Quality Assurance Approved Report No RR-131

Assessing Temperature Effects on Earth Pressure CellsDaigle, L.; Zhao, J.Q.IRC-RR-131February 2003http://irc.nrc-cnrc.gc.ca/ircpubsAssessing Temperature Effects on Earth Pressure CellsResearch Report 131AuthorLyne Daigle, M.Sc., P.Eng.AuthorJack Q. Zhao, Ph.D., P.Eng.QualityAssuranceBalvant Rajani, Ph.D.Group Leader, Buried Utilities ResearchApprovedDon Taylor, Ph.D., P.Eng.Director, Urban Infrastructure RehabilitationReport No:RR-131Report Date:February 17, 2003Program:Urban Infrastructure Rehabilitationv + 31 pagesCopy No. 1 of 10 copiesAbstractVibrating-wire earth pressure cells are often used to measure soil pressure in fills and embankments or contact pressure between soil and buried structures. The cell consists of two metal plates welded on their periphery and filled with oil. Changes in applied pressure result in variations in fluid pressure in the cell, which is detected by a transducer, one type of which is the vibrating wire. The vibrating wire pressure cell is commonly used due to its long-term reliability. Geotechnical instrumentation companies provide each cell with a formula to calculate pressure based on frequency and temperature readings.Small diameter (76 mm) pressure cells installed in the field, around and in the vicinity of a concrete culvert, were responding in a manner that seemed to be strongly affected by temperature variations. Laboratory work was subsequently done to confirm the behaviour of the cells. Temperature calibration work was carried out for a series of 76 mm diameter cells and several 228 mm cells. The calibration was run with temperatures ranging from –10°C to 30°C with and without the effect of applied pressure on the cells. From this work, and from data gathered from different field sites, it was found that temperature calibration factors, given on the manufacturer’s calibration sheet specific to each cell, largely underestimated the temperature effect. It was also found that the correction factors were dependent not only on temperature, but also on the pressure applied to the cell. The results showed that the temperature calibration curve, given as a linear correction on the calibration datasheet, became parabolic as pressure was applied to the cell.Recommendations are provided for minimising the temperature effect on sensitive pressure cells as well as for more accurately correcting temperature effects on cell readings. In addition, suggestions for improvement in the manufacture and calibration of pressure cells before they are installed in the field are presented.Keywords: earth pressure cells, vibrating wire, soil pressure, temperature calibration.AcknowledgementsAssistance from Mr. Haoze (Ryan) Deng and Mr. Omar Tabba in performing the laboratory work is appreciated.Table of Contents1.Introduction (1)2.Earth Pressure Cells (2)3.Literature Review (4)4.Testing Program and Laboratory Set up (6)4.1 Testing equipment (6)4.2 Unloaded steady state temperature calibration (10)4.3 Loaded steady state temperature calibration (11)4.4 Unloaded transient temperature calibration (12)5.Results and Discussion (13)5.1 Temperature and pressure readings (13)5.2 Unloaded steady state temperature (14)5.3 Loaded steady state temperature (17)5.4 Unloaded transient temperature (22)5.5 Results from field sites (24)6.Summary, Conclusions and Recommendations (27)REFERENCES (30)List of TablesTable 4.1. List of pressure cells tested (6)Table 5.1. Summary of unloaded steady state calibration of 76 mm pressure cells (200 kPa) (15)Table 5.2. Summary of unloaded steady state calibration of 228 mm pressure cells (17)Table 5.3. Summary of loaded steady state calibration of 76 mm pressure cells (200 kPa) (19)Table 5.4. Summary of loaded steady state calibration of 228 mm pressure cells (21)List of FiguresFigure 2.1. Sketch of pressure cells (2)Figure 4.1. Schematic of test pressure chamber (8)Figure 4.2. Schematic of the test set up for cell calibration under pressure (9)Figure 4.3. Pressure cell layout for unloaded condition (10)Figure 4.4. Temperature-time pattern of the environmental chamber for steady state condition (11)Figure 4.5. Temperature-time pattern of the environmental chamber for steady state condition of loaded cells (12)Figure 4.6. Temperature-time patterns of the environmental chamber for transient temperature conditions (13)Figure 5.1. Typical result of 76 mm cell under unloaded steady state calibration (15)Figure 5.2. IU versus temperature for 76 mm cell # 1 (18)Figure 5.3. Loaded temperature calibration for 76 mm cell #1 (19)Figure 5.4. Loaded temperature calibration of 228 mm cell 3-A (20)Figure 5.5. Temperature calibration of loaded 228 mm cell 4-A (22)Figure 5.6. Transient temperature effect on a 76 mm cell (23)Figure 5.7. Results from a 76 mm cell installed in the field (25)Figure 5.8. Results from a 228 mm cell installed in the field (26)1.IntroductionVibrating-wire earth pressure cells are commonly used to measure contact pressure on engineered structures or directly buried in the ground to measure the soil pressure. Valuable data can be obtained from these sensors to confirm design assumptions and to provide information to improve future designs of embankments, retaining walls, piers, buried pipes and culverts, among others.A hydraulic pressure cell, in simple terms, consists of two metal plates welded together on their periphery and filled with a relatively thin layer of fluid. Changes in applied pressure result in fluid pressure variations, which are sensed by a transducer. Frequency (or internal unit) readings are converted to pressure readings using equations given by the manufacturer. These equations include the initial installation conditions (pressure, temperature and barometric pressure) as well as a pressure calibration factor and a temperature calibration factor that are specific to each cell.From our own experience and from a review of the scientific literature (Yang et al. 2001, Félio and Bauer 1986, Dunnicliff and Green 1988) it was found that calibration of earth pressure cells should not be overlooked to enable the collection of reliable data, especially when cells are installed in a medium that experiences seasonal temperature variations. Although many other factors influence the accuracy of the pressure cells readings, this report will focus on the influence of temperature variations.The objective of the calibration testing program carried out at the NRC laboratories was originally intended to determine suitable temperature calibration factors or equations that could be used for 76 mm diameter pressure cells already installed in the field in the Ottawa region. Therefore, most of the work was carried out on these smaller cells rather than the more commonly used 228 mm diameter pressure cells.The results of the calibration tests are presented and discussed in this report. Data obtained from two different field sites are also included in the discussion. Recommendations are provided for carrying out reliable temperature calibration data before pressure cells are installed in the field and to minimise the temperature effect.2. Earth Pressure CellsEarth pressure cells tested in this study were hydraulic vibrating-wire cells. Most of them had a measuring range of 200 kPa. Cells with a diameter of 76 mm had an aspect ratio (cell diameter/cell thickness) of 7.6 whereas cells with a 228 mm diameter had a ratio of 38.3 and 23.2 depending on the manufacturer. From the aspect ratio, it is expected that the boundary effect should be more severe on the smaller diameter cells. Figure 2.1 shows the dimensions of the two cells tested. On each cell the pressure pad is connected to the thermistor and transducer housing by a length of stainless steel tubing. Lugs (or eyelets) are usually present around the circumference of the pad to allow the installation of the cell on a contact surface.External pressure on the pad of the cell induces a pressure change in the internal fluid. This pressure change is measured by a vibrating-wire transducer by means of a small diaphragm, located in the transducer housing, to which the vibrating wire is attached. Deflection of the diaphragm causes a variation in the wire tension and changes its resonant frequency when the measuring device (datalogging system or readout unit) plucks the wire.Pressure cell pad6 to 10 mm Pressure cell padthermistor housingEyelet 10 mmFigure 2.1. Sketch of pressure cells.Different models of pressure cells are available depending on whether they are installed on rigid contact surfaces or embedded in the soil. Models can also vary depending on the cell stiffness and sensitivity. Ideally, pressure cell design should be adjustable to fit different types of installation and to match the surrounding soil stiffness, but this approach would be difficult to implement considering that soil stiffness is not a constant parameter. It would also be far too expensive (Dunnicliff and Green 1988). Manufacturers have instead a few different types of models that fit a range of needs.Equations given in instruction manuals to convert frequency or internal unit readings (proportional to squared frequency) to a pressure reading have the following general form:)()()(010101B B T T CT R R CF P −−−−−=∆ [1a])()()(01012021B B T T CT F F CF P −−−−−=∆ [1b]where:?P is the pressure variation in kPa,CF is the calibration factor in kPa/Internal units or kPa/Hertz 2,CT is the temperature calibration factor in kPa/°C,R 1, R 0 are the current and initial readings in internal units,F 1 , F 0 are the current and initial readings in Hertz and,B 1 , B 0 are the current and initial barometric pressures in kPa.The CF and CT values, measured for each cell by the manufacturer, are given on the calibration sheet along with the temperature and barometric pressure at which the initial calibration was performed. In the manufacturer’s literature about the cells, it is stated that the pressure cells should be installed in relatively stable temperature environments, though they give the thermal correction for significant temperature variations. On one of the technical specification sheets, the operating temperature range is indicated to be between –20°C and 80°C. Most of the time, it is not clearly mentioned on the datasheet that the temperature calibration factor does not apply to the whole pressure cell unit but only to the transducer part (Dunnicliff 1997), in this case, the vibrating-wire. The same comment can be made with regard to instruction manuals where explanations for the use of the thermal calibration factor are more detailed.3. Literature ReviewMany papers emphasise the need for laboratory calibration of pressure cells either by fluid or soil pressure. Pressures in field conditions can however be over- or under-registered by the pressure cells depending on the soil/cell stiffness ratio, the different boundary conditions and other factors which are specific to the field installation and difficult to reproduce in laboratory conditions (Theroux et al. 2001; Theroux 2000; Garnier et al. 1999, Weiler and Kulhawy 1982).Dunnicliff and Green’s (1988) book includes a chapter on the measurement of total stress in soil which relates to the use of embedded earth pressure cells and contact earth pressure cells. They summarise a large number of studies and describe in detail the many factors that affect the measurement of stress in soil. Among those factors, the aspect ratio, the soil/cell stiffness ratio, the size of the cell, the non-uniform and point loads, the placement and temperature variation effects are mentioned. Beside each of these factors, a corrective method is proposed and the one suggested for temperature effects is:“Design cell for minimum sensitivity to temperature; if significant temperature change is likely, measure temperature and apply correction factor determined during calibration.” They further recognise that temperature calibration of cells in unloaded condition is not likely to give the same results as a temperature calibration of a confined cell which is likely more susceptible to temperature effects. They recommend the use of a cell with a layer of fluid as thin as possible to minimise the temperature effect on the pressure cells. They also point out that the influence of temperature is potentially greater for a contact pressure cell due to a possibly greater temperature variation of the structure on which it is installed.Temperature effects for contact earth pressure cells are thoroughly discussed in Sellers (2000) and Yang et al. (2001) papers. Based on some assumptions and approximations, Sellers gives equations [2a] and [2b] to determine a thermal correction factor for embedment and contact pressure cells, respectively.5.1= [2a]EKtCT/R0.3= [2b]EKtRCT/where :CT is the temperature calibration factor in kPa/°C,E is the soil elastic modulus in GPa,K is the thermal expansion of fluid inside cell in 10-6/°C,t is the thickness of the fluid inside the cell in mm and,R is the radius of the cell in mm.Yang et al. (2001) describe and discuss results obtained from contact pressure cells installed on the roof of a cast-in-place concrete box culvert. In spite of the temperature correction recommended by the manufacturer, the pressure data collected over a four-year period still showed a very strong relationship with the seasonal temperature variations. They used the theoretical correction, equation [2b], proposed by Sellers (2000) which reduced the temperature-induced variation but only to one order of magnitude less than the correction factor obtained by the empirical approach. This approach involves finding the relationship between the recorded temperature and measured pressure assuming the vertical soil pressure remained constant over the range of observed temperatures. For one of the pressure cells used, the temperature coefficient factor given by the manufacturer was 0.113 kPa/°C, that obtained with equation [2b] was 0.582 kPa/°C and that derived from observed data was 6.99 kPa/°C.Félio and Bauer (1986) and Coyle and Bartoskewitz (1976) also give other examples where a significant temperature influence on the pressure results was observed. In both cases, pressure cells were placed in contact with a structure, the first case being on a bridge abutment and the second case on the back face of a retaining wall.In a more recent note (Dunnicliff 1997), the temperature sensitivity of earth pressure cells was acknowledged. He mentioned however that contrary to contact pressure cells, embedment installations were rarely affected by temperature variations.4. Testing Program and Laboratory Set upA testing program was established to verify the effect of temperature variations under three different sets of conditions. The first series of tests were carried out to check the applicability of the temperature factor, given on the calibration datasheet for unloaded cells, at different temperature values between –10°C and 30°C, representative of field conditions in the Ottawa region, in a steady state mode. The second series of tests was conducted, with the same temperature range, to verify if the level of pressure acting on a cell would enhance or modify the temperature effects on the cell readings. For the last series, the unloaded procedure was repeated but in a transient temperature mode. Table 4.1 lists the pressure cells tested under each condition.Table 4.1. List of pressure cells tested.Testing conditionPad diameter (mm)Full scale range (kPa)# of cells testedUnloaded Steady state T°Loaded Steadystate T°Unloaded Transient T°762005X XX7620019*X 2282003X X (1 cell)X 2281732X X X 2287501X XX 22810002X X 22815003XX*These pressure cells were installed in the field after the unloaded temperature calibration.4.1 Testing equipmentAn environmental chamber (inside dimensions: 915 mm x 915 mm x 1100 mm) was used to provide a temperature-controlled environment. This chamber is equipped with an air-circulating fan to provide a uniform temperature distribution inside. Temperature is controlled by a Yokogawa model UP-550controller, which can be programmed with complex temperature versus time patterns. The inside chamber temperature is read by a resistance temperature detector (RTD) located at the back of the chamber. An additional temperature probe was used to measure the inside temperature of thechamber at its center. The environmental chamber is connected to a computer. The chamber temperature (from the RTD) is graphically displayed on the computer screen by means of HP VEE®test and measurement application software.The data acquisition system was composed of a Campbell Scientific (CSI) CR10X measurement and control module, a 16-channel relay multiplexer (AM416 from CSI), a vibrating-wire interface module (AVW1 from CSI) for both frequency and temperature measurements of the pressure cells and a storage module (SM716 from CSI) for data backup. This data acquisition system was programmed to take readings from the vibrating wire sensors, a thermocouple and a temperature probe every minute. Communication to the data acquisition system was provided through the PC208W software (from CSI) installed on a laptop. This software permitted viewing of real time and time history data in graphical format.For the second series of calibration tests, a pressure chamber was used to apply load on the cell. This steel chamber had an inside diameter of 380 mm and was designed to accommodate a 76 mm diameter cell (Fig. 4.1). The pressure chamber was filled with dry Ottawa sand and the cell’s pressure pad was positioned in the center of the chamber at mid-height during a test. The steel tubing connecting the pressure pad to the transducer was routed through a slot in the wall of the pressure chamber. Hence, the vibrating-wire transducer was situated outside the pressure chamber. A rubber membrane was placed against the interior of the chamber steel cover. A pressurised oil line was connected to an opening in the cover, which allowed the injection of oil between the membrane and the cover to create a uniform pressure on the sand surface. A vibrating-wire piezometer was connected to a second opening in the cover to measure the oil pressure.The overall dimensions of the pressure chamber were adapted to fit inside the environmental chamber. The oil pressure line was connected to a compressed air line through an air-oil interface chamber. Loading on the cell was controlled by the air pressure control valve. The complete test pressure chamber set up is shown in Figure 4.2.Figure 4.1. Schematic of test pressure chamber.4.2 Unloaded steady state temperature calibrationMany cells could be tested at once under the unloaded steady state temperature condition (Fig. 4.3). Each cell’s pressure pad was laid on a plywood square to ensure the pressure cell would remain stable during the test. Figure 4.4 shows the temperature-time pattern that was used for the environmental chamber. The five temperature set points were 30°C, 20°C, 10°C, 0°C and –10°C. For each set point, a 3-hour plateau was sufficient to achieve temperature equilibrium of the unloaded cells since they were in direct contact with the air circulating inside the chamber.Figure 4.3. Pressure cell layout for unloaded condition.-15-10-5051015202530350200400600800100012001400160018002000Time (minutes)T e m p e r a t u r e (°C )Figure 4.4. Temperature-time pattern of the environmental chamber for steady state condition.4.3 Loaded steady state temperature calibrationThe use of the pressure chamber for the loaded steady state condition only permitted testing of one cell at a time. The temperature-time pattern that was used for the unloaded tests could no longer be used because of the much longer time needed for the cell, surrounded by sand, to reach temperature equilibrium. In addition, time limitations only permitted to programming of four temperature set points during a cooling cycle. The same set points were not repeated during a heating cycle as under the unloaded condition (except for three of the four 228 mm cells). The total time of a calibration run was 66 hours for each loading level (Fig. 4.5) compared to 30 hours for the unloaded steady condition.Calibrations of five 76 mm pressure cells with 3 or 4 levels of applied pressures ranging from 1.1 kPa to 132.6 kPa were carried out. Tests on two 228 mm pressure cells were also conducted, though as mentioned earlier, the pressure chamber was not designed for this size of cells. These test results are only preliminary.-15-10-50510152025303505001000150020002500300035004000Time (minutes)T e m p e r a t u r e (°C )Figure 4.5. Temperature-time pattern of the environmental chamber for steady state condition ofloaded cells.4.4 Unloaded transient temperature calibrationFor the third series of tests, the set up was the same as the unloaded condition. The transient temperature condition was achieved by gradually lowering the temperature of the environmental chamber from 30°C to –10°C and, after a fixed period at –10°C, the temperature was gradually raised back, at the same rate, to 30°C. Two temperature rates were selected. Based on field temperature data collected from the buried surface of a concrete culvert, a rate of 2°C/hour was used.This relatively high temperature change would mainly occur during the spring and fall seasons. A higher temperature rate of 5°C/hour was also selected. A pressure cell embedded in soil may not be subjected to such a high temperature variation and this situation will not likely happen for a contact pressure cell either. The period when the cell is most susceptible to fast temperature change is during its installation, when the baseline readings are taken. Since a reliable baseline value is critical to the analysis of subsequent data, a transient temperature calibration with a temperature rate of 5°C/hour was found appropriate to determine the effect of fast temperature change on the cells. Figure 4.6shows the temperature-time patterns that were used for the environmental chamber. The time set at –10°C was 3 and 6 hours, for the 2°C/hour and 5°C/hour temperature rates respectively. Pressure cells with 76 mm and 228 mm diameters were tested under both steady state and transient temperature cycles.-15-10-505101520253035050010001500200025003000Time (minutes)T e m p e r a t u r e (°C )Figure 4.6. Temperature-time patterns of the environmental chamber for transient temperatureconditions.5. Results and Discussion5.1 Temperature and pressure readingsIn the previous section, the environmental chamber temperature was presented in the form of temperature-time pattern. This term was used to avoid the confusion between the desired temperature,which is the temperature pattern programmed in the controller and the actual chamber temperature.The latter is obtained by two different sensors, the first one being the temperature measured by the resistance temperature detector (RTD) inside the chamber and used by the controller to make adjustments to the heating or cooling rate in order to match the desired temperature. The accuracy of the RTD temperature is within ±0.01°C. The temperature probe, hooked up to the data logging system, gives a second measurement of the chamber temperature. Its accuracy is specified to be within ±0.2°C for the temperature range used. This measurement is the one used in all subsequent results when referring to the environmental chamber temperature. This explains the minor temperature differences between the programmed temperature and the reading of the temperature probe in stable temperature conditions.The temperature of the pressure cell is measured by the thermistor built in the sensor. Its accuracy is estimated to be within ±0.2°C when measured with the CR10 unit. For the cells used in this study, the thermistor was located close to the vibrating–wire transducer at some distance from the pressure pad (Fig. 2.1).For the loaded condition, the temperature of the pressure pad was measured by a thermocouple installed on the surface of the pad (Fig. 4.1). The accuracy of thermocouple readings is generally between ±0.2°C.In the subsequent text, the term “apparent pressure” represents the variation in frequency readings of a pressure cell that is caused solely by temperature changes while loading conditions remain constant.5.2 Unloaded steady state temperatureA calibration result for one 76 mm pressure cell is shown in Figure 5.1 as a typical result obtained for the 76 mm cells. The reference temperature was taken as 20°C where the apparent loading was adjusted to zero. The datasheet temperature correction factor of 0.0336 kPa/°C would cause an apparent pressure variation of 1.3 kPa (=0.0336x40°C) for the studied temperature range. An apparent pressure variation of 59.2 kPa was measured in the laboratory for the same temperature range. This variation represents 30.3% of the full-scale reading of the pressure cell. In addition, the data collected suggest that a quadratic equation would give a more accurate correction than the linear correction equation.-15-10-55101520253035Temperature (°C)A p p a r e n t p r e s s u r e (k P a )Figure 5.1. Typical result of 76 mm cell under unloaded steady state calibration.Table 5.1 is a summary of 76 mm pressure cell calibration results in the unloaded steady state. The ratio of the thermal factor (CT) calculated from laboratory data to the one taken from the datasheet varies from 11 to 85. The percentage of the apparent pressure variation over the full-scale range due to a temperature difference of 40°C varies from 23.7% to 45.1%. In all cases, the degree of correlation between laboratory data and a second degree equation was higher than with a linear equation. Values of the coefficient of determination (r 2) for the quadratic equations were all above 0.9995 whereas r 2 values for linear corrections were between 0.9929 and 0.9963. The calibration of the five 76 mm cells summarised in the table were carried out specifically for this study whereas the calibration of 19 other 76 mm cells was done as a verification process before installing them in the field. The ratio of the thermal factors (laboratory/datasheet) varied between 4.8 and 38.5 for these additional cells. The percentage of the apparent pressure variation over the full-scale range was calculated over a 40°C temperature variation. This percentage varied between 13% and 50%.Table 5.1. Summary of unloaded steady state calibration of 76 mm pressure cells.Cell #CT fromLab. linearGoodnessCT from lab./CT% of pressure variationdatasheet (kPa/ºC)calibrationfactor (kPa/ºC)of fit(linear)datasheet over full-scale range for?T of 40°C10.0336 1.5260.99414530.320.0390 1.3230.99293426.230.0266 2.2610.99638545.140.05960.6350.99491123.750.0218 1.7710.99618135.5Such a high temperature effect on pressure measurement is critical. When cells are subjected to temperature variations, the apparent pressure caused by such temperature variations could be as high as the physical pressure acting on the cell, which is an undesirable behavior for any type of sensor. Thermal factors given by the manufacturer are measured for the vibrating-wire transducers only. Thermal factors of the whole pressure cell assembly are sometimes assumed to be in close agreement with that of the transducer, which for these 76 mm pressure cells was clearly not the case.Summary results obtained for 228 mm pressure cells are presented in Table 5.2. Cells obtained from two different sources are noted as #–A and #–B in the table. Cells with different ranges were tested. Readings were corrected for temperature variation according to the datasheet CT to obtain the apparent pressure variation. The percentage of the apparent pressure variation, over the measuring range (full-scale) and due to a temperature difference of 40°C, appears to be related to the cell measuring range. The ratio of thermal factors (CT) is indicated only for cells with an apparent pressure variation of 1% or more which was only the case for cells with lower measuring range. This ratio varied from 1.25 to 5.61 for those cells. The difference between a linear and quadratic correction for the 228 mm cells was not significant considering the reduced effect of temperature variation on the cell’s readings. This is especially true for cells with a measuring range higher than 200 kPa. Based on unloaded steady state calibration, two of the five low range (200 kPa or 173 kPa) cells gave high apparent pressure variations of 6.8% and 5.3%, respectively. The use of cell 2-A, with an apparent pressure variation of 3.0% is questionable depending on the anticipated temperature variation and pressure that would be measured with that cell. A pressure variation of 1% or less over the full-scale range for a temperature difference of 40°C was judged adequate.。