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22

4. DATA REDUCTION

4.1 Pressure Calculation

The basic units utilized by Geokon for measurement and reduction of data from Vibrating Wire
Earth Pressure Cells are "digits". Geokon Readouts display "digits" in the Earth Pressure Cell
reading position. Calculation of digits is based on the following equation:

Digits =

�

1

Period

�

2

x 10

-3

Or

Digits

=

Hz

2

1000

Equation 2 - Digits Calculation

To convert digits to pressure the following equation applies:

Pressure

=

(Current Reading - Initial Reading)

×

Calibration Factor

Or

P = (R

1

- R

0

)

×

G

Equation 3 - Convert Digits to Pressure

The Initial Reading (R

0

) is normally obtained during installation (usually the zero reading). The

Calibration Factor (G, usually in terms of psi or kPa per digit) comes from the supplied
Calibration Sheet (a typical calibration sheet is shown in Figure 18). To convert the output to
other engineering units, multiply the Calibration Factor by the conversion multiplier listed in
Table 2.

From

→

To

↓

psi

"H

2

O

'H

2

O

mm H

2

0

m H

2

0

"HG

mm HG

atm

mbar

bar

kPa

MPa

psi

1

.036127 .43275 .0014223 1.4223

.49116 .019337 14.696 .014503 14.5039 .14503

145.03

"H

2

O

27.730

1

12

.039372

39.372

13.596

.53525

406.78

.40147

401.47

4.0147

4016.1

'H

2

O

2.3108

.08333

1

.003281

3.281

1.133

.044604 33.8983 .033456 33.4558

.3346

334.6

mm H

2

0

704.32

25.399 304.788

1

1000

345.32

13.595

10332

10.197

10197

101.97

101970

m H

2

0

.70432 .025399 .304788

.001

1

.34532 .013595 10.332 .010197 10.197

.10197

101.97

"HG

2.036

.073552 .882624 .0028959 2.8959

1

.03937

29.920 .029529 29.529

.2953

295.3

mm HG

51.706

1.8683 22.4196 .073558

73.558

25.4

1

760

.75008

750.08

7.5008

7500.8

atm

.06805 .0024583 .0294996 .0000968

.0968

.03342 .0013158

1

.0009869 .98692 .009869

9.869

mbar

68.947

2.4908 29.8896 .098068

98.068

33.863

1.3332

1013.2

1

1000

10

10000

bar

.068947 .0024908 .0298896 .0000981 .098068 .033863 .001333 1.0132

.001

1

.01

10

kPa

6.8947

.24908 2.98896 .0098068 9.8068

3.3863

.13332 101.320

.1

100

1

1000

MPa

.006895 .000249 .002988 .00000981 .009807 .003386 .000133 .101320

.0001

.1

.001

1

Table 2 - Engineering Units Multiplication Factors

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Summary of Contents for 4800 Series

Page 1: ...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 ...

Page 2: ......

Page 3: ...re 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 t...

Page 4: ......

Page 5: ...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 wi...

Page 6: ...EERING 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 EQUA...

Page 7: ...t 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 sho...

Page 8: ...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 t...

Page 9: ...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 p...

Page 10: ...dard 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 Vi...

Page 11: ...s 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 transduc...

Page 12: ...ottom 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 t...

Page 13: ... 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 7Ω per 1 000 feet 48 5Ω per 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 normall...

Page 14: ... 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 t...

Page 15: ... 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 Gr...

Page 16: ...ete 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 c...

Page 17: ...d 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...

Page 18: ...en 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 Mod...

Page 19: ... 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 Pu...

Page 20: ...uence 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 c...

Page 21: ...sible to take the reading in a bucket of water that is at the ground temperature Piezometer Saturation The piezometer filter and sensor are saturated at the factory and sealed with Mylar tape Do not remove the tape until just before the sensor is installed in the ground The filter is saturated by drawing a vacuum on the sensor and then allowing water to flow into the sensor when the vacuum is rele...

Page 22: ... which more than one layer of cables must be placed in a given array the cables should be separated from each other by a vertical interval of not less than 50 mm 2 of hand compacted sand or selected fine embankment material Since the elongation capability of electrical cable is quite substantial it is not necessary to install the cable with any S shaped meanders During the backfill of trenches in ...

Page 23: ...oise from the power cable and this will likely cause a problem obtaining a stable reading Contact the factory concerning filtering options available for use with the Geokon dataloggers and readouts should difficulties arise 2 5 Initial Readings Initial readings must be taken and carefully recorded along with the barometric pressure and temperature at the time of installation Take the initial readi...

Page 24: ...s of the sensor conductors with blue representing the shield bare To turn the GK 404 on press the ON OFF button on the front panel of the unit The initial startup screen will be displayed After approximately one second the GK 404 will start taking readings and display them based on the settings of the POS and MODE buttons The unit display from left to right is as follows The current Position Set b...

Page 25: ...il it locks into place 3 2 2 Connecting Sensors with Bare Leads Attach the GK 403 2 flying leads to the bare leads of a Geokon vibrating wire sensor by connecting each of the clips on the leads to the matching colors of the sensor conductors with blue representing the shield bare 3 2 3 Operating the GK 405 Press the button labeled POWER ON BLUETOOTH A blue light will begin blinking signifying that...

Page 26: ...2 Connecting Sensors with Bare Leads Attach the GK 403 2 flying leads to the bare leads of a Geokon vibrating wire sensor by connecting each of the clips on the leads to the matching colors of the sensor conductors with blue representing the shield bare 3 3 3 Operating the GK 403 1 Turn the display selector to position B 2 Turn the unit on 3 The readout will display the vibrating wire output in di...

Page 27: ... effect of cable resistance is usually insignificant 2 Look up the temperature for the measured resistance in Table 6 in Appendix B For 4800HT models use Table 7 in Appendix C Alternately the temperature could be calculated using Equation 5 in Appendix B For example a resistance of 3400 ohms is equivalent to 22 C When long cables are used the cable resistance may need to be taken into account Stan...

Page 28: ...on multiplier listed in Table 2 From To psi H2 O H2 O mm H2 0 m H2 0 HG mm HG atm mbar bar kPa MPa psi 1 036127 43275 0014223 1 4223 49116 019337 14 696 014503 14 5039 14503 145 03 H2 O 27 730 1 12 039372 39 372 13 596 53525 406 78 40147 401 47 4 0147 4016 1 H2 O 2 3108 08333 1 003281 3 281 1 133 044604 33 8983 033456 33 4558 3346 334 6 mm H2 0 704 32 25 399 304 788 1 1000 345 32 13 595 10332 10 1...

Page 29: ...mally corrected pressure is 203 5 kPa 204 12 25 x 0 03852 Note that this correction for temperature applies only to the pressure transducer not to the entire cell surrounded by soil or soil and concrete each with its own different temperature coefficient of expansion Commercially it is not practical to measure this effect without incurring huge expenses The effect is usually small at depths where ...

Page 30: ...24 Figure 18 Sample Model 4800 Calibration Sheet ...

Page 31: ...and GK 405 might be able to read the pressure cells To convert the Channel A period display to digits use Equation 2 Is there a source of electrical noise nearby Most probable sources of electrical noise are motors generators transformers arc welders and antennas Make sure the shield drain wire is connected to ground Connect the shield drain wire to the readout using the blue clip Green for the GK...

Page 32: ... infinite infinite infinite White infinite infinite N A 3000Ω at 25 C infinite Green infinite infinite 3000Ω at 25 C N A infinite Shield infinite infinite infinite infinite N A Table 3 Sample Resistance Vibrating Wire Sensor Lead Grid SENSOR NAME Red Black White Green Shield Red Black White Green Shield Table 4 Resistance Work Sheet ...

Page 33: ...mial expression Linearity 0 5 FSR standard 0 1 FSR optional Overrange 1 5 x Rated Pressure Operating Temperature 20 to 80 C Excitation Frequency Range 1400 3500Hz Output Frequency Range 2000 3000Hz Cell Dimensions active area 150 250 mm 6 10 230 mm OD 9 OD 230 mm OD 9 OD 125 mm OD 5 OD Coil Resistance 150 Ω Material 316 Stainless Steel Weight 2 3 kg 5 lbs 2 3 kg 5 lbs 4 7 kg 10 3 lbs 2 7 kg 6 lbs ...

Page 34: ...3 107 9 123 77 66K 36 8006 4 1363 44 331 5 84 105 2 124 72 81K 35 7618 5 1310 45 321 2 85 102 5 125 68 30K 34 7252 6 1260 46 311 3 86 99 9 126 64 09K 33 6905 7 1212 47 301 7 87 97 3 127 60 17K 32 6576 8 1167 48 292 4 88 94 9 128 56 51K 31 6265 9 1123 49 283 5 89 92 5 129 53 10K 30 5971 10 1081 50 274 9 90 90 2 130 49 91K 29 5692 11 1040 51 266 6 91 87 9 131 46 94K 28 5427 12 1002 52 258 6 92 85 7 ...

Page 35: ...241 1 139 116 9 171 62 1 203 35 6 235 18 090 12 4 543 44 1 432 76 539 9 108 235 3 140 114 5 172 61 0 204 35 1 236 17 255 13 4 368 45 1 385 77 525 0 109 229 7 141 112 1 173 59 9 205 34 5 237 16 463 14 4 201 46 1 340 78 510 6 110 224 3 142 109 8 174 58 8 206 33 9 238 15 712 15 4 041 47 1 297 79 496 7 111 219 0 143 107 5 175 57 7 207 33 4 239 14 999 16 3 888 48 1 255 80 483 2 112 213 9 144 105 3 176 ...

Page 36: ... Thickness D D 1 Formulas Consider a circular cell of radius R containing a liquid film of a thickness D coefficient of thermal expansion Kppm C and bulk modulus G For a temperature rise of 1 C the expansion YT of the liquid film is given by the equation YT KD Equation 7 Expansion of Liquid for a Temperature Rise of 1 C Expansion of the liquid is resisted by the confinement of the surrounding medi...

Page 37: ...Edge The difference being PR 1 ν2 2 4 π E Equation 12 Difference in Deformation The above formulas apply to pressures acting on a free surface However in the confined case Y at the edge of the cell can be assumed to be nearly zero Therefore Y at the center is assumed to be the same as shown in Equation 12 If the average Y across the cell is assumed to be half this value and if the deformation of t...

Page 38: ...tion the term 1 ν2 can be replaced by 0 91 since ν usually lies between 0 25 and 0 35 The total embedment is given by P 1 5 EKD R psi o C Equation 15 Total Embedment And for contact pressure cells P 3 EKD R psi o C Equation 16 Total Embedment for Contact Pressure Cells Some typical values of the various parameters are Liquid K x 10 6 ºC G x 106 psi Oil 700 0 3 Mercury 180 3 6 Water 170 0 3 Glycol ...

Page 39: ...o C Soil medium stiffness E 10000 psi ν 0 3 P 0 138 psi o C Coarse Sand E 50000 psi ν 0 3 P 0 69 psi o C For an oil filled concrete stress cell nine inches in diameter and D 0 020 inches totally embedded in Concrete E 5 x 106 psi ν 0 25 P 22 7 psi o C Completely rigid medium P 210 psi o C For the same cell filled with mercury instead of oil Concrete P 5 8 psi o C Completely rigid medium P 650 psi ...

Page 40: ...y F H Factors Affecting Stress Cell Measurement in Soil J Geotech Eng Div ASCE Vol 108 No GT12 Dec pp1529 1548 3 Lazebnik G E Monitoring of Soil Structure Interaction Chapman Hall pp 224 4 Fujiyasu Y and Orihara K Elastic Modulus of Weathered Rock Proc of the 5th Intl Symp on Field Measurements in Geomechanics Singapore 1999 p 183 ...

Page 41: ...ssure Calculation with Second Order Polynomial Where R is the reading digits channel B A B and C are coefficients Figure 18 shows a typical calibration sheet of a transducer that has a very little nonlinearity The figure under the Linearity FS column is Calculated Pressure True Pressure Full Scale Pressure x 100 G R1 R0 P F S x 100 Equation 18 Linearity F S on Calibration Sheet Note The linearity ...

Page 42: ...ressure P set to zero If the field zero reading is not available calculate C using the zero pressure reading on the calibration sheet In the above example the value of C would be derived from the equation 0 A 8981 2 B 8981 from which C 1053 Equation 19 Calculating C Using the Zero Pressure Reading from the Calibration Sheet It should be noted that where changes of earth pressures are being monitor...

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