-In Pressure Cell.............................................................................................................6
2. INSTALLATION ....................................................................................................................................................7
2.1 PRELIMINARY TESTS............................................................................................................................................7
2.2 PRESSURE CELL INSTALLATION...........................................................................................................................7
2.2.1 Installation of Model 4800 Earth Pressure Cells Inside Fills and Embankments ......................................7
2.2.2 Installation of Model 4810 Contact ("Fat Back") Pressure Cell ..............................................................10
2.2.3 Installation of Model 4815 Hydraulic Load Cell......................................................................................12
2.2.4 Installation of Model 4820 Jackout Pressure Cell in Slurry Trenches .....................................................13
2.2.5 Installation of Cells to Measure Earth Pressure at the Base of Footings, Floor Slabs, Pavements, Etc..14
2.2.6 Installation of Push-In Pressure Cells to Measure Lateral Earth Pressures............................................15
2.3 CABLE INSTALLATION AND SPLICING ................................................................................................................16
2.4 ELECTRICAL NOISE............................................................................................................................................17
2.5 INITIAL READINGS.............................................................................................................................................17
3. TAKING READINGS...........................................................................................................................................18
3.1 GK-404 READOUT BOX.....................................................................................................................................18
3.1.1 Operating the GK-404 ..............................................................................................................................18
3.2 GK-405 READOUT BOX.....................................................................................................................................19
3.2.1 Connecting Sensors with 10-pin Bulkhead................................................................................................19
3.2.2 Connecting Sensors with Bare Leads........................................................................................................19
3.2.3 Operating the GK-405 ..............................................................................................................................19
3.3 GK-403 READOUT BOX (OBSOLETE MODEL)....................................................................................................20
3.3.1 Connecting Sensors with 10-pin Bulkhead................................................................................................20
3.3.2 Connecting Sensors with Bare Leads........................................................................................................20
3.3.3 Operating the GK-403 ..............................................................................................................................20
3.4 MEASURING TEMPERATURES.............................................................................................................................21
4. DATA REDUCTION ............................................................................................................................................22
4.1 PRESSURE CALCULATION ..................................................................................................................................22
4.2 TEMPERATURE CORRECTION.............................................................................................................................23
4.3 BAROMETRIC CORRECTION ...............................................................................................................................23
5. TROUBLESHOOTING........................................................................................................................................25
APPENDIX A. SPECIFICATIONS.........................................................................................................................27
A.1 EARTH PRESSURE CELLS ..................................................................................................................................27
A.2 STANDARD TEMPERATURE THERMISTOR..........................................................................................................27
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION.....................................................................28
APPENDIX C. HIGH TEMPERATURE THERMISTOR LINEARIZATION..................................................29
APPENDIX D. TEMPERATURE EFFECT ON EARTH PRESSURE AND CONCRETE STRESS CELLS 30
D.1 FORMULAS........................................................................................................................................................30
D.2 EXAMPLES ........................................................................................................................................................33
APPENDIX E. NON LINEARITY AND THE USE OF A SECOND ORDER POLYNOMIAL TO IMPROVE
THE ACCURACY OF THE CALCULATED PRESSURE..................................................................................35

FIGURES
FIGURE 1-STRESS REDISTRIBUTION,WEAK SOIL WITH STIFF CELL............................................................................. 2
FIGURE 2-STRESS REDISTRIBUTION,STRONG SOIL WITH STIFF CELL.......................................................................... 2
FIGURE 3-STRESS REDISTRIBUTION,STIFF SOIL WITH WEAK CELL............................................................................. 2
FIGURE 4-MODEL 4800 RECTANGULAR EARTH PRESSURE CELL................................................................................. 4
FIGURE 5-MODEL 4800 CIRCULAR EARTH PRESSURE CELL ........................................................................................ 4
FIGURE 6-MODEL 4810 CONTACT PRESSURE CELL ..................................................................................................... 5
FIGURE 7-MODEL 4815 HYDRAULIC LOAD CELL ........................................................................................................ 5
FIGURE 8-MODEL 4820 JACKOUT PRESSURE CELL ...................................................................................................... 6
FIGURE 9-MODEL 4830 PUSH-IN PRESSURE CELL ....................................................................................................... 6
FIGURE 10 -MODEL 4800 EARTH PRESSURE CELL INSTALLATION................................................................................ 8
FIGURE 11 -ATTACHMENT OF MODEL 4810 TO CONCRETE FORM ...............................................................................10
FIGURE 12 -MODEL 4810 CONTACT PRESSURE CELL INSTALLATION ..........................................................................11
FIGURE 13 -MODEL 4815 HYDRAULIC LOAD CELL MEASURING LOADS ON A TUNNEL LINING...................................12
FIGURE 14 -MODEL 4820 JACKOUT PRESSURE CELL INSTALLATION...........................................................................13
FIGURE 15 -MODEL 4800-1-1P EARTH PRESSURE CELL INSTALLATION......................................................................14
FIGURE 16 -LEMO CONNECTOR TO GK-404 ................................................................................................................18
FIGURE 17 -LIVE READINGS –RAW READINGS............................................................................................................19
FIGURE 18 -SAMPLE MODEL 4800 CALIBRATION SHEET .............................................................................................24
FIGURE 19 -RADIUS (R) AND THICKNESS (D) ..............................................................................................................30
TABLES
TABLE 1-RATIOS FOR TWO GROUT MIXES................................................................................................................... 9
TABLE 2-ENGINEERING UNITS MULTIPLICATION FACTORS ........................................................................................22
TABLE 3-SAMPLE RESISTANCE ...................................................................................................................................26
TABLE 4-RESISTANCE WORK SHEET...........................................................................................................................26
TABLE 5-EARTH PRESSURE CELL SPECIFICATIONS.....................................................................................................27
TABLE 6-THERMISTOR RESISTANCE VERSUS TEMPERATURE......................................................................................28
TABLE 7-THERMISTOR RESISTANCE VERSUS TEMPERATURE FOR HIGH TEMPERATURE MODELS...............................29
TABLE 8-TYPICAL VALUES OF VARIOUS CELL PARAMETERS .....................................................................................32
EQUATIONS
EQUATION 1-TERZAGHI’S PRINCIPLE OF EFFECTIVE STRESS ....................................................................................... 1
EQUATION 2-DIGITS CALCULATION............................................................................................................................22
EQUATION 3-CONVERT DIGITS TO PRESSURE .............................................................................................................22
EQUATION 4-TEMPERATURE CORRECTION FOR THE TRANSDUCER ONLY...................................................................23
EQUATION 5-RESISTANCE TO TEMPERATURE .............................................................................................................28
EQUATION 6-HIGH TEMPERATURE RESISTANCE TO TEMPERATURE............................................................................29
EQUATION 7-EXPANSION OF LIQUID FOR A TEMPERATURE RISE OF 1°C....................................................................30
EQUATION 8-COMPRESSION OF LIQUID.......................................................................................................................30
EQUATION 9-EXPANSION OF LIQUID ...........................................................................................................................30
EQUATION 10 -DEFORMATION AT THE CENTER ...........................................................................................................31
EQUATION 11 -DEFORMATION AT THE EDGE ...............................................................................................................31
EQUATION 12 -DIFFERENCE IN DEFORMATION ............................................................................................................31
EQUATION 13 -AVERAGE TOTAL EXPANSION OF THE CELL.........................................................................................31
EQUATION 14 -COMBINED EQUATIONS........................................................................................................................31
EQUATION 15 -TOTAL EMBEDMENT ............................................................................................................................32
EQUATION 16 -TOTAL EMBEDMENT FOR CONTACT PRESSURE CELLS .........................................................................32
EQUATION 17 -PRESSURE CALCULATION WITH SECOND ORDER POLYNOMIAL ...........................................................35
EQUATION 18 -“LINEARITY (%F.S.)” ON CALIBRATION SHEET ...................................................................................35
EQUATION 19 -CALCULATING CUSING THE ZERO PRESSURE READING FROM THE CALIBRATION SHEET ...................36

1
1. INTRODUCTION
1.1 Theory of Operation
Earth Pressure Cells, sometimes called Total Pressure Cells or Total Stress Cells are designed to
measure stresses in soil or the pressure of soil on structures. Cells will respond not only to soil
pressures but also to ground water pressures or to pore water pressure, hence the term total
pressure or total stress. A simultaneous measurement of pore water pressure (µ), using a
piezometer, is necessary to separate the effective stress (σ') from the total stress (σ) as defined by
Terzaghi's principle of effective stress:
σ' =σ- µ
Equation 1 - Terzaghi’s Principle of Effective Stress
These parameters coupled with the soil strength characteristics will determine soil behavior
under loads.
Earth pressure cells of the type described here are the hydraulic type; two flat plates are welded
together at their periphery and are separated by a small gap filled with a hydraulic fluid. The
earth pressure acts to squeeze the two plates together thus building up a pressure inside the fluid.
If the plates are flexible enough (i.e., if they are thin enough relative to their lateral extent), then
at the center of the plate the supporting effect of the welded periphery is negligible and it can be
stated that at the center of the cell the external soil pressure is exactly balanced by the internal
fluid pressure.
This is true only if the deflection of the plates is kept to a minimum and thus it is important that
the cell be stiff. This in a practical sense means that the fluid inside the cell should be as
incompressible as possible and that the pressure transducer required to measure the fluid pressure
should also be stiff having very little volume change under increasing pressure.
Tests conducted by various researchers (as reported by Dunnicliff, 1988) have shown that the
introduction of a flat stress cell into a soil mass will alter the stress field in a way dependent on
the relative stiffness of the cell, with respect to the soil, and also with respect to the aspect ratio
of the cell, i.e., the ratio of the width of the cell to its thickness. A thick cell will alter the stress
more than a thin cell. For these reasons, a thin, stiff cell is best and studies have shown an aspect
ratio of at least 20 to 1 to be desirable.
Ideally, the cell ought to be as stiff (compressible) as the soil, but in practice this is difficult to
achieve. If the cell is stiffer (less compressible) than the soil then it will over register the soil
pressure because of a zone of soil immediately around the cell which is "sheltered" by the cell
and therefore does not experience the full soil pressure. This can be represented schematically as
shown in Figure 1.

2
Cell
0
Mean
Stress
Figure 1 - Stress Redistribution, Weak Soil with Stiff Cell
As can be seen there is a stress concentration at the rigid rim but in the center of the cell the soil
stress is only slightly higher than the mean soil stress, i.e., only slightly higher than the stress
which would obtain were the cell not present.
In a stronger soil, the destressed zone around the edge of the cell is more extensive; therefore, the
degree of over registration of the mean stress is greater at the center of the cell. This is
represented schematically in Figure 2.
Cell
0
Mean
Stress
Figure 2 - Stress Redistribution, Strong Soil with Stiff Cell
In a stiff soil the cell may be less stiff (more compressible) than the soil, in which case the cell
will under register the mean soil stress as the stresses in the soil tend to "bridge" around the cell.
This is represented schematically in Figure 3.
Cell
0
Mean
Stress
Figure 3 - Stress Redistribution, Stiff Soil with Weak Cell

3
Tests conducted at the University of Ohio (Ohio, USA) with several different soil types have
shown that for Geokon cells the maximum degree of over or under registration amounts to 15%
of the mean soil stress.
Other factors should be kept in mind. The inherent variability of soil properties, which give rise
to varying soil stresses at different locations, and a corresponding difficulty in getting a good
sample of the mean stress from a limited number of cell locations. In addition, the response of
the cell to its immediate surroundings depends mostly on how closely the soil mass immediately
around the cell has the same stiffness or compressibility or the same degree of compaction as the
undisturbed soil mass. Installation methods will need to pay particular attention to this
detail.
1.2 Earth Pressure Cell Design
Earth Pressure Cells are constructed from two stainless steel plates welded together around the
periphery to leave a narrow space between them. This space is completely filled with de-aired
hydraulic oil, which is connected hydraulically to a pressure transducer. The pressure transducer
converts the oil pressure into an electrical signal, which is transmitted through a signal cable to
the readout location.
In general, Geokon Earth Pressure Cells use an all welded construction; this means the space
confining the oil is entirely metal and does not require any o-rings, which tend to trap air and
reduce the cell stiffness. The oil is de-aired using a Nold DeAerator,which materially improves
the fluid stiffness and the performance of the cell. The pressure transducer normally employed is
the Geokon Model 4500H, which is available in several different pressure ranges (see Appendix
A.1). The cable is attached to the transducer in a sealed, waterproof manner. For earth pressure
cells located inside a soil mass, the cable may be armored and provided with strain relief at the
cell to reduce the likelihood of pullout.
Located inside the vibrating wire pressure transducer housing is a thermistor for the
measurement of temperature at the cell location. In addition, a tripolar plasma surge arrestor
inside the transducer housing protects the vibrating wire pluck and read coils from electrical
transients such as may be induced by direct or indirect lightning strikes.
Alternative pressure transducers with voltage (0-100 mV, 0-5 VDC, 0-10 VDC) or current (4-20
mA) output are also available for dynamic readout capability. Consult the factory for additional
information.

4
1.3 Earth Pressure Cell Construction
1.3.1 Model 4800 Earth Pressure Cells
Model 4800 Earth Pressure Cells may be rectangular or circular in shape. The standard
size for the rectangular Model 4800 is 150 mm ×250 mm (6" ×10"), for the circular it is
230 mm (9") in diameter. Standard thickness for both styles is 6 mm (aspect ratio ≈40).
For laboratory tests, smaller, thinner cells can be manufactured. Contact the factory for
additional information.
Pressure Cell Transducer Housing Instrument Cable
(4 conductor, 22 AWG)
Side View
Top View
6"
150 mm
10"
250 mm
Figure 4 - Model 4800 Rectangular Earth Pressure Cell
Pressure Cell Transducer Housing Instrument Cable
(4 conductor, 22 AWG)
Side View
Top View
9"
230 mm
Figure 5 - Model 4800 Circular Earth Pressure Cell

5
1.3.2 Model 4810 Contact ("Fat Back") Pressure Cell
Model 4810 Earth Pressure Cells are designed for measuring soil pressures on structures.
One of the plates is thick and designed to bear against the external surface of the structure
in a way that will prevent flexure of the cell. The other plate is thin and reacts to the soil
pressure.
Pressure Cell Transducer Housing Instrument Cable
(4 conductor, 22 AWG)
Side View
Top View
9"
230 mm
Mounting Lugs (4 places)
Thin Pressure Sensitive Plate
Figure 6 - Model 4810 Contact Pressure Cell
1.3.3 Model 4815 Hydraulic Load Cell
Model 4815 Hydraulic Load Cell has been used for the measurement of loads in piles and
of concentrated loads on tunnel linings. The pressure transducer housing is connected
directly and perpendicular to the thick back plate.
Figure 7 - Model 4815 Hydraulic Load Cell

6
1.3.4 Model 4820 Earth Pressure "Jackout" Cell
Model 4820 Earth Pressure Cells are designed specifically for the measurement of soil
pressures on the back side of slurry walls. The pressure transducer housing is connected
directly and perpendicular to the thick back plate.
Pressure Cell
Transducer Housing
Instrument Cable
(4 conductor, 22 AWG)
Bottom View Side View
6"
150 mm
Back Plate (with mounting holes)
5"
125 mm
(6 places, 6.75 mm ID)
Mounting Hole
Figure 8 - Model 4820 Jackout Pressure Cell
1.3.5 Model 4830 Push-In Pressure Cell
Model 4830 Push-In Pressure Cells are designed to be pushed in place for the
measurement of total pressures in soils and earth fills. The semiconductor pressure
transducer enables measurement of dynamic pressures. A thread is provided on the end of
the cell to allow for installation using lengths of pipe or drill rods.
Figure 9 - Model 4830 Push-In Pressure Cell

7
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 sheet, which shows the relationship between readout digits and
pressure, as well as the initial no load zero reading. (Figure 18 in Section 4 shows a typical
calibration sheet.)The cell electrical leads (usually the red and black leads) are connected to a
readout box (see Section 3) and the zero reading given on the calibration sheet is compared to the
current zero reading. 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
gage leads should be approximately 180 ohms, ± 5%. Check the resistance between the two
thermistor wires (usually white and green). Using Table 6 in Appendix B, convert the resistance
to temperature. Compare the result to the current ambient temperature. (For Model 4800HT see
Table 7 in Appendix C.) Resistance between any conductor and the shield should exceed 20
megohms. Remember to add cable resistance when checking (22 AWG stranded copper leads are
approximately 14.7Ωper 1,000 feet (48.5Ωper km), multiply by two for both directions).
2.2 Pressure Cell Installation
2.2.1 Installation of Model 4800 Earth Pressure Cells Inside Fills and Embankments
Earth pressure cells are normally installed with the flat surfaces horizontal to measure
vertical stresses. However, they can be placed at other orientations, inside the fill, to
measure stresses in other directions e.g., a cell placed with the flat surfaces vertical will
measure horizontal stresses in a direction perpendicular to the plates of the cell. They are
sometimes placed at angles of 45 degrees.
Experience has shown that attempts to measure earth pressures in fills frequently meets
with failure. The problem is twofold. First, the stress distribution in the fill can be
inherently variable due to varying properties of the ground and varying degrees of
compaction of the ground. Thus, the soil stress at one location may not be typical of the
surrounding locations. Secondly, a cell installed directly in the fill could result in the
creation of an anomalous zone immediately around the cell where there may be a
different, more fine-grained material, under a lesser degree of compaction. (The material
around the cell may be poorly compacted because of the need to avoid damage to the
cell.)

8
In an earth fill, this zone of poor compaction would not be expected to be a problem since
the earth above might be expected to move downwards to fill the voids and consolidate
the ground. However, under the influence of rainwater and vibration, any spaces in the
soil immediately around, and especially under, the cell may grow, causing the cell to
become completely decoupled from the soil around it. In such situations, the internal soil
stresses go around the cell instead of through it. The cell will then register only a very
low pressure, which does not change much as the loads increase. This situation occurs
frequently.
2.2.1.1 Weak Grout Method
One way to avoid the problem is to cast the cell inside a weak grout. A method used
successfully in South Africa, by Oosthuizen et al, essentially uses the techniques similar
to the one described in Section 2.2.5. Installation of the cells begins when the fill has
reached a height of one meter above the instrument level. The Instrument location and the
cable trenches are excavated one meter deep, the instrument pocket, with 45° sloping
sides (Figure 10).
Figure 10 - Model 4800 Earth Pressure Cell Installation

9
The cells (Model 4800-1-1P, complete with pinch tubes and lugs) are positioned on a thin
layer of non-shrink, sand cement grout, and are nailed in position using the lugs on the
cells provided for this purpose. The excavated pocket is then backfilled to a depth of 300
mm with a weak concrete in 100 mm layers, vibrated with a poker vibrator. After 24
hours, the cells are pressurized by pinching the pinch tubes until the pressure in the cell,
displayed on a connected Readout Box, starts to change.
The instrument location containing the grouted cells and the cable trench is then
backfilled in 250 mm layers, using the same material as the main fill placed by hand and
compacted with pneumatic or gasoline backfill tampers, or vibratory trench rollers. After
this, standard construction filling and compaction practices can continue.
Earth Pressure Cells clusters, placed according to the methods outlined above, may be
installed either in trenches, below the temporary embankment grade, or in ramps above
the temporary embankment grade. In dams, for example, it is usually convenient to install
in trenches in the impervious rolled fill core, and in ramps in the filter zones and
compacted rockfill shell zones. In earth embankments, it is convenient to install in
trenches. By doing so, adequate degrees of compaction of the backfill can be more easily
obtained without damage to the cell clusters or cable arrays. As the cells are being
covered and compacted, repeated readings should be taken to ensure that the cells are
continuing to function properly.
See Section 2.3 for cable installation and protection.
Application
Grout for Medium to Hard
Soils
Grout for Soft Soils
Materials
Weight
Ratio by
Weight
Weight
Ratio by
Weight
Water
30 gallons
2.5
75 gallons
6.6
Portland
Cement
94 lbs.
(One sack)
1
94 lbs.
(One sack)
1
Bentonite
25 lbs.
(as required)
0.3
39 lbs.
(as required)
0.4
Notes
The 28-day compressive strength of this
mix is about 50 psi, similar to very stiff to
hard clay. The modulus is about 10,000 psi
The 28-day strength of this mix is about
4 psi, similar to very soft clay.
Table 1 - Ratios for Two Grout Mixes.
2.2.1.2 Alternative Method
In this method, the pressure cell used to monitor vertical earth pressures is placed directly
in the fill. The procedures are similar to those in Section 2.2.1.1, except that the pressure
cell does not have a pinch tube and the layer of weak grout is dispensed with. Instead, the
cell is placed on a pad of quick-setting mortar. This is done to ensure uniform contact
with the soil at the bottom of the trench. The cell is then covered by soil placed in
300 mm layers and compacted as before.

10
2.2.2 Installation of Model 4810 Contact ("Fat Back") Pressure Cell
This section details installation instructions for Model 4810 Earth Pressure Cells, which
are used for the measurement of earth pressures on structures. In backfills for piers, piles,
bridge abutments, retaining walls, culverts and other structures the cells may be installed
either inside a concrete structure being poured or directly on the surface of an existing
structure. For slurry walls, the Model 4820 Earth Pressure Cell is used as described in
Section 2.2.4.
2.2.2.1 Installation in Poured Concrete
When pouring concrete the cells can be held to the forms using nails through the lugs
welded to the edge of the cell. Position the cell so that the thin pressure sensitive plate is
directly against the concrete form. Nail the plates to the form lightly in such a manner
that they engage the concrete sufficiently and will not pull out of the concrete when the
forms are removed. Route the cable inside the concrete to a convenient readout location
or to a block out inside where excess cable can be coiled. Protect the cable from damage
during concrete placement and vibration, by tying it to adjacent rebars. See Figure 11.
Concrete Form
Excess Cable
(coiled inside blockout)
Double Headed Nails
(through mounting lugs, 4 places)
Pressure Cell
Side View Front View
Figure 11 - Attachment of Model 4810 to Concrete Form

11
2.2.2.2 Installation on Existing Structures
The lugs welded to the edge of the cell can be used to hold the cell against the structure
using nails, lag bolts, tie wire, etc. Even if the surface is smooth, but especially when the
surface is rough or irregular, a mortar pad between the cell and the structure is required.
See Figure 12.
Mortar Pad
Pipe Straps & Conduit
Zone with large aggregate removed
Concrete Nails
(4 places)
Side View Front View
Figure 12 - Model 4810 Contact Pressure Cell Installation
Use the lugs on the cell as a template to locate the position for drilling holes for the
installation of expanding anchors or install the anchors nearby and use wire to hold the
cells in place. Alternately, the cell may be nailed in place using the lugs as a guide.
Mix up some quick-setting cement mortar or epoxy cement. Trowel this onto the surface
then push the cell into the cement so that the excess cement extrudes out of the edges of
the cell. Hold the cell in place while the cement sets up then complete the installation by
adding the lag bolts (using the expansion anchors) and tightening or nailing the cell in
place. Protect the cell, transducer housing, and cable from direct contact with large
chunks of rock by covering them with a fine grained fill material from which all pieces
larger than about 10 mm (0.5") have been removed. This material is kept near the cell and
cable as the fill is placed. Additional cable protection can be achieved by using metal
conduit strapped to the surface of the structure.

12
2.2.3 Installation of Model 4815 Hydraulic Load Cell
A particular installation, shown in Figure 13, used the Model 4815 Hydraulic Load Cell
to measure the concentrated load on a tunnel lining from an existing wooden pile
(supporting a building above) that had been cut short by the tunnel excavation in frozen
ground. The load cell was designed to measure any increase of load on the tunnel lining
that might occur when, at the end of tunnel construction, the ground was allowed to thaw
out. The load cell was positioned below the bottom of the pile and temporarily held in
place with lugs and a mortar pad until the shotcrete tunnel lining was sprayed.
Figure 13 - Model 4815 Hydraulic Load Cell Measuring Loads on a Tunnel Lining

13
2.2.4 Installation of Model 4820 Jackout Pressure Cell in Slurry Trenches
The Jackout Pressure first needs to be assembled into the Jackout frame. The assembly is
shown in Figure 14. The support plate has a circular hole cut in it and bolt holes to fit the
Jackout Pressure Cell (JOPC), and is connected to one end of a double-acting hydraulic
jack by means of steel struts. The support plate and reaction plate are cambered top and
bottom to prevent them from snagging on the sides of the slurry trench. The reaction plate
is attached to the other side of the double-acting hydraulic jack. The jack is attached
firmly to the rebar cable and arranged so that the plates are free to move outwards. The
hydraulic line and signal cable are tied off to one of the rebars at intervals of one meter (~
three feet).
When the rebar cage has been lowered to its proper depth, the jack is activated, forcing
the two plates out against the trench walls.
Figure 14 - Model 4820 Jackout Pressure Cell Installation
Observation of the pressure indicated by the JOPC (see Section 3 for readout
instructions) will indicate when the cell has made contact with the wall. Pump up the jack
until the JOPC reading indicates a pressure roughly 70 KPa (10 psi) greater than the
slurry pressure at JOPC depth. This ensures that the cell is bearing against the walls of
the trench, and that the concrete grout pressure will not close the jack, which could allow
the reaction plates to move away from the trench walls. Check the JOPC reading from
time to time, because the pressure might bleed away if the walls of the trench are soft and
yielding. Repressurize as needed. Leave the jack pressurized until the grout has set up.

14
2.2.5 Installation of Cells to Measure Earth Pressure at the Base of Footings, Floor
Slabs, Pavements, Etc.
Experience has shown that attempts to measure contact earth pressures on the base of
footings, floor slabs, pavements, etc., frequently meets with failure. The problem is
twofold. First, the contact stress distribution can be inherently variable due to varying
properties of the ground and varying degrees of compaction of the ground. Thus the
contact stress at one location may not be typical of the surrounding locations. Secondly, a
cell installed as described in Section 2.2.1 could result in the creation of an anomalous
zone immediately around the cell where there may be a different, finer grained material,
under a lesser degree of compaction. (The material around the cell may be poorly
compacted because of the need to avoid damage to the cell.)
In an earth fill, this zone of poor compaction would not be a problem, since the earth
above would move downwards to fill the voids and consolidate the ground. However,
where there is a concrete slab immediately above the cell, this consolidation may not take
place. In fact, under the influence of rainwater and vibration, the spaces around the cell
may grow, causing the cell to become completely decoupled from the concrete above. In
such a situation, the concrete slab bridges over the gap and the loads in the concrete go
around the cell instead of through it. The cell registers only a very low pressure, which
does not change as the loads increase.
The best way to avoid the problem is to cast the cell inside the concrete if possible. This
can often be done when the initial concrete bonding layer is spread over the surface of the
ground. At this time a Model 4800-1-1P Earth Pressure Cell with a pinch tube, is pressed
into the bonding layer so that it rests against the ground below. A weighted tripod can be
used to hold the stress cell in place until the concrete hardens. The pinch tube is arranged
to protrude above the bonding layer and, when the concrete has hardened, it is used to
pressurize the cell and ensure good contact between the cell and the surrounding
concrete. See Figure 15. The advantage of this method is its simplicity and that it permits
the ground below the concrete to be completely compacted in the normal way.
Model 4850-2 Concrete Stress Cell
Pinch Tube
Concrete Footing
Concrete Bonding Layer (mud mat)
or
Compacted Subgrade
Figure 15 - Model 4800-1-1P Earth Pressure Cell Installation
Model 4800-1-1P