INVESTIGATION OF BRIDGE FOUNDATION SITES IN KARST TERRANE VIA MULTI-ELECTRODE ELECTRICAL RESISTIVITY

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1 INVESTIGATION OF BRIDGE FOUNDATION SITES IN KARST TERRANE VIA MULTI-ELECTRODE ELECTRICAL RESISTIVITY Dennis R. Hiltunen 1 and Mary J. S. Roth 2 1 Associate Professor of Civil Engineering, The Pennsylvania State University, University Park, Pennsylvania, USA; 2 Associate Professor of Civil Engineering, Lafayette College, Easton, Pennsylvania, USA; Abstract Resistivity data was collected on several bridge foundation sites in Pennsylvania, in close proximity to geotechnical boring locations. Earth resistivity tomograms determined from multi-electrode measurements were plotted against drilling data and notable similarities were discovered. Resistivity profiles were found to match layering structure of drilling data in many cases, and resistivity values were indicative of materials observed in boring logs. In addition, resistivities could adequately predict transitions in material and top of rock profile. It appears that multi-electrode electrical resistivity is a useful tool for developing basic layer structure in karst terrane. Introduction Personnel of the Pennsylvania Department of Transportation (PennDOT) have described current site investigation practices for bridge foundations as inadequate, particularly in karst terrane where rock conditions are often highly variable. Bridges in Pennsylvania are largely founded on piles or other deep foundations. Highly variable subsurface conditions result in great uncertainty in deep foundation design and construction. PennDOT would greatly benefit from a better definition of subsurface conditions, including top of rock profile and quality of rock. Typical PennDOT practice for bridges is to conduct two to three geotechnical borings per substructure unit in order to select and design a foundation. Both shallow and deep foundations are employed, and the decision is based primarily on the depth and quality of bedrock encountered in the borings. Shallow foundations are typically used where bedrock is shallow and of sufficient quality, while deep foundations are used for the remainder (and majority) of cases. For deep foundations, steel H-piles driven to refusal are typical. The design load capacity of these end-bearing piles is typically based upon an allowable stress in steel (6 to 9 ksi). Due to uncertainty in subsurface conditions, experience with this process has led PennDOT to also conduct exploratory drilling at selected sites as part of the construction contract. Based upon judgments gathered from the original design borings, these exploratory borings are conducted on a small grid pattern over the area of a selected substructure unit, and they employ either traditional geotechnical boring equipment or air rotary drilling techniques. The intent is to reveal a more detailed subsurface model for the location prior to foundation construction, and the information is often able to provide this more detailed model. Because of significant lateral variability in karst terrane, this detailed model can be significantly different from that revealed by the two or three borings conducted for the original design. In this case, significant foundation design changes may be required after the construction contract has been awarded and begun, leading to significant overall cost increases for PennDOT. Also, because contractors are aware of the large uncertainty, contingencies can also be incorporated within the original agreement. It would appear that a more detailed subsurface model would be beneficial at the design stage. In addition, it would be desirable that these improved models not require the expense of exploratory geotechnical borings conducted on a small grid pattern. Vision Engineering geophysics is a promising solution to this problem. Engineering geophysics uses methods adapted from seismological and petroleum industries for characterization of shallow subsurface ground conditions, and enables seeing between boreholes or from instruments along the ground

2 surface. Methods are analogous to medical techniques such as an X-ray or MRI. The geophysical method explored in this study is multi-electrode earth resistivity, a nondestructive, in situ test procedure used to determine the subsurface s electrical resistivity profile. Electrical resistivity is an important physical parameter that can be used as a tool to characterize material changes with depth. The vision of the research is to develop a characterization technique that will establish a credible subsurface model for a site for the design stage of a project. The model should be of sufficient detail such that significant and costly design changes during the construction contract are minimized, and contingencies for uncertain subsurface conditions are reduced. The methodology should be efficient with respect to both money and time, and capable of implementation in a wide variety of terrane and other surface and site conditions. The hypothesis is that images of electrical resistivity produced from geophysical measurements at a site can be used in conjunction with traditional geotechnical borings to establish a credible model. The purpose of this paper is to document a study where multi-electrode earth resistivity was employed to characterize the subsurface in mantled karst terrane. Resistivity data was collected on several bridge foundation sites in Pennsylvania, in close proximity to geotechnical boring locations. Resistivity profiles determined from multi-electrode measurements were plotted against drilling data and notable similarities were discovered. Electrical Resistivity The electrical resistivity method has been used in site characterization for about a century. Most earth materials are either good insulators or dielectrics, i.e., they do not conduct electricity very well. Rather, electricity is conducted through the subsurface via interstitial water. Rock typically has a significantly higher resistivity than soil because it has a smaller primary porosity and fewer interconnected pore spaces. It is thus drier. Earth materials such as clay tend to hold more moisture and generally conduct electricity much better; their resistivity values are typically much lower than that of rock. Thus, resistivity methods typically work well in characterizing karst terrane because of the high contrast in resistivity values between carbonate rock and moist, clayey residual soil overlying it. Concept Numerous articles and textbooks are available describing the resistivity method and its applications. For example, Dunscomb and Rehwoldt (1999) and Roth and Nyquist (23) provide discussion of historical perspective, method summary, and current capabilities. Resistivity measurements (figure 1) are made by introducing current into the subsurface through two current electrodes (C1 and C2), and measuring the voltage difference with two potential electrodes (P1 and P2). From the magnitude of the introduced current, measured voltage, and a factor that accounts for the geometric arrangement of the electrodes, a resistivity value is calculated. The calculated resistivity value is not a true assessment of subsurface resistivity, but is instead an apparent value. Apparent resistivity is defined as the resistivity value that would be obtained if the subsurface were homogeneous. To obtain more accurate estimates for resistivities of inhomogeneous subsurface materials, measured values are compared with values calculated from an assumed model of the subsurface. This is typically an iterative procedure using inversion techniques in which the subsurface model is modified until a reasonable match is obtained. Traditional Four-Electrode Systems A traditional four-electrode system consists of a power source, current meter, voltage meter, and four electrodes. Two survey methods are commonly used. First, in a sounding survey, the spacing between electrodes is increased between measurements, while the centerline of the electrode group remains fixed. As the electrodes are spread further apart, resistivites of deeper subsurface materials are obtained. The data from a sounding survey is typically interpreted by comparing the measured results to results calculated using a one-dimensional model of a layered subsurface system. The depth of investigation is governed by array type (the geometrical arrangement of the C1, C2, P1, and P2 electrodes), electrode spacing, and the specific subsurface materials present. Second, in a profiling survey, the spacing between electrodes is fixed, and the electrode group is moved horizontally along a line between measurements. Resulting measurements can be used to locate

3 variation in the subsurface along the measurement line. Interpretation of data obtained using this approach involves a simple plot of measurements as a function of distance along the line, followed by observation of variations of interest. Multi-Electrode Systems Recent development of multi-electrode earth resistivity testing has substantially improved investigation capabilities. Rather than moving equipment between data points, multi-electrode systems collect multiple data points with stationary equipment. These systems consist of multiple (usually 2 or more) electrodes (figure 2) connected to a switching device, a power source, a current meter, a voltage meter, and a data recorder (see figure 3 where these are combined within two units). Electrodes are spaced at equal distances along a survey line, and the switching device is used to automatically select combinations of four electrodes for each measurement. In the most recent devices now available, it is possible to apply current to two electrodes, and then simultaneously measure voltages across multiple pairs of electrodes, significantly reducing test time still further. The resulting data set consists of a combination of soundings and profiles, resulting in a two-dimensional survey of the subsurface materials. The depth of investigation using these methods is a function of line length, array type, and the subsurface materials present. Depths typically range from one-third to one-fifth of the length of the line. A pseudo section is a simple plot of results from a multi-electrode test, where resistivity values are plotted at a horizontal location coinciding with the midpoint of the four electrodes responsible for the measurement, and at a depth proportional to the spread of the four electrodes. The process by which a geologic cross section of the subsurface is developed involves comparing pseudo sections of measured test data with pseudo sections of data calculated with an assumed model of the subsurface. This iterative inversion process continues until an acceptable match is found between the measured and theoretical pseudo sections. Once a match is found, the subsurface model producing the match is accepted as the best representation of actual subsurface conditions. This inversion process is automated via commercial computer programs, allowing for observation of approximate site conditions shortly following completion of test measurements. Previous Studies in Karst Several studies reported in the literature have applied electrical resistivity techniques to site characterization of karst terrane. As shown in the following paragraphs, use of these techniques has proven quite satisfactory. Werner (1984) in a review of electrical resistivity methods for sinkhole prediction indicates that the problem of detection of sinkhole potential resolves to two aspects of local geology: the presence of a subterranean cavity and the presence of weakness, usually represented by fractures, in the overlying material. Neither condition is easily detected directly by the resistivity method unless the features are very large. However, increased weathering along zones of weakness and changes in ground-water drainage caused by cavities provide volumes of rock material with electrical resistivities different than that of the country rock, and these differences can be easily detected. Lambert (1997) conducted a dipole-dipole survey in eastern Missouri to determine depth to top of bedrock and to locate voids within bedrock. He concludes that dipole-dipole surveying appears to be a viable tool for detecting and delineating karst features, particularly because of new equipment that allows for quick data collection and new modeling software that allows for more accurate interpretations. He cautions that the technique is a non-intrusive, indirect method of exploration, and should be used to supplement a subsurface exploration program that includes drilling or test pits. Zhou, Beck, and Stephenson (1999) employed dipole-dipole electrical resistivity tomography during a site investigation on the Mitchell Plain of southern Indiana. They indicate that sinkholes are likely to form in depressions in the bedrock surface as a result of subterranean erosion of unconsolidated sediment by flowing groundwater. Electrical resistivity was used to identify such depressions, and they report that useful information was provided for identifying potential sinkhole collapse areas. They caution that their case study also demonstrates that data must be analyzed and interpreted cautiously, even with aid of boring data. Dunscomb and Rehwoldt (1999) summarize the history and theory of electrical resistivity methods, including development of modern multi-electrode technology, and they present several case studies of

4 two-dimensional resistivity profiling. They conclude that the method has very good resolving abilities in karst terrain to image geologic features such as pinnacled bedrock surfaces, overhanging rock ledges, fracture zones, and voids within the rock mass and within soil overburden. They caution that twodimensional techniques only provide a slice of the subsurface, and karst features are generally highly variable in the third dimension. Thus, they recommend future experimentation with three-dimensional tomography. Also, although the technique has been shown to provide excellent information on subsurface features, there are resolution limits with respect to potentially significant but small karst features. Roth, et al. (1999) present a case study of the reliability of multi-electrode resistivity testing for geotechnical investigations in karst terranes. In this case, comparisons between boring and resistivity results demonstrate that multi-electrode testing appears to be a valuable tool for geotechnical exploration in areas of karst overlain by clay soils. They suggest that by using intrusive methods to confirm interpretation of results, the test can predict depth to bedrock and determine trends in bedrock surface. However, they caution that reliability of the method is still in question with regard to locating and determining the size of possible voids. Three-dimensional variability and effects of line orientation both have significant influence on results and require further study. They suggest that use of threedimensional resistivity tests may overcome some of these effects. Kaufmann and Quinif (21) used combined array two-dimensional electrical resistivity tomography to delineate cover-collapse sinkhole prone areas in southern Belgium. Simulations and field tests led to use of dipole-dipole and Wenner-Schlumberger arrays in a combined array inversion procedure to obtain a better image of the subsurface. A three-dimensional model of limestone bedrock was built using the twodimensional results, and this model was compared to cone penetration (CPT) and boring results as well as locations and alignments of known cover-collapse sinkholes in and around the survey area. This comparison showed the validity of the proposed model and its usefulness to infer potential sinkhole collapse areas. Roth and Nyquist (23) evaluated multi-electrode resistivity testing in karst by comparing over 14 resistivity tests at two sites with results from 51 borings. The results demonstrated that multi-electrode resistivity can reliably map depth to bedrock with excellent repeatability, although there was some smoothing in areas where the true bedrock surface is highly irregular. Multi-electrode resistivity was not as effective at locating the size of voids, although resistivity results can provide useful information concerning void location. The likelihood of locating a void is increased if multiple orientations of lines are used and if lines are spaced at intervals of approximately 5 m or less. Karst Site Results Multi-electrode resistivity testing was conducted at five test sites in mantled karst terrane located along the new Interstate I-99 corridor between Bald Eagle and State College, Pennsylvania. Testing was conducted via a Sting R1/Swift system and manufactured by Advanced Geosciences, Inc. (AGI) of Austin, TX. The system employed 28 electrodes configured in a dipole-dipole array. By way of example, data from two structures are presented herein. Structures 318 and 319 will be two parallel two-lane bridges that carry I-99 over route US 322. Abutment 1 of Structure 318 will be supported on pile foundations, while Pier 2 of Structure 319 will be supported on shallow foundations bearing on rock. Test results for Structure 318, Abutment 1 are shown in figures 4 and 5. Figure 4 is a resistivity tomogram for one line conducted over the planned location of the abutment. The line was 162 ft long and employed an electrode spacing of 6 ft. The resistivity contours outline a pinnacle of higher values between horizontal distances of 5 to 8 ft. Shown in figure 5 are material profiles determined from two geotechnical borings conducted along the same line as the resistivity test. Figure 5(a) logs the materials at a horizontal line distance of 67 ft, i.e., near the top of the pinnacle. Overburden soils are followed by a transition zone of soil and rock fragments, followed by dolomite at approximately a depth of 17 ft. Figure 5(b) logs the materials at a horizontal line distance of 85 ft, i.e., near the right edge of the pinnacle. Here overburden soils are followed by a transition zone of soil and rock fragments, followed by dolomite at approximately a depth of 3 ft. The borings reflect similar materials at these two locations, but the depth to dolomite is considerably larger in the second boring. These data appear to verify at two

5 points the pattern of behavior in resistivity values, i.e., there is a zone of shallower dolomite between line distances of 5 and 8 ft. Test results for Structure 319, Pier 2 shown in figures 6 and 7 provide a scenario very similar to Structure 318. Figure 6 is a resistivity tomogram for one line conducted over the planned location of the pier. The line was 18 ft long and employed an electrode spacing of 4 ft. The resistivity contours outline a pinnacle of higher values between horizontal distances of 1 to ft. Shown in figure 7 are material profiles determined from two geotechnical borings conducted along the same line as the resistivity test. Figure 7(a) logs the materials at a horizontal line distance of 15 ft, i.e., near the top of the pinnacle. Overburden soils are followed by limestone at approximately a depth of 6 ft. Figure 7(b) logs the materials at a horizontal line distance of 37 ft, i.e., to the right edge of the pinnacle. Here overburden soils and a mixture of soil and rock fragments are followed by limestone at approximately a depth of 21 ft. As with Structure 318, the borings reflect similar materials at these two locations, but the depth to limestone is considerably larger in the second boring. As before, these data appear to verify at two points the pattern of behavior in resistivity values, i.e., there is a zone of shallower limestone between line distances of 1 and ft. Summary and Conclusions The purpose of this study was to employ multi-electrode resistivity testing to characterize the subsurface in mantled karst terrane. There is great need for a reliable testing method to map competent rock and estimate pile tip elevations during design of a structure. Data was presented for two bridge foundation sites. For each site, resistivity profiles were compared against material profiles available from geotechnical drilling. The resistivity profiles were found to match layering structure of the drilling data, particularly the top of rock profile. Based upon these findings, it can be concluded that multi-electrode resistivity is a useful tool for developing basic layer structure and top of rock profile in karst terrane. Acknowledgements The authors gratefully acknowledgement that the research described herein was funded by the Pennsylvania Department of Transportation via the University Based Research, Education, and Technology Transfer Program Work Order 82, Geotechnical Site Investigation for Bridge Foundations. References Dunscomb, M. H. and Rehwoldt, E. (1999), Two-Dimensional Profiling; Geophysical Weapon of Choice in Karst Terrain for Engineering Applications, Hydrogeology and Engineering Geology of Sinkholes and Karst 1999, Proceedings of the Seventh Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst, Hershey, Pennsylvania, April 1-14, pp Kaufmann, O. and Quinif, Y. (21), An Application of Cone Penetration Tests and Combined Array 2D Electrical Resistivity Tomography to Delineate Cover-Collapse Sinkhole Prone Areas, Geotechnical and Environmental Applications of Karst Geology and Hydrology, Proceedings of the Eigth Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst, Louisville, Kentucky, April 1-4, pp Lambert, D. W. (1997), Dipole-Dipole D.C. Resistivity Surveying for Exploration of Karst Features, The Engineering Geology and Hydrogeology of Karst Terranes, Proceedings of the Sixth Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst, Springfield, Missouri, April 6-9, pp Roth, M. J. S., Mackey, J. R., Mackey, C., and Nyquist, J. E. (1999), A Case Study of the Reliability of Multi-Electrode Earth Resistivity Testing for Geotechnical Investigations in Karst Terrains, Hydrogeology and Engineering Geology of Sinkholes and Karst 1999, Proceedings of the Seventh Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst, Hershey, Pennsylvania, April 1-14, pp

6 Roth, M. J. S. and Nyquist, J. E. (23), Evaluation of Multi-Electrode Earth Resistivity Testing in Karst. Geotechnical Testing Journal, ASTM, Vol. 26, pp Werner, E. (1984), Sinkhole Prediction - Review of Electrical Resistivity Methods, Sinkholes: Their Geology, Engineering and Environmental Impact, Proceedings of the First Multidisciplinary Conference on Sinkholes, Orlando, Florida, October 15-17, pp Zhou, W., Beck, B. F., and Stephenson, J. B. (1999), Application of Electrical Resistivity Tomography and Natural-Potential Technology to Delineate Potential Sinkhole Collapse Areas in a Covered Karst Terrane, Hydrogeology and Engineering Geology of Sinkholes and Karst 1999, Proceedings of the Seventh Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst, Hershey, Pennsylvania, April 1-14, pp C1 P1 P2 C2 Figure 1. Four-Electrode Resistivity Test Schematic

7 Figure 3. Resistivity Test System Figure 2. Resistivity Test Electrode

8 Resistivity Test #3 North South Depth (feet) Distance (feet) Resistivity (Ohm-ft) Figure 4. Resistivity Tomogram, Structure 318, Abutment 1, Test 3 Soil Profile Soil Profile Topsoil Sandy Silt with Clay 5 Silt with Sand 5 Sandy Clay/ Clayey Sand/ 1 15 Silt/Clay/ Silt/Sand/ Dolomite 2 Silt with Dolomite End of Boring at 39.' EndofBoringat51.' (a) Distance = 67 ft (b) Distance = 85 ft Figure 5. Geotechnical Boring Results, Structure 318, Abutment 1

9 Resistivity Test #7 East West Depth (feet) Distance (feet) Resistivity (Ohm-feet) Figure 6. Resistivity Tomogram, Structure 319, Pier 2 Soil Profile Soil Profile Unsampled Topsoil Dolomite/Clay 5 1 Sand/ Limestone Frag Silt/Sand/Clay 2 Limestone 2 Limestone End of Boring at 31' End of Boring at 41' (a) Distance = 15 ft (b) Distance = 37 ft Figure 7. Geotechnical Boring Results, Structure 319, Pier 2