GEOPHYSICAL INVESTIGATION OF THE SUCCESS ...

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Success Dam is a zoned earth-fill embankment located about 70 miles northeast of Bakersfield,. CA. Stability analyses indicate that there is a potential for ...
Hunter. L.E., Asch, T., Powers, M., Burton, B.L., and Haines, S.S., 2007, Geophysical investigations of the Success Dam foundation: an overview. Denver, Colorado, Proceedings of SAGEEP 2007, 21-30.

GEOPHYSICAL INVESTIGATION OF THE SUCCESS DAM FOUNDATION: AN OVERVIEW Lewis E. Hunter, U.S. Army Corps of Engineers, Sacramento, California 95814 Michael H. Powers, Seth Haines, Theodore Asch, Bethany L. Burton, Crustal Imaging and Characterization Team, U.S. Geological Survey, Box 25046, MS-964, Denver Federal Center, Denver, Colorado 80215

Abstract Success Dam is a zoned earth-fill embankment located about 70 miles northeast of Bakersfield, CA. Stability analyses indicate that there is a potential for large-scale deformation of the dam during relatively low levels of earthquake shaking. At least three earthquakes in the past 150 years, and prior to dam construction, are believed to have been large enough to create a dam failure. To better understand material behavior, the U.S. Army Corps of Engineers has been investigating properties of the dam and its foundation. This includes extensive field explorations and detailed engineering studies using a variety of analytical techniques to estimate the response of the dam and foundation to earthquake shaking. Although a large amount of data have been acquired since 1992, these data are largely point data from borings. A geophysical investigation was designed to provide a continuous image of the foundation toe. This investigation employed direct-current (DC) resistivity, seismic refraction tomography (P- and S-wave), audio-magnetotellurics (AMT), and self-potential (SP). The purpose of the DC resistivity and seismic refraction tomography was to produce 2-D imagery across the foundation to investigate depth to bedrock and the occurrence of beds potentially susceptible to liquefaction. DC resistivity was used to look at the conductivity relationships in the subsurface. The resistivity data produced a higher-resolution image relative to seismic refraction tomography, which looks at compressional and shear properties of the material. AMT was applied to look considerably deeper (several 100s of m) in order to confirm depth to bedrock and investigate for deep faults. The goal of this paper is to provide technical background on the site and to highlight how these data have been used by the engineers in redesigning the new dam. Specific details on the geophysical methods are presented by the co-authors in two other papers in this session (Asch et al., 2007; Powers et al., 2007).

Introduction Success Dam is a zoned earth-fill structure located on the Tule River about six miles east of Porterville and 70 miles north of Bakersfield, California (Figure 1). The crest of the dam is about 3,400 feet long with a maximum width and height of about 880 feet and 145 feet, respectively. The reservoir behind the dam has a gross pool of 80,000 acre-feet, and construction was completed in 1961 (U.S. Army Corps of Engineers, 1961). Structural integrity of the dam has been investigated since 1992, including extensive field explorations using a range of analytical techniques, soil borings, and shearwave tests to estimate the response of the dam and foundation during earthquake shaking (e.g., Llopis et

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al., 1997; Sherer, 1996; URS Corporation, 2003; Wahl and Llopis, 1982). These efforts have revealed lateral heterogeneity with regions of soft sand in the older alluvium and terrace deposits located under parts of the dam.

Figure 1. Location of Success Dam in Tulare County, California. Sherer (1996) provides a detailed discussion on the geology of the Success Dam area. He found that the embankment is situated on slope wash, fan, alluvial, and terrace deposits as well as cemented older alluvium and undifferentiated metamorphic and deformed Mesozoic marine, igneous, volcanic, and sedimentary rocks. The metamorphic bedrock has been subjected to severe lateral compression resulting in tight folds that strike northwest and a well-developed bedrock fabric (schistocity and cleavage). Subsequent deformation has produced additional joint sets that vary depending on the rock type and deformation history. The Seismic Hazard Assessment (URS Corporation, 2004) and other earthquake hazard studies have determined that Success Dam has the potential to fail due to earthquake induced liquefaction of loose alluvial foundation materials. Records indicate that at least three such earthquakes occurred in the past 150 years. The predicted strength loss of the foundation and embankment materials during an earthquake may result in excess deformational weakening, leading to a release of the reservoir. As a result, a new dam is being designed that will be located downstream of the existing dam. To investigate the new dam foundation, recent drilling and geophysical activities have targeted the area under the footprint of the new dam. Of particular interest are the existence, location, and extent of any materials

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with the potential for liquefaction. To further investigate these conditions, a surface geophysical investigation using non-intrusive profiling techniques was conducted. The methodologies employed include compressional (P) and shear (S) wave seismic refraction, direct-current (DC) resistivity, selfpotential (SP), and audio-magnetotellurics (AMT). These techniques were applied along the toe of the dam (Figure 2).

Figure 2. Aerial photograph of Success Dam showing the locations of the geophysical surveys. Seismic transects – thick blue lines; DC resistivity – red lines; and AMT – yellow lines. The goal of this investigation was to image the shallow subsurface along a line approximating the axis of the new dam to investigate heterogeneities in the alluvial fill, profile the top of consolidated bedrock, image the occurrence of weathered bedrock materials, and infer fault zones in the valley floor. Technical details of the survey are provided by the U.S. Geological Survey (2006), whereas this paper provides a general overview of the techniques with an emphasis on the usefulness of the results for improving our understanding of the dam foundation.

Discussion The combined results from our geophysical surveys at Success Dam are shown in Figure 3. Four profiling techniques were used, each following approximately the same path covering about 4200 feet

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along the downstream toe of the dam (Figures 2 and 3). Figure 4a shows seismic P-wave refraction models, Figure 4b shows seismic S-wave refraction models, and Figure 4c shows the Poisson's ratios inferred from these models. All of the seismic results are described in more detail by Powers et al. (2007). Figure 4d shows results from DC resistivity profiling and Figure 4e shows AMT results, both are described in more detail by Asch et al. (2007). Figure 4f shows a generalized interpretation based on the combined geophysical results, and Figure 4g shows a geologic cross section based on lithologic logs from the available boreholes.

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Figure 3. Location of geophysical survey lines. Success Dam from west embankment looking to the east showing general locations of the geophysical transects along the downstream toe This paper’s discussion focuses on how these data were applied to achieve maximum benefit for geotechnical evaluations at Success Dam. The survey was designed so that multiple techniques could be used to achieve a more comprehensive understanding of subsurface conditions. This combination yields compatible results that increased our confidence in the geotechnical interpretation. The seismic techniques yield information on bulk mechanical properties, while DC resistivity and AMT evaluate changes in electrical properties related to saturation and stratigraphy. Our data clearly demonstrate how seismic methods are complemented by the DC resistivity (Figure 4). Our resistivity images can distinguish between the recent alluvium and older alluvium according to mineralogical differences that make the older alluvium more conductive, and the seismic data provide useful information on material saturation, consolidation, and depth to competent bedrock. The benefit of the S-wave data is the ability to calculate the Vp/Vs and Poisson’s ratios that can be directly related to geotechnical properties (cf. Powers et al. 2007). Furthermore, different bedrock horizons were identified in the Vp and Vs plots that indicate differences in material properties that likely relate to variations in consolidation, cementation, and weathering (Figure 4f). Such a zone is identified by an anomaly in the Poisson’s ratio near well 1F-05-16 (Figure 4c), and in slightly lower conductivity

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Figure 4. Composite of the geophysical data and generalized interpretations from the downstream toe of Success Dam. (A) Vp, (B) Vs, (C) Poisson’s ratio, (D) DC resistivity, (E) AMT, (F) generalized interpretation based on geophysical data shown in A-D, the SP date of Asch et al. (2007) and the engineer’s geologic cross section, and (G) engineer’s geologic cross section derived from lithologic logs. Black lines above 3C indicate areas of interest identified from Poisson’s ratio. Wells discussed in text are plotted on 3F, and dashed lines indicate faults interpreted from the seismic, AMT, SP, and borehole logs.

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Figure 4 (continued). Composite of the geophysical data and generalized interpretations from the downstream toe of Success Dam. (A) Vp, (B) Vs, (C) Poisson’s ratio, (D) DC resistivity, (E) AMT, (F) generalized interpretation based on geophysical data shown in A-D, the SP date of Asch et al. (2007) and the engineer’s geologic cross section, and (G) engineer’s geologic cross section derived from lithologic logs. Black lines above 3C indicate areas of interest identified from Poisson’s ratio. Wells discussed in text are plotted on 3F, and dashed lines indicate faults interpreted from the seismic, AMT, SP, and borehole logs.

values in the high-conductivity zone in the DC resistivity data (Figure 4d), and the upper boundary of this zone corresponds with the base of the conductive regions as demonstrated in the AMT data (Figure 4e). There are five zones of interest identified by letters in the Poisson’s ratio plot (Figure 4c): a. The top of the reddish zone near elevation 550 feet between wells 1F-05-22 and 1F-05- 09; b. The bright red oval between elevations 450 and 550 feet near well 1F-05-10; c. The reddish layer between 500 and 550 feet elevation extending from well 1F-05-10 to well 1F05-16; d. The near-surface reddish layer below water table (elevation 500 - 550 ft) between wells 1F-05-04 to well 1F-05-02; e. Possibly the deep red oval near 400 feet elevation just east of STA 30+00 if a fault plane is interpreted through this location. Engineering analyses of these zones indicate that they likely do not present a potential for liquefaction based on their particle size and density as determined from laboratory tests. However, the profiling techniques provide the ability to study the spatial relationships among such layers, which can be a great advantage when compared to point data acquired from lithologic logs. Also, as noted above, the anomalous zones indicate that something is different about these areas. A combined approach of drilling and geophysical sampling provides for detailed imaging of the subsurface with point data to confirm the geophysics. Although the values in the plots do not indicate a potential hazard, it needs to be recognized that these anomalies represent spatially averaged values. As such, the zones responsible for such anomalies could actually be thin, with very low Vs values, or thicker, with only moderate Vs values. The Vp/Vs and Poisson’s ratio thus provide a synthesized view that highlights areas where more detailed analyses are warranted. Also of interest to our investigation is the ability to identify potential faults. It is speculated by Asch et al. (2007) that the blocky nature in the AMT data is a result of the 164 ft sampling interval and occurrence of faults. Such may be evident by the sharp transition near station 30+00, which is supported by the SP investigation that characterized a sheet source in the area (U.S. Geological Survey, 2006). Another fault is believed to exist near well 1F-05-10. This corresponds to the deepest part of the channel that likely represents preferential erosion of the fault zone and is supported by field observations at well 1F-05-10 that terminated in breccia. This well is artesian, yielding water that is about 10° F warmer and more saline than other wells along the dam toe. Additionally, hydrothermal alteration was identified in nearby well 1F-05-15 leading us to interpret an east-dipping fault in which well 1F-05-10 terminated that brought hydrothermal fluids to 1F-05-15. An ancillary goal of these geophysical surveys was to provide a proof-of-concept example of the utility of these types of surveys. Ideally, they would be done in the early stages of foundation evaluation. The full survey of greater than 4300 feet along the toe of the dam utilizing 3-profiling techniques and SP was completed for the same cost as drilling about 10 boreholes with a significantly reduced timeframe and lower oversight requirements. Thus, by collecting and using geophysical data to establish ground truth and identify anomalous zones, the drilling program can be designed to realize significant cost savings. Furthermore, geophysical data from previous shear-wave tests (Llopis et al., 1997; URS

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Corporation, 2003) show a good correspondence with the refraction Vs data (Powers et al., 2007). This indicates that the Vs profile technique used in this study provides realistic data yielding confidence in the profile results. Another benefit is that the S-wave survey provides data that is directly useable by the geotechnical engineers. At Success Dam, the Vs data were used as input parameters in the program FLAC (Itasca Consulting Group, 2005), which is used to simulate material behavior during earthquake shaking. Of particular importance is the behavior of the dam during an Operational Based Earthquake (OBE) and Maximum Credible Earthquake (MCE) (Figure 5). An OBE event is one that has a 50% probability of occurring in 100 years, and an MCE event is the largest that is determined possible. URS Corporation (2004) estimates significant damage and possible failure of the dam if it experiences a horizontal acceleration of 0.08 g to more than 0.1 g during an earthquake event. They further defined the OBE as an Mw 8.0 event on the San Andreas Fault (72 mi) away capable of producing a peak horizontal acceleration of 0.10 g. The MCE was defined as an Mw 6.8 event on the Premier Fault (13 mi) away capable of producing peak horizontal acceleration of 0.18 g to 0.28 g. Such data help to provide control on the lateral variability used for model inputs. Typically, the engineer running the model uses data from 2 or 3 cross-hole shear wave tests. These data provide general estimates that are then extrapolated across the dam’s toe.

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X-displacement contours -3.00E+00 -2.50E+00 -2.00E+00 -1.50E+00 -1.00E+00 -5.00E-01 0.00E+00

Operating Basis Earthquake (OBE). San Andreas Fault @ 72 miles. Magnitude = 8.0 0.1g acceleration

Operating Basis Earthquake (OBE) Deformations of new Success Dam Maximum Credible Earthquake (MCE) Premier Fault @ 13 miles. Magnitude = 6.8 0.28g acceleration

B

Maximum Credible Earthquake (MCE) Deformation of new Success Dam Figure 5. Examples of FLAC deformation models generated using shear wave data generated by this survey. For A) an operational based earthquake and B) a maximum credible earthquake.

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Conclusions The geophysical survey conducted at Success Dam provides valuable subsurface information that engineers are using to evaluate geotechnical properties. The seismic refraction tomography survey indicates consolidated and unconsolidated materials in the subsurface that correlate well with known geology. The 2-D resistivity inversion results indicate conductive and resistive materials that also correlate with the known geology. These methods complement each other in providing information on consolidation and mineralogy to determine or confirm material types and to delineate their interfaces. Together they provide useful information on the top of consolidated bedrock and the nature and extent of recent and old alluvium that will be encountered at the site of the new dam. By using profiling techniques the geophysical data provide the engineers with useful information on lateral variations in the stratigraphy and bedrock surface. Our investigation was also able to delineate suspected faults below the toe of the dam. Some lessons learned by this investigation are listed below. • • • • • •

The simultaneous application of electrical and seismic methods yields complementary data sets that alone provide intriguing results but incomplete answers. Augmenting P-wave seismic refraction data with S-wave refraction data provides additional capabilities for evaluating geotechnical properties. S-wave seismic refraction data provide results consistent with traditional downhole shear-wave tests. These techniques are cost-effective alternatives to downhole shear-wave testing in the future. S-wave refraction data can be used in modeling to provide a more comprehensive look than does using point data from cross-hole shear wave tests. Geophysical profiling techniques represent cost-effective alternatives to multiple soil-borings and provide added advantages by generating two-dimensional images that indicate spatial relationships. A cost-effective approach for evaluating a large dam foundation should include reconnaissance level geophysical profiling surveys early in the program followed by development of a borehole-testing program that selectively drills to confirm geophysical data and investigate anomalous zones of interest.

References Asch, T.H, Burton, B.L., Powers, M.H., Rodriguez, B.D., Bedrosian, P. and Hunter, L.E., 2007. Electrical characterization of Success Dam in Porterville, California: Denver, Colorado, Proceedings of SAGEEP 2007, this volume. Itasca Consulting Group, 2005. FLAC, fast Lagrangian analysis of continua, version 5.0. Minneapolis, Minnesota, Itasca Consulting Group. Llopis, J. L., Lee, L. T., and Green, R. A., 1997. In situ geophysical investigation to evaluate dynamic soil properties at Success Dam, California: U.S. Army Corps of Engineers Waterways Experiment Station, Technical Report GL-97-8, June 1997. Powers, M.H., Burton, B.L. and Haines, S., 2007. Compressional and shear wave seismic refraction tomography at Success Dam, Porterville, California: Denver, Colorado, Proceedings of SAGEEP 2007, this volume.

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Sherer, S.G., 1996. Geotechnical investigations for safety of dams study, Success Dam: Department of Interior, Bureau of Reclamation report for the U.S. Army Corps of Engineers, Sacramento District, 116 pp. URS Corporation, 2003. Success Dam Geophysical Investigation, Porterville, California: Letter Report, Geophysical Investigation, Prepared for U.S. Army Corps of Engineers, Sacramento District, Contract No. DAC205- 01-D-0003, 68 pp. URS Corporation, 2004. Deterministic and probabilistic seismic hazard analysis for Success Dam, California. Prepared for U.S. Army Corps of Engineers, Sacramento District, 119 pp. U.S. Army Corps of Engineers, 1961. Foundation Report, Success Dam Project, Tule River, California, Vol. I. Sacramento, California, U.S. Army Engineer District, Sacramento 157 pp. U.S. Geological Survey, 2006. Draft Report: Success Dam, Porterville, California, geophysical characterization: seismic refraction tomography, DC resistivity, self potential and audio magnetotellurics: Denver, Colorado, U.S. Geological Survey, Administrative Report, January 2006. Wahl, R.E. and Llopis, J.L., 1982. Final report: In situ seismic investigation of Success Dam, October 1982: Geotechnical Laboratory, U.S. Army Engineer Waterways Experiment Station, Prepared for U.S. Army Corps of Engineers, Sacramento District, 71 pp.

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