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Geokon 4850 Instruction Manual

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

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Summary of Contents for Geokon 4850

  • Page 1 Model 4850 N.A.T.M. Style V.W. Concrete Stress cells No part of this instruction manual may be reproduced, by any means, without the written consent of Geokon ® The information contained herein is believed to be accurate and reliable. However, Geokon ®...
  • Page 3 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.

  • Page 5: Table Of Contents

    2. INSTALLATION ..............................3 2.1 P ............................3 RELIMINARY ESTS 2.2 S ..........................3 TRESS NSTALLATION 2.2.1 Installing the Model 4850-1 ........................3 2.2.2 Installing the Model 4850-2 ........................5 2.3 I ............................... 5 NITIAL EADINGS 2.4 R ..........................6 RESSURIZING THE 2.4.1 Standard Re-Pressurization Technique .......................
  • Page 6 FIGURES 1 - G ......................... 1 IGURE ROUND EACTION URVE 2 - M 4850 C ......................2 IGURE ODEL ONCRETE TRESS 3 - M 4850-1 I ........................4 IGURE ODEL NSTALLATION 4 - M 4850-1 I ....................... 4 IGURE ODEL...

  • Page 7: Introduction

    1. INTRODUCTION 1.1 Theory of Operation The “New Austrian Tunneling Method”, or N.A.T.M., calls for the support of a tunnel by the rapid application of shotcrete to the freshly exposed ground. The theory behind this method of support, particularly useful in weaker ground, is that if the inherent strength of the ground can be preserved, it will be almost self-supporting and will require much less artificial support in the form of concrete or steel.

  • Page 8: Stress Cell Design And Construction

    Side View Figure 2 - Model 4850 Concrete Stress Cell The vibrating wire sensor is a standard Geokon Model 4500H transducer inside an all welded housing. The sensor is hermetically sealed and is connected via waterproof connectors to an electrical cable leading to the readout location. The sensor housing also incorporates a thermistor which permits measurement of temperature at the cell location.

  • Page 9: Installation

    2.2.1 Installing the Model 4850-1 The Model 4850-1 is designed to measure tangential stresses in the lining. Figure 3 shows one method of installation using short pieces of steel rebar grouted inside short boreholes and protruding into the area where the lining will be placed.

  • Page 10: Figure 3 - Model 4850-1 Installation

    Rebar Dowels (grouted into boreholes) Shotcrete Lining Tie Wire Model 4850-1 Pinch Tube Instrument Cable Junction Box Figure 4 - Model 4850-1 Installation Detail...

  • Page 11: Installing The Model 4850-2

    2.2.2 Installing the Model 4850-2 The Model 4850-2 is designed to measure radial pressures on the tunnel lining. To accommodate irregularities in the rock surface, it is necessary to fill the space between the rock surface and the cell with quick setting mortar.

  • Page 12: Re-Pressurizing The Cell

    2.4 Re-Pressurizing the Cell 2.4.1 Standard Re-Pressurization Technique After shotcreting, the cells temperature and initial reading can be read again. Once the temperature has stabilized to ambient then the cells can be inflated using the pinch tube and a special set of accessory pliers. The cell is first connected to the readout and then the pliers are used to squeeze the pinch tube flat beginning at the capped end.

  • Page 13: Remote Re-Pressurization Technique

    (i.e., over three meters). In this case it is possible to use the Geokon remote pinching apparatus as shown in Figure Figure 7 - Model 4850 with Remote Pinching Apparatus A short pinch tube is pinched by a hydraulic piston on the end of a hydraulic line leading to a hydraulic pump.

  • Page 14: Cable Installation

    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.

  • Page 15: Taking Readings

    20 hours on two AA batteries. It is designed for the readout of all Geokon vibrating wire gauges 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 stress cell (embedded thermistor) with a resolution of 0.1 °C.

  • Page 16: Gk-405 Readout Box

    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 17: Gk-403 Readout Box (Obsolete Model)

    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 18: Data Reduction

    4. DATA REDUCTION 4.1 Pressure Calculation To convert digits to pressure the following equation applies; Pressure = (Current Reading - Initial Reading) × Calibration Factor P = (R – R )×G Equation 1 - Convert Digits to Pressure The Initial Reading is normally obtained during installation (usually the zero reading). The Calibration Factor (usually in terms of PSI or MPa per digit) comes from the supplied calibration report.

  • Page 19: Troubleshooting

    5. TROUBLESHOOTING Maintenance and troubleshooting of Vibrating Wire Concrete Stress 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 20: Appendix A. Specifications

    6. APPENDIX A. SPECIFICATIONS A.1 Stress Cells Model: 4850-1 4850-2 Tangential Radial Ranges: 7 MPa (1000 psi) 2 MPa (300 psi) 20 MPa (3000 psi) 3.5 MPa (500 psi) 5 MPa (750 psi) Sensitivity: 0.025% FSR Accuracy: 0.10% FSR Linearity: 0.25% FSR (standard)

  • Page 21: 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) -273.15 °C Equation 3 - Resistance to Temperature Where; T = Temperature in °C. LnR = Natural Log of Thermistor Resistance. A = 1.4051 ×...

  • Page 22: Appendix C. Temperature Effect On Earth Pressure And Concrete Stress Cells

    APPENDIX C. 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 23 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 4, 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 24: Table 3 - Typical Values Of Various Cell Parameters

    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 25: Examples

    C.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 26 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.”...

  • Page 27: Appendix D. Typical Calibration Report

    APPENDIX D. TYPICAL CALIBRATION REPORT Figure 11 - Typical Calibration Report...

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