Role of integrated geophysical surveying for the risk assessment of levee systems: Lessons from the East Japan Earthquake. Tomio INAZAKI, Public Works Research Institute, Tsukuba, JAPAN Summary Integrated geophysical surveying plays an important role for the risk assessment of levee systems not only for the seepage but also for the seismic hazard, because only the continuous geophysical surveying can detect anomalies in the levee systems effectively. Geophysical properties adopted for the vulnerability characterization are S-wave velocity and resistivity obtained by means of surface wave method using Land Streamer, capacitively-coupled resistivity method using OhmMapper, or supplemental Slingram electromagnetic survey method, respectively. Crossplot analysis and zoning in S-wave velocity versus resistivity was deployed for the characterization. We have conducted the production surveys at 20 levees in Japan, and successfully identified anomaly structures in and beneath the levee systems. Among them, two sites were damaged by the East Japan Earthquake just at the anomaly parts identified beforehand, which eventually demonstrated the practical usefulness of the geophysical investigation for the seismic risk assessment of levee systems. Introduction Levee system is one of the oldest man-made structures as a symbol of civilization to protect habitation areas and farm lands from inundation by flood waters or to confine the stream flow to its regular channel. Flood risk potential is still high even in developed countries including Japan. Actually, Japan is nervously facing the coming rainy season because the levee systems were severely damaged at many places by the East Japan Earthquake of magnitude (Mw) 9.0, which occurred at 05:46 UTC on Friday, 2011 March 11 (Fig. 1). It was the largest earthquake Japan ever had in its history, and caused a devastating tsunami disaster in northeast Japan. Actually, tsunami waves traveled up to 10 km inland by overtopping maximum 15 m high tsunami walls or through river channels followed by rupturing 5 m high river levees. Accordingly, coastal areas more than 500 square km were inundated along the Pacific coast region and more than 20,000 casualties were lost or missed by tsunami. As well known, the tsunami attacked ill guarded Fukushima Nuclear Power Plant and triggered fatal nuclear accident. The nuclear accident forced more than 200,000 local residents to evacuate to the outside of the 30 km exclusion circle zone. Furthermore, the accident exposed worldwide the hazardous nature of nuclear power generation technology. On the other hand, the accident anew reminded us the importance of “scientific” risk assessment based on detailed geological and geophysical

investigations. This is the primary lesson should be learned from the East Japan Earthquake. Second feature of the East Japan Earthquake was its long duration time of strong motion. For instance, strong ground motion was recorded during more than 2 minutes at Tokyo, 400 km far from the epicenter. This unusual length of strong motion caused serious ground failure as typified by liquefaction in many places in the Kanto Plain, the largest tectonic sedimentary basin in Japan. Consequently, more than 1,000 sections of earthen river levee systems were severely damaged (Fig. 2). Ministry of Land, Infrastructure and Transportation (MLIT) is undertaking rushed repair of failure levees before the rainy season beginning from June. Concurrently to the repair work, drilling survey is to be conducted to clarify the internal conditions of levee systems. As easily imagined, drilling is not an appropriate method to interpret heterogeneous structure. In contrast, geophysical survey can provide 2-D or 3-D structure involving discontinuity. We have demonstrated usefulness of geophysical methods to the safety assessment of levee systems through the comparative testing at 20 actual sites

Figure 1. A map showing the epicenter and seismogenic zone of the 2011 East Japan Earthquake. PGA contours are drawn in reference to K-NET data.

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Integrated geophysical surveying for the risk assessment of levee systems

and confirmed the advantages of integrated method or combination of seismic and electrical method (Inazaki, 2007). We could successfully identify anomaly structures in and beneath the levee systems by means of the method. Among them, two sites located in the Kanto Plain (Fig. 2) were heavily damaged by the East Japan Earthquake just at the anomaly parts identified beforehand, which eventually demonstrated the practical usefulness of the geophysical investigation for the risk assessment of levee systems. This indicates that we geophysicists should have driven the expansion of both research and routine survey for the safety assessment of levee systems using integrated geophysical method more powerfully. This is the secondary lesson learned from the earthquake. Integrated Geophysical Investigation for Levee Systems Integrated geophysical investigation for the safety assessment of levee systems comprises multiple methods applied to the same target at different stages. It enables to identify anomalies in the levee body and underlying substrata by combining individual survey results. In principle, each geophysical method provides us the spatial distribution of single geophysical property, such as seismic velocity, resistivity, or gravity potential. Further, each obtained geophysical property is a function of many physical properties. For instance, resistivity is a function of porosity, pore fluid conductivity, water saturation condition,

and grain size (complement to pore size) as described as Archie’s equation. For the safety assessment of levee systems, two major geotechnical parameters, vulnerability on seepage and seismic resistance are required to be evaluated along the levee. As schematically illustrated in Figure 3, seepage characteristics of unconsolidated soil materials and bearing layers is mainly influenced by grain size and stiffness. That is, the coarser in grain size and the looser in stiffness, the more unsafe in seepage vulnerability. Similarly, seismic resistance is also characterized by grain size and stiffness but in different way. Namely, the softer in stiffness and the finer in grain size, the more unsafe in seismic resistance vulnerability. Then when the relationship between these physical properties and geophysical properties of soils and sediments is clarified, we can directly estimate the vulnerability condition from the geophysical properties. Figure 4 (a) shows the close relationship between S-wave velocity and N-value (blow counts) measured by Standard Penetration Test (Inazaki, 2006). N-values are often used as an indicator of the stiffness. Similar relationship between

Figure 2. A map showing two survey sites and major levee failure parts (red circle). More than 1,000 sections of levee systems in the Kanto Plain were severely damaged by the earthquake even situated 300 km far from the epicenter.

Figure 4. Crossplots of geotechnical and geophysical properties. (a): Relationship between S-wave velocity and N-value as an indicator of stiffness (Inazaki, 2006); (b) : Correlation between resistivity and specific grain size D20 (Inazaki, et al., 2011).

Figure 3. Schematic diagram assessing seepage vulnerability (left) and seismic resistance (right) based on general relationship between geophysical and soil properties on crossplot of S-wave velocity and resistivity data (Inazaki, et al., 2011).

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Integrated geophysical surveying for the risk assessment of levee systems

resistivity and the specific grain size (D20) is also shown in Fig. 4 (b). As shown in Fig. 3 and Fig. 4, we have utilized S-wave velocity and resistivity as geophysical properties because of their relatively high correlation with geotechnical properties of the target soils and sediments. Geophysical survey methods for the levee safety assessment, therefore, are required to measure S-wave velocity and resistivity. We then chose a high-resolution surface wave method (Hayashi & Suzuki, 2004) using Land Streamer (Inazaki, 1999) for the S-wave measurement (LS_SW), capacitively-coupled resistivity (CCR) survey using OhmMapper (Geometrics, 2001), and supplemental Slingram electromagnetic (EM) survey method for the resistivity measurement. Geophysical Characterization of Levee Failure As described above, two sites were severely damaged by the East Japan Earthquake, where integrated geophysical surveys had been conducted, and levee failure occurred just at the part that we had detected anomaly. Conversely, the actual levee failures gave us valuable information on the

vulnerability assessment process, or how to interpret geophysical survey results. This is the third lesson learned from the earthquake. Figure 5 exemplifies a geophysical interpretation of the levee failure caused by the earthquake. The failure occurred on the left bank of the levee along Kokai River, about 300 km away from the epicenter of the earthquake (Kokai_35L in Fig.2). Top of the levee settled down about 1 m with slope failure. Fissures and accompanying liquefaction were observed on the ground adjacent to the levee in and around the damaged zone. S-wave profile along the levee is characterized as low in levee body about 140 m/s and moderate but partly low in the substrata (Fig.5 (a)). Compared with S-wave profile, resistivity section shows significant heterogeneous structure in the levee body (Fig.5 (b)). This indicates that coarse or potentially permeable materials were used partly to embank the levee body. As clearly shown in Fig. 5 (d), the levee failure just occurred on an abandoned channel at 35.0 K. However, no clear sign on the failure was observed at other two zones where abandoned channel underlay. Figure 5 (c) is an interpreted

Figure 5. Comparison of integrated geophysical survey results along the levee of Kokai River, about 300 km away from the epicenter of the 2011 East Japan Earthquake (Kokai_35L in Fig.2). Field survey was conducted in 2005, and had depicted anomaly zones in levee body. Levee failure took place at the shaded part about 100 m in width. Top of the levee settled down about 1 m with slope failure. Fissures and accompanying liquefaction were observed on the ground adjacent to the levee body in the failure zone. (a): S-wave velocity structure reconstructed from LS_SW data; (b): Resistivity profile along the levee inverted from CCR data; (c): Seismic resistance vulnerability section classified into 4 categories based on S-wave velocity and resistivity; (d): Landform along the levee interpreted from aerial photos. Note that the levee failure occurred just on an abandoned channel (revised from Inazaki, 2007).

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Integrated geophysical surveying for the risk assessment of levee systems

section on the vulnerability of seismic resistance along the levee. The section is classified into 4 categories based on S-wave velocity and resistivity value. Here threshold values of 180 m/s in S-wave velocity and 90 Ω-m in resistivity were adopted for the classification empirically. Consequently, the failure part was clearly distinguished as low S-wave velocity and low resistivity zone both in levee body and substrata. This interpreted section demonstrates the advantage of crossplot analysis but also the capability of integrated geophysical surveying. Figure 6 is another case of geophysical interpretation for the levee failure. Slope sliding at the riverside surface took place on the levee along Edo River, about 320 km far from the epicenter of the earthquake (Edo_58L in Fig.2), at the shaded part about 200 m in width. About 50 cm thick slope surface slid down at lower half part. No evidence indicating liquefaction was observed around the slide zone. As clearly shown, two high resistivity anomaly zones were delineated in the lower part of levee body around 58.0 km in distance (Fig.6 (b)). S-wave profile along the levee showed generally plain except at surficial part at 57.5 km (Fig.6 (a)). However, it is hard to interpret the vulnerability in seismic resistance from the classification section (Fig.6 (c)) based on the basic concept as shown in Fig.3. Then we calculated a virtual impedance values from the S-wave velocity and resistivity (Fig.6 (d)). It still has discrepancy but the characteristic feature of the slide at lower half of the slope may result from the high resistivity anomalies. As shown in Figure 6 (e), the levee system has asymmetric shape. So it can be explainable by the overlying structure of thin clayey materials on sandy layer inferred from the geophysical profiles.

Conclusions Geophysical investigation method is expected to play an important role for the safety assessment, but it was needful to demonstrate its usefulness. We have successfully shown its usefulness to seepage assessment. In addition, reexamination of survey results for levee failure sites caused by the East Japan Earthquake revealed practical usefulness of the geophysical investigation for the seismic risk assessment of levee systems. Further applications of the integrated geophysical method to damaged levees are essential to find appropriate survey parameters and to establish the optimal criteria for both seepage and seismic safety assessment.

Figure 6. Comparison of integrated geophysical survey results along the levee of Edo River, about 320 km away from the epicenter of the 2011 East Japan Earthquake (Edo_58L in Fig.2). Field survey was conducted in 2005, and had depicted high resistivity anomaly zones in levee body. Slope sliding at the riverside levee surface took place at the shaded part about 200 m in width. About 50 cm thick slope surface slid down at lower half part. No evidence indicating liquefaction was observed around the slide zone. (a): S-wave velocity structure reconstructed from LS_SW data; (b): Resistivity profile along the levee inverted from CCR data; (c): Seismic resistance vulnerability section classified into 4 categories based on S-wave velocity and resistivity; (d): Virtual impedance section calculated from S-wave velocity and resistivity; (e): Inferred geological section across the levee at slide part (revised from Inazaki, 2007).

(e)

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Integrated geophysical surveying for the risk assessment of levee systems

Geometrics, 2001, OhmMapper TR1 operation manual, Geometrics Inc., 147p.

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