Geokon 4500 Series User manual

©2021,GEOKON. All rights reserved.
Document Revision: PP | Release date: 1/28/21
Model 4500 Series
Vibrating Wire Piezometer
Instruction Manual

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 specifi-
cation, misapplication, misuse or other operating conditions outside of GEOKON's
control. Components that wear or 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 merchant-
ability 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 instal-
lation 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.
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.
The GEOKON® wordmark and logo are registered trademarks with the United States Patent and Trademark Office.

I
TABLE OF CONTENTS
1. THEORY OF OPERATION ...................................................................................................................1
2. QUICK START INSTRUCTIONS .................................................................................................2
3. PRIOR TO INSTALLATION...............................................................................................................3
3.1 SATURATING FILTER TIPS .........................................................................................................3
3.1.1 SATURATING LOW AIR ENTRY (STANDARD) FILTERS ....................................................3
3.1.2 SATURATING HIGH AIR ENTRY CERAMIC FILTERS..........................................................3
3.1.3 SATURATING MODEL 4500C FILTER TIPS.............................................................................4
3.2 ESTABLISHING AN INITIAL ZERO READING .............................................................4
3.2.1 RECOMMENDED METHOD FOR ESTABLISHING AN INITIAL ZERO READING......5
3.2.2 ALTERNATIVE METHOD ONE ......................................................................................................5
3.2.3 ALTERNATIVE METHOD TWO .....................................................................................................5
3.2.4 ALTERNATIVE METHOD THREE .................................................................................................6
3.3 CHECKING THE PIEZOMETER PERFORMANCE.......................................................6
4. INSTALLATION..............................................................................................................................................7
4.1 INSTALLATION IN STANDPIPES OR WELLS.............................................................7
4.2 INSTALLATION IN BOREHOLES.............................................................................................7
4.3 INSTALLATION IN FILLS AND EMBANKMENTS....................................................9
4.4 INSTALLATION BY PUSHING OR DRIVING INTO SOFT SOILS ............. 10
4.5 MODEL 4500H AND MODEL 4500HH TRANSDUCER.................................... 11
4.6 SPLICING AND JUNCTION BOXES ................................................................................. 11
4.7 LIGHTNING PROTECTION........................................................................................................ 12
4.8 FREEZING PROTECTION............................................................................................................ 13
5. TAKING READINGS ............................................................................................................................. 14
5.1 GK-404 VIBRATING WIRE READOUT............................................................................ 14
5.1.1 OPERATING THE GK-404 ........................................................................................................... 14
5.2 GK-405 VIBRATING WIRE READOUT............................................................................ 15
5.2.1 CONNECTING SENSORS WITH 10-PIN BULKHEAD CONNECTORS
ATTACHED ....................................................................................................................................... 15
5.2.2 CONNECTING SENSORS WITH BARE LEADS................................................................... 15
5.2.3 OPERATING THE GK-405 ........................................................................................................... 15
5.3 MEASURING TEMPERATURES ........................................................................................... 15
6. DATA REDUCTION ................................................................................................................................17
6.1 PRESSURE CALCULATION...................................................................................................... 17

II
6.2 TEMPERATURE CORRECTION.............................................................................................. 17
6.3 BAROMETRIC CORRECTION (REQUIRED ONLY ON UNVENTED
TRANSDUCERS) ................................................................................................................................18
6.4 MODEL 4500SV, VENTED PIEZOMETERS................................................................. 19
6.5 ENVIRONMENTAL FACTORS ................................................................................................ 20
7. TROUBLESHOOTING........................................................................................................................... 21
APPENDIX A. SPECIFICATIONS................................................................................................... 23
A.1 4500 SPECIFICATIONS .............................................................................................................. 23
A.2 4500CR SPECIFICATIONS....................................................................................................... 23
A.3 THERMISTOR ...................................................................................................................................... 23
A.4 STANDARD PIEZOMETER WIRING ................................................................................. 23
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION ....................... 24
B.1 3KΩ THERMISTOR RESISTANCE...................................................................................... 24
B.2 10KΩ THERMISTOR RESISTANCE .................................................................................. 25
APPENDIX C. IMPROVING THE ACCURACY OF THE CALCULATED
PRESSURE ..................................................................................................................... 26
APPENDIX D. TYPICAL CALIBRATION REPORT....................................................... 27
APPENDIX E. MODEL 4500AR PIEZOMETER .............................................................. 28
APPENDIX F. PIEZOMETER PRESSURE AND WATER LEVEL................... 29

III
FIGURES
FIGURE 1: MODEL 4500S VIBRATING WIRE PIEZOMETER ...........................................1
FIGURE 2: 4500C SATURATION .......................................................................................4
FIGURE 3: TYPICAL LEVEL MONITORING INSTALLATION .............................................7
FIGURE 4: TYPICAL BOREHOLE INSTALLATIONS ...........................................................8
FIGURE 5: HIGH AIR ENTRY FILTER ..............................................................................10
FIGURE 6: LOW AIR ENTRY FILTERS ONLY ..................................................................10
FIGURE 7: TYPICAL SOFT SOILS INSTALLATION ..........................................................11
FIGURE 8: TYPICAL MULTI-PIEZOMETER INSTALLATION ...........................................12
FIGURE 9: RECOMMENDED LIGHTNING PROTECTION SCHEME ..............................13
FIGURE 10: GK-404 READOUT .......................................................................................14
FIGURE 11: LEMO CONNECTOR TO GK-404 .................................................................14
FIGURE 12: GK-405 READOUT .......................................................................................15
FIGURE 13: VENTED PIEZOMETERS .............................................................................19
FIGURE 14: TYPICAL CALIBRATION REPORT ................................................................27
FIGURE 15: 4500AR PIEZOMETER .................................................................................28

IV
TABLES
TABLE 1: CEMENT / BENTONITE / WATER RATIOS ......................................................9
TABLE 2: ENGINEERING UNITS MULTIPLICATION FACTORS.....................................17
TABLE 3: 4500 VIBRATING WIRE PIEZOMETER SPECIFICATIONS ............................23
TABLE 4: 4500CR VIBRATING WIRE PIEZOMETER SPECIFICATIONS .......................23
TABLE 5: STANDARD PIEZOMETER WIRING ................................................................23
TABLE 6: 3KΩ THERMISTOR RESISTANCE....................................................................24
TABLE 7: 10KΩ THERMISTOR RESISTANCE..................................................................25
TABLE 8: 4500AR WIRING CHART..................................................................................28

V
EQUATIONS
EQUATION 1: DIGITS CALCULATION .............................................................................. 17
EQUATION 2: CONVERT DIGITS TO PRESSURE ............................................................ 17
EQUATION 3: TEMPERATURE CORRECTION................................................................. 18
EQUATION 4: BAROMETRIC CORRECTION.................................................................... 18
EQUATION 5: CORRECTED PRESSURE CALCULATION................................................ 19
EQUATION 6: 3KΩ THERMISTOR RESISTANCE............................................................. 24
EQUATION 7: 10KΩ THERMISTOR RESISTANCE........................................................... 25
EQUATION 8: SECOND ORDER POLYNOMIAL EXPRESSION ...................................... 26
EQUATION 9: LINEARITY CALCULATION ....................................................................... 26

VI

MODEL 4500 SERIES VIBRATING WIRE PIEZOMETER | THEORY OF OPERATION | 1
1. THEORY OF OPERATION
GEOKON Model 4500 Vibrating Wire Piezometers are intended primarily for long-
term measurements of fluid depths and pore pressures in standpipes, boreholes,
embankments, pipelines, and pressure vessels. Several different models are
available to suit a variety of geotechnical applications. Calibration data is
supplied with each piezometer.
All GEOKON vibrating wire piezometers utilize a sensitive stainless steel
diaphragm (with the exception of model 4500C, which employs bellows) to
which a vibrating wire element is connected. During use, changing pressures on
the diaphragm cause it to deflect. This deflection is measured as a change in
tension and frequency of vibration of the vibrating wire element. The square of
the vibration frequency is directly proportional to the pressure applied to the
diaphragm. A filter is used to keep out solid particles and prevent damage to the
sensitive diaphragm. Standard filters are 50-micron stainless steel. High≅air
entry value filters are available upon request.
Two coils, one with a magnet insert, the other with a pole piece insert, are
installed near the vibrating wire. In use, a pulse of varying frequency (swept
frequency) is applied to these coils, causing the wire to vibrate primarily at its
resonant frequency. When the excitation ends, the wire continues to vibrate.
During vibration, a sinusoidal signal is induced in the coils and transmitted to the
readout box where it is conditioned and displayed.
Portable readout units are available to provide the excitation, signal
conditioning, and readout of the instrument. Datalogger systems, which allow
remote, unattended data collection of multiple sensors, are also available.
Contact GEOKON for additional information.
1:
FIGURE 1: Model 4500S Vibrating Wire Piezometer
All exposed components are made of corrosion resistant stainless steel. If proper
installation techniques are used, the device should have an unlimited life.
In salt water, it may be necessary to use special materials for the diaphragm and
housing. The 4500INCO and 4500TI series piezometers are specifically designed
to be used in this type of environment. For example, both models incorporate
enhanced seals at the cable entry and filter connection; the 4500INCO uses a
custom, dual O-ring seal, while the 4500TI is of an all-welded construction.

2| QUICK START INSTRUCTIONS | GEOKON
2. QUICK START INSTRUCTIONS
For those familiar with Geotechnical instrumentation and its installation, the
following quick start instructions may be used. For more detailed instructions
see Section 3.
1. Prior to installation, allow the piezometer to come to thermal equilibrium for
a minimum of 15 minutes. (Alternatively, if the instrument is attached to a
readout box, wait until the piezometer reading has stabilized.)
2. Record the piezometer reading, barometric pressure, and temperature while
the piezometer is experiencing zero (atmospheric) pressure. This is what is
known as the "initial zero" reading.
3. Verify that the initial zero reading for the piezometer is compatible with the
factory supplied zero reading on the calibration report.
4. Carefully measure and mark the cable where it will lie at the top of the
borehole, well, or standpipe, once the piezometer has reached the desired
depth. (The piezo diaphragm lies ¾ of an inch above the tip of the
piezometer.)
5. Saturate the piezometer filter (see Section 3.1).
Warning! Do not allow the piezometer to freeze once the filter stone has
been saturated!
6. For installation in standpipes or wells, see Section 4.1. For boreholes, see
Section 4.2. For fills and embankments, see Section 4.3.

MODEL 4500 SERIES VIBRATING WIRE PIEZOMETER | PRIOR TO INSTALLATION | 3
3. PRIOR TO INSTALLATION
3.1 SATURATING FILTER TIPS
Warning! Do not allow the piezometer to freeze once the filter stone has been
saturated!
See Section 4.8 for information about protecting the piezometer from freezing.
Most filter tips can be removed for saturation and then reassembled. To
maintain saturation, the unit should be kept underwater until installation. If the
piezometer is used in a standpipe where it will be raised and lowered frequently,
the filter housing may loosen over time, and a permanent filter assembly may be
required. The removable filter may be fixed permanently by prick punching the
piezometer tube approximately ⅟16" to ⅛" behind the filter assembly joint.
Salts in the water can be deposited into the filter stone causing it to become
clogged if it is allowed to dry out completely. Filter stones may be replaced with
screens for standpipe installations. Screens available from GEOKON are less likely
than standard filters to collect salt and become clogged.
3.1.1 SATURATING LOW AIR ENTRY (STANDARD) FILTERS
For accurate results, total saturation of the filter is necessary. As the piezometer
is lowered into the water, water is forced into the filter, compressing the air in
the space between the filter stone and the pressure sensitive diaphragm. After a
period, this air will dissolve into the water, filling the filter and the space above it
entirely with water.
To speed up the saturation process, remove the filter from the piezometer by
carefully twisting and pulling on the filter housing assembly (or unscrewing the
point of the piezometer for model 4500DP). Hold the piezometer with the filter
facing up and fill the space above the diaphragm with water. Slowly replace the
filter housing, allowing the water to squeeze through the filter stone as it is
installed. For piezometers with a range of less than 10 psi, take readings with a
readout box while reinstalling the filter housing to ensure the piezometer is not
overranged.
3.1.2 SATURATING HIGH AIR ENTRY CERAMIC FILTERS
Because of the high air entry characteristics of the ceramic filter, de-airing is
particularly important. Different air entry values require different saturation
procedures.
ONE BAR FILTERS
1. Remove the filter from the piezometer by carefully twisting and pulling on
the filter housing assembly.
2. Boil the filter assembly in de-aired water.
3. Reassemble the piezometer under the surface of a container of de-aired
water. Use a readout box while installing the filter to monitor the diaphragm
pressure. If the piezometer begins to overrange, allow the pressure to
dissipate before pushing further.
4. Be sure that no air is trapped in the transducer cavity.
TWO BAR AND HIGHER FILTERS
The proper procedure for de-airing and saturating these filters is somewhat
complex; therefore, it is recommended that saturation be done at the factory by
GEOKON. If saturation must be done in the field, carefully follow the instructions
below:

4| PRIOR TO INSTALLATION | GEOKON
1. Place the assembled piezometer, filter down, in a vacuum chamber that has
an inlet port at the bottom for de-aired water.
2. Close off the water inlet and evacuate the chamber. The transducer should
be monitored while the chamber is being evacuated.
3. When maximum vacuum has been achieved, allow de-aired water to enter
the chamber until it reaches an elevation a few inches above the piezometer
filter.
4. Close off the inlet port.
5. Release the vacuum.
6. Observe the transducer output. It may take up to 24 hours for the filter to
completely saturate and the pressure to rise to zero.
7. After saturation, the transducer should be kept in a container of de-aired
water until installation. If de-aired at the factory a special cap is applied to
the piezometer to maintain saturation.
3.1.3 SATURATING MODEL 4500C FILTER TIPS
WARNING! The filter housing is not removable on the 4500C. Any attempt to
remove the filter stone or the housing will destroy the transducer!
If the pressure to be measured is less than 5 psi the filter stone must be
saturated. A hand operated vacuum pump and short length of half inch surgical
tubing is required. Hand pumps and tubing are available from the factory. (A
hand pump that has been used successfully is the MityvacII® by Lincoln
Industries Corp. of St. Louis, MO.)
The saturation procedure is as follows:
1. Attach the tube to the transducer as shown in Figure 2.
2. Fill the tubing with approximately two inches (five centimeters) of water.
3. Attach the other end of the tube to the hand vacuum pump.
4. While holding the transducer so that the water rests on the filter, but does
not enter the pump, squeeze the hand pump to initiate a vacuum inside the
tubing. This will draw the air out of the filter and the area behind it, replacing
it with water. A vacuum of 20 to 25" Hg. (50 to 65 cm Hg.) is enough for
proper air evacuation.
FIGURE 2: 4500C Saturation 3.2 ESTABLISHING AN INITIAL ZERO READING
Vibrating wire piezometers differ from other types of pressure sensors in that
they indicate a reading when no pressure is exerted on the sensor.
Note: It is imperative that an accurate initial zero reading be obtained for each
piezometer, as this reading will be used for all subsequent data reduction.
Generally, the initial zero reading is obtained by reading the instrument prior to
installation. There are several different ways of taking an initial zero reading. The
essential element in all methods is that the piezometer needs to thermally
stabilize in a constant temperature environment while the pressure on the
piezometer is barometric only. Because of the way the piezometer is
constructed, it usually takes 15 to 20 minutes for the temperature of all the
different elements to equalize.
A question may arise as to what to do with the filter stone while taking zero
readings. It will not matter if the filter stone is saturated when using a standard
stainless steel filter. However, if the piezometer is equipped with a ceramic high
air entry filter stone, then it must be saturated while taking the zero readings.

MODEL 4500 SERIES VIBRATING WIRE PIEZOMETER | PRIOR TO INSTALLATION | 5
It will be necessary to measure the barometric pressure only if the piezometer is
unvented and it will be installed in a location that is subject to barometric
pressure changes that would require correction, such as in an open well. A
piezometer sealed in place at depth could be recording pressures in
groundwater that is not hydraulically connected to the atmosphere, for which
barometric pressure compensation would be inappropriate. See Section 6.3 for
more information on Barometric corrections.
Calibration data is supplied with each gauge, a factory zero reading taken at a
specific temperature and absolute barometric pressure is included. (See
Appendix D for a sample calibration report.) Zero readings at the site should
coincide with the factory readings within 50 digits, after barometric and
temperature corrections are made. Barometric pressures change with elevation
at a rate of approximately 3.45 kPa (½ psi) per 300 meters (1,000 ft). The factory
elevation is +580 feet. All stated barometric readings represent absolute
pressure uncorrected for height above sea level. A thermistor is included inside
the body of the piezometer for the measurement of temperature.
NOTE REGARDING THE 4500C: The construction of this very slender
vibrating wire transducer requires a miniaturization of the internal parts, which
consequently are somewhat delicate. Handle the transducer with care
during the installation procedure. Despite taking every precaution to ensure
that the transducer arrives unharmed, it is possible for the zero to shift during
shipment due to rough handling. However, tests have shown that though the
zero may shift, the calibration factors do not change. Therefore, it is doubly
important that an initial no load zero reading be taken prior to installation.
3.2.1 RECOMMENDED METHOD FOR ESTABLISHING AN INITIAL ZERO
READING
1. Saturate the filter stone per Section 3.1. Warning! Do not allow the
piezometer to freeze once the filter stone has been saturated!
2. Replace the filter stone.
3. Hang the piezometer in the borehole at a point just above the water.
4. Wait until the piezometer reading has stopped changing.
5. Take the zero and temperature readings.
3.2.2 ALTERNATIVE METHOD ONE
1. Place the piezometer under water in a bucket.
2. Allow 15 to 20 minutes for the temperature of the unit to stabilize.
3. Use the instrument cable to lift the piezometer out of the water. Do not
handle the piezometer housing; body heat from the hands could cause
temperature transients.
4. Immediately take a zero and temperature reading.
3.2.3 ALTERNATIVE METHOD TWO
1. Allow 15 to 20 minutes for the temperature of the unit to stabilize.
2. Lift the piezometer by the cable only. Do not handle the piezometer
housing; body heat from the hand could cause temperature transients.
3. Take a zero and temperature reading.
(If this method is chosen, be sure that the piezometer is protected from sunlight
or sudden changes of temperature. Wrapping it in some insulating material is
recommended.)

6| PRIOR TO INSTALLATION | GEOKON
3.2.4 ALTERNATIVE METHOD THREE
1. Lower the piezometer to a known depth marked on the piezometer cable.
(The diaphragm inside the piezometer is located approximately 15 mm (¾")
from the tip.)
2. Use a dip meter to accurately measure the depth to the water surface.
3. After temperature stabilization, read the piezometer pressure.
4. Using the factory calibration constants and a knowledge of the pressure of
the water column above the piezometer (height times density), calculate the
equivalent zero pressure reading if linear regression is used, or the factor C
if the second order polynomial is used.
3.3 CHECKING THE PIEZOMETER PERFORMANCE
If a rough check of the piezometer performance is needed, the following
procedure is recommended:
1. Lower the piezometer to a point near the bottom of a water-filled borehole,
or below the surface of a body of water.
2. Allow 15 to 20 minutes for the piezometer to come to thermal equilibrium.
3. Using a readout box, record the reading at the current depth.
4. Raise the piezometer by a measured increment.
5. Record the reading on the readout box at the new depth.
6. Using the factory calibration factor, calculate the change in water depth.
7. Compare the calculated change in depth with the measured depth
increment. The two values should be roughly the same.
ALTERNATIVE METHOD USING A DIP METER:
1. Lower the piezometer tip to a measured depth below the water surface.
2. Allow 15 to 20 minutes for the piezometer to come to thermal equilibrium.
3. Using a readout box, record the reading at that level.
4. Calculate the elevation of the water surface using the given calibration
factor.
5. Compare the calculated elevation to the elevation measured using the dip
meter.
THINGS THAT CAN AFFECT THIS CHECKING PROCEDURE:
■If the density of the water is not one gram/cubic centimeter.
■If the water is saline or turbid.
■The water level inside the borehole may vary during the test. This is due to
the displacement of water caused by the cable as it is raised and lowered in
the borehole. The smaller the borehole is, the greater the displacement will
be. For example, a Model 4500S-50KPA piezometer lowered 50 feet below
the water column in a one-inch (0.875-inch ID) standpipe will displace the
water level by more than four feet.

MODEL 4500 SERIES VIBRATING WIRE PIEZOMETER | INSTALLATION | 7
4. INSTALLATION
4.1 INSTALLATION IN STANDPIPES OR WELLS
1. Saturate the filter stone and establish an initial zero reading by following the
steps outlined in Section 3.1 and Section 3.2.
Warning! Do not allow the piezometer to freeze once the filter stone has
been saturated!
2. Mark the cable where the top of the well or standpipe will reside once the
piezometer has reached the desired depth. (The piezometer diaphragm is
located ¾ of an inch above the tip of the piezometer.)
3. Lower the piezometer into the standpipe/well.
4. Be sure the cable is securely fastened to prevent the piezometer from
sliding further into the well and causing an error in the readings.
3:
FIGURE 3: Typical Level Monitoring Installation
It is not recommended that piezometers be installed in wells or standpipes
where an electrical pump or cable is nearby. Electrical interference from these
sources can cause unstable readings. If unavoidable, it is recommended that the
piezometer be placed inside a piece of steel pipe. In situations where packers
are used in standpipes, special care should be taken to avoid cutting the cable
jacket with the packer, as this could introduce a possible pressure leak in the
cable.
4.2 INSTALLATION IN BOREHOLES
GEOKON piezometers can be installed in cased or uncased boreholes, in either
single or multiple piezometer configurations. If pore pressures in a particular
zone are to be monitored, careful attention must be paid to the borehole sealing
technique.
The borehole should extend six to 12 inches below the proposed piezometer
location. Boreholes should be drilled without using drilling mud, or by using a
material that degrades rapidly with time, such as Revert™. Wash the borehole
clean of drill cuttings. Backfill the borehole with clean fine sand to a point six

8| INSTALLATION | GEOKON
inches below the desired piezometer tip location. The piezometer can then be
lowered into position. (Preferably, the piezometer will be encapsulated in a
canvas bag containing clean, saturated sand.) While holding the instrument in
position, (a mark on the cable is helpful) fill the borehole with clean fine sand to
a point six inches above the piezometer.
Three methods of isolating the zone to be monitored are detailed below.
INSTALLATION A
Immediately above the area filled with clean fine sand, known as the "collection
zone", the borehole should be sealed by an impermeable bentonite cement
grout mix, or with alternating layers of bentonite and sand backfill, tamped in
place for approximately one foot, followed by common backfill (see Figure 4).
If multiple piezometers are to be used in a single hole, the bentonite and sand
should be tamped in place below and above the upper piezometers, as well as at
interval between the piezometer zones. When using tamping tools special care
should be taken to ensure that the piezometer cable jackets are not cut during
installation, as this could introduce a possible pressure leak in the cable.
INSTALLATION B
The borehole is filled from the collection zone upwards with an impermeable
bentonite grout.
4:
FIGURE 4: Typical Borehole Installations
INSTALLATION C
It should be noted that since the vibrating wire piezometer is essentially a no
flow instrument, collection zones of appreciable size are not required. The
piezometer can be placed directly in contact with most materials, provided that
the fines are not able to migrate through the filter. The latest thinking is that it is
not necessary to provide sand zones and that the piezometer can be grouted
directly into the borehole using a bentonite cement grout only. However, good
results have been obtained by placing the piezometer inside a canvas bag filled
with sand before grouting.
The general rule for installing piezometers in this way is to use a bentonite grout
that mimics the strength of the surrounding soil. The emphasis should be on

MODEL 4500 SERIES VIBRATING WIRE PIEZOMETER | INSTALLATION | 9
controlling the water to cement ratio. This is accomplished by mixing the
cement with the water first. The most effective way of mixing the two
substances is to use a drill rig pump to circulate the mix in a 50- to 200-gallon
barrel or tub.
Make drilling mud using any kind of bentonite powder combined with Type I or
Type II Portland cement. The exact amount of bentonite needed will vary
somewhat. Table 1 shows two possible mixes for strengths of 50 psi and 4 psi.
TABLE 1: Cement / Bentonite / Water Ratios
Add the measured amount of clean water to the barrel then gradually add the
cement in the correct weight ratio. Slowly add the bentonite powder so that
clumps do not form. Keep adding bentonite until the watery mix turns to an oily/
slimy consistency. Let the grout thicken for five to 10 minutes. Add more
bentonite as required until it is a smooth, thick cream, similar to pancake batter,
which is as heavy as it is feasible to pump.
When pumping grout (unless the tremie pipe is to be left in place), withdraw the
tremie pipe after each batch, by an amount corresponding to the grout level in
the borehole.
CAUTION! If the grout is pumped into the hole, rather than tremie piped, there
is a danger that the piezometer will be overranged and damaged. Pumping
directly into the bottom of the borehole should be avoided. It is good practice to
read the piezometer while pumping.
For more details on grouting, refer to "Piezometers in Fully Grouted Boreholes"
by Mikkelson and Green, FMGM proceedings Oslo 2003. Copies are available
from GEOKON.
4.3 INSTALLATION IN FILLS AND EMBANKMENTS
GEOKON piezometers are normally supplied with direct burial cable suitable for
placement in fills such as highway embankments and dams, both in the core
and in the surrounding materials.
For installations in non-cohesive fill materials, the piezometer may be placed
directly in the fill, or, if large aggregate sizes are present, in a saturated sand
pocket in the fill. If installed in large aggregate, additional measures may be
necessary to protect the cable from damage.
In fills such as impervious dam cores, where subatmospheric pore water
pressure may need to be measured, (as opposed to the pore air pressure), a
ceramic tip with a high air entry value is often used. This type of filter should be
carefully placed in direct contact with the compacted fill material. (See Figure 5).
Cables are normally installed inside shallow trenches with the fill material
consisting of smaller size aggregate. This fill is carefully hand compacted around
the cable. Bentonite plugs are placed at regular intervals to prevent migration of
water along the cable path. In high traffic areas and in materials that exhibit
pronounced "weaving", heavy-duty armored cable should be used.
1: 50 PSI Grout for Medium to Hard Soils 4 PSI Grout for Soft Soils
Amount Ratio by Weight Amount Ratio by Weight
Water 30 gallons 2.5 75 gallons 6.6
Portland Cement 94 lb. (one sack) 1 94 lb. (one sack) 1
Bentonite 25 lb. (as required) 0.3 39 lb. (as required) 0.4
Note:
The 28 day compressive strength of this mix is about
50 psi,similar to very stiff/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.

10 | INSTALLATION | GEOKON
5:
FIGURE 5: High Air Entry Filter
In partially saturated fills (if only the pore air pressure is to be measured), the
standard tip is satisfactory. It should be noted that the standard coarse tip (low
air entry) measures the air pressure when there is a difference between the pore
air pressure and the pore water pressure. The difference between these two
pressures is due to the capillary suction in the soil. The consensus is that the
difference is normally of no consequence to embankment stability.
The coarse tip filter is suitable for most routine measurements. Both the
installation shown in Figure 5 and the installation shown in Figure 6 may be
used with the standard piezometer filter.
6:
FIGURE 6: Low Air Entry Filters ONLY
4.4 INSTALLATION BY PUSHING OR DRIVING INTO SOFT SOILS
The Model 4500DP piezometer is designed to be pushed into soft soils. In soft
soils, it can be difficult to keep a borehole open. The 4500DP may eliminate the
need for a borehole altogether. The unit is connected directly to the drill rod
(AW, EW, or other) and pressed into the ground, either by hand or by means of
the hydraulics on the rig (see Figure 7). The units can also be driven into the soil,
but there is a possibility that the driving forces may shift the zero reading.
The ground conditions need to be relatively soft for the 4500DP to be effective.
Soft soils (like clays or silts) with SPT blow counts under 10 are ideal. In stiffer
soils, it is possible to drill a hole and then push the 4500DP only a few feet below
the bottom of the hole. If the soil is too stiff, the sensor may overrange or break.