WATER CONTENT REFLECTOMETER CALIBRATIONS FOR FINAL COVER SOILS

Transcription

1 WATER CONTENT REFLECTOMETER CALIBRATIONS FOR FINAL COVER SOILS by K. C. Kim and Craig H. Benson Geo Engineering Report No Geo Engineering Program University of Wisconsin-Madison Madison, Wisconsin USA November 18, 2002

2 i EXECUTIVE SUMMARY This study dealt with calibration of low-frequency (40 MHz) time domain reflectometry (TDR) probes (referred to as water content reflectometers, or WCRs) for measuring the volumetric water content of soils used in landfill final covers. Elements of the study consisted of (i) identifying a suitable calibration cell, (ii) developing calibrations for 28 final cover soils, and (iii) evaluating how calibrations are affected by properties of the soil (electrical conductivity and index properties). Electrical conductivity of the soils was measured using conventional high frequency TDR. A suitable size calibration cell was identified by making WCR measurements in PVC cells of different diameter that were filled with salt solutions or DI water. Cell size had an appreciable effect on the WCR period for cells having a diameter less than 150 mm. For larger cells, the period was only marginally influenced by cell size. The change in period was less than 0.3% for all solutions as the diameter was increased from 150 and 300 mm. In addition, nearly identical WCR calibrations were obtained for Sacramento gravelly clay using 150 and 300-mm-diameter cells. An ANCOVA analysis also showed that calibrations for Sacramento gravelly clay obtained using the 150 and 300-mm-diameter cells were not statistically different. Thus, 150-mm-diameter cells were used for all calibrations. Linear and polynomial calibrations relating volumetric water content to WCR period were developed for each soil. Both calibrations provided equally good fits to the data. Therefore, linear calibrations were used for all subsequent analyses. The

3 ii slope of the calibrations ranged between and ms -1 and the minimum time (T min ) was close to 0.8 ms. The linear calibrations had R 2 between and , a standard error in volumetric water content ranging between and , and a bias in volumetric water content ranging between and General calibrations based on soil type were also developed by pooling the data for similar soils. Calibrations were developed for sands, dense sands, silty and clayey sands, and clays. These calibrations can be used in lieu of soil specific calibrations if errors in volumetric water content on the order of are acceptable. The effect of soil electrical conductivity and index properties on WCR calibrations was investigated. Soils with higher electrical conductivity had lower calibration slopes due to greater attenuation of the signal during transmission in the soil. Soils with higher electrical conductivity tended to have higher clay content, organic matter, liquid limit, and plasticity index. The effects of temperature and dry unit weight on WCR calibrations were conducted with clayey and silty soils. The period was found to increase as the temperature and the density increase, with greater sensitivity in fine-textured plastic soils. For typical variations in temperature, errors in volumetric water content on the order of 0.04 can be expected for wet soils and 0.01 for drier soils if temperature corrections are not applied. Errors on the order of 0.03 (clays) and 0.01 (silts) can be expected for typical variations in dry unit weight (±2 kn/m 3 ).

4 iii ACKNOWLEDGEMENTS Financial support for this study was provided in part by the US Department of Energy (DOE) through the Fernald Environmental Management Project (FEMP) and by the National Science Foundation (Grant No. CMS ). Soils used in the study were provided by FEMP and the United States Environmental Protection Agency s (USEPA) Alternative Cover Assessment Program. The opinions expressed herein are solely those of the authors and do not reflect the polices or opinions of DOE, NSF, or USEPA.

5 iv TABLE OF CONTENTS ABSTRACT... i ACKNOWLEDGEMENT...iii TABLE OF CONTENTS...iv LIST OF FIGURES...vi LIST OF TABLES... x 1. INTRODUCTION BACKGROUND BASIC PRINCIPLES OF TIME DOMAIN REFLECTOMETRY Volumetric Water Content Relationship Between Volumetric Water Content and Dielectric Constant Measuring the Dielectric Constant with Conventional TDR Equations Relating Volumetric Water Content and Applications Soil Electrical Conductivity WATER CONTENT REFLECTOMETERS Basic Principles Effects of Electrical Conductivity and Clay Content Effect of Temperature Applications MATERIALS AND METHODS SOILS INSTRUMENTATION Water Content Reflectometer (CS 615) TDR Probe (CS 605)... 33

6 v Datalogger CALIBRATION CELLS PROCEDURE FOR CALIBRATING WCRS Preparation of Specimens WCR Measurement Temperature Effect Tests SOIL ELECTRICAL CONDUCTIVITY MEASUREMENTS Procedure Probe Calibration RESULTS AND DISSCUSSION GENERAL CHARACTERISTICS Soil Specific Calibrations General Calibrations RELATIONSHIP BETWEEN SOIL PROPERTIES AND SLOPE OF CALIBRATION CURVE Electrical Conductivity Clay Content and Fines Atterberg Limits EFFECT OF TEMPERATURE EFFECT OF DRY UNIT WEIGHT SUMMARY REFERENCES APPENDIX A APPENDIX B APPENDIX C APPENDIX D

7 vi LIST OF FIGURES Fig Main components used in a reflectometer (after Benson and Bosscher 1999) (a) and cross-section of coaxial cable (b)... 7 Fig TDR system with cable tester and three-rod probe in soil (after Jones et al. 2002)... 8 Fig TDR waveform analysis. Distance is based on V p =c (after Suwansawat 1997) Fig Calibration curves for relationship between dielectric constant (K a ) and volumetric water content (θ) Fig Attenuation of conventional TDR signal due to soil electrical conductivity (from Jones et al. 2002) Fig Electric circuit used for WCRs (after Bilskie 1997) Fig CS 615 WCR calibration curves showing effect of electrical conductivity (CSI 1996) Fig Effect of temperature on oscillator period for an open-reflection oscillator. Tests conducted using a silt loam at a volumetric water content of 0.14 (from Campbell and Anderson 1998) Fig WCR calibration curve for cover soils used at Rocky Mountain Arsenal (from RVO 1997) Fig Locations where soils used in study were obtained. Asterisks indicate non- ACAP sites Fig Soils exhibiting plasticity on Casagrande s (1948) plasticity chart Fig Location of soils on USDA texture triangle Fig CS 615 WCR probe used in study Fig Period measured with probes A-E in (a) DI water and (b) 0.1 M NaCl Fig Calibration curves for Monticello clayey silt for probes A-E Fig Three-rod CS 605 probe used in study... 36

8 vii Fig Wiring diagram for CR 10 used for conventional TDR system (from Suwansawat 1997) Fig Wiring diagram for CR 10 and WCR (CS 615 probe) Fig Effect of cell size on the WCR period: (a) DI water, (b) 0.01 M NaCl, and (c) 0.1 M NaCl Fig WCR calibration curves for Sacramento gravelly clay determined using 150-mm and 300-mm diameter cells. Default calibration curve from CSI is also shown Fig Schematic diagram of calibration cell with WCR probe Fig Electrical conductivity calibration curve for CS 605 probe with 100 mm rods Fig General calibrations for sands and dense sands (a) and silts (b) Fig General calibrations for silty and clayey sands (a) and clays (b) Fig Comparison of soil type calibrations and corresponding parameters Fig Comparison of general calibration curves for the organic clayey sands, clayey and silty sands, and clays Fig Effect of electrical conductivity at saturation on the slopes of the WCR calibration curve Fig Effect of clay content (a) and fines (b) on the slopes of calibration curves. C=2 µm clay content (%) and F=fines content (%) Fig Effect of clay content (a) and fines content (b) on the electrical conductivity at saturation Fig Effect of liquid limit (a) and plastic index (b) on slope of WCR calibration curves Fig Effect of liquid limit (a) and plastic index (b) on the electrical conductivity at saturation Fig Effect of temperature on WCR period for Sacramento gravelly clay (a) and Boardman silt (b) at a volumetric water content of

9 viii Fig Relationships between WCR calibration slope α and temperature for Sacramento gravelly clay and Boardman silt Fig Errors in volumetric water content using actual temperature data: Sacramento gravelly clay (a) and Boardman silt (b) Fig Errors in volumetric water content vs. temperature difference (T-20 o C): (a) Sacramento gravelly clay and (b) Boardman silt Fig Effect of dry unit weight on WCR period for Sacramento gravelly clay and Boardman silt at a volumetric water content of Fig Error in volumetric water content as a function of deviation in dry unit weight for Sacramento gravelly clay and Boardman silt at a volumetric water content of Fig. B.1. WCR calibrations for Sacramento soils Fig. B.2. WCR calibration for Monticello soil Fig. B.3. WCR calibrations for Omaha soils Fig. B.4. WCR calibrations for Helena soils Fig. B.5. WCR calibrations for Polson soils Fig. B.6. WCR calibrations for Boardman soils Fig. B.7. WCR calibrations for Albany soils Fig. B.8. WCR calibration for Altamont soil Fig. B.9. WCR calibrations for Cedar Rapids soils Fig. B.10. WCR calibrations for Monterey soils Fig. B.11. WCR calibrations for Newberry and Phelan soils Fig. B.12. WCR calibrations for Apple Valley soils Fig. B.13. WCR calibrations for Fernald soils Fig. D.1. Daily soil temperature data at the depth of mm from field site Sacramento, CA

10 Fig. D.2. Daily soil temperature data at the depth of mm from field site Boardman, OR ix

11 x LIST OF TABLES Table 2.1 Temperature sensitivity of the WCR probe (from Campbell and Anderson 1988) Table 2.2 Index properties of soils at Rocky Mountain Arsenal Table 3.1 Properties of soils used in study Table 3.2 Results of ANCOVA analysis of calibration curves for Monterey clayey silt obtained with probes A-E Table 3.3 Results of ANCOVA analysis on calibration curves from 150 and 300- mm-diameter cells Table 4.1 Summary of parameters, bias, and standard error for linear and polynomial WCR calibrations Table 4.2 Parameters of electrical conductivity equation Table 4.3 Summary of bias and standard error for specific and general calibration of WCRs Table 4.4 Temperature sensitivities of Sacramento gravelly clay and Boardman silt... 73

12 1 SECTION ONE INTRODUCTION The time domain reflectometry (TDR) method is used widely for measuring volumetric water content (θ) and bulk electrical conductivity (σ e ) of soil. Volumetric water content is indirectly obtained from a measurement of apparent dielectric constant (K a ) based on the propagation velocity of electromagnetic wave traveling along a probe placed in the soil. Soil electrical conductivity is inferred from attenuation of the TDR signal. Water is the principal factor affecting a TDR signal. However, many studies have shown that chemical, mineralogical, and physical properties of the soil can also affect TDR measurements (Ledieu et al. 1986, Dasberg et al. 1992, Roth et al. 1992). Although TDR has become widely used, the method has some practical disadvantages. First, conventional TDR, which operates at frequencies in the GHz range, is costly. Special instruments, coaxial multiplexers, and coaxial cable are required. Second, cable lengths in conventional TDR are required to be relatively short (typically 20 to 30 m) due to signal attenuation (Jones et al. 2002). As a result, operating TDR systems at large field sites can be costly and cumbersome. An alternative approach is to employ a probe that operates in a lower frequency range, such as the Water Content Reflectometer (WCR) manufactured by Campbell Scientific, Inc. of Logan, Utah. WCRs employ lower frequency electromagnetic energy (30-60MHz) and incorporate all of the electronic components directly into the probe. WCRs can be also connected to a datalogger

13 2 using conventional shielded data cable. Thus, WCRs are inexpensive to implement and can be extended over relatively long cable lengths (up to 100 meters) (CSI 1996). However, because WCRs operate in the MHz range, the calibration between volumetric water content and dielectric constant is more strongly affected by soil bulk electrical conductivity than in conventional TDR (CSI 1996, Campbell and Anderson 1988). For this reason, soil specific calibrations are generally required. This study had two primary objectives. The first objective was to determine WCR calibrations for a variety of different soils used for landfill final covers. The soils were obtained primarily from field sites in USEPA s Alternative Cover Assessment Program (ACAP), where they are being used extensively for hydrological monitoring. The second objective was to evaluate how soil properties and electrical conductivity affect WCR calibrations. The methods and findings of this study are described in this report. Principles of conventional TDR and WCRs are presented in Section 2. Materials for the experiments and the methods are described in Section 3. Section 4 describes the volumetric water content calibrations for various soils, and how the calibrations are sensitive to the physical and electrical properties of the soil. A summary of the results and conclusions are presented in Section 5.

14 3 SECTION TWO BACKGROUND 2.1 BASIC PRINCIPLES OF TIME DOMAIN REFLECTOMETRY Time domain reflectometry (TDR) is an electromagnetic method that uses a metallic cable tester or reflectometer attached to probe to measure and monitor engineering properties of soil, such as volumetric water content and electrical conductivity (Topp et al. 1980, Benson and Bosscher 1999, Jones et al. 2002). The TDR technique is similar to RADAR, in that RADAR consists of a transmitter emitting an electromagnetic pulse to the objects of interest and a receiver that listens for a reflected pulse (Suwansawat and Benson 1999, Benson and Bosscher 1999). The same concept is used to investigate soil properties using TDR. Fellner-Feldegg (1969) initially applied the TDR technique to measure the electrical properties of organic liquids. Topp et al. (1980) proposed using TDR to measure the dielectric constant and water content of soil. Lidieu et al. (1986) and Topp et al. (1988) illustrated how to measure the electrical conductivity of soil using TDR. Subsequently, a variety of investigators have evaluated various aspects of the TDR method, and how it can be used to monitor the hydrologic behavior of soil. A review of these studies can be found in Benson and Bosscher (1999) and Jones et al. (2002).

15 Volumetric Water Content Relationship Between Volumetric Water Content and Dielectric Constant Measurements of volumetric water content using TDR are possible because water has greater dielectric constant (81) that that of air (1) and soil solids (3-10) (Topp et al. 1980, Look and Reeves 1992, Benson and Bosscher 1999). A change in the volume fraction of water causes a changes in the dielectric constant of soil. Consequently, a device sensitive to dielectric constant can be used to measure volumetric water content. Whalley (1993) suggests that the apparent dielectric constant (K a ) of soil is related to the dielectric constants of air (K o ), water (K w ), and soil solids (K s ) by: K = f K + f K + f K (2.1) a o o w w s s where f o, f w, and f s are the volume fractions of air, water, and soil solids, respectively. Volumetric water content is obtained from Eq. 2.1 by substituting soil properties for the volume fractions and noting that K o =1:? d Ka - Ks - n Gs? w? = (2.2) K -1 w where γ d is the dry unit weight, γ w is the unit weight of water, G s is the specific gravity of soil solids, and n is porosity. Because K s is unknown and varies with soil

16 5 composition, Eq. 2.2 must be calibrated for each soil type. In most practical applications, this calibration has the form (Benson and Bosscher 1999):? = a K a + ß (2.3) where α and β are constants obtained by calibration. Measurement of the dielectric constant of soil with TDR is based on the propagation velocity (V p ) of an electromagnetic pulse displaced along a transmission line (or TDR probe) of known length. The relationship between propagation velocity and dielectric constant is (Topp et al. 1980): V p = K ' 1+ c 1+ tan 2 2 d (2.4) where c is the speed of an electromagnetic wave in free space (3 x 10 8 m/s), K is the real part of the complex dielectric constant, and tanδ is the loss tangent. The dielectric loss for most soils is small and does not affect the propagation velocity at the high frequency associated with conventional TDR (Bilskie 1997, Suwansawat 1997, Benson and Bosscher 1999). Thus, tanδ is assumed to be negligible (i.e., tanδ << 1) and Eq. 2.4 is simplified to: V p ' (2.5) = K c

17 6 Thus, the dielectric constant of soil can be obtained directly from a measurement of the propagation velocity (V p ) of electromagnetic pulses. Because Eq. 2.5 is an approximation, the real part of the dielectric constant (K ) is replaced by K a, which is referred to as the apparent dielectric constant of soil Measuring the Dielectric Constant with Conventional TDR Conventional TDR systems employ a metallic cable tester to measure the propagation velocity of electromagnetic waves in soil. A cable tester is composed of four components: a pulse generator, a coaxial cable, a sampler, and an oscilloscope (Fig 2.1). Cable testers were developed by the telecommunications industry to detect locations of faults in cables (O Conner 1999). The signal propagation velocity (V p ), along with a reflection at discontinuity fault, is used to identify the fault location. The amplitude of the reflection provides information about the type of damage to the cable. Similar principles are used to measure volumetric water content. A pulse generator creates a series of square voltage pulses that are launched into a probe embedded in soil consisting of two or three conductors. When a pulse encounters the probe, a portion of the pulse is reflected back to the cable tester. The oscilloscope or the cable tester reports the magnitude of the reflection as a function of apparent distance or time on the oscilloscope (Suwansawat 1997, Benson and Bosscher 1999, Jones et al. 2002) (Fig. 2.2). The cable tester employed in most studies is the Tektronix 1502B manufactured by Tektronix, Inc. of Beaverton Oregon. The Tektronix 1502B generates a square voltage wave with an amplitude of

18 7 (a) (b) Fig Main components used in a reflectometer (after Benson and Bosscher 1999): (a) and cross-section of coaxial cable (b).

19 Fig TDR system with cable tester and three-rod probe in soil (after Jones et al. 2002). 8

20 V and a rise time < 200 ps. This wave is launched into cable every 200 µs (Tektronix 1993). A conventional TDR cable tester typically displays a graph of voltage versus distance along the probe (referred to as a waveform) rather than voltage versus time. The distance that is reported on the oscilloscope depends on the setting that defines V p for the reflectometer (Suwansawat and Benson 1999, Benson and Bosscher 1999). The propagation velocity is usually set to c (3 x 10 8 m/s) when a cable tester is used for measuring volumetric water content. When the propagation velocity is set to c, the apparent length of the probe (L a ) can be determined from the waveform corresponding to K a =1 (Suwansawat and Benson 1999). If the true length of the probe (L p ) is known, the apparent dielectric constant can be determined by evaluating L a from the reflected waveform using Eq 2.5 and distance-time relationships (Fig. 2.3): L K (2.6) a 2 a = ( ) Lp A calibration relating K a and θ is readily determined by measuring L a for a series of water contents (Suwansawat and Benson 1999).

21 10 TDR Probe Coaxial Cable Soil Lp Beginning of Probe Voltage (mv) TDR Trace La End of Probe Distance (cm) Fig TDR waveform analysis. Distance is based on V p =c (after Suwansawat 1997).

22 Equations Relating Volumetric Water Content and Applications Numerous investigations have reported empirical relationships between θ and K a. Several of these relationships are shown in Fig The first relationship was reported by Topp et al. (1980): ? = -5.3 x x 10 Ka x 10 Ka x 10 K a (2.7) This relationship became known as Topp s Universial Equation. More recent studies have shown that Eq. 2.7 is not universal. Soil specific calibration curves are needed for soils that are highly conductive, have high organic content, or contain magnetic materials (Dasberg et al. 1992, Roth et al. 1992, Sawansawat 1997). A procedure for developing calibration curves is described in Suwansawat and Benson (1999) Soil Electrical Conductivity Soil electrical conductivity (σ e ) can be determined from attenuation of the TDR waveform (Topp et al. 1988, Zegelin et al. 1989, Benson and Bosscher 1999). Free ions in the soil solution provide a path for electrical conduction, which results in attenuation of the signal applied to the probe. Attenuation reduces the amplitude and affects the shape of the reflected signal displayed on the oscilloscope (Nadler et al. 1991, White et al. 1994, CSI 1996, Benson and Bosscher 1999, Jones et al. 2002) (Fig. 2.5).

23 12 Dielectric Constant (K a ) Topp et al. (1980) Ledieu et al. (1986) Roth et al. (1992) for mineral soils Roth et al. (1992) for organic soils Volumetric Water Content Fig Calibration curves for relationship between dielectric constant (K a ) and volumetric water content (θ).

24 ds/m 0.2 Reflection coefficient ds/m V f ds/m -0.8 V 0 V1 12 ds/m Travel time (nsec) Fig Attenuation of conventional TDR signal due to soil electrical conductivity (from Jones et al. 2002).

25 14 Several models exist for computing electrical conductivity from the TDR waveform. Zegelin et al. (1989) evaluated the electrical conductivity models, and found that the Giese-Tiemann (G-T) method (Giese and Tiemann 1975) is the most accurate method for measuring soil electrical conductivity. The Giese-Tiermann (G- T) equation can be written as: Kp 1-? K f p 2V0 s e = ( ) = ( -1) (2.8) Z 1+? Z V c f c f where K p is a probe constant, Z c is the cable impedance (usually 50 Ω), ρ f is final reflection coefficient (ρ f =V f /V 0-1), V 0 is the incident pulse voltage, and V f is the return voltage at a long distance from the end of probe (>10L a, Nadler et al. 1991, Benson and Bosscher 1999, Jones et al. 2002). 2.2 WATER CONTENT REFLECTOMETERS Basic Principles The water content reflectometer (CS 615) manufactured by Campbell Scientific, Inc. employs TDR principles for measuring volumetric soil water content without the Tektronix 1502B cable tester. WCRs operate at a lower frequency (30-60 MHz) compared to conventional TDR, which allows the use of low-cost electronic components that are small enough to be embedded in the head of the probe. The WCR manufactured by CSI is based on probes developed and tested by Campbell and Anderson (1998). They evaluated three types of simple oscillators that could

26 15 measure the round trip travel time for an electromagnetic wave traversing two parallel rods. They found that an open reflection oscillator provided the best results. The circuit for the open reflection oscillator employed in WCRs is shown in Fig The line driver controls the output at one of two discrete values. When the circuit switches state, electromagnetic pulses with a band of frequencies travel along the rods. The pulses are reflected at end of rods back to the line driver circuit. A threshold circuit detects part of the returning signal, and switches the line driver to the other state. The line driver then launches another set of pulses down the rods. This process is repeated, causing a periodic change in state of the line driver, or an oscillation (Bilskie 1997). The period of oscillation (T) is obtained by inverting the oscillation frequency of the line driver. The period (T) of the oscillator represents the combination of travel time along the rods in the probe and time delays in the circuitry: L Ka T = 2[(tpd + tc ) + 2 ] (2.9) c where t pd is propagation delay of the amplifier, t c is the charging time for stray capacitance, and L is the length of the probe rods (Bilskie 1997, Campbell and Anderson 1998). The factor 2 is in Eq. 2.8 because one complete cycle corresponds to two trips along the rods and through the circuit (i.e., the two states of the line driver). WCRs generate a frequency in free air of 40 MHz. However, the final output

27 16 SQUARE WAVE OUTPUT SCALING CIRCUIT LINE DRIVER 30 cm STAINLESS STEEL RODS THRESHOLD CIRCUIT POWER CONTROL LINE FOR CIRCUIT ENABLE Fig Electric circuit used for WCR (after Bilskie 1997).

28 17 transmitted by the probe is a square wave with lower frequency (i.e., 600 to 1500 Hz) to permit data recording using low cost dataloggers. The datatogger records the frequency of the square wave or the average period of the wave. The period measurement frequently is used for water content measurements because it provides better resolution Effects of Electrical Conductivity and Clay Content The amount of organic matter and clay content in a soil can alter the response of the WCRs and increase the electrical conductivity (CSI 1996). The response of WCRs is sensitive to attenuation of the signal, and the degree of attenuation increases with the electrical conductivity of the soil. The rise time increases and the amplitude diminishes as the attenuation increases. Reflections that are attenuated and have longer rise time take longer to exceed the threshold level in the threshold circuit (CSI 1996). Thus, the frequency decreases and the period increases as the electrical conductivity of the soil increases. Accordingly, a soil specific calibration equation is needed for WCRs. Calibration curves reported by CSI (1996) are shown in Fig. 2.7 for soils having electrical conductivities between 0.8 to 3.0 ds/m. The slope of the calibration curve increases with decreasing electrical conductivity.

29 ds/m 1.8 ds/m 3.0 ds/m Volumetric Water Content Period Measurement (msec) Fig CS 615 WCR calibration curves showing effect of electrical conductivity (CSI 1996).

30 Effect of Temperature Campbell and Anderson (1998) evaluated the response of the open-reflection oscillator employed for WCRs to changes in temperature. The response in air and in a silt loam was evaluated at volumetric water contents of 0.03, 0.14, and 0.28 for temperatures between 10 o C and 30 o C. The effect of temperature on the period is shown in Fig. 2.8 for the silt loam at a volumetric water content of The temperature sensitivity at different volumetric water contents is summarized in Table 2.1. The period measurement in air is insensitive to changes in temperature, indicating that the electric components in the probe are not responsible for the temperature sensitivity in soil. The temperature sensitivity increases with increasing volumetric water content Applications WCRs are being used to monitor volumetric water contents in alternative cover soils at Rocky Mountain Arsenal (RVO 1997). The probes have been placed in four soils ranging from clay loam (CL) to sandy clay loam (SC-CL). Characteristics of the soils are shown in Table 2.2. Calibration curves for the soils are shown in Fig The calibration curves for the soils at Rocky Mountain Arsenal fall below those reported by Campbell Scientific. These fine textured soils probably have higher electrical conductivity than the soils tested by Campbell Scientific, causing the period to be longer for a given volumetric water content.

31 Fig Effect of temperature on oscillator period for an open-reflection oscillator. Tests conducted using a silt loam at a volumetric water content of 0.14 (from Campbell and Anderson 1998). 20

32 21 Table 2.1. Temperature sensitivity of WCR probes (from Campbell and Anderson 1998). Media dθ/dt 1 (m 3 /m 3 per o C) Air -1.5 x 10-5 Soil at θ= x 10-4 Soil at θ= x 10-4 Soil at θ= x dθ/dt is the change in water content per degree change in soil.

33 22 Table 2.2. Index properties of soils at Rocky Mountain Arsenal. Layer Description 1 Dry Density (Mg/m 3 ) Sand (%) Silt (%) Clay (%) USCS 2 Clay loam CL Loam/Clay loam CL Sandy clay loam CL-SC Sandy clay loam SC-CL 1 USDA=United States Department of Agriculture textural classification system. 2 USCS=Unified Soil Classification System.

34 Volumetric Water Content Clay loam (CL) Loam/Clay Loam (CL) Sandy Clay Loam (CL-SC) Sandy Clay Loam (SC-CL) CSI Default Calibration θ=0.335t T θ=0.2382t T Period (msec) Fig WCR calibration curve for cover soils used at Rocky Mountain Arsenal (from RVO 1997).

35 24 SECTION THREE MATERIALS AND METHODS 3.1 SOILS Twenty-eight soils were used in this study. Twenty-three of the soils are from field sites in USEPA s ACAP. Five soils are from other studies being conducted at the University of Wisconsin-Madison. Locations from which the soils were obtained are shown in Fig Index properties, compaction characteristics, loss on ignition (LOI), texture, and classifications according to the United Soil Classification System (USCS) and the United States Department of Agriculture (USDA) textural classification system of the soils are shown in Table 3.1. The soils that were used represent a variety of soil types ranging from sand to clay (two gravelly sands, two sands, a loamy sand, six sandy loams, three sandy clay loams, four silty clay loams, four silty loams, three loams, a silty clay and two clay loams). Liquid limits and plasticity indices for the plastic soils are plotted on Casagrande s (1948) plasticity chart (Fig. 3.2). Most of the fine textured soils are low to moderate plasticity. The exception is the highly plastic soil from Helena, MT. Locations of soils on USDA texture triangle are shown in Fig More information on these soils is available in Gurdal (2002).

36 Fig Locations where soils used in study were obtained. Asterisks indicate non-acap sites. 25

37 26 Table 3.1. Properties of soils used in study. Location Description g df 1 (kn/m 3 ) g dm 1 (kn/m 3 ) W opt 1 (%) LL 2 PI 2 G s 2 Gravel 3 (%) Sand 3 (%) Silt 3 (%) Clay 3 (%) LOI 2 (%) USDA 2 USCS 2 Sacramento, CA Gravelly Clay Gravelly loam SC Clayey Silt Silty loam CL Silty Sand N/A Sandy loam SM Monticello, UT Clayey Silt N/A Clay loam CL Silty Clay Silty clay loam CL Omaha, NE Sand 15.8 N/A N/A NP NP Sand SP Clayey Sand Silty clay loam CL Helana, MT Clayey Sand Sandy clay loam SC Silty Sand Sandy loam SC Silt Silty loam CL-ML Polson, MT Boardman, OR Albany, GA Silty Sand NV NP Sandy loam SM Silty Sand NV NP Sandy loam SM Sandy Silt Silty loam ML Clayey Sand Sandy loam SC Clayey Sand Sandy loam SC

38 27 Table 3.1. Properties of Soils Used in Study (continued). Location Description g df 1 (kn/m 3 ) g dm 1 (kn/m 3 ) W opt 1 (%) LL 2 PI 2 G 2 Gravel 3 (%) Sand 3 (%) Silt 3 (%) Clay 3 (%) LOI 2 (%) USDA 2 USCS2 Altamont, CA Claystone Silty clay loam CL Silty Sand Sandy loam SC Cedar Rapids, IA Sandy Clay Loam CL Clay N/A Loam CL Clayey Silt Clay loam CH Sand 16.6 N/A N/A NP NP Sand SP-SM Monterey, CA Clay N/A Silty clay CH Clayey Sand Sandy clay loam SC Newberry, CA 4 Gravelly Sand N/A N/A Gravelly sand SW Phelan, CA 4 Gravelly Sand N/A N/A Gravelly sand SW Apple Valley, 4 Gravelly Sand N/A N/A Gravelly sand SW CA Fernald, Silty clay Silty clay loam ML-CL OH 4 Silty clay N/A Silty clay loam ML-CL 1 γ df =field dry unit weight, γ dm =maximum dry unit weight, w opt =optimum water content. 2 LL=liquid limit, PI=plastic index, G s =specific gravity of solids, USDA=United States Department of Agriculture textural classification system, USCS=Unified Soil Classification System, LOI=loss of ignition, N/A=not available. 3 Based on size definitions in Unified Soil Classification System 4 Non-ACAP site

39 28 60 Plasticity Index Sacramento Monticello Omaha Helena Polson Boardman Albany Fernald Cedar Rapids Monterey Altamont Silty clays; clayey silts and sands (CL-ML) U-line Medium plastic inorganic clays Low plastic inorganic clays; sandy and silty clays (CL) A-line Inorganic clays of high plasticity (CH) Liquid limit Fig Soils exhibiting plasticity on Casagrande s (1948) plasticity chart.

40 29 Sacramento Monticello Omaha Helena Polson Boardman Albany Altamont Cedar Rapids Monterey Newberry Phelan Apple Valley Fernald clay CLAY (%) SILT (%) 40 silty clay sandy clay sandy clay loam clay loam silty clay loam sand loamy sand sandy loam loam silt loam silt SAND (%) Fig Location of soils on USDA texture triangle.

41 INSTRUMENTATION Water Content Reflectometer (CS 615) CS 615 WCRs manufactured by CSI were used in the study. These WCRs consist of two stainless steel rods spaced at 32 mm. Each rod has a diameter of 3.2 mm and is 300 mm long (Fig. 3.4). A circuit board sealed in an epoxy block is attached to the end of the rods. A shielded 4-conductor cable is connected to the circuit board to supply 12 VDC power, to enable the probe, and to monitor the output. The WCRs are directly connected to the single-ended analog input channel on a datalogger (CSI 1996). An initial test was conducted to assess probe-to-probe variability. Five probes (labeled A, B, C, D, and E) were tested in DI water, 0.1 M NaCl, and in soil. All tests were conducted in a PVC calibration cell (see discussion in Sec. 3.3). For each probe, the period was read 5 to 10 times in each material. Box plots of the data from the tests with DI water and NaCl solution are shown in Fig A similar range of periods was obtained for each probe, and the average period differs by at most 0.2% (i.e., probe A vs. probe B). A one-way ANOVA was conducted to assess whether the means were statistically similar at the 0.05 significance level. An F-statistic of was obtained for DI water (F critical =2.650) and for 0.1 M NaCl (F critical =3.060), indicating that the hypothesis of similar means cannot be rejected in either case (i.e., the probes yield the same period). The tests with soil were conducted using Monticello clayey silt prepared at a dry unit weight of 15.5 kn/m 3. The soil was compacted in a calibration cell following

42 Fig CS 615 WCR probe used in study. 31

43 Period (msec) DI Water (a) Probe A Probe B Probe C Probe D Probe E Period (msec) M NaCl (b) Probe A Probe B Probe C Probe D Probe E Fig Periods measured with probes A-E in (a) DI water and (b) 0.1 M NaCl.

44 33 the methods in Sec Calibrations obtained with each probe are shown in Fig. 3.6.The calibrations are almost identical. An ANCOVA analysis was conducted to determine if the calibrations were statistically similar. The ANCOVA analysis compared a general calibration curve (obtained by pooling all the data) to the calibrations obtained from each probe. The null hypothesis is the slopes obtained from the probes are the same. Relative large F-statistics and small p-values result in the null hypothesis being rejected. Results of the ANCOVA analysis are summarized in Table 3.2. The F-statistics are small and the p-values are large for the probe factor compared to those of volumetric water content. Thus, the null hypothesis is accepted and the calibrations are not statistically different. Because the tests with DI water, 0.1 M NaCl, and Monticello clayey silt all showed that each probe provided the same readings, a single probe (Probe C) was used for all subsequent calibrations TDR Probe (CS 605) Three-rod TDR probes (CS 605) manufactured by CSI were used for the TDR measurements. The CS 605 is comprised of three stainless-steel rods 4.8 mm in diameter and 300 mm long attached to a coaxial cable without a balun (Fig. 3.7). The rods are in a parallel and planar arrangement, with an inter-rod spacing of 23 mm. The probe head is an epoxy block. The reference impedance for the CS 605 probe is 50 Ω. The probe was connected to a Tektronix 1502B cable tester.

45 Probe A Probe B Probe C Probe D Probe E Volumetric Water Content Period (msec) Fig Calibration curves for Monticello clayey silt for probes A-E.

46 35 Table 3.2. Results of ANCOVA analysis of calibration curves for Monterey clayey silt obtained with probes A-E. Probe Variable F-Statistic P-Value Probe A Volumetric Water Content 1502 < Probe Probe B Volumetric Water Content 1354 < Probe Probe C Volumetric Water Content 1394 < Probe Probe D Volumetric Water Content 1469 < Probe Probe E Volumetric Water Content 1696 < Probe

47 Fig Three-rod CS 605 probe used in study. 36

48 Datalogger A CR 10 datalogger manufactured by CSI was used to collect data from the WCRs and the conventional TDR system. The datalogger programs are in Appendix A. PC208W software from CSI was used for programming the datalogger and for downloading of data to a personal computer. A diagram showing how the CR 10 was connected to conventional TDR system is shown in Fig A similar diagram for the WCRs is shown in Fig A CSI SDM 1502 telecommunications interface was used as the interface between the Tektronix 1502B and the CR 10 datalogger. 3.3 CALIBRATION CELLS Preliminary tests were conducted to determine an adequate size cell for calibrating the CS 615 WCRs. PVC cells having diameters of 50, 75, 100, 150, and 300 mm and a height of 500 mm were used for the tests. The cells were filled with DI water, 0.01 M NaCl, or 0.1 M NaCl to bracket the range of electrical conductivity that might be encountered for soils. These solutions have electrical conductivities of 0, 0.12, and 1.06 S/m, respectively. A test consisted of placing a probe in the center of a PVC cell containing a given solution, and reading the period. Each test was repeated 10 times. The test results are shown in Fig For all solutions, the period increases as the diameter of the cell increases from 50 to 150 mm, with greater changes in the more dilute solutions. The increase in the period with increasing cell diameter reflects the decreasing influence of the cell wall and the air outside the cell wall, both

49 Fig Wiring diagram for CR 10 used for conventional TDR system (from Sawansawat 1997). 38

50 39 Connected to PC computer through SC 32A interface Color Red Green Orange Black Clear CR V H1 C1 G G Fig Wiring diagram for CR 10 and WCR (CS 615 probe).

51 DI Water Period (msec) (a) M NaCl Period (msec) (b) Period (msec) M NaCl 50 mm 75 mm 100 mm 150 mm 300 mm (c) Fig Effect of cell size on the WCR period: (a) DI water, (b) 0.01 M NaCl, and (c) 0.1 M NaCl.

52 41 of which have low apparent dielectric constant (K pvc =3, K air =1). The change in period as the diameter increases from 150 and 300 mm is small (0.3% for DI water, and less for the salt solutions). Thus, PVC cells having a diameter of 150 and 300 mm were considered acceptable. An additional test with soil was conducted to determine if different calibration curves would be obtained using 150 and 300-mm-diameter cells. The tests were conducted using the gravelly clay soil from the Sacramento ACAP site prepared at a range of volumetric water contents using tap water. Soil specimens were prepared in both cells at a target dry unit weight of 15.8 kn/m 3 using the method described in Sec The base of the PVC cell was maintained at least 150 mm from the end of rods to reduce any effect of the PVC base. Calibrations curves obtained from both cells are shown in Fig Similar calibrations were obtained from both cells, although the calibration from the 300-mm cell falls slightly below that obtained from the 150-mm cell. An ANCOVA analysis was conducted to determine if the calibrations obtained from the 150-mm and 300- mm cells were statistically similar. Results of the ANCOVA analysis are summarized in Table 3.3. The F-statistics are small and the p-values are large for the cell size factor compared to those for volumetric water content. That is, the null hypothesis is accepted and the two calibrations are not statistically different. Thus, the 150-mmdiameter cell was used for all calibrations in the study. The 150-mm cell was also selected for pragmatic reasons, because less effort was required to prepare test specimens.

53 mm PVC Container 300 mm PVC Container Volumetric Water Content CSI Default Calibration Period (msec) Fig WCR calibration curves for Sacramento gravelly clay determined using 150-mm and 300-mm diameter cells. Default calibration curve from CSI is also shown.

54 43 Table 3.3. Results of ANCOVA analysis on calibration curves from 150 and 300-mm-diameter cells. Variable F-Statistic P-Value Volumetric Water Content 582 < Cell Size

55 PROCEDURE FOR CALIBRATING WCRs Preparation of Specimens Approximately 10 kg of soil was used for each WCR calibration. The soil was air-dried, crushed to break up agglomerates of clay, and then sieved through an US No. 4 sieve (4.75 mm openings). The water content of the sieved soil was adjusting by spraying the soil with the required amount of tap water, while continuously mixing the soil to ensure the water content was uniform. The moistened soil was compacted into a 150-mm-diameter PVC calibration cell using the impact hammer for standard Proctor compaction. The soil was compacted into lifts 30 mm thick, with the number of blows per lift varied between 15 and 20 to reach the target dry unit weight. The target dry unit weight for most calibrations was the average field unit weight for the soil reported in Gurdal (2002) and tabulated in Table 3.1. However, a series of tests was conducted using soils compacted at several dry unit weights to assess the effect of dry unit weight on the WCR calibrations. The top of the calibration cell was sealed with plastic after compaction to prevent evaporation. The specimen was then allowed to equilibrate for 24 hours before testing with a WCR probe WCR Measurement The WCR calibration for each soil was obtained using water contents ranging between air dryness and saturation. Target volumetric water contents of 0.00, 0.08, 0.16, 0.24, 0.32, and 0.40 were used. The CS 615 probe was placed in the center of the PVC container and pushed vertically into the soil until the rods were completely

56 45 embedded. A diagram showing how the WCR probe was placed in the soil is shown in Fig Movement of the rods from side to side during insertion was minimized to the greatest extent possible to prevent air-voids between rods and the soil. The probe was inserted at least three times at different orientations for each water content. Three or four readings were collected at each volumetric water content. After testing with the WCR probe, samples were collected at the top, middle, and bottom of the container for direct measurement of volumetric water content Temperature Effect Tests A series of tests were conducted to assess how temperature affects WCR calibrations. These tests were conducted by cooling or warming the soil in the calibration cell, and then measuring the period with the WCR. The calibration cell was cooled using a refrigerator, and warmed using an oven. Soil temperature was measured using a thermometer embedded in the soil in close proximity (20 mm) to the mid-point of the WCR rods. The period was then measured following the procedure described in Sec SOIL ELECTRICAL CONDUCTIVITY MEASUREMENTS Procedure The calibration cell used for measuring soil electrical conductivity with TDR was a PVC cell having a diameter of 150 mm and height of 200 mm. This cell is larger than the minimum cell size (150 mm diameter and 150 mm height) for TDR measurements identified by Suwansawat and Benson (1999).

57 46 WCR Probe Cable to Datalogger Stainless Steel Rods Soil 32 mm 59 mm 500 mm PVC Cell 150 mm 150 mm Fig Schematic diagram of calibration cell with WCR probe.

58 47 Approximately 1.5 kg of soil was used for preparing specimens for measuring soil electrical conductivity. These specimens were prepared using a method similar to the WCR calibration, except the number of blows per lift for soil compaction was varied between 10 and 15 to reach the target dry unit weight. After compaction, a CS 605 probe with rods 100 mm long was inserted vertically until the rods were completely surrounded by soil. Soil electrical conductivity was measured three or four times for each volumetric water content, with each measurement corresponding to a different orientation of insertion. After removing the probe, samples were collected from the specimen to determine its volumetric water content Probe Calibration The probe constant (K p ) for the CS 605 probe was obtained by calibrating the probe using 0.005, 0.01, and 0.02 M CaCl 2 solutions. Electrical conductivity of these solutions was measured using a Conductivity/ o C Meter manufactured by Cole- Parmer Inc. The calibration curve for the CS 605 probe with 100 mm rods is shown in Fig The slope of the line is the probe constant, K p. Least squares linear regression yielded K p =1.652 m -1 with R 2 =0.99. This probe constant is slightly higher than the probe constant reported by CSI (1996) (K p =1.63 m -1 ).

59 CaCl M Electrical Conductivity (S/m) CaCl M CaCl M 1 K p =1.652 m (1-ρ)/[Z (1+ρ)] (S) c Fig Electrical conductivity calibration curve for CS 605 probe with 100 mm rods.

60 49 SECTION FOUR RESULTS AND DISCUSSION 4.1 GENERAL CHARACTERISTICS Soil Specific Calibrations WCR calibrations were made for 28 soils. Linear and polynomial relationships between volumetric water content (?) and period (T) were fit to each data set. The linear function is:? a(t - T ) (4.1) = min and the polynomial function is: 2? = at + bt+ c (4.2) In Eq. 4.1, α is a constant and T min is the period corresponding to?=0. In Eq. 4.2, a, b, and c are constants. Eqs. 4.1 and 4.2 are valid for T>T min. The parameters, bias, and standard error for the linear and polynomial calibration equations are summarized in Table 4.1. The coefficient of determination (R 2 ) for the polynomial WCR calibrations ranges between and For the linear WCR calibrations, R 2 ranges between and Thus, a strong correlation exists in the data and reliable calibrations were obtained from both equations. Although the R 2 for the

61 50 Table 4.1. Summary of parameters, bias, and standard error for linear and polynomial WCR calibrations. Location Soil Type a (ms -1 ) T min (ms) Linear Calibration Polynomial Calibration R 2 Bias SE A b c R 2 Bias SE Sacramento, CA Monticello, UT Omaha, NE Helena, MT Polson, MT Boardman, OR Albany, GA Gravelly Clay Clayey Silt Silty Sand Clayey Silt Silty Clay Sand Clayey Sand Clayey Sand Silty Sand Silt Silty Sand Silty Sand Sandy Silt Clayey Sand Clayey Sand