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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)
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Table of Contents
Related Manuals for Geokon 4820
Summary of Contents for Geokon 4820
- Page 1 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.
- Page 3 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...
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Page 5: Table Of Contents
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 .......... - Page 6 ..................... 5 IGURE ODEL ONTACT RESSURE 7 - M 4815 H ......................5 IGURE ODEL YDRAULIC 8 - M 4820 J ...................... 6 IGURE ODEL ACKOUT RESSURE 9 - M 4830 P ....................... 6 IGURE ODEL RESSURE 10 - M 4800 E ................
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Page 7: Introduction
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. -
Page 8: Figure 1 - Stress Redistribution
Mean Stress Cell 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. -
Page 9: Earth Pressure Cell Design
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. -
Page 10: Earth Pressure Cell Construction
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. -
Page 11: Model 4810 Contact ("Fat Back") Pressure Cell
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. -
Page 12: Model 4820 Earth Pressure "Jackout" Cell
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. -
Page 13: Installation
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. -
Page 14: Figure 10 - Model 4800 Earth
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. -
Page 15: Table 1 - Ratios For Two Grout
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. -
Page 16: Installation Of Model 4810 Contact ("Fat Back") Pressure Cell
For slurry walls, the Model 4820 Earth Pressure Cell is used as described in Section 2.2.4. -
Page 17: Figure 12 - Model 4810 Contact
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. -
Page 18: Installation Of Model 4815 Hydraulic Load Cell
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. -
Page 19: Installation Of Model 4820 Jackout Pressure Cell In Slurry Trenches
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. -
Page 20: Installation Of Cells To Measure Earth Pressure At The Base Of Footings, Floor Slabs, Pavements, Etc
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. -
Page 21: Installation Of Push-In Pressure Cells To Measure Lateral Earth Pressures
If the sensor is to be installed and then removed for use at other sites, the saturation process should be performed at each installation. Geokon can supply the necessary portable equipment to accomplish this. -
Page 22: Cable Installation And Splicing
Splice kits recommended by Geokon incorporate casts, which are placed around the splice and are then filled with epoxy to waterproof the connections. When properly made, this type of splice is equal or superior to the cable in strength and electrical properties. Contact Geokon for splicing materials and additional cable splicing instructions. -
Page 23: Electrical Noise
50 or 60 Hz (or other frequency) noise 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. -
Page 24: Taking Readings
20 hours continuously on two AA batteries. It is designed for the readout of all Geokon vibrating wire gages and transducers, and is capable of displaying the reading in either digits, frequency (Hz), period (µs), or microstrain (µε). The GK-404 also displays the temperature of the load cell (embedded thermistor) with a resolution of 0.1 °C. -
Page 25: Gk-405 Readout Box
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). -
Page 26: Gk-403 Readout Box (Obsolete Model)
3.3.2 Connecting Sensors with Bare Leads Attach the GK-403-2 flying leads to the bare leads of a Geokon vibrating wire sensor by connecting each of the clips on the leads to the matching colors of the sensor conductors, with blue representing the shield (bare). -
Page 27: Measuring Temperatures
When long cables are used, the cable resistance may need to be taken into account. Standard 22 AWG stranded copper lead cable is approximately 14.7Ω/1000' or 48.5Ω/km, multiply by two for both directions. Note: Geokon readout boxes will read the thermistor and display temperature in °C automatically. -
Page 28: Data Reduction
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 = �... -
Page 29: Temperature Correction
4.3 Barometric Correction The pressure transducer used in Geokon Vibrating Wire Earth Pressure Cells is evacuated and hermetically sealed and will respond to barometric pressure fluctuation. If a correction for these... -
Page 30: Figure 18 - Sample Model
Figure 18 - Sample Model 4800 Calibration Sheet... -
Page 31: Troubleshooting
5. TROUBLESHOOTING Maintenance and troubleshooting of Vibrating Wire Pressure Cells is confined to periodic checks of cable connections. Once installed, the cells are usually inaccessible and remedial action is limited. Consult the following list of problems and possible solutions should difficulties arise. Consult the factory for additional troubleshooting help. -
Page 32: Table 3 - Sample Resistance
Vibrating Wire Sensor Lead Grid - SAMPLE VALUES Black White Green Shield ≅180Ω infinite infinite infinite ≅180Ω Black infinite infinite infinite 3000Ω at 25°C White infinite infinite infinite 3000Ω at 25°C Green infinite infinite infinite Shield infinite infinite infinite infinite Table 3 - Sample Resistance Vibrating Wire Sensor Lead Grid - SENSOR NAME/## : Black... -
Page 33: Appendix A. Specifications
APPENDIX A. SPECIFICATIONS A.1 Earth Pressure Cells 4800 4800 4810 4820 Earth Pressure Model: Earth Pressure Cell Contact Pressure Jack-Out Cell (rectangular) Cell Pressure Cell (circular) 70 kPa (10 psi) 70 kPa (10 psi) 350 kPa (50 psi) 350 kPa (50 psi) -
Page 34: Appendix B. Thermistor Temperature Derivation
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION Thermistor Type: YSI 44005, Dale #1C3001-B3, Alpha #13A3001-B3 Resistance to Temperature Equation: A+B ( LnR ) +C(LnR) -273.2 Equation 5 - Resistance to Temperature Where; T = Temperature in °C. LnR = Natural Log of Thermistor Resistance. A = 1.4051 ×... -
Page 35: Appendix C. High Temperature Thermistor Linearization
APPENDIX C. HIGH TEMPERATURE THERMISTOR LINEARIZATION Resistance to Temperature Equation for US Sensor 103JL1A: A+B ( LnR ) +C(LnR) -273.2 +D(LnR) Equation 6 - High Temperature Resistance to Temperature Where; T = Temperature in °C. LnR = Natural Log of Thermistor Resistance. A = 1.127670 ×... -
Page 36: Appendix D. Temperature Effect On Earth Pressure And Concrete Stress Cells
APPENDIX D. TEMPERATURE EFFECT ON EARTH PRESSURE AND CONCRETE STRESS CELLS The following theoretical treatment is by no means rigorous — there are some questionable assumptions and approximations — but it should give some idea of the magnitude of the thermal effect to be expected on hydraulic earth pressure cells, buried in soil, or installed at the contact between soil and structure, and on concrete stress cells embedded in concrete. -
Page 37: Equation 10 - Deformation At The Equation 11 - Deformation At The
Liquid pressure inside the cell causes deformation of the surrounding medium. The amount of deformation can be quantified by modification of formula found in Equation 7, where the deformation (Y), produced by a uniform pressure (P), acting on a circular area, (R) radius, on the surface of a material with modulus of elasticity (E) and Poisson’s ratio (ν), is given by: At the center of the cell: 2 PR (1-ν... -
Page 38: Table 8 - Typical Values Ofv
If one side of the cell lies in contact with a rigid structure, e.g., a concrete retaining wall or a concrete bridge footing, then: Y = 0.73 PR (1-ν ) x 0.5/E = 0.36 PR (1-ν P (D/G + 0.36 R (1-ν )/E) = KD Where (E) pertains to the soil material. -
Page 39: Examples
D.2 Examples For an oil-filled cell, nine inches diameter, and D = 0.060 inches, totally embedded in: (For contact pressure cells, multiply the values for P by two.) Plastic Clay: E = 3000 psi ν = 0.3 P = 0.042 psi / Soil, medium stiffness: E = 10000 psi ν... - Page 40 References: [1] Roark, R.J. and Young, W.C. “ Formulas for Stress and Strain,” McGraw Hill, fifth edition, 1982, p 519. [2] Weiler, W.A. and Kulhawy, 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.”...
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Page 41: Appendix E. Non Linearity And The Use Of A Second Order Polynomial To Improve The Accuracy Of The Calculated Pressure
APPENDIX E. NON LINEARITY AND THE USE OF A SECOND ORDER POLYNOMIAL TO IMPROVE THE ACCURACY OF THE CALCULATED PRESSURE Most vibrating wire pressure transducers are sufficiently linear (± 0.2 % FS) that use of the linear calibration factor satisfies normal requirements. However, it should be noted that the accuracy of the calibration data, which is dictated by the accuracy of the calibration apparatus, is always ±... -
Page 42: Eading From The Calibration Sheet
Note. If the polynomial equation is used it is important that the value of C, in the polynomial equation, be taken in the field, following the procedures described in Section 2.5. The field value of C is calculated by inserting the initial field zero reading into the polynomial equation with the pressure, P, set to zero.