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Development of an electrical resistivity cone for groundwater contamination studies Weemees, Ilmar Andrew 1990

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DEVELOPMENT OF AN ELECTRICAL RESISTIVITY CONE FOR GROUNDWATER CONTAMINATION STUDIES By ILMAR ANDREW WEEMEES B.A.Sc, The University of B r i t i s h Columbia, 1987 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES Department of C i v i l Engineering We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1990 <© Ilmar Andrew Weemees, 1990 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of CIVIL The University of British Columbia Vancouver, Canada Date Serf 28 , tffO DE-6 (2/88) ABSTRACT i i The evaluation of groundwater q u a l i t y has become increasingly important as more i n d u s t r i a l waste and s o l i d domestic refuse comes into contact with groundwater. Often the quantity and extent of contamination i s determined by d i r e c t sampling of the groundwater and s o i l . An a l t e r n a t i v e method of detecting contaminated groundwater i s by noting the e l e c t r i c a l r e s i s t i v i t y of the contaminated s o i l . The f e a s i b i l i t y of logging r e s i s t i v i t y while conducting cone penetrometer t e s t i n g has been investigated i n t h i s research. To t h i s end a two stage program was devised, consisting of lab t e s t i n g and then f i e l d t e s t s of a working t o o l . Lab t e s t i n g was c a r r i e d out using a prototype probe designed to evaluate the f e a s i b i l i t y of the project. The lab t e s t i n g consisted of determining the r e s i s t i v i t y of a number of d i f f e r e n t s o i l , e l e c t r o l y t e , and organic contaminant mixtures while varying the configuration of the probe. On the basis of lab t e s t i n g the necessary requirements for the module dimensions and ele c t r o n i c s were chosen and were f i n e tuned by f i e l d t e s t s . The module i t s e l f consists of an insulated four electrode array and i s mounted behind a standard 15 sq cm piezo-cone (CPTU). Upon completion of the development phase the instrument was tested at four d i f f e r e n t s i t e s . From f i e l d t e s t i n g i t was determined that the r e s i s t i v i t y cone (RCPTU) was able to accurately map changes i n groundwater chemistry on the basis of i i i r e s i s t i v i t y measurements. The r e s u l t s of the r e s i s t i v i t y t e s t i n g were v e r i f i e d by groundwater sampling. I t was also found that changes i n l i t h o l o g i c a l properties, as determined by the cone penetration t e s t (CPT), could influence the r e s i s t i v i t y . Basic guidelines f o r the use of the RCPTU i n contaminant investigations are presented. i v TABLE OF CONTENTS PAGE ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i ACKNOWLEDGEMENTS v i i i 1. INTRODUCTION 1 1.1 Rationale for R e s i s t i v i t y Cone Development 1 1.2 Scope of Research 4 2. ELECTRICAL CONDUCTION PHENOMENA 6 2.1 Introduction 6 2.2 E l e c t r i c a l Conduction i n Pore Water 7 2.3 E l e c t r i c a l Conduction i n Multi-Phase Systems 9 2.4 Frequency Dependent Behavior 12 2.4.1 Induced P o l a r i z a t i o n 12 2.4.2 D i e l e c t r i c P o l a r i z a t i o n 14 3. INSTRUMENT DEVELOPMENT 16 3.1 E x i s t i n g R e s i s t i v i t y Cone Technology 16 3 . 2 Lab Testing Equipment 17 3.3 Design Requirements and Considerations 18 3.3.1 Number of Electrodes 18 3.3.2 Electrode Spacing 21 3.3.3 Ex c i t a t i o n Frequency 21 3.3.4 Measurement Range 22 3.3.5 Probe Materials 22 3.4 R e s i s t i v i t y Module Description 23 3.5 R e s i s t i v i t y Cone Ca l i b r a t i o n 25 3.6 BAT Water Sampling System Description and Use 26 3.7 F i e l d Testing Procedures 27 3.8 Data Reduction 29 4. SITE DESCRIPTION AND TEST RESULTS 31 4.1 McDonald Farm Research Site 31 4.2.1 Site Description 31 4.2.2 Stratigraphic and R e s i s t i v i t y P r o f i l e 35 4.2 Fraser Valley Glaciomarine Deposits 37 4.2.1 Strong P i t 38 4.2.1.1 Si t e Description 38 4.2.1.2 Stratigraphic and R e s i s t i v i t y P r o f i l e 38 4.2.2 Langley 40 4.2.2.1 Site Description 40 4.2.2.2 Stratigraphic and R e s i s t i v i t y P r o f i l e 42 4.2.3 Colebrook 42 4.2.3.1 Site Description 42 4.2.3.2 Stratigraphic and R e s i s t i v i t y P r o f i l e 43 V PAGE 5. INTERPRETATION AND DISCUSSION OF RESULTS 45 5.1 Repeatability of Results 45 5.2 P r o f i l i n g Capacity 47 5.3 E f f e c t of S o i l Lithology 50 5.3.1 General Aspects 50 5.3.2 Cone Parameter Relations to S o i l R e s i s t i v i t y 51 5.3.2.1 F r i c t i o n Ratio Relationship 51 5.3.2.2 Cone Bearing Relationship 53 5.3.2.3 Pore Pressure Relationship 53 5.4 Determination of Pore F l u i d R e s i s t i v i t y 55 5.5 Influence of Electrode Spacing on Measured 57 R e s i s t i v i t y 6. APPLICATION OF THE RESISTIVITY CONE 63 6.1 A p p l i c a b i l i t y of R e s i s t i v i t y for Contaminant Detection 65 6.2 Use of the RCPTU i n Contamination Problems 68 6.3 Other Possible Applications 70 6.3.1 Corrosion Assessment 70 6.3.2 Water Quality Assessment 71 6.3.3 S o i l C l a s s i f i c a t i o n 71 7. CONCLUSIONS AND RECOMMENDATIONS 72 REFERENCES 75 LIST OF TABLES TABLE PAGE 4.1 I n - s i t u t e s t i n g program 32 4.2 Summary of Typical R e s i s t i v i t y Measurements of Fluids and Bulk S o i l - F l u i d Mixtures 67 v i i LIST OF FIGURES FIGURE PAGE 3.1 Inner and outer electrode normalized r e s i s t i v i t y measurements versus frequency 20 3.2 UBC R e s i s t i v i t y Cone 24 4.1 General location of the UBC research s i t e s 33 4.2 McDonald Farm Research Site 34 4.3 R e s i s t i v i t y cone sounding at McDonald Farm 36 4.4 R e s i s t i v i t y cone sounding at Strong P i t 39 4.5 R e s i s t i v i t y cone sounding at Langley 41 4.6 R e s i s t i v i t y cone sounding at Colebrook 44 5.1 A comparison of two adjacent r e s i s t i v i t y logs. made 11 days apart for (a) The inner electrodes, and b) The outer electrodes 46 5.2 Stratigraphic and r e s i s t i v i t y p r o f i l e of the McDonald Farm s i t e 48 5.3 A comparison of the r e s i s t i v i t y (outer electrodes) and the f r i c t i o n r a t i o f o r two soundings from the McDonald Farm s i t e 52 5.4 Observed r e l a t i o n s h i p between apparent formation factor and cone bearing normalized with respect to horizontal e f f e c t i v e stress 54 5.5 A comparison between the r e s i s t i v i t y of pore f l u i d samples and the r e s i s t i v i t y measured by the RCPTU 56 5.6 A comparison between the r e s i s t i v i t y measured by the inner and outer electrodes at McDonald Farm 59 5.7 A comparison between the r e s i s t i v i t y measured by the inner and outer electrodes at Langley 60 5.8 A comparison between the r e s i s t i v i t y measured by the inner and outer electrodes at Colebrook 61 5.9 A comparison between the r e s i s t i v i t y measured by the inner and outer electrodes at Strong P i t 62 ACKNOWLEDGEMENTS v i i i I wish to thank Dr. Campanella f o r t h i s choice of a research project and h i s continued i n t e r e s t i n t h i s project. I would l i k e to thank my fellow students i n the In - s i t u Testing Group: Dave Brown, Ross Hitchman, John Sully, Damika Wickremesinghe, and from Geological Engineering, Mark Pritchard for t h e i r assistance i n carrying out the necessary f i e l d work for t h i s t h e s i s . Many thanks to Harold Schrempp for h i s work i n machining the r e s i s t i v i t y cone module and es p e c i a l l y Scott Jackson, who designed and b u i l t the c i r c u i t r y for the ex c i t a t i o n , measurement and data logging of the resistance measurements f o r the r e s i s t i v i t y cone module. I would also l i k e to thank Art Brookes for h i s continual r o l e i n maintaining the i n - s i t u t e s t i n g vehicle, without which none of the f i e l d work would be possible. I wish to acknowledge the f i n a n c i a l support of the Natural Sciences and Engineering Research Council, Canada, i n the form of a research assistantship. 1. INTRODUCTION 1 1.1 Rationale for R e s i s t i v i t y Cone Development The detection of contaminated s o i l and groundwater has become an important aspect of both the groundwater and geotechnical industry. The degradation of groundwater q u a l i t y has a d i r e c t e f f e c t on the qu a l i t y of l i f e i n a region by possibly threatening both the health of the l o c a l population and of w i l d l i f e habitats. Therefore, the rapid detection and containment of contaminated s o i l i s of wide i n t e r e s t . Examples of p o t e n t i a l contamination sources are: a c i d i c sludge as a byproduct from o i l reclamation (Greenhouse and Slaine, 1986), leachate from sanitary l a n d f i l l s (MacFarlane et a l , 1983), a c i d i c leachate from mine t a i l i n g s dumps (Morin et a l , 1982), or accidental s p i l l s . Contaminants may be detected by either d i r e c t sampling and analysis of groundwater and s o i l samples or by i n d i r e c t methods that operate on the premise that the presence of contaminants w i l l change some other measurable s o i l or groundwater property. A common i n d i r e c t method i s the measurement of the subsurface e l e c t r i c a l conductivity. The e l e c t r i c a l conductivity i s a measure of the ease that an e l e c t r i c a l current may be passed through a medium, i n t h i s case s o i l . Often these measurements are presented i n terms of s o i l r e s i s t i v i t y (or bulk r e s i s t i v i t y ) , which i s the inverse of s o i l conductivity. In most cases contaminants influence the bulk r e s i s t i v i t y of the s o i l because they change the e l e c t r i c a l properties of the groundwater. The e l e c t r i c a l r e s i s t i v i t y w i l l decrease with an increase i n dissolved s o l i d s , and increase i f in s u l a t i n g contaminants are present i n the groundwater. Generally for non-cohesive s o i l s v i r t u a l l y a l l the e l e c t r i c a l conduction takes place through the pore f l u i d with the s o i l matrix acting as an insu l a t o r . In many instances contaminant s i t e s contain a mixture of aqueous phase (usually conductive) and non-aqueous phase (in s u l a t i n g contaminants). Usually such mixtures w i l l produce highly conductive plumes since the influence of the conductive contaminants i s generally greater than that of the non-conducting contaminants. Given that s o i l r e s i s t i v i t y measurements are a v a l i d method of detecting contaminants i t becomes necessary to choose what method i s most suitable to make such measurements. Generally, there are two methods, galvanic measurement, and electromagnetic (EM) induction. Galvanic measurements are made by d i r e c t contact with the ground while for EM measurements no contact with the ground i s necessary. Both can be employed from the surface or i n boreholes. EM measurements have the advantage of being able to make measurements i n p l a s t i c cased holes. The objective of t h i s research i s to make rapid and accurate measurements of s o i l r e s i s t i v i t y using a modified cone penetrometer. The cone penetrometer t e s t (CPT) has gained wide acceptance as the most applicable logging t o o l for s o i l i n the geotechnical industry (Campanella and Robertson, 1982). With the addition of the r e s i s t i v i t y logging c a p a b i l i t y i t i s l i k e l y that the CPT w i l l see more use f o r hydrogeological investigations. A cone penetrometer with the c a p a b i l i t i e s of r e s i s t i v i t y logging would be appealing f o r the following reasons: 1) By being pushed into the ground the measuring electrodes are i n intimate contact with the ground, as opposed to the case where a r e s i s t i v i t y probe i s deployed i n a mud f i l l e d borehole. 2) Direct measurements are more accurate than surface measurements. Surface methods are commonly used to measure e l e c t r i c a l s o i l r e s i s t i v i t y (Telford et a l , 1976), but require at l e a s t a 5 to 10% e l e c t r i c a l contrast between contaminated and uncontaminated s o i l to successfully map a contaminant plume (Benson et a l , 1985), assuming that there are no l i t h o l o g i c a l v a r i a t i o n s . A r e s i s t i v i t y cone can measure r e s i s t i v i t y to a res o l u t i o n of 1% and at the same time record changes i n l i t h o l o g y . 3) The development of instrumentation for measuring galvanic r e s i s t i v i t y i s much easier to design and implement than for electromagnetic induction. 4) Valuable information from the cone penetrometer i s also recorded i n addition to the r e s i s t i v i t y data. 1.2 Scope of Research 4 Very l i t t l e published technical l i t e r a t u r e i s available concerning the design and use of e l e c t r i c a l r e s i s t i v i t y cones though both D e l f t (Van de Graff and Zuidberg, 1985) and Fugro-McClelland (Horsnell, 1988) have successfully operated such probes. D e l f t have used t h e i r r e s i s t i v i t y cone f o r density measurements i n offshore sands. The Fugro-McClelland r e s i s t i v i t y cone has been used to p r o f i l e conductive contaminants (Horsnell, 1988) and also used i n the determinination of c o r r o s i v i t y p o t e n t i a l . With there being such l i t t l e published work on the operation and i n t e r p r e t a t i o n of r e s i s t i v i t y data, the purpose of t h i s research i s to f i l l t h i s gap and independently determine how applicable the r e s i s t i v i t y cone penetration t e s t (RCPTU) i s for detecting the presence of contamination. To meet t h i s objective a research program was developed and i s summarized as follows: 1. Gather available l i t e r a t u r e regarding e l e c t r i c a l conduction i n s o i l and water and also r e s i s t i v i t y t e s t i n g methods, s p e c i f i c a l l y downhole t e s t s . 2. Set up a s i m p l i f i e d lab r e s i s t i v i t y t e s t i n g apparatus to aid i n the s e l e c t i o n of a f i n a l design for the r e s i s t i v i t y cone. This step i s meant to check the v i a b i l i t y of making accurate r e s i s t i v i t y measurements and to minimize the number of changes that would be needed for the eventual f i e l d prototype. 5 3. F i e l d t e s t the r e s i s t i v i t y cone to ensure i t i s c o l l e c t i n g accurate data and then compile data from various s i t e s . 4. Determine how the r e s i s t i v i t y measurements are affected by d i f f e r e n t s o i l types and groundwater conditions. 5. Outline the application of the r e s i s t i v i t y cone to contaminant studies and for other purposes. Each of these steps are examined i n the following chapters of t h i s t h e s i s . Conclusions and recommendations f o r further work are also presented. 6 2. ELECTRICAL CONDUCTION PHENOMENA 2.1 Introduction An e l e c t r i c charge can be transferred by three d i f f e r e n t processes (Telford, 1976). The f i r s t , being the most f a m i l i a r , i s the transfer of a charge by the flow of free electrons i n a m e t a l l i c conductor. This charge transfer i s referr e d to as e l e c t r o n i c , or ohmic conduction. Another method of charge transfer i s e l e c t r o l y t i c conduction, the transfer of charge by the migration of cations and anions i n an e l e c t r o l y t e i n response to an e l e c t r i c a l f i e l d . This method of charge transfer i s dominant at the frequencies used i n galvanic r e s i s t i v i t y measurements. The f i n a l method of charge transfer i s referred to as d i e l e c t r i c conduction. With d i e l e c t r i c conduction a current i s produced due to changing e l e c t r o n i c , i o n i c or molecular p o l a r i z a t i o n caused by the ap p l i c a t i o n of a time varying e l e c t r i c f i e l d . Conduction, i n the most general terms, may be described by the following r e l a t i o n (Hearst ,1985). [2.1] J =[er+<^£+ i(<r-u/£)]E 2 where: J - current density (A/m ) E - e l e c t r i c f i e l d strength (V/m) °" - conductivity (S/m) 8. - d i e l e c t r i c p e r m i t t i v i t y (F/m) 1 0 - angular frequency i - 1/^1 The form of the above equation can be s i m p l i f i e d depending upon the e x c i t a t i o n frequency. Both the conductivity and d i e l e c t r i c p e r m i t t i v i t y are complex quantities that are frequency dependent. In the case of low frequency r e s i s t i v i t y measurements the r e l a t i o n between the applied e l e c t r i c f i e l d and the current density s i m p l i f i e s to the following r e l a t i o n : [2.2] J = <rE = E/p where: p = r e s i s t i v i t y (ohm-m) This r e l a t i o n s h i p w i l l be v a l i d at low frequencies, up to several k i l o h e r t z , providing the charges are not bound. Using t h i s r e l a t i o n s h i p the low frequency r e s i s t i v i t y (which i s sometimes referred to as the DC r e s i s t i v i t y ) , or conversely conductivity may be determined. The factors which influence r e s i s t i v i t y at low frequencies i n pore f l u i d and i n saturated s o i l w i l l be examined i n the following sections. 2.2 E l e c t r i c a l Conduction i n Pore Water At low frequencies conduction i n pore water takes place by e l e c t r o l y t i c conduction. A current i s produced when the ions migrate due to the application of an e l e c t r i c f i e l d . In general, the more ions present i n the pore f l u i d the greater the conductivity. However, a number of other factors a f f e c t the conductivity of a p a r t i c u l a r ion i n an e l e c t r o l y t e . K e l l e r (1982) points out that the acceleration of ions i n an e l e c t r i c f i e l d i s opposed by viscous drag. This l i m i t s the maximum v e l o c i t y of the ions, hence i t s conductivity. The amount of drag i s a function of temperature, i o n i c concentration, ion s i z e , and valence. Since f l u i d v i s c o s i t y i s a function of temperature, the conductivity of a p a r t i c u l a r ion w i l l increase as the temperature increases. While conductivity increases with increased i o n i c concentration there i s an increased tendency towards c o l l i s i o n s which retard the migration of ions to some extent. I t would seem reasonable that smaller ions would be more conductive since there would be less viscous drag. However, Hearst (1985) states that the r e l a t i o n s h i p between si z e and mobility i s not that simple since charged ions tend to solvate ( a t t r a c t a layer of water molecules). Conductivity i s also dependent on the valence of the ion. The greater the valence the more charge transferred per ion, r e s u l t i n g i n a higher conductivity. V i s c o s i t y also decreases with pressure so that conductivity w i l l increase at higher pressures, however t h i s i s a n e g l i g i b l e consideration with the pressures involved i n cone t e s t i n g . By assuming a l i n e a r r e l a t i o n s h i p between the concentration of ions and conductivity, a reasonable estimate of the t o t a l dissolved s o l i d s (TDS) i n a solution can be made. (CRC, 1982). [2.3] C =crK where: C - concentration of t o t a l dissolved s o l i d s (mg/1) O - conductivity (^S/cm) and K i s a constant with the following ranges. K < 0.55 high s a l i n i t y (TDS>2g/l ,6<pH<9) K = 0.55 - 0.7 low s a l i n i t y (TDS<2g/l ,6<pH<9) K > 0.7 for highly a c i d i c or caustic pore waters The high value of K for low pH pore f l u i d s i s due to the high mobility of hydrogen ions i n comparison to other ions. D i f f e r e n t ions have d i f f e r e n t conductivity factors, the conductivity factor being the r e l a t i v e contribution to the t o t a l conductivity of a solution. Using conductivity factors the concentration of each constituent may be estimated i f the r e l a t i v e quantities of contaminants i n the groundwater are not changing 2.3 E l e c t r i c a l Conduction i n Multi-Phase Systems A study by Urish (1983) determined that bulk conductivity of uncontaminated s o i l i s a combination of a number of factors: conductivity of the matrix and pore f l u i d , porosity, t o r t u o s i t y (which i s dependent on the geometric packing and shape of the grains), the t o t a l i n t e r s t i t i a l surface area of the pores per un i t pore volume of the sample, the s p e c i f i c surface conductivity of the grains (which i s dependent on the pore water r e s i s t i v i t y ) , i o n i c composition of the pore water, pore water pH, cation exchange capacity (CEC) of the matrix minerals. The bulk and conductivity i n a contaminated multi-phase system becomes more complicated since four components must be considered: s o i l p a r t i c l e s , aqueous phase l i q u i d s (APL), non-aqueous phase l i q u i d s (NAPL), and a i r . The measured bulk r e s i s t i v i t y of the s o i l i s a r e s u l t of conduction (or lack of conduction) through the above mentioned components and how the components in t e r a c t with each other. If each s p e c i f i c contributor to conductivity was considered the r e l a t i o n s h i p between f l u i d conductivity and bulk conductivity would be extremely complicated. Formulas that describe the r e s i s t i v i t y i n multi-phase systems are referred to as mixing laws. Fortunately simplifying assumptions can be made. A i r ,NAPLs, and sand can be considered equivalent with respect to t h e i r conductivity, i n most cases they are considered in s u l a t o r s . Therefore the simplest case i s the assumption that a l l conduction takes place through the pore f l u i d . This assumption i s the premise of Archie's Formula (Archie, 1942, and Tel f o r d et a l , 1976), the simplest of the mixing laws. Archie's Formula assumes that bulk r e s i s t i v i t y i s d i r e c t l y r e l a t e d to pore water r e s i s t i v i t y and the geometry of the pore spaces i n the s o i l (or rock). A term commonly used to r e l a t e s o i l r e s i s t i v i t y to pore f l u i d r e s i s t i v i t y i s the formation factor, which i s a function of the pore geometry. Archie's Formula i s given as: [2.4] F = Ph/pf = an" m where: F = i n t r i n s i c formation factor (°b = bulk r e s i s t i v i t y (ohm-m) P-f = f l u i d r e s i s t i v i t y (ohm-m) a,m = constants for a given s o i l n = porosity For unconsolidated s o i l a m 1, and m i s dependent on s o i l type. For sands the value of m i s approximately 1.5, and for various clays authors have found that m = 1.8 to 3 (Jackson et a l , 1978). Archie's Formula has been recognized to be an ov e r s i m p l i f i c a t i o n but i s s t i l l v a l i d under the condition that the pore f l u i d r e s i s t i v i t y i s very low or there are no clay minerals present i n the s o i l . This i s because the bulk r e s i s t i v i t y can be a function of factors, other than pore geometry and f l u i d r e s i s t i v i t y , which would tend to decrease the observed formation factor. For these reasons the measured formation factor i s referred to as the apparent formation factor. The i n t r i n s i c formation factor of a s o i l i s a function only of the pore geometry, which has been found to be a function of p a r t i c l e shape. Jackson et a l . (1978) researched the e f f e c t of p a r t i c l e s i z e , d i s t r i b u t i o n and shape on the formation factor and found m to be only a function of grain shape. Thus, m i s a measure of pore t o r t u o s i t y , with m increasing as the s o i l p a r t i c l e s become more elongated. Other more complicated mixing laws have been formulated to consider the e f f e c t of mineralogical conduction due to the presence of clay. An excellent summary of these r e l a t i o n s may be found i n Jordan and Campbell (1986). These r e l a t i o n s are d i f f i c u l t to put into practice due to the necessity of measuring the cation exchange capacity of the s o i l . However an awareness of these rel a t i o n s h i p s i s important i n understanding what factors may influence r e s i s t i v i t y . The e f f e c t of surface conduction i s important i n clayey s o i l s and organic material which have a high cation exchange capacity. Attempts have been made by some authors to use r e s i s t i v i t y as a method of determining hydraulic conductivity. The r a t i o n a l e being that i f porosity can be estimated from r e s i s t i v i t y measurements then hydraulic conductivity could also be estimated. I f the Kozeny - Carmen equation i s used as a representation of the factors that influence hydraulic conductivity i t can be seen that hydraulic conductivity depends, to some extent, on the square of the pore radius, a s p e c i f i c surface term and the representative grain s i z e . A l l these factors are measures of pore siz e , hence there should be some r e l a t i o n to the formation factor. However r e l a t i o n s developed between formation factor and hydraulic conductivity have been contradictory. The fac t that the apparent and i n t r i n s i c formation factor are often d i f f e r e n t makes t h i s approach of determining hydraulic conductivity unfavorable. 2.4 Frequency Dependent Behavior Frequency dependent e l e c t r i c a l behavior i n s o i l s may be ascribed to two d i f f e r e n t p o l a r i z a t i o n processes. The f i r s t being induced p o l a r i z a t i o n , and the second d i e l e c t r i c p o l a r i z a t i o n . Both e f f e c t s r e s u l t i n a current out of phase with the voltage therefore requiring the conductivity, or p e r m i t t i v i t y to be described as a complex quantity. Induced p o l a r i z a t i o n i s noted at frequencies of less than one hertz while d i e l e c t r i c p o l a r i z a t i o n i s noted i n the mega-hertz range. 2.4.1 Induced P o l a r i z a t i o n At low frequencies complex conduction may occur due to the p o l a r i z a t i o n of ions. P o l a r i z a t i o n takes place due to zones of unequal i o n i c transport properties (Hearst, 1985). Clays have f i x e d negative charges so that the conduction of cations i n clayey s o i l s i s favored over that of anions. This leads to zones of unequal charge d i s t r i b u t i o n . In s o i l s containing clay t h i s leads to what i s referred to as the membrane p o l a r i z a t i o n . When a current i s applied to a clayey s o i l i o n i c concentration gradients develop i n regions where clay p a r t i a l l y blocks pore spaces. This behavior i s i n d i c a t i v e of variable clay content. The r e l a t i o n s h i p between p o l a r i z a t i o n and the state of the clay i s complex. Bodmer et a l . (1968) noted that p o l a r i z a b i l i t y increases to a maximum at 5 to 9 percent clay content and then decreases. Besides confirming the presence of clay minerals i n the s o i l p o l a r i z a t i o n i s also i n d i c a t i v e of contamination i n s o i l s that have an i n i t i a l high p o l a r i z a t i o n . Increased pore f l u i d s a l i n i t y w i l l decrease the p o l a r i z a b i l i t y since conductive paths w i l l be chosen over capacitive paths of current flow. Bodmer et a l (1968) noted that i n sand clay mixtures that the p o l a r i z a b i l i t y i s roughly proportional to the cation exchange capacity (CEC) of the clay. The presence of contaminants that would decrease the CEC capacity of the clay could be noted by observing a decrease i n p o l a r i z a b i l i t y of the s o i l . Examples of such contaminants are hydrocarbons (Olhoeft, 1985), water soluble organic cations (Hughes, 1986), and also chlorides (Scott-Fleming et a l . 1983). Measurements of p o l a r i z a t i o n are accomplished i n either the time domain or frequency domain (Telford, 1976). In the time domain, once the source voltage i s removed, the observed voltage decay curve i s integrated. In the frequency domain two measurements of r e s i s t i v i t y are made at low frequencies one decade apart. Another method, referred to as complex r e s i s t i v i t y , uses r e s i s t i v i t y measurements over a large frequency range. Using the complex r e s i s t i v i t y method Towle et a l (1985) noted an inverse r e l a t i o n s h i p between grain s i z e and a c r i t i c a l frequency, which i s defined as the peak i n the complex r e s i s t i v i t y spectrum. 2.4.2 D i e l e c t r i c P o l a r i z a t i o n At low frequencies conduction of free e l e c t r o l y t e s i s independent of frequency. As the frequency i s increased an increasing proportion of the charge transfer i n pore f l u i d i s due to displacement currents. In the mega-hertz range the d i e l e c t r i c p e r m i t t i v i t y of water, and other polar f l u i d s , i s frequency dependent due to time dependent orien t a t i o n of dipoles. At higher frequencies the d i e l e c t r i c p e r m i t t i v i t y i s a function of frequency due to various resonances of atoms and electrons. As previously noted displacement currents dominate conduction currents at high frequencies, the frequency range where displacement currents dominate depends on the conductivity of the s o i l . The advantage of measuring the d i e l e c t r i c p e r m i t t i v i t y of the s o i l i s that i t i s c l o s e l y r e l a t e d to the water content of the s o i l . The d i e l e c t r i c constant of water i s weakly influenced by s a l i n i t y and temperature f o r most s o i l s . This i s because of the very high d i e l e c t r i c p e r m i t t i v i t y of water, being highly polar, i n comparison to the d i e l e c t r i c constant of the s o i l matrix. Thus the i n - s i t u measurement of the d i e l e c t r i c constant of the s o i l could be used to determine water content, and hence void r a t i o i s cohesionless s o i l s . 3. INSTRUMENT DEVELOPMENT The successful development of the r e s i s t i v i t y module was contingent on two factors: (1) understanding the underlying p r i n c i p l e s of conductivity i n s o i l s and; (2) the completion of the lab t e s t i n g program. This chapter commences with a b r i e f o utline of currently a v a i l a b i l i t y r e s i s t i v i t y cone technology. Available l i t e r a t u r e concerning these too l s was so sparse that a lab t e s t i n g program was necessary to determine an appropriate design f o r the UBC r e s i s t i v i t y cone. The lab t e s t i n g apparatus w i l l be described and r e s u l t s from that program w i l l be used i n the section of t h i s chapter concerning design c r i t e r i a . 3.1 E x i s t i n g R e s i s t i v i t y Cone Technology E l e c t r i c a l measurements of s o i l and pore water have been used successfully i n a number of instances to map changes i n groundwater composition. Normally, surface r e s i s t i v i t y surveys are c a r r i e d out to measure s o i l r e s i s t i v i t y . Surface methods require at least a 5 to 10% e l e c t r i c a l contrast between contaminated and uncontaminated s o i l to successfully map a contaminant plume (Benson, 1985), assuming there are no l i t h o l o g i c a l v a r i a t i o n s . By measuring r e s i s t i v i t y during cone penetration t e s t i n g much more accurate determinations of r e s i s t i v i t y can be made while obtaining l i t h o l o g i c a l information. The recognition of t h i s has led to the development of a small number of r e s i s t i v i t y cone penetrometers. However, there has been l i t t l e published about t h e i r use at d i f f e r e n t s i t e s and of t h e i r actual description. The following i s a b r i e f description of these cones. 1) D e l f t : This cone was developed i n 1970 /s to be used i n conjunction with water sampling to determine s o i l porosity (Van de Graff and Zuidberg, 1985). The cone has 8 electrodes. I t i s presumed that there are two set of four electrodes. Data i s c o l l e c t e d at discreet i n t e r v a l s of 20 cm to provide a p r o f i l e of r e s i s t i v i t y with depth. The input signal i s at a frequency of 10 hertz. 2) Fugro: The Fugro cone contains two c i r c u l a r electrodes set f i v e cm apart (Horsnell, 1988). The electrodes are i s o l a t e d from the rest of the cone body by a ceramic material. The e x c i t a t i o n signal i s applied at an unspecified low frequency. 3) ConeTec: This module has two 10 mm wide electrodes set 7.5 cm apart. The e x c i t a t i o n current i s applied at 1000 Hz. I t i s a modular design b u i l t to be f i t t e d on a standard Hogentogler piezocone. The probe i s a f u l l y d i g i t a l , microprocessor controlled unit. This module was developed a f t e r the UBC r e s i s t i v i t y cone. 3.2 Lab Testing Equipment The lab t e s t i n g equipment was quite simple but e f f e c t i v e i n making r e s i s t i v i t y measurements. The probe part consisted of four rings that s l i d along a p l a s t i c cylinder. This allowed the electrodes to be placed at varying distances to note the e f f e c t of changing the electrode spacing. The source was b a s i c a l l y the same constant current c i r c u i t that was i n the f i n a l unit, with the power being supplied from a signal generator. This allowed for measurements over a wide frequency range to study the e f f e c t of varying the frequency. The voltage output from the electrodes was noted on an oscilloscope. The electrode assembly was submerged i n a large tank of water and Potassium Chloride (KC1) was added to the water i n the tank to create solutions of d i f f e r e n t conductivity. 3.3 Design Requirements and Considerations The following i s an outline of the requirements for the r e s i s t i v i t y probe. Options i n the design are considered and the benefits and problems involved with each option are considered i n the l i g h t of the re s u l t s obtained from from the lab t e s t s . 3.3.1 Number of Electrodes Most r e s i s t i v i t y probes have used eith e r two or four electrode arrays. Usually surface r e s i s t i v i t y methods use four electrodes, with the current applied at the outermost electrodes and the p o t e n t i a l measured with the inner electrodes. With a two electrode probe the p o t e n t i a l i s measured across the electrodes applying the current. For the UBC probe i t was decided to use a four electrode design while measuring p o t e n t i a l across the inner and outer electrodes. The purpose of t h i s configuration was to observe i f there was any difference i n the r e s u l t s between the two sets of electrodes. The four electrode probe also has the advantage of being able to make low frequency measurements without the inner electrodes becoming polarized. P o l a r i z a t i o n occurs due to a buildup of ions at the electrodes which produces an impedance i n ser i e s with that of the surrounding s o i l . At the electrodes oxidation-reduction (redox) reactions must take place for there to be a transfer of charge. (Keller, 1982). The redox reaction rates are c o n t r o l l e d by the concentration of reactions that accumulate as the current i s transferred. As the ions cannot react they accumulate at the electrodes causing the p o l a r i z a t i o n . Since p o l a r i z a t i o n i s a function of current the problem may be ameliorated by decreasing the current passing through the electrodes. This can be done by employing a second set of electrodes to measure voltage. Since the impedance through the voltage measuring c i r c u i t i s high the current passing through the electrodes i s very small. Therefore, such an array can be operated at low frequencies without becoming polarized. The e f f e c t of p o l a r i z a t i o n i s i l l u s t r a t e d i n Figure 3.1 where the r e s i s t i v i t y , normalized with respect to the r e s i s t i v i t y measured at 1000 Hz, i s plotted versus frequency f o r three d i f f e r e n t electrode configurations. Using a four electrode system, with the electrodes set 3.5 cm apart, the output i s independent of frequency, showing that there i s no p o l a r i z a t i o n of the inner electrodes. The two electrode configuration shows a frequency dependent output c h a r a c t e r i s t i c 20 2.00 FREQUENCY (Hz) Figure 3.1 Inner and outer e l e c t r o d e normalized r measurements versus frequency of electrode p o l a r i z a t i o n . As 1000 Hz i s approached the e f f e c t of p o l a r i z a t i o n decreases, t h i s i s because the ions do not have time to accumulate at the electrodes. Therefore, a two electrode configuration can be successfully employed i f the e x c i t a t i o n frequency i s adequate. The advantage of a two electrode configuration over a four electrode probe i s ease of f a b r i c a t i o n . 3.3.2 Electrode Spacing Three factors are involved when determining the spacing used. Smaller distances between the electrodes allow for the possible detection of thinner layers of contrasting r e s i s t i v i t y . Wider spacing provides a greater penetration into the s o i l and should give a more accurate determination of i n - s i t u r e s i s t i v i t y . With a four electrode array, i f the distance between the p o t e n t i a l and current electrodes i s too small the p o t e n t i a l electrodes w i l l become polarized i f a very low e x c i t a t i o n frequency i s employed. From a number of lab t e s t s at d i f f e r e n t electrode spacings i t was found that f o r 5 mm wide electrodes, spacings as small as 15mm from centre to centre were adequate i n providing accurate determinations of r e s i s t i v i t y . 3.3.3 E x c i t a t i o n Frequency Alternating current e x c i t a t i o n supplies are used i n order to avoid electrode p o l a r i z a t i o n . An upper l i m i t to the frequency chosen for the probe i s a few thousand hertz to avoid inductive coupling. A few p r a c t i c a l l i m i t a t i o n s were also encountered. Sixty hertz should be avoided since noise from power sources may a f f e c t the instrumentation. The frequency must high enough to ensure accurate AC to RMS conversion. As seen from Figure 3.1 t h i s frequency must be high enough to avoid electrode p o l a r i z a t i o n of the outer electrodes. I t i s also notable that the e f f e c t of electrode p o l a r i z a t i o n i s d i f f e r e n t f o r varying f l u i d r e s i s t i v i t i e s . The frequency chosen f o r the probe was one k i l o - h e r t z . This frequency meets a l l the desired c r i t e r i a and i s also within the range 25-3000 Hz suggested by the ASTM (D1125-82) for conductivity measurements of water. 3.3.4 Measurement Range E l e c t r i c a l r e s i s t i v i t y of the ground can vary over a range of many orders of magnitude. With both conducting and in s u l a t i n g contaminants being considered a wide measurement range i s necessary. With very conductive contaminants the measured voltages across the electrodes may be too small to accurately determine changes i n r e s i s t i v i t y , due to a low signal to noise r a t i o . The maximum input current i s li m i t e d by the power supply. 3.3.5 Probe Materials The material from which the probe i s constructed must be re s i s t a n t to abrasion and able to withstand reactive chemicals. The parts must also be e a s i l y replaced. The electrode material must be reasonably durable, have a high conductivity, and be non-oxidizing. Lab and f i e l d t e s t i n g was done with a number of d i f f e r e n t metals: brass, aluminum, copper, and beryllium copper. I t was found that the r e s u l t s from the f i r s t three of the formentioned metals were e s s e n t i a l l y the same. The beryllium copper electrodes however became e a s i l y polarized due to the formation of an oxidized surface. Brass was chosen as i t i s reasonably abrasion r e s i s t a n t and i s a good conductor. Aluminum and copper are not durable enough for f i e l d use. I t i s l i k e l y that s t a i n l e s s s t e e l would also be adequate as an electrode material since i t i s both very durable and a good conductor. 3.4 R e s i s t i v i t y Module Description The r e s i s t i v i t y module, as shown i n F i g . 3.2, consists of a four electrode array. The electrodes are made of brass and are i s o l a t e d by p l a s t i c i n s u l a t i n g sections and are sealed by 0-rings. The i n s u l a t i o n and the electrodes are f i e l d replaceable. The spacing between the centre of the electrodes i s 25 mm, with the width of the electrodes being 5 mm. Simultaneous and continuous measurements are made with the inner and outer electrodes. Conversion of the measured AC voltage to RMS voltage takes place downhole. The electrode furthest from the t i p i s the current source. This electrode i s set at the centre of the i n s u l a t i o n to maximize the distance from the cone body. The cone body i s grounded and w i l l tend to draw some current towards i t . Ideally a l l the current should go to the ground electrode, the electrode closest to the t i p . The magnitude of F i g . 3.2 UBC Resistivity Cone. the constant current source, operating at 1000 Hz, i s cont r o l l e d from the surface. Since the peak applied current i s very small, t y p i c a l l y i n the order of 150 micro-amps, the po t e n t i a l measured across the electrodes must be amplified downhole to the point where they f a l l into the range usable by the data a q u i s i t i o n system. 3.5 R e s i s t i v i t y Cone C a l i b r a t i o n The RCPTU measures e l e c t r i c a l resistance between the electrodes. The resistance w i l l increase i f the electrodes are set farther apart or i f the electrode surface area i s decreased. The r e s i s t i v i t y i s the actual s o i l parameter and thus i t s value should be independent of the probes electrode geometry as long as the probes i n s e r t i o n into the s o i l does not change the i n -s i t u r e s i s t i v i t y . To convert from resistance to r e s i s t i v i t y a lab c a l i b r a t i o n was made for both the outer and inner electrode p a i r s . This was accomplished by placing the probe i n an open c y l i n d r i c a l chamber that completely surrounded the module. The chamber was f i l l e d with water and potassium chloride (KC1) was added i n quantities such that measurements of resistance could be made at a number of d i f f e r e n t e l e c t r o l y t e concentrations. The r e s i s t i v i t y of the solution was noted with a portable conductivity meter (Omega CDH-30) and the values were compared to the resistances measured by the probe. The conductivity meter was c a l i b r a t e d with a 0.01 M solution of KC1 and then checked with a 0.10 M solution. From the c a l i b r a t i o n of the cone a l i n e a r r e l a t i o n between the resistance and the r e s i s t i v i t y was derived. For the dimensions given i n F i g . 3.2 the c a l i b r a t i o n factor, K, (Eq. 3.3), was found to be 0.100 m for the outer electrodes and 0.838 m f o r the inner electrodes. In an Ertec (1987) report they note that K i s dependent on the penetration of the e l e c t r i c f i e l d into the ground, or a l t e r n a t i v e l y stated K w i l l vary with d i f f e r e n t s o i l types. One way of providing some l e v e l of confidence as to the v a l i d i t y of the c a l i b r a t i o n factor i s by comparing the r e s i s t i v i t y r e s u l t s determined by both the inner and outer electrode sets. Differences i n measured r e s i s t i v i t y values between the two electrode sets w i l l be discussed i n Chapter 5. For cohesive s o i l s lab r e s i s t i v i t y measurements could be made on tube samples for comparison. 3.6 BAT Water Sampling System Description and Use Besides making measurements of the r e s i s t i v i t y of tube samples the only other way of gauging the r e l i a b i l i t y of the r e s i s t i v i t y cone i s to measure the conductivity of the pore f l u i d at the s i t e of a RCPTU t e s t . The easiest way of doing t h i s was with the BAT ground water sampling system developed by Torstensson (1984). This system was designed to have a f i l t e r t i p pushed into the s o i l sediments with cone penetration equipment. In t h i s case a s t e e l BAT t i p , coupled with a f r i c t i o n reducer was pushed into the s o i l with AWL rods. At one metre i n t e r v a l s a hypodermic syringe sampling system i s lowered through the rods, penetrating a rubber septum at the t i p , and extracts a sample. The small t i p volume i s quickly purged and a second sample i s taken, from which the conductivity i s measured by using an Omega CDH-3 0 portable conductivity meter. The BAT and RCPTU r e s i s t i v i t y values should correlate quite well as they are both approximately sampling a s i m i l a r volume of s o i l . In the case of the cone the outer electrodes are 7.5 cm apart and for the BAT t i p the length of the porous f i l t e r i s approximately the same distance. I f measurements of the conductivity are made immediately a f t e r the water sample i s drawn from the ground no temperature corrections are necessary to compare the r e s i s t i v i t y of the pore water to that measured by the RCPTU. I f the samples are stored and the conductivity measured at a l a t e r time the i n -s i t u temperature at that depth must be known so that the r e s i s t i v i t y values may be corrected to a common temperature. 3.7 F i e l d Testing Procedures A l l the f i e l d work involved i n the research was ca r r i e d out from the UBC i n - s i t u t e s t i n g vehicle, of which a description i s provided by Campanella and Robertson (1981). Cone data was co l l e c t e d using the UBC i n - s i t u t e s t i n g d i g i t a l data a q u i s i t i o n system, which has the c a p a b i l i t y to simultaneously record eight channels. Seven channels (bearing, f r i c t i o n , lower pore pressure, upper pore pressure, temperature, r e s i s t i v i t y 1 , r e s i s t i v i t y 2 ) where recorded during penetration at i n t e r v a l s of 25 mm. The preparations involved i n r e s i s t i v i t y t e s t i n g are the si m i l a r to those involved with the standard piezocone t e s t . These procedures are outlined i n Robertson and Campanella (1988). The only additional preparatory step for the RCPTU i s that the AC ex c i t a t i o n voltage i s set. This e x c i t a t i o n voltage controls the amount of current supplied to the electrode array according to the r e l a t i o n : [3.1] I = V / 47.5kQ. where: V = input voltage (RMS v o l t s at 1kHz) I = e x c i t a t i o n current (RMS amps) Hence, an appropriate choice of voltage must be made based on the desired range of r e s i s t i v i t i e s to be measured, where the r e s u l t i n g r e s i s t i v i t y w i l l follow the r e l a t i o n s : [3.2] R = V/(I*A) where: R = resistance (ohm) I = current (RMS amp) V = output voltage (RMS volt) A = amplifier gain ( 50 for outer electrode pair, 500 for inner electrode pair) [3.3] p = K*R = (1/conductivity) where: p = r e s i s t i v i t y (ohm-m) K = c a l i b r a t i o n factor The maximum allowable input voltage i s 10 V without corrupting the sinusoidal nature of the output s i g n a l . This r e s u l t s i n a maximum allowable input current of 210 A. The allowable input voltage can be also l i m i t e d to a lower value i f the r e s i s t i v i t y output voltage from the DAS exceeds 7.5 V RMS. Therefore, during te s t i n g , i f the output voltage s t a r t s to exceed 7 V RMS penetration should be stopped and the input voltage should be lowered to an appropriate l e v e l . Care must be taken to note the input voltage p r i o r to each t e s t and then any subsequent changes. From the s i t e s investigated f o r t h i s research i t was found that an input voltage 5 V RMS provided the best accuracy without exceeding the range of the DAS. This resulted i n a possible range of measurement of 0 to 119 ohm-m. If higher r e s i s t i v i t i e s were expected at a s i t e a lower input voltage should be chosen. The frequency of the input signal must be maintained at 1000 Hz since a v a r i a t i o n i n the frequency w i l l a l t e r the observed c a l i b r a t i o n of the electrodes. This i s because the behavior of the elect r o n i c s i s frequency dependent due to the capacitive coupling of the electrodes to the probe c i r c u i t r y . I d e a l l y the r e s i s t i v i t y module should be regularly maintained. This e n t a i l s cleaning the electrodes a f t e r each t e s t and p e r i o d i c a l l y cleaning and inspecting the O-ring seals. A f t e r prolonged use the i n s u l a t i o n and the electrodes should be replaced due to abrasional reduction i n t h e i r outer diameter which may a f f e c t the repeatablity of the r e s i s t i v i t y measurements. 3.8 Data Reduction In addition to the normal corrections necessary to cone data some data manipulation must be done to the r e s i s t i v i t y data. For purposes of p l o t t i n g , the r e s i s t i v i t y data must be o f f s e t 71 cm upward to the same datum as the cone t i p . The r e s i s t i v i t y data from the cone i s i n the form of a measured voltage from across the electrodes. The voltage i s converted to a resistance based on the constant current chosen for the p a r t i c u l a r t e s t . The conversion to r e s i s t i v i t y i s based on the c a l i b r a t i o n done i n water. The r e s i s t i v i t y provided from t h i s c a l i b r a t i o n should be f a i r l y representative of the i n - s i t u c a l i b r a t i o n factor. The a p p l i c a b i l i t y of the c a l i b r a t i o n factor i s discussed l a t e r i n the r e s u l t s . R e s i s t i v i t y i s also temperature dependent. The re s u l t s i n t h i s research have not been corrected to a common temperature for comparison. The temperature changes between s i t e s are not large enough to s i g n i f i c a n t l y change the r e s u l t s . Water sample r e s i s t i v i t y values have been corrected to the same temperature as the i n - s i t u r e s i s t i v i t y measurements. No temperature c a l i b r a t i o n has been made. I t i s assumed since that almost a l l conduction i s e l e c t r o l y t i c that the same temperature correction for a f l u i d s o l u t i o n holds for any p a r t i c u l a r s o i l (2% /degree C e l s i u s ) . This could be tested for d i f f e r e n t s o i l s by forming samples around the cone i n the c a l i b r a t i o n chamber. When penetration takes place the probe does heat up due to f r i c t i o n , but the temperature of the probe w i l l not a f f e c t the measurements. I f comparisons of r e s i s t i v i t y at a s i t e are being made over a long period of time the i n - s i t u temperature should be noted i f seasonal s o i l temperature conditions change appreciably. 4. RESEARCH SITES AND FIELD RESULTS 31 A l l f i e l d t e s t i n g was done from the UBC i n - s i t u t e s t i n g v e h i c l e . Testing started on Oct 31, 1989. The f i r s t s i x tests were needed to f i n e tune the probe e l e c t r o n i c s . The f i r s t t e s t with the f i n a l working configuration was RES 89-7. A table showing the t e s t schedule i s presented i n Table 4.1. A map showing the location of the t e s t s i t e s i s shown i n Figure 4.1. Most of the t e s t i n g was done at the nearby McDonald Farm research s i t e . The f i n a l three t e s t s were conducted i n deposits of g l a c i a l o r i g i n i n the lower Fraser Valley. A d e s c r i p t i o n of each s i t e i s provided along with a d e s c r i p t i o n of the s t r a t i g r a p h i c and r e s i s t i v i t y p r o f i l e from each s i t e . 4.1 McDonald Farm 4.1.1 S i t e Description This s i t e i s located at an abandoned farm on the north side of Sea Island as shown on the area map (Figure 4.1). A more det a i l e d map of the s i t e showing the location of the various t e s t s i s presented i n Figure 4.2. The elevation of the s i t e i s +1.6 m with variat i o n s i n elevation of no more than 0.5 m. The area i s mostly covered with low grass and l i g h t deciduous vegetation. Sea Island i s part of the prograding Fraser River de l t a . The lowlands i n t h i s area are underlain by a complex sequence of Table 4.1 m - s l t u te s t i n g program S i t e Test No. Date Coordinates McDonald Farm RES 89-5 Feb 21/89 210.4,28.7 RES 89-7 Mar 6/89 29.5,0.2 RES 89-8 Mar 6/89 29.5,-99.9 RES 89-9 Mar 6/89 29,5,-50.2 RES 89-10 Mar 17/89 29.5,-0.5 RES 89-11 Mar 29/89 214.1,2.2 RES 89-12 Mar 29/89 141.0,0.2 RES 89-13 Mar 29/89 424,3,-16.1 BAT 89-2 Apr 3/89 215.1,2.2 Strong P i t RES 89-14 Apr 6/89 n/a Langley RES 89-15 Apr 6/89 n/a colebrook RES 89-16 Apr 18/89 n/a Fig. 4.1 General Location of the U.B.C. Research S i t e s co CO Figure 4.2 McDonald Farm research s i t e g l a c i a l , f l u v i a l , and marine deposits up to a depth of 300 m, with the more recent Fraser River sediments accounting f o r up to 200 m of t h i s sequence. The groundwater table at the s i t e varies from 1 to 2 metres depending on r a i n f a l l and t i d a l influence. During the winter some surface ponding occurs due to the low permeability of the surface overbank s i l t deposits, thus l i m i t i n g vehicle access. 4.1.2 Stratigraphic and R e s i s t i v i t y P r o f i l e An interpreted r e s i s t i v i t y cone p r o f i l e (RES89-7) representative of the s i t e i s presented i n Figure 4.3. T y p i c a l l y on the s i t e from the surface to a depth of 2 to 4 m overbank sandy to clayey s i l t can be expected. This i s followed by a sand horizon to a depth of 15 m. This horizon consists of d e l t a i c and d i s t r i b u t o r y channel f i l l sand and s i l t y sand. This has resulted i n a highly variable s o i l r e l a t i v e density, h o r i z o n t a l l y and v e r t i c a l l y across the s i t e . The sand i s medium to coarse grained with t h i n layers of medium to f i n e sand. Sieve analysis from a nearby SPT show that the sand i s coarse grained with increasing fines content near the surface. At a depth of 15 m the sand becomes f i n e r and i s occasionally interbedded with s i l t . This t r a n s i t i o n layer grades into a clayey s i l t . From the r e s i s t i v i t y p r o f i l e the high s a l i n i t y of the pore water i s evident (25 to 2 ohm-m). This i s due to the i n f i l t r a t i o n of brackish water found near the mouth of the FRICTION RATIO SLEEVE FRICTION CONE BEARING PORE PRESSURE RESISTIVITY INTERPRETED i g . 4.3 R e s i s t i v i t y Cone Sounding at McDonald Farm, Vancouver I n t e r n a t i o n a l A i r p o r t , B.C. CO Fraser River. The s a l i n i t y of the pore water increases with depth, as i l l u s t r a t e d by the decreasing r e s i s t i v i t y , u n t i l reaching a l i m i t i n g value i n the clayey s i l t of about 2 ohm-m. The groundwater table i s noted by the sudden drop i n r e s i s t i v i t y at 1.2 m. 4.2 Fraser Valley Glaciomarine Deposits The current surface geology of the Fraser Valley i s dominated by glaciomarine clay deposited during the Fraser Deglaciation. At the time of deposition the lowland areas were depressed by more than 200 m by the weight of the c o r d i l l e r a n ice sheet. During the recession glaciomarine d e l t a i c deposits of the Capilano and Fort Langley Formation were deposited. While the two formations were deposited at the same time there are some features that make them d i s t i n c t (Clague and Luternauer, 1982). The d e l t a i c deposits of the Capilano Formation were deposited from melt water from the Coast Mountains so t h e i r mineralogy i s dominated by g r a n i t i c rock types. Fort Langley deposits were derived from rock of the Cascade Mountains to the east of the Fraser Valley. Capilano sediments, which contain no i c e contact sediments, were deposited i n a more saline environment. The Capilano sediments consist of raised deltas, i n t e r t i d a l and beach deposits and glaciomarine sediments. With the resultant u p l i f t of the land and leaching these clays would have a greater tendency to become very s e n s i t i v e . The Fort Langley glaciomarine clays were deposited closer to the decaying g l a c i a l i c e front and thus tended to have an o r i g i n a l l y lower s a l i n i t y . The farther east i n the Fraser Valley the lower the s a l i n i t y of the depositional environment. 4.2.1 Strong P i t 4.2.1.1. S i t e Description The Strong P i t research s i t e i s located within an abandoned gravel p i t approximately 4 km west of Abbotsford A i r p o r t (see location on Figure 4.1). The s i t e i s on the north side of a va l l e y which i s occupied by Pepin Creek. The v a l l e y was incised by g l a c i a l outwash and contains outwash sand and gravel and i c e contact deposits of the Sumas Formation. The south side of the v a l l e y contains i ce contact deposits, implying that s i t e was ju s t west of the farthest l o c a l advance of Sumas Ice during the Fraser G l a c i a t i o n . Immediately to the north of the s i t e Fort Langley glaciomarine stoney clay and sand i s present at the surface. The close proximity of the Fort Langley clay to the s i t e would suggest that the clay tested at the s i t e belongs to the same formation. The variable depth to clay throughout the s i t e may be due to i r r e g u l a r erosion by the Sumas melt water. 4.2.1.2 Stratiqraphic and R e s i s t i v i t y P r o f i l e The RCPTU p r o f i l e , as shown i n Figure 4.4, can be divided into three zones. The f i r s t 1.5 m consist of f r e e l y draining f FRICTION RATIO SLEEVE FRICTION CONE BEARING PORE PRESSURE RESISTIVITY INTERPRETED Rf(%) (MPo) q (MPo) U(m of voter) p f o h m - m ) PROFILE O 5 0 0.25 0 1 7.5 15 O 150 0 b 125 i g - 4.4 R e s i s t i v i t y Cone Sounding at Strong P i t , A l d ergrove, B.C. 10 U3 sand and gravel which was o r i g i n a l l y 10 m thick at the s i t e . Below the f i l l i s overconsolidated stoney clay. The boundary between the f i l l and the clay i s c l e a r l y marked by the sudden increase i n pore pressure and drop i n r e s i s t i v i t y . There i s a perched water table at t h i s point. The presence of a harder desiccated layer at the surface of the clay (1.5 to 1.8 m) r e s u l t s i n a higher cone bearing and s l i g h t l y lower r e s i s t i v i t y . The lower r e s i s t i v i t y could be due to surface water f i l l i n g of small f i s s u r e s i n the clay. From 1.8 to 6.8 m the r e s i s t i v i t y i s f a i r l y constant at a value of 35 ohm-m. At 6.8 m there i s a d i s t i n c t increase i n the r e s i s t i v i t y of the clay. This would be due to the proximity of the sand layer. The sand layer contains fresher water and thus would tend to leach the adjacent clay to some extent. The l a s t unit i n the p r o f i l e i s a medium dense sand. In the sand the r e s i s t i v i t y i s higher due to the lack of conducting clay minerals but more so due to higher pore water r e s i s t i v i t y . In the sand layer the more denser parts of the layer, higher Qc, also have the highest r e s i s t i v i t i e s . 4.2.2 Langley 4.2.2.1 Si t e Description The Langley s i t e i s located approximately 100 m west of the B.C. R a i l overpass along the Trans Canada Highway i n the municipality of Langley. This area i s part of the Surrey upland. The area i s underlain by Capilano sediments consisting of glaciomarine clay with numerous t h i n interbeds of s i l t and f i n e sand. 42 4.2.2.2 Stratiqraphic and R e s i s t i v i t y P r o f i l e A r e s i s t i v i t y cone p r o f i l e i s presented on figure 4.5. The s i t e stratigraphy consists of clay overlain by 2.3 m of f i l l and desiccated clay. The beginning of the clay layer i s marked by an increase i n pore pressure and drop i n r e s i s t i v i t y as the s o i l becomes saturated. Generally the r e s i s t i v i t y f a l l s between 6 and 9 ohm-m, increasing with depth, with occasional peaks of up to 15 ohm-m. The clay i s normally consolidated with occasional narrow interbeds of f i n e sand. These t h i n layers are noted by increased cone bearing and low pore pressures. These sand layers also produce pronounced increases i n the r e s i s t i v i t y . The sand layers noted by bearing and pore pressure do not account for a l l the r e s i s t i v i t y highs, such as at 12.5 m. These high values are most l i k e l y due to lower pore water s a l i n i t i e s i n the clay or could be sand layers that are too narrow f o r the cone bearing resistance to respond to. 4.2.3 Colebrook 4.2.3.1 Si t e Description The Colebrook s i t e i s located immediately west of the south abutment of the Highway 99 railway overpass. The area i s located within the Nicomekl-Serpentine Valley at an elevation of several metres above sea-level. The v a l l e y has been subjected to marine sedimentation throughout most of the Quaternary Period (Armstrong, 1984) with the Capilano Formation being the most recent. The Capilano sediments i n t h i s area consist of glaciomarine clay overlying p r o g l a c i a l d e l t a i c sand and gravel. 4.2.3.2 Stratigraphic and R e s i s t i v i t y P r o f i l e A r e s i s t i v i t y cone p r o f i l e from the s i t e (Fig. 4.6) shows that the stratigraphy consists mainly of marine clay overlying dense sand. At the surface there i s 0.5 m of t o p s o i l . In the clay the r e s i s t i v i t y i s f a i r l y constant, perhaps with a small tendency toward decreasing, between 4.5 and 25 m. The r e s i s t i v i t y from 4.5 to 14 m has some minor v a r i a b i l i t y , while from 14 to 25 m the p r o f i l e i s very uniform. From the water table to a depth of 4.5 m the r e s i s t i v i t y i s much lower suggesting either a change i n l i t h o l o g y or a change i n pore f l u i d r e s i s t i v i t y . This layer of lower r e s i s t i v i t y may be due to deposition of dissolved s o l i d s since the surface i s a zone of groundwater discharge. The sand layer below the clay has artesian pore pressures therefore there would be some component of upward flow through the clay layer. The upper clay could also have a greater organic content. Organic s o i l s tend to have a very high CEC, and higher CEC s o i l s are more conductive. Leaching of the clay by t h i s process would explain the r e l a t i v e l y high r e s i s t i v i t y for t h i s s o i l , considering the o r i g i n a l s a l i n e depositional environment. A lowered pore water s a l i n i t y may cause the clay to be very s e n s i t i v e . DEPTH (meters) 1 • o o O 2 o m h o x w z o P 2 C - o > s ft ~ i I I I i I 1 5 o cv r- Z) -o I- 30 m m • • • • • • * i i i i 5. INTERPRETATION AND DISCUSSION OF RESULTS A number of observations may be made on the basis of the r e s i s t i v i t y cone p r o f i l e s c o l l e c t e d from the research s i t e s . Questions regarding; (1) The r e p e a t a b i l i t y of the RCPTU, (2) The p r o f i l i n g c a p a b i l i t y of the instrument, (3) The e f f e c t of changing s o i l l i t h o l o g y on r e s i s t i v i t y , (4) The determination of pore f l u i d r e s i s t i v i t y from bulk r e s i s t i v i t y , and (5) The e f f e c t of electrode spacing on the measured r e s i s t i v i t y , w i l l be examined i n t h i s chapter. 5.1 Repeatability of Results With the development of new instrumentation i t i s important to v e r i f y the v a l i d i t y of the r e s u l t s by checking the r e p e a t a b i l i t y of the measurements. Repeatability provides confidence i n the r e s u l t s , enabling the comparison of reasonably small changes i n groundwater q u a l i t y from d i f f e r e n t t e s t locations. Two t e s t s , one metre apart, were conducted at McDonald Farm eleven days apart. The r e s i s t i v i t i e s measured from the two holes by both the inner and outer electrodes are superimposed on Figure 5.1a and 5.1b. These p r o f i l e s indicate good r e p e a t a b i l i t y for both the inner and outer electrodes. The p r o f i l e s shown i n Figure 5.1 only deviate i n the upper clayey F i g . 5.1 A Comparison of Two Adjacent Resistivity Logs Made 11 Days Apart for (a) The Inner Electrodes, and (b) The Outer Electrodes. s i l t , t h i s i s due to water table flucuations from r a i n f a l l at the s i t e between the time the two soundings were made. The groundwater table i s closer to the surface for RES 89-10. There i s also a sharp peak for RES 89-10, where the groundwater table f o r RES 89-7 was. Due to the highly variable nature of the sand at the s i t e the two p r o f i l e s do not completely match up but the same peaks and troughs are evident from both t e s t s . 5.2 P r o f i l i n g Capability The r e s i s t i v i t y cone i s id e a l for r a p i d l y determining cross sec t i o n a l p r o f i l e s of groundwater q u a l i t y . T y p i c a l l y t h i s would be done to delineate the boundaries of a contaminant plume. While there i s no contaminant plume as such at McDonald Farm the s i t e does provide a si m i l a r application - a s a l t water to fresh water in t e r f a c e . The interface between s a l t water and fresh water i s not d i s t i n c t . There i s a zone of hydrodynamic dispersion (Bear and V e r r u i j t , 1988) which r e s u l t s i n a more gradual t r a n s i t i o n between fresh and brackish water. The s a l t water, due to i t s higher density w i l l tend to migrate below the fresh water. The s a l t water to fresh water interface at McDonald Farm i s further complicated by seasonal fluctuations i n r i v e r s a l i n i t y . During periods of low flow i n the winter the r i v e r water at McDonald Farm becomes very brackish ( =0.64 ohm-m i n early February) with a s a l i n i t y i n order of that of sea water ( =0.2 ohm-m, sea water average, Tel f o r d et a l , 1976). When the r i v e r RESISTIVITY (ohm—m) 0 10 20 30 i i i i I i i i i I i i i i i RESISTIVITY (ohm-m) 0 10 20 30 i i i i » i i i t I i i i i » RESISTIVITY (ohm—m) 0 10 20 30 I I I 1 I I I I I I I I I i I 10m from River 60m from River 110m from River F i g . 5.2 Stratigraphic and Resistivity Profile of McDonald Farm Site. oo flow i s increased due to either prolonged r a i n f a l l i n the winter or snow melt i n the spring and summer the s a l i n i t y drops considerably (^  = 14.8 ohm-m measured on May 9, 1989). This v a r i a b i l i t y i n s a l i n i t y would be ra p i d l y r e f l e c t e d i n the groundwater adjacent to the r i v e r . Farther away from the r i v e r the s a l i n i t y of the groundwater would not have that much seasonal v a r i a b i l i t y . Figure 5.2 i l l u s t r a t e s a cross section through three RCPTU soundings, each separated by 50 m, i n a l i n e perpendicular to the r i v e r bank. The figure shows the r e s i s t i v i t y f o r each t e s t along with a 6 ohm-m contour and the stratigraphy. The location of the holes (RES 89-7,8,9) i s shown on the s i t e map (Fig. 4.2). As i l l u s t r a t e d by the cross section there i s a decrease i n the r e s i s t i v i t y as the r i v e r i s approached, as would be expected for the case of s a l t water in t r u s i o n . Below a depth of approximately 11 m there i s very l i t t l e d ifference between the r e s i s t i v i t i e s from the three t e s t s . This would be the case i f there was very l i t t l e groundwater movement below t h i s depth. The r e s i s t i v i t y of the s i l t y clay does not vary at a l l across the whole s i t e since i t was deposited i n a marine environment and there has been no subsequent groundwater flow through the clay. Near the surface, there tends to be an increase i n the r e s i s t i v i t y with depth of the overbank s i l t below the water table. This may be due to decreasing amount of conductive clay minerals with depth i n the overbank deposit. 5.3 E f f e c t of S o i l Litholocrv 5.3.1 General Aspects As discussed i n the section on conduction i n soil-pore f l u i d systems a number of d i f f e r e n t factors a f f e c t to bulk r e s i s t i v i t y of the s o i l . By f a r the most important factor i s the r e s i s t i v i t y of the pore f l u i d . At low pore water r e s i s t i v i t i e s the a f f e c t of surface conduction i s i n s i g n i f i c a n t i n comparison to e l e c t r o l y t i c conduction i n the pore f l u i d . The s i t u a t i o n i s analogous to the t o t a l resistance measured by two r e s i s t o r s i n p a r a l l e l , where the r e s i s t o r s represent surface conduction and pore water conduction. For example, at McDonald Farm (hole RES 89-7), the clayey s i l t layer at 11.8 m does not provide an appreciable r e s i s t i v i t y contrast with the sand bounding t h i s layer. This i s because a much greater proportion of conduction i n both s o i l s takes place through the pore water where the s o i l r e s i s t i v i t y was only about 4 ohm-m. When the pore water r e s i s t i v i t y i s higher the e f f e c t s of surface conduction become more apparent. This was p a r t i c u l a r l y true at the Strong P i t s i t e and the Colebrook s i t e where the clays have bulk r e s i s t i v i t i e s of 35 and 25 ohm-m respectively, and the sands have values of 110 and 70 ohm-m. In clay minerals surface conduction i s related to the CEC capacity of the s o i l . S o i l s with high CEC are clays, the more active clays w i l l have a greater CEC, and organic s o i l s (Olhoeft, 1985). The r e s i s t i v i t y measurement i s also i n d i c a t i v e of ground water flow regimes. At the Colebrook s i t e recharge from ground water with a lower amount of dissolved s o l i d s than i n the clay layer r e s u l t s i n a greater r e s i s t i v i t y . This was ju s t the opposite at McDonald Farm where brackish pore water gives very low r e s i s t i v i t i e s r e s u l t i n g i n almost no difference between sand and clay r e s i s t i v i t y . 5.3.2 Cone Parameter Relations to S o i l R e s i s t i v i t y With the simultaneous measurement of cone bearing, f r i c t i o n , and pore pressure, comparisons may be made with the r e s i s t i v i t y to see how changes i n mechanical s o i l properties a f f e c t the r e s i s t i v i t y . 5.3.2.1 F r i c t i o n Ratio Relationship The f r i c t i o n r a t i o (R^), sleeve f r i c t i o n stress divided by cone bearing stress expressed as a percentage, w i l l increase with increasing K Q and increasing fines content. In sand deposits where K Q i s constant R f may be used to note increases i n f i n e s content. The presence of fines i n a sandy s o i l may a f f e c t the r e s i s t i v i t y i n two ways: (1) increased fines content w i l l decrease porosity since the fine s w i l l occupy void space between the sand grains. Decreasing the porosity has the e f f e c t of increasing the r e s i s t i v i t y ; and (2) the presence of fine s may indicate the presence of conducting RESISTIVITY ( o h m - m ) 0 5 10 15 20 25 30 ^ I i i i i i i i i i i i i i i i j . i j j i i i i i i i i 6-i 8 CL Q 10 RESISTIVITY (ohm-m) FRICTION RATIO 1 2 | I 1 i | l I i " | i I i | l I I | I I I 0.0 0.2 0.4 0.6 0.8 1.0 FRICTION RATIO RESISTIVITY (ohm-m) 0 5 10 15 20 25 30 4 | i i i i i i i i i i i i i j j_ >_ i i i i iII i i i i i i i 6-I 8 U J Q 10-12 RESISTIVITY (ohm-m) FRICTION RATIO « i r i | i i i | i i i | i i i | i i i 0.0 0.2 0.4 0.6 0.8 1.0 FRICTION RATIO F i g . 5.3 A Comparison of the Resistivity (Outer Electrodes) and the Friction Ratio for Two Soundings from the McDonald Farm Site. ui clay minerals, which would r e s u l t i n a decrease i n r e s i s t i v i t y . Figure 5.3 ( McDonald Farm, RES 89-5,7) i l l u s t r a t e s that increased f r i c t i o n r a t i o i n sandy s o i l s w i l l tend to decrease the r e s i s t i v i t y . Generally the peaks i n the r e s i s t i v i t y match the troughs i n the f r i c t i o n r a t i o . This would suggest that, at le a s t at t h i s s i t e , that fines i n the sand w i l l decrease the r e s i s t i v i t y . As the r e s i s t i v i t y decreases t h i s e f f e c t becomes less pronounced, t h i s being due to the domination of pore f l u i d conduction over other conduction mechanisms at low r e s i s t i v i t i e s . The r e l a t i o n s h i p between f r i c t i o n r a t i o and r e s i s t i v i t y was noted by ERTEC (1987). They normalized t h e i r conductivity data as a function of f r i c t i o n r a t i o to remove the e f f e c t s of l i t h o l o g i c a l change from t h e i r data. However, the complex nature of s o i l r e s i s t i v i t y make the a p p l i c a t i o n of such corrections uncertain. 5.3.2.2 Cone Bearing Relationship Cone bearing i n sands has been shown to be re l a t e d to horizontal e f f e c t i v e stress, compressibility, and r e l a t i v e density (Robertson and Campanella, 1988). Relations between r e l a t i v e density and cone bearing normalized by horizontal e f f e c t i v e stress have been proposed so i t should be reasonable to expect there i s a s i m i l a r c o r r e l a t i o n between formation factor and normalized cone bearing, since the formation factor i s r e l a t e d to s o i l porosity by Archie's Formula. In low r e s i s t i v i t y s o i l s t h i s r e l a t i o n s h i p has been shown to be quite accurate (Jackson, 1978). Figure 5.4 r e l a t e s apparent formation CONE BEARING / HORIZONTAL EFFECTIVE STRESS (Ko=0.55) Fig. 5.4 Observed Relationship Between Apparent Formation Factor and Cone Bearing Normalized with Respect to Horizontal Effective Stress. factor to the normalized cone bearing. While the r e l a t i o n s h i p i s not that strong i t s t i l l i l l u s t r a t e s that increased cone bearing, or decreased r e l a t i v e density w i l l increase the formation factor. 5.3.2.3 Pore Pressure Relationship No d i r e c t r e l a t i o n s h i p between any pore pressure parameters and r e s i s t i v i t y has been made. In general high pore pressures are i n d i c a t i v e of a high fines content, thus a lower r e s i s t i v i t y . In normally consolidated clayey s o i l s , d i l a t i v e behavior, which would be i n d i c a t i v e of sand layers, may also be r e f l e c t e d by changes i n the r e s i s t i v i t y . 5.4 Determination of Pore F l u i d R e s i s t i v i t y From Figure 5.5 i t can be seen that the pore f l u i d r e s i s t i v i t y determined from BAT samples and the bulk r e s i s t i v i t y measured from the cone r e l a t e quite well, with a formation factor between three and four. This c o r r e l a t i o n i s expected on the basis of Archie's Formula. Given t h i s , i t should be possible to make reasonable estimates of pore f l u i d r e s i s t i v i t i e s from the r e s i s t i v i t y cone. The two quantities are rel a t e d by the formation factor. Therefore, the pore f l u i d r e s i s t i v i t y may be estimated by determining the formation factor by either: (1) s i t e s p e c i f i c c o rrelations between s o i l and pore f l u i d r e s i s t i v i t y ; or (2) Archie's Formula assuming that an accurate estimate of the s o i l porosity i s known. Fig. 5.5 A Comparison Between the Resistivity of Pore Fluid Samples and the Resistivity Measured by the RCPTU. The estimation of pore f l u i d r e s i s t i v i t y i n clay i s more d i f f i c u l t due to the ef f e c t s of surface conduction. In clay the apparent formation factor i s very dependent on the pore f l u i d r e s i s t i v i t y , clay mineral content and type. From Colebrook the apparent formation factor was found to be 1.43 at a depth of 10.6 m on the basis of a water sample obtained from a BAT probe. This i s considerably lower than the range of 3 to 4 that was noted i n the sand at McDonald Farm. The s i l t y clay at McDonald Farm has an apparent formation factor of 4 to 5. According to Archie's Formula the formation factor for a clay with a void r a t i o of 1 should be 4, assuming m=2. So that at the McDonald Farm s i t e Archie's Formula was applicable since the pore f l u i d r e s i s t i v i t y was very low. For n=0.6,m=2, F - i n t r i n s i c = 2.8 for Colebrook as compared to the F-apparent = 1.43. This difference i s due to clay mineral surface conduction. 5.5 Influence of Electrode Spacing on Measured R e s i s t i v i t y The UBC r e s i s t i v i t y cone has been equipped to make simultaneous measurement of r e s i s t i v i t y from the inner and outer electrodes. This section deals with the comparison of the re s u l t s of the inner and outer electrodes and suggests why there should be differences between the two measurements i n d i f f e r e n t s o i l types. When the electrodes are i n a homogeneous - i s o t r o p i c medium the electrodes should respond i n a s i m i l a r manner to that of the case of water immersion. However, s o i l i s r a r e l y homogeneous and i s o t r o p i c , so that the response of the electrodes w i l l be dependent on the state of the s o i l and the changes to the s o i l caused by penetration. When the electrodes enter a layer of contrasting r e s i s t i v i t y the f u l l response due to the change i n r e s i s t i v i t y w i l l not be noted u n t i l the probe has f u l l y penetrated the layer. Comparisons have been made between the magnitude of the r e s u l t s of the two electrode p a i r s and t h e i r responsiveness to narrow layers. The following observations were made at the t e s t s i t e s and are presented i n Figures 5.6 to 5.9. In these figures the r e s i s t i v i t i e s determined from the inner and outer electrodes are superimposed and compared to the s o i l type, since i t i s the s o i l type that appears to cause the differences i n the r e s u l t s . On the basis of these observations some suggestions are presented as to why there should be a difference between the two r e s i s t i v i t y measurements. 1. Clean sands at McDonald Farm (Fig. 5.6): The r e s i s t i v i t y measured by the outer electrodes i s greater i n the medium dense sand. In s i l t y sands there appears to be very l i t t l e difference at McDonald Farm. 2. Langley (Fig. 5.7): The inner and outer electrodes give almost exactly the same r e s u l t i n clay with narrow sand interbeds. 59 RESISTIVITY (oh m—m) 10 20 SOIL TYPE - 30 INNER ELECTRODES OUTER ELECTRODES F i g . 5.6 A comparison between the r e s i s t i v i t y measured by the inner and outer electrodes at McDonald Farm. 60 Fig. 5.7 A comparison between the r e s i s t i v i t y measured by the inner and outer electrodes at Langley. 61 RESISTIVITY (ohm-m) SOIL TYPE 0 50 100 FILL; PEAT & ORGANIC SILT CLAY SAND •- INNER ELECTRODES - OUTER ELECTRODES F i g . 5.8 A comparison between the r e s i s t i v i t y measured by the inner and outer electrodes at Colebrook. 62 RESISTIVITY ( o h m - m ) SOIL TYPE 25 50 75 100 125 SANDY GRAVEL O.C. SILTY CLAY SAND F i g . 5.9 INNER ELECTRODES OUTER ELECTRODES A comparison between the r e s i s t i v i t y measured by the inner and outer electrodes at Strong P i t . 3. Colebrook (Fig. 5.8): The r e s u l t s of the two electrodes are the same i n clay. However the inner electrodes give s u b s t a n t i a l l y greater r e s i s t i v i t y i n the loose sand layer. 4. Strong P i t (Fig. 5.9): The r e s u l t s are the same i n clay and the sand layer with the exception of the l a s t 40 cm, where the sand i s looser, the inner electrodes give a higher r e s i s t i v i t y . In dense sands there i s a narrow zone of d i l a t i o n adjacent to the cone. For loose sands there i s an increase i n density adjacent to the cone. If there i s d e n s i f i c a t i o n i n the sand adjacent to the cone the r e s i s t i v i t y measured by the inner electrodes would be greater. For the case of a dense sand the inner electrodes would measure a lower r e s i s t i v i t y . In any case the larger the spacing between the electrodes the greater the penetration of the e l e c t r i c f i e l d into the s o i l and the more representative the r e s i s t i v i t y measurements should be of the undisturbed s o i l . I t has been observed that i n s i l t and clay, both NC and OC, that the inner and outer electrodes give consistent r e s i s t i v i t y r e s u l t s . Adjacent to the cone, when pushing through clay, there i s a zone of remolding. This remolding of the clay i s at constant volume and hence constant water content. Since the water content of the s o i l i s constant the r e s i s t i v i t y should not change. This i s i n f a c t the behavior noted i n a l l the s i l t y clays tested at the four s i t e s . By measuring r e s i s t i v i t y at a number of d i f f e r e n t spacings one can use the r e s i s t i v i t y measurements to note the a f f e c t of sand disturbance by the cone. Figure 5.5 suggests, for the inner electrodes, that disturbance has caused the formation factor to be more uniform. In general, for clays, c l o s e l y spaced electrodes w i l l give true values of undisturbed r e s i s t i v i t y . In sands the value of the r e s i s t i v i t y i s affected by s o i l disturbance from penetration. This e f f e c t decreases with increased electrode spacing because more of the current t r a v e l s through undisturbed s o i l further from the cone. Therefore, f o r contaminant studies i t i s preferable to have close electrode spacing. This i s because the narrow spacing w i l l indicate the presence of narrow layers and the influence of sand density i s reduced, making i t easier to estimate pore f l u i d conductivity on the basis of measured s o i l r e s i s t i v i t i e s . 65 6. APPLICATIONS OF THE RESISTIVITY CONE The purpose of t h i s research was the development of a r e s i s t i v i t y cone for contaminant detection. Due to d i f f i c u l t i e s i s accessing such s i t e s no t e s t i n g at contaminant s i t e s was done. The following section outlines f o r what types of contaminants the r e s i s t i v i t y method would be applicable and describes how the RCPTU should be deployed i n a s i t e i n v e s t i g a t i o n . The r e s i s t i v i t y cone can be used for other applications which are described at the end of t h i s chapter. 6.1 A p p l i c a b i l i t y of R e s i s t i v i t y for Contaminant Detection There are two general groups of contaminants; 1) Aqueous phase l i q u i d s (APLs); and 2) Non-aqueous phase l i q u i d s (NAPLs). Aqueous phase l i q u i d s consist of both conducting (ionic) and soluble organic (insulating) contaminants. The easiest contaminants to detect by the r e s i s t i v i t y method are conducting aqueous phase l i q u i d s . Sources of such contamination may be l a n d f i l l s ( i n d u s t r i a l , sanitary, f l y ash) or mine t a i l i n g s . P y r i t e produces acid water when oxidized, 3+ + releasing Fe and H . The best conductors are solutions of most inorganic s a l t s , acids, and bases. Contamination may also be due to s a l t water i n f i l t r a t i o n of aquifers. Non-ionic aqueous phase l i q u i d s w i l l not be detected by the r e s i s t i v i t y method. 66 Non-aqueous phase l i q u i d s (NAPLs) are i n s u l a t i n g organic contaminants. The presence of NAPLs decreases the bulk r e s i s t i v i t y by blocking pathways of conduction through the pore space of the s o i l . NAPLs can be either c l a s s i f i e d as DNAPLs, contaminants that are denser than water, or f l o a t i n g NAPLs, that have a density less than that of water. DNAPLs w i l l sink under a gravity gradient to some low permeability layer or w i l l remain i n r e s i d u a l saturation. Light NAPLs w i l l f l o a t on the surface of the water table and may be d i f f i c u l t to detect since the r e s i s t i v i t y above the water table would be very high regardless of the presence of contamination. However, f i e l d work at an appropriate t e s t s i t e would be necessary to evaluate the effectiveness of the RCPTU at detecting such contaminants. By assuming NAPLs are insulators Archie's Formula (Equation 2.4) can be used to approximately estimate the amount of contamination by assuming the r e s i s t i v i t y of the contaminant i s equivalent to that of a i r . For the r e s i s t i v i t y method to note a s u f f i c i e n t change there would need to be at le a s t 5% NAPL saturation. However, i n most cases conductive contaminants w i l l be the target of a s i t e investigation. This i s because most i n d u s t r i a l waste w i l l have a large component of dissolved s o l i d s along with possible NAPLs. Very small amounts of some organic chemicals are very hazardous. I f such contaminants are the only source of contamination the r e s i s t i v i t y cone would not be able to detect Table 6.1 Summary of Typical Resistivity Measurements of Fluids and Bulk Soil-Fluid Mixtures p f,ohm-m (fluid) ,ohm-m (bulk soil) Seawater 0.2 -Drinking Water >15 -McDonald Farm Clay Colebrook Site Clay 401 @ 232 Ave., Railway Site Clay B.C. Highway Strong Pit Clay 0.3 18.2 1.5 25 8 35 McDonald Farm Sand Colebrook Site Sand Strong Pit Sand 1.5-6 5-20 70 115 Typical Landifll Leachate 0.5-10 100% Ethylene Dichloride (ED) 20400 50% ED/50% 150 ohm-m fluid in Wedron 7020 sand 696 30% ED/70% 150 ohm-m fluid in Wedron 7020 sand 335 17% ED/83% 150 ohm-m fluid in Wedron 7020 sand 273 them. The r e s i s t i v i t y cone i s used for determining contrasts i n r e s i s t i v i t y . I f the natural groundwater i s very brackish, such as i n a marine delta, i t may be d i f f i c u l t to detect conductive contaminants. On the other hand i f the natural groundwater i s highly conductive i t becomes easier to detect i n s u l a t i n g NAPLs. Table 6.1 provides a convenient summary of r e s i s t i v i t i e s of a number of s o i l types, s o i l contaminant mixtures, and f l u i d s . Having a knowledge of the s o i l type and contamination at a s i t e one can use t h i s information to help decide i f the r e s i s t i v i t y method i s appropriate. 6.2 Use of the RCPTU i n Contamination Problems By i t s e l f the RCPTU i s a valuable t o o l i n contamination s i t e i nvestigations. Besides providing information on the extent of groundwater contamination the cone may also be used for determining: hydrogeological properties, s t r a t i g r a p h i c p r o f i l e s , and geotechnical properties. The following i s a suggested outline for an RCPTU investigation. The CPTU can quickly determine steady state pore pressures i n drained s o i l s . Given that the CPTU provides accurate measures of pore pressure, soundings should be p e r i o d i c a l l y stopped to allow an accurate determination of the equilibrium pore pressure. In sandy s o i l s equilibrium w i l l be reached almost instantaneously thus allowing many such measurements to be made. Dissipa t i o n of excess pore pressure i n non-plastic s i l t y s o i l s i s f a i r l y rapid. In p l a s t i c clays i t would not be p r a c t i c a l to determine the equilibrium pore pressure due to the low permeability of such s o i l s . By repeating t h i s procedure i n a number of holes at a s i t e the d i r e c t i o n and magnitude of the hydraulic gradient may be determined. The hydraulic conductivity can be determined for f i n e grained s o i l s by pore pressure d i s s i p a t i o n s . D e t a i l s on t h i s procedure may be found i n Robertson and Campanella (1988). This procedure works well for normally consolidated s i l t y clays and clays. In the case that such s o i l s are encountered i t would be worthwhile doing a d i s s i p a t i o n t e s t since i t i s a cost e f f e c t i v e method of determining hydraulic conductivity. For drained s o i l s an i n i t i a l estimate of hydraulic conductivity can be made on the basis of s o i l type c l a s s i f i c a t i o n determined from the CPT parameters. Porosity may also be estimated from s o i l type c l a s s i f i c a t i o n s and r e l a t i v e density estimates. A l t e r n a t i v e l y the porosity may also be determined by using a mixing law. In the case of low r e s i s t i v i t y sand Archie's Formula may be used to determine porosity providing the r e s i s t i v i t y of the pore f l u i d i s also known. A rapid assessment of the three dimensional extent of groundwater contamination can be made a f t e r a serie s of cone t e s t s . Sections showing stratigraphy and r e s i s t i v i t y should be, prepared. By comparing the r e s i s t i v i t y at. c e r t a i n points to contaminant concentrations determined from d i r e c t water sampling the r e s i s t i v i t y values may be used semi^quantitatively to give an i n d i c a t i o n of contaminant concentrations at other points. The highly accurate s t r a t i g r a p h i c p r o f i l e that the CPT provides allows f o r the i d e n t i f i c a t i o n of s o i l layers with high hydraulic c o n d u c t i v i t i e s that may be pathways for contaminants. Also s o i l s with low hydraulic conductivities which may act as aquitards are also i d e n t i f i e d . At the same time the information obtained from the CPT can also be used to determine geotechnical parameters. On the basis of an i n i t i a l i n v e s t i g a t i o n with the RCPTU planning f o r other forms of investigation, such as piezometers and sampling wells, can be made. The RCPTU can also be used for period i c monitoring of a s i t e to note the advance of any contaminants. I f baseline r e s i s t i v i t y t e sts are done changes i n the groundwater chemistry can be e a s i l y noted. 6.3 Other Possible Applications of the RCPTU While the r e s i s t i v i t y cone was designed with the the purpose of contaminant detection i t may be applied to other areas. Three applications are b r i e f l y outlined i n the following section. 6.3.1 Corrosion Assessment S o i l s with low r e s i s t i v i t i e s have high concentrations of dissolved s o l i d s which would accelerate corrosion. A standard adopted i n the United Kingdom (Geological Society Engineering Group Working Party, 1988) defines severely corrosive s o i l as having a r e s i s t i v i t y of less than 10 ohm-m, and moderately corrosive s o i l s having a r e s i s t i v i t y i n the range of 10 to 100 ohm-m. S o i l s with a r e s i s t i v i t y of greater than 100 ohm-m are of l i t t l e concern with respect to corrosion. 6.3.2 Water Quality Assessment As noted e a r l i e r there i s a strong l i n e a r r e l a t i o n s h i p between TDS and f l u i d conductivity. By using a mixing law the pore water r e s i s t i v i t y may be estimated from the d i r e c t measurement of the bulk r e s i s t i v i t y . For sands, by using a formation factor of 3.5, an i n i t i a l estimate of the pore f l u i d r e s i s t i v i t y can be made. The l i m i t of t o t a l dissolved s o l i d s for potable water i s 500 mg/1 (water r e s i s t i v i t y aprx = 12 ohm-m) hence for most sand aquifers the minimum bulk r e s i s t i v i t y should be i n the range of 35 to 50 ohm-m. 6.3.3 S o i l C l a s s i f i c a t i o n The r e s i s t i v i t y cone can be used as a t o o l f or s o i l c l a s s i f i c a t i o n where s o i l horizons have d i f f e r e n t s a l i n i t i e s or water contents. The high water content of peats may make r e s i s t i v i t y measurements an id e a l method of d i f f e r e n t i a t i n g such s o i l from other s o i l s with lower water contents. R e s i s t i v i t y measurements are ide a l for detecting the presence of frozen ground. There can be no mobility of e l e c t r o l y t e s i n i c e , therefore i t acts as an insulator and ice content could perhaps be i n f e r r e d by using Archie's Formula. R e s i s t i v i t y measurements would also be sen s i t i v e to the presence of gas i n s o i l . 72 7. CONCLUSIONS AND RECOMMENDATIONS I t was found, through extensive f i e l d t e s t s , that the i n i t i a l design of the UBC RCPTU was successful i n r a p i d l y and accurately determining r e s i s t i v i t y . The r e p e a t a b i l i t y of the measurements and the favorable comparisons to d i r e c t groundwater sampling proved the v a l i d i t y of the r e s u l t s . While i n t h i s research no actual contaminated s i t e s were tested the t e s t i n g done at the McDonald Farm s i t e served well to show the a b i l i t y of the RCPTU to make a de t a i l e d p r o f i l e of the bulk r e s i s t i v i t y , which i s representative of changes i n the amount of t o t a l dissolved s o l i d s i n the groundwater. I t was noted that changes i n s o i l porosity and fine s content could influence the bulk r e s i s t i v i t y . The four electrode module with simultaneous measurements of r e s i s t i v i t y from the outer and inner electrodes showed that penetration causes l o c a l i z e d changes i n s o i l density which i n turn influences the measured bulk r e s i s t i v i t y of sandy s o i l s . Due to the e l e c t r i c a l l y i n s u l a t i v e properties of organic contaminants they should also be detectable by the r e s i s t i v i t y method. Besides being a t o o l for contaminant detection the RCPTU may also be used for corrosion assessment, drinking water q u a l i t y assessment, and as an aid i n s t r a t i g r a p h i c logging. On the basis of the r e s u l t s presented i t appears that the speed, economy, and r e l i a b i l i t y of the RCPTU make i t i d e a l for many contaminant investigations i n unconsolidated s o i l . Lab and f i e l d t e s t i n g with simultaneous measurement of r e s i s t i v i t y across two sets of electrodes showed that a two electrode probe operating at a frequency of no less than 1000 Hz i s adequate. For contaminant applications close spacing between the electrodes i s desirable as i t gives good re s o l u t i o n of t h i n layers and tends to reduce the influence of s o i l porosity i n measurements of sand r e s i s t i v i t y . I t i s recommended that further f i e l d work at d i f f e r e n t s i t e s with various s o i l conditions coupled with a more comprehensive water sampling program be undertaken. Testing i n d i f f e r e n t s o i l types w i l l give an i n d i c a t i o n of what background r e s i s t i v i t i e s could be expected when t e s t i n g at a contaminant s i t e . Ideally, t e s t i n g should take place at a well documented s i t e of organic contamination to prove that the r e s i s t i v i t y method i s vi a b l e way to detect the presence of contaminants. Further research w i l l l e g i t i m i z e the RCPTU as a p r a c t i c a l t o o l i n contaminant investigations. Further r e s i s t i v i t y f i e l d t e s t i n g coupled with s o i l sampling would allow for comparisons between changes i n r e s i s t i v i t y and changes i n s o i l water content, percentage of clay, and clay type. Numerical modeling could be used to determine the distance the e l e c t r i c a l f i e l d penetrates into the s o i l . Modeling could be useful i n i l l u s t r a t i n g the influence of s o i l disturbance and of electrode spacing on observed r e s i s t i v i t y measurements. Research into the subject of the corrosion of buried structures would be appropriate since the measurement of bulk r e s i s t i v i t y i s one way of assessing s o i l corrosion p o t e n t i a l . Research into the v i a b i l i t y into making i n - s i t u d i e l e c t r i c p e r m i t t i v i t y measurements could be examined. D i e l e c t r i c measurements have application i n both contaminant detection and the determination of s o i l water content. This former app l i c a t i o n would be a p r a c t i c a l approach i n determining i n - s i t u void r a t i o of cohesionless s o i l s . REFERENCES Archie, G.E. 1942. The e l e c t r i c a l r e s i s t i v i t y log as an aid i n determining some reservoir c h a r a c t e r i s t i c s , Transactions American I n s t i t u t e of Mineral Metallurgy Engineering 146, pp. 54-62. Armstrong, J.E. 1984. Environmental and Engineering Applications of the S u r f i c i a l Geology of the Fraser Lowland, B r i t i s h Columbia. 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