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Geokon 4850, Instruction Manual

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Geokon 4850 Instruction Manual Download Page 26

 

20 

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.” 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. 

 

 

 

Summary of Contents for 4850

Page 1: ...ed 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 ...

Page 2: ......

Page 3: ...nnel 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 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 ...

Page 4: ......

Page 5: ...READOUT BOX 9 3 1 1 Operating the GK 404 9 3 2 GK 405 READOUT BOX 10 3 2 1 Connecting Sensors 10 3 2 2 Operating the GK 405 10 3 3 GK 403 READOUT BOX OBSOLETE MODEL 11 3 3 1 Connecting Sensors 11 3 3 2 Operating the GK 403 11 3 4 MEASURING TEMPERATURES 11 4 DATA REDUCTION 12 4 1 PRESSURE CALCULATION 12 4 2 TEMPERATURE CORRECTION 12 4 3 BAROMETRIC CORRECTION 12 5 TROUBLESHOOTING 13 6 APPENDIX A SPE...

Page 6: ...NS 14 TABLE 2 THERMISTOR RESISTANCE VERSUS TEMPERATURE 15 TABLE 3 TYPICAL VALUES OF VARIOUS CELL PARAMETERS 18 EQUATIONS EQUATION 1 CONVERT DIGITS TO PRESSURE 12 EQUATION 2 TEMPERATURE CORRECTION FOR TRANSDUCER ONLY 12 EQUATION 3 RESISTANCE TO TEMPERATURE 15 EQUATION 4 EXPANSION OF LIQUID FOR A TEMPERATURE RISE OF 1 C 16 EQUATION 5 COMPRESSION OF LIQUID 16 EQUATION 6 EXPANSION OF LIQUID 16 EQUATIO...

Page 7: ...f inherent support and ground deformation Thus to prevent any support deformation or tunnel closure at all would require a support pressure exerted on the tunnel walls equal to the original in situ ground stress A strong lining with characteristics of curve A would allow only a small amount of ground deformation but might because it is too strong be uneconomical A thinner lining which would allow ...

Page 8: ...red and has cooled off to the ambient temperature During concrete curing temperatures very often rise and will cause the cell to expand in the still green concrete On cooling the cell contracts leaving a space between it and the surrounding concrete which if allowed to remain would prevent the transmission of pressures from the concrete to the cell Pinch Tube Stress Cell Transducer Housing Instrum...

Page 9: ...he temperature Compare the measured resistance with the values given in Table 2 of Appendix B Resistance between any conductor and the shield should exceed 20 megohm 2 2 Stress Cell Installation Cells are positioned on the wall of the tunnel in two ways one way to measure tangential stresses and the other to measure radial 2 2 1 Installing the Model 4850 1 The Model 4850 1 is designed to measure t...

Page 10: ...tion The pinch tube is bent so that it will protrude from the lining after it has been placed Or it can be wrapped in foam plastic etc so that it can be dug out and retrieved after shotcreting Tie Wire Shotcrete Lining Rebar Dowels Pinch Tube Model 4850 1 Instrument Cable Junction Box grouted into boreholes Figure 4 Model 4850 1 Installation Detail ...

Page 11: ...lace the cell must be gripped firmly using the previously installed hardware See Figure 5 The cable is routed to the readout location and held firmly in place by tying it off to other rebars nails etc driven into the ground or to the reinforcing mesh of one is used At the readout location the cable can be coiled inside a box cast inside the shotcrete lining as before The pinch tube should be bent ...

Page 12: ...o a pressure if so desired but this is not necessary Length of Tube Pinched Decrease Knee in Reading or increase in pressure Stop Here Figure 6 Cell Re Pressurization Graph As the cell expands inside any space that may exist the pressure rise accompanying each pinch will be small only one or two digits But as soon as the cell starts to fill the space the pressure rise with each pinch will become l...

Page 13: ...cooled the tube is pinched by applying pressure with the hydraulic pump The pinching effect begins at around 4 MPa 600 psi when the pinched tube begins to crush and continues to about 10 MPa 1450 psi when the tube is completely flattened The maximum burst pressure of the hydraulic tube is 17 MPa 2500 psi Connect the stress cell to the Model GK 404 or GK 405 readout box channel B while pinching Sto...

Page 14: ...factory 2 6 Electrical Noise Care should be exercised when installing instrument cables to keep them as far away as possible from sources of electrical interference such as power lines generators motors transformers arc welders etc Cables should never be buried or run with AC power lines The instrument cables will pick up the 50 or 60 Hz or other frequency noise from the power cable and this will ...

Page 15: ... 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 by the PO...

Page 16: ...s into place 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 2 Operating the GK 405 Press the button labeled POWER ON A blue light will begin blinking signifying that the Remote Module is wait...

Page 17: ...ition B 2 Turn the unit on 3 The readout will display the vibrating wire output in digits The last digit may change one or two digits while reading 4 The thermistor reading will be displayed above the gauge reading in degrees centigrade 5 Press the Store button to record the value displayed If the no reading displays or the reading is unstable see Section 5 for troubleshooting suggestions The unit...

Page 18: ...ed it can be made using the factors supplied on the calibration report and Equation 2 However there can be much larger temperature effects caused by the mismatch between temperature coefficients of the cell and surrounding concrete This effect is not quantifiable in the laboratory but a theoretical treatment is given in appendix C Temperature Correction Current Temperature Initial Temperature Ther...

Page 19: ... sure the shield drain wire is connected to ground whether using a portable readout or datalogger Make sure to connect the clip with the blue boot to the shield drain wire Does the readout work with another stress cell If not the readout may have a low battery or be malfunctioning Symptom Stress Cell Fails to Read Is the cable cut or crushed This can be checked with an ohmmeter Nominal resistance ...

Page 20: ...andard 0 1 FSR optional Operating Temperature 30 to 70 C Frequency range 1400 3500Hz Dimensions 100 200 mm 4 8 150 250 mm 6 10 Pinch Tube Length 600 mm Material 303 304 Stainless Steel Electrical Cable 2 twisted pair 4 conductor 22 AWG Foil shield PVC jacket nominal OD 6 3 mm 0 250 Table 1 Specifications Consult the factory for other sizes or options available A 2 Thermistor see Appendix B also Ra...

Page 21: ... 83 107 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 ...

Page 22: ... Thickness D C 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 4 Expansion of Liquid for a Temperature Rise of 1 C Expansion of the liquid is resisted by the confinement of the surrounding medi...

Page 23: ...Edge The difference being PR 1 ν2 2 4 π E Equation 9 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 9 If the average Y across the cell is assumed to be half this value and if the deformation of the...

Page 24: ...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 12 Total Embedment And for contact pressure cells P 3 EKD R psi o C Equation 13 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 25: ...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 26: ...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 27: ...21 APPENDIX D TYPICAL CALIBRATION REPORT Figure 11 Typical Calibration Report ...

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