Tentative Report Analysis on Acceleration Data of Dams Collected by JCOLD *

Transcription

1 June 14, ICOLD Stavanger Technical Committee (B) SEISMIC ASPECTS OF DAM DESIGN Tentative Report Analysis on Acceleration Data of Dams Collected by JCOLD * (For New Bulletin Seismic Interpretation of Integrated Observation Data ) Takashi SASAKI, JAPAN Member of Technical Committee (B) SEISMIC ASPECTS OF DAM DESIGN 1. Introduction Securing of the seismic performance of dams is an extremely important issue. As Japan is one of the most earthquake-prone countries in the world, various kinds of efforts regarding the issue have been carried out. In particular, seismic recordings have been conducted in existing dams since they can verify the characteristics of earthquake input and dam response empirically. The seismic records have been utilized for the advancement of dam safety management, design and construction. The strong motion records are highly valuable; however, it takes years, plus a great deal of work and expense, to record such data appropriately. The data can be used more effectively by providing it to other entities or researchers once it has been recorded, and this is commonly understood to conform to the intended purpose of the seismic recordings. It will also contribute to the further advancement of seismic safety evaluation and the design of dams. The Japan Commission on Large Dams (JCOLD) released databases of earthquake recordings in 1978 (Earthquake Records on Rock Foundations, ) and 2002 (Acceleration Records on Dams and Foundations No.2, ). In view of this background, the Japan Commission on Large Dams, with support from dam owners and other interested parties, has organized the committee on Acceleration Records on Dams and Foundations and updated the dam earthquake records database to promote technical improvement and the advancement of evaluation of the seismic performance of dams and dam engineering. The JCOLD committee has collected these time histories of accelerations, recorded from October 2000 to September 2012 newly. The recordings in data book titled Acceleration Records on Dams and Foundations No.3 were restricted to acceleration records at dam foundations, dam crests, ground and so on whose peak acceleration at the dam foundation exceeds 25 cm/s 2. They include data which were recorded before the target collection period. In addition to the above activity, the JCOLD committee has carried out analysis of fundamental strong motion characteristics at dam sites, acceleration response characteristics of dams, and natural periods of dams and so on. This tentative report show the outlines of data book of No.3 and the characteristics of acceleration data of dams and foundations. Table-1-(1) shows the list of earthquake records by organizations, Table-1-(2) shows the number of earthquake records by dam Types, Table-1-(3) shows the number of earthquake records by range of peak acceleration, Table-1-(4) shows the distinguished earthquake records by earthquake type, Fig.-1-(1) shows the location map of epicenter for object earthquake, Fig.-1-(2) shows the location map of dams targeted and Fig.-1-(3) shows the example of seismograph location in the case of a concrete gravity dam *This report is based on the technical paper Report of JCOLD committee on Acceleration Records on Dams and Foundations, Journal of JCOLD, April 2015 which is written in Japanese. 1

2 Table-1-(1) List of earthquake records by organizations (recorded in report of No.3 1) ) Organization Number of dams 2) Number of earthquake Number of records 3) accelerograms 4) Ministry of Land, Infrastructure, Government (117) Transport and Tourism Local government (177) Subtotal (294) Ministry of Agriculture, Forestry Government (81) and Fisheries Local government (0) Subtotal (81) Incorporated Administrative Agency Japan Water Agency (21) Subtotal (21) Hokkaido Electric Power Co., Inc (0) Tokyo Electric Power Co., Inc (62) Kansai Electric Power Co., Inc (0) Electric power company and others Chubu Electric Power Co., Inc (0) Shikoku Electric Power Co., Inc (0) Electric Power Development Co.,Ltd (36) East Japan Railway Company (24) Subtotal (122) Total (518) Tohoku Number of (2011) 5) earthquakes 6) 250 1) Acceleration Records on Dams and Foundations No.3, which contains accelerograms in the period from Jan until Sep and some records after the period. 2) Number of dams where the accelerograms were recorded. 3) Number of earthquake records of dams. 4) Number of accelerograms. It counted each direction component of records. 5) Number of total accelerograms which recorded by the 2011 off the Pacific coast of Tohoku Earthquake 6) Number of earthquakes compiled in No.3 published by JCOLD 2

3 Table-1-(2) Number of Earthquake Records by Dam Types Dam Type Symbol Number of Dams 1) Number of Records Number of Componet of Records Concrete Gravity G Concrete Arch A Rockfill R Earthfill E Combined GF Concrete Facing FC Asphalt Facing FA Hollow Gravity HG Buttress B Total 計 ) Number of dams which collected earthquake records Dam Type Table-1-(3) Number of Earthquake Records by Range of Peak Acceleration Number of Dams 1) Acceleration (cm/s 2 ) 2) Over 500 cm/s 2 500~ 400 cm/s 2 400~ 300 cm/s 2 300~ 200 cm/s 2 200~ 100 cm/s 2 Table-1-(4) Distinguished Earthquake Records by Earthquake Type Under 100 cm/s 2 Concrete Gravity Concrete Arch Rockfill Earthfill Combined Concrete Facing Asphalt Facing Hollow Gravity Buttress Total ) Number of dams which collected earthquake records 2) Maximum acceralation value at dam foundation Number of Records Total Inland Eathquake Subduction Zone Earthquake Earthquake Type Earthquake Name (Epicenter) M Depth of Hypocenter Distance of Epicenter Name of Dam (km) (km) strike-slip fault 2000 Western Torrori Pref Kasho G 531 reverse fault 2004 Niigataken Chuetsuoki Kawanishi E 559 reverse fault 2008 Iwate-Miyagi Nairiku Aratozawa R 1024 normal fault 2011/4/11 (Eastern Fukushima Pref) Takashiba G 265 inter-plate 2003 Tokachioki Urakawa G Tohoku Minamikawa G Sengosawa E 315 intra-plate 2003/5/26 (Nothern Miyagi Pref) Tase G /4/7 (Off East Coast Miyagi Pref) Takou G 373 Dam Type Max. Acc. at Dam Foundation (cm/s 2 ) 3

4 Number of earthquakes N250 Period: 1978/2/20~2013/9) Fig.-1-(1) Location map of epicenter Location of dam Fig.-1-(2) Location map of dams T1 T1 G1 K1 M1 G2 F1 G2 T1 G1 M1 F1 T : Crest, M : Dam Body, F : Dam Foundation (Basement), G : Ground Fig.-1-(3) Example of seismograph location, Concrete gravity dam 4

5 2. Analysis of Observed Earthquake Records 2.1 Overview The targets of analyses were broadly divided into 3 categories: characteristics of ground motion, dam amplification ratio, and natural period of dams and damping characteristics. Contents of analyses are shown in Table In this tentative report, a part of results of analysis conducted by JCOLD are shown below. Table-2.1-(1) Analysis of Observed earthquake record at dams Type Field Details Seismic Motion at Foundation Response Amplification Ratio Natural Period Damping Ratio 1) Seismic Motion characteristics at dam foundation 2) Distance attenuation characteristics - 1) Dam Height and Natural Period 2) Dam Crest Width and Natural Period 3) Ground motion Level and Natural Period 4) Water Level and Natural Period - Compares records on dam foundations and abutments as well as on dam foundations and downstream free rock surfaces, and considers the existence of any correlations. Conducts comparisons between maximum acceleration values and the distance of earthquake sources. And compares acceleration on dam foundations, on soil foundations and undergrounds. Seeks response amplification ratios using maximum accelerations at dam foundations and crests, then considers the relationship between these ratios and dam height and crest width. Seeks and estimates 1st natural period and correlations with dam heights Considers dam crest width and estimated 1st natural period correlations. Seeks and considers correlations between maximum acceleration in dam foundations and estimated 1st natural periods for representative dams for which multiple earthquake records had been observed. Seeks and considers correlations between water level and estimated 1st natural periods for representative dams for which water level data existed. Seeks and considers damping ratios using the half power method for representative dams for which multiple records existed. 5

6 2.2 Seismic Motion Characteristics Characteristics of Seismic motion in Bedrock Dams with earthquake records for bedrock outside of the dam body were extracted and contrasted against records for dam foundations. Dam extraction was determined from seismometer location and dams with seismometers located in abutments, abutment limb tunnels, and downstream free rock surfaces were selected. The number of selected dams, earthquakes, and records are shown below. Notes 1), 2) Table (1) Earthquake Records Gravit Arch Rockf Type of Dam y ill Number Dams of With Downstream Records Number of Earthquakes Earthquake Records Stream Directions Dam Axial Direction Vertical Direction Earth sets 93 sets 93 sets 37 sets 29 sets 29 sets 90 sets 79 sets 72 sets 32 sets 32 sets 32 sets Note 1) Discrepancies in numbers of stream direction, axial, and vertical records are due to absence of observed record entries. Note 2) Gravity dams include GF (combined dams) and HG (hollow gravity dams), and rockfill dams include AF (asphalt facing dams). type and direction of acceleration. Furthermore, frequency analysis of timed acceleration records was conducted in order to determine a Fourier spectrum for 18 dams for which downstream free rock surface records were available. This spectrum was then contrasted against dam body foundations. Here, the former results are shown in Fig Maximum acceleration (F') in bedrock outside the dam body was compared against maximum acceleration in the foundation (F1), and generally increasing tendencies were confirmed in all directions. Dispersal is comparatively large, although this is thought to be influenced by the presence or absence of restrictions caused by the dam body. The comparisons of maximum accelerations and the amplification ratios (F'/F1) between bedrock outside of dam bodies (F') and dam foundation (F1) were arranged in relation to stream, dam axis, and vertical directions. Results were organized according to dam 6

7 Stream Directions Stream Directions Peak Acc. at bedrock (gal) Amplitude (F /F1) Dam Axial Direction Dam Axial Direction Peak Acc. at bedrock (gal) Amplitude (F /F1) Vertical Direction Vertical Direction Peak Acc. at bedrock (gal) Amplitude (F /F1) Fig (1) Comparison of Peak Acceleration in dam foundation, abutments, and free rock surface (Gravity dams) (1) Fig (2) Comparison of Peak Acceleration in dam foundation, abutments, and free rock surface (Gravity dams) (2) 7

8 Stream Directions Stream Directions Peak Acc. at bedrock (gal) Amplitude (F /F1) Dam Axial Direction Dam Axial Direction Peak Acc. at bedrock (gal) Amplitude (F /F1) Vertical Direction Vertical Direction Peak Acc. at bedrock (gal) Amplitude (F /F1) Fig (3) Comparison of Peak Acceleration in dam foundation, abutments, and free rock surface (Arch dams) (1) Fig (4) Comparison of Peak Acceleration in dam foundation, abutments, and free rock surface (Arch dams) (2) 8

9 Stream Directions Stream Directions Peak Acc. at bedrock (gal) Amplitude (F /F1) Dam Axial Direction Dam Axial Direction Peak Acc. at bedrock (gal) Amplitude (F /F1) Vertical Direction Vertical Direction Peak Acc. at bedrock (gal) Amplitude (F /F1) Fig (5) Comparison of Peak Acceleration in dam foundation, abutments, and free rock surface (Rockfill dams) (1) Fig (6) Comparison of Peak Acceleration in dam foundation, abutments, and free rock surface (Rockfill dams) (2) 9

10 Stream Directions Stream Directions Peak Acc. at bedrock (gal) Amplitude (F /F1) Dam Axial Direction Dam Axial Direction Peak Acc. at bedrock (gal) Amplitude (F /F1) Vertical Direction Vertical Direction Peak Acc. at bedrock (gal) Amplitude (F /F1) Fig (7) Comparison of Peak Acceleration in dam foundation, abutments, and free rock surface (Earthfill dams) (1) Fig (8) Comparison of Peak Acceleration in dam foundation, abutments, and free rock surface (Earthfill dams) (2) 10

11 2.2.2 Distance attenuation characteristics of acceleration on dam foundations Notable earthquakes, from which seismic records were observed in the foundations of 5 or more dams, were selected. All earthquakes selected occurred in 2000 or more recently. Results are shown in Table (1), below. Relationships between observed values and hypocenter distances were graphed for observed values in dam foundations and observed values for surface and sub-surface soil (equivalent to foundation in engineering terms) obtained through kik-net (a strong-motion seismograph network by the National Institute for Earth Science and Disaster Prevention, JAPAN, Distance attenuation characteristics in dam foundations and ground were then compared. Furthermore, the target is set as accelerations observed on the ground surface of 10gal or higher for the kik-net data and additional information is provided on the observation point (V S, depth of point observed underground). Also note that hypocenter distances as shown in the graphs below are defined in Fig (1). Table (1) List of Earthquakes No. Time Mj *1) Name of Earthquake Number of dam EQ. Type *2) Depth of Hypocenter (km) Western Tottori Pref. 23 A Geiyo 8 α(c) Miyagi North 15 α(d) Tokachi-oki 17 B Mid Niigata Pref. 11 A Kyushu North West 24 A Noto Hanto 16 A Niigataken Chuetsu-oki 15 A Iwate-Miyagi Nairiku 24 A *3) Pacific Coast of Tohoku 57 B Niigata Chubu 6 A Miyagi East 18 α(c) Fukushima East 5 A 6.4 *1 Mj is a magnitude of the earthquake defined by the Japan Meteorological Agency. *2 A refers to inland earthquakes, B refers to interplate earthquakes, and α refers to intraplate earthquakes. In addition, C refers to intraplate earthquakes with a hypocenter depth of less than 60km are shown, and D refers to those with a depth of more than 60km are shown. *3 The magnitude of the earthquake of the Pacific Ocean off the coast of the Tohoku region is shown in a moment magnitude of M W. 11

12 Epicentral Distance Observed Point Hypocenter Depth Hyposentral Distance (depth considered) Fault Plane Fig (1) Hypocenter Distance Definitions The relationship between maximum acceleration values in dam foundations and soil (surface/underground), and hypocenter distance are shown in Fig (2) through Fig (14). Each figure shows earthquakes by type, and the numbers are consistent with Table-1. Observed values for dam foundations are categorized under either concrete dams or fill dams (earthfill dams and rockfill dams). Furthermore, average elastic wave velocity V S values (km/s) are notated for all data. For observed values in underground, average depth D(m) is notated alongside average elastic wave velocity V S '. 12

13 [Type A] Observed Values Observed Values Hypocentral Distance (km) Hypocentral Distance (km) Fig (2) No. 1 Observed values from the 2000 Tottori Seibu earthquake (V S =0.2km/s, V S =2.0km/s, D=190m) Fig (4) No. 6 Observed values from the 2005 Fukuoka earthquake (V S =0.3km/s, V S =1.9km/s, D=160m) Observed Values Observed Values Hypocentral Distance (km) Hypocentral Distance (km) Fig (3) No. 5 Observed values from the 2004 Chuetsu earthquakes (V S =0.2km/s, V S =1.5km/s, D=320m) Fig (5) No. 7 Observed values from the 2007 Noto earthquake (V S =0.2km/s, V S =1.5km/s, D=200m) 13

14 Observed Values Observed Values Hypocentral Distance (km) Hypocentral Distance (km) Fig (6) No. 8 Observed values from the 2007 Chuetsu offshore earthquake (V S =0.2km/s, V S =1.4km/s, D=270m) Fig (8) No. 11 Observed values from the 1983 Niigata Chubu earthquake (V S =0.2km/s, V S =1.5km/s, D=290m) Observed Values Observed Values Hypocentral Distance (km) Hypocentral Distance (km) Fig (7) No. 9 Observed values from the 2008 Iwate-Miyagi Nairiku earthquake (V S =0.2km/s, V S =1.5km/s, D=170m) Fig (9) No. 13 Observed values from the April 2011 Fukushima earthquake (V S =0.2km/s, V S =1.6km/s, D=380m) 14

15 [Type B] [Type α] Observed Values Observed Values Hypocentral Distance (km) Hypocentral Distance (km) Fig (10) No. 4 Observed values from the 2003 Hokkaido earthquake (V S =0.2km/s, V S =1.3km/s, D=160m) Fig (12) No. 2 Observed values from the 2001 Geiyo earthquake (V S =0.3km/s, V S =2.0km/s, D=140m) Observed Values Observed Values Hypocentral Distance (km) Hypocentral Distance (km) Fig (11) No. 10 Observed values from the 2011 Tohoku earthquake (V S =0.2km/s, V S =1.4km/s, D=260m) Fig (13) No. 3 Observed values from the 2003 Miyagi earthquake (V S =0.2km/s, V S =1.4km/s, D=260m) 15

16 Observed Values Hypocentral Distance (km) Fig (14) No. 12 Observed values from the 2005 Miyagi earthquake (V S =0.2km/s, V S =1.4km/s, D=280m) Generally, the following conclusions can be drawn from Fig (2) through Fig (14). -Distance attenuation characteristics can be seen between hypocenter distance and maximum acceleration for both dam foundations and soil (surface/underground). -Traditionally, dam foundation data is plotted as a center point between surface and underground data. However, in these results, dam foundation data generally overlaps with underground data. -No clear differences can be seen in distance attenuation characteristics between the type of dam (concrete dams, fill dams). 16

17 2.3 Response amplification ratio Numbers of relevant observed records are shown in Table (1). Response ratios of accelerations were calculated using dam crest maximum acceleration (T) divided with foundation maximum acceleration (F). Correlation figures for stream direction are shown below. Gravity dams: Fig (1) through (3) Arch dams: Fig (1) through (3) Rockfill dams: Fig (1) through (3) Earth dams: Fig (1) through (3) For concrete gravity dams, a somewhat larger response ratio was calculated as dam height rose. The presence of spillway facilities on many tall concrete gravity dam crests could be a factor of influence on amplification. Table-2.3-(1) Relevant earthquake observational records Gravity Arch Rockfill Earth Stream 190 sets 35 sets 175 sets 98 sets Directions Dam Axial 198 sets 34 sets 175 sets 98 sets Direction Vertical Direction 180 sets 34 sets 175 sets 98 sets On correlation diagrams that use maximum foundation acceleration and dam width/height as a horizontal axis, examples were set for heights of 50m and under, over 50m and under 100m, and 100m and over. Then, the influence of maximum foundation acceleration on dam amplification ratio was examined. As earthfill dams over 50m are rare, examples were set with closer parameters for 30m and under, over 30m and under 50m, and 50m and over. On correlation diagrams that use dam height as a horizontal axis, examples were set for maximum foundation accelerations under 50gal, over 50gal and under 100gal and 100gal and over. For concrete gravity dams, rockfill dams, and earthfill dams, a tendency for response ratio to decline as maximum foundation acceleration grows can be seen. There are few observed earthquake records for arch dams, and as maximum foundation accelerations are within the 100gal range, correlation between maximum foundation acceleration and response ratio is unclear. 17

18 (1) Concrete gravity dam (2) Arch dam Response Amplitude Ratio Under 50m 50m+, under 100m Response Amplitude Ratio 50m+, under 100m Foundation Peak Acceleration Fig (1) Relationship between maximum foundation acceleration and response ratio (Concrete gravity dam, stream directions) Foundation Peak Acceleration Fig (1) Relationship between maximum foundation acceleration and response amplification ratio (Arch dam, stream directions) Response Amplitude Ratio Under 50gal 50gal+, under 100gal Response Amplitude Ratio Under 50gal 50gal+, under 100gal Dam Height (m) Fig (2) Relationship between dam height and response amplification ratio (Concrete gravity dam, stream directions) Dam Height (m) Fig (2) Relationship between dam height and response amplification ratio (Arch dam, stream directions) Response Amplitude Ratio Under 50m 50m+, under 100m Response Amplitude Ratio 50m+, under 100m Dam Width / Dam Height Dam Width / Dam Height Fig (3) Relationship between dam height/width and response amplification ratio (Concrete gravity dam, stream directions) Fig (3) Relationship between dam height/width and response amplification ratio (Arch dam, stream directions) 18

19 (3) Rockfill dam (4) Earth dam Response Amplitude Ratio Under 50m 50m+, under 100m 100m+ Response Amplitude Ratio Under 30m 30m+, under 50m 50 + Foundation Peak Acceleration Fig (1) Relationship between maximum foundation acceleration and response amplification ratio (Rockfill dam, stream direction) Foundation Peak Acceleration Fig (1) Relationship between maximum foundation acceleration and response amplification ratio (Earth dam, stream directions) Response Amplitude Ratio Under 50gal 50gal+, under 100gal 100gal+ Response Amplitude Ratio Under 50gal 50gal+, under 100gal 100gal+ Dam Height (ml) Fig (2) Relationship between dam height and response amplification ratio (Rockfill dam, stream direction) Dam Height (ml) Fig (2) Relationship between dam height and response amplification ratio (Earth dam, stream directions) Response Amplitude Ratio Under 50m 50m+, under 100m 100m+ Response Amplitude Ratio Under 30m 30m+, under 50m 50m+ Dam Width / Dam Height Fig (3) Relationship between dam height/width and response amplification ratio (Rockfill dam, stream direction) Dam Width / Dam Height Fig (3) Relationship between dam height/width and response amplification ratio (Earth dam, stream directions) 19

20 2.4 Natural Period Characteristics Dam Height and Natural Period In order to find 1st natural periods of dams, transfer functions between dam foundations and dam crests have been calculated for stream, dam axis, and vertical directions. Numbers of relevant earthquake records are as shown in Table (1). Discrepancies in the number of records for stream, dam axis, and vertical items are due to inability to determine transfer function peaks and other reasons. Table (1) Relevant Earthquake Records Gravity Arch Rockfill Earthfill Stream 189 sets 35 sets 174 sets 93 sets Directions Dam Axial 197 sets 34 sets 173 sets 97 sets Direction Vertical Direction 179 sets 34 sets 174 sets 94 sets natural period are generally proportionally related. Additionally, as the records collected newly include some showing comparatively high accelerations in dam foundations, it is clear that the regression formula produced has a somewhat longer period than previous one. Dispersion can be seen in each correlation when focusing on individual dam types. It is thought that this represents strong influence from seismometer locations in concrete dams (for example, those tower structures on crests), dam body physical properties and water levels, and non-linear responses in rockfill and earthfill dams. Processing to nullify these factors would be desirable. But here, said processing has not been carried out, and data analysis is left in a simple state. Correlation diagram figure numbers for stream direction are shown below. In these figures, new regression formula that passes through the origin, average values and average values ±σ (standard deviation), have both been graphed. And in same figures, the previous regression formula (Matsumoto et al. 2005) has also been shown. All figures show the regression new formula used in this case with thick lines, and shows the previous formula (Matsumoto, 2005) using fine lines. -Gravity dams Fig (1) -Arch dams Fig (2) -Rockfill dams Fig (3) -Earthfill dams Fig (4) A summary of regression formulas concerning dam height and initial natural periods is shown in Table (2). An overview of these items shows that in accordance with previous research findings, dam height and initial 20

21 Table (2) Dam height and initial natural period Field Stream direction correlation diagram Regression Formula Prior 1st Natural Period Gravity Dam axial direction New Dam Height (m) Fig (1) Relationship between dam height and Vertical initial natural period (Gravity dams: Stream Directions) direction Arch Stream direction Dam axial direction Prior New 1st Natural Period Dam Height (m) Vertical Fig (2) Relationship between dam height and direction initial natural period (Arch dams: Stream Directions) Stream Prior Rockfill direction Dam axial direction New T H T H st Natural Period Vertical direction T H Dam Height (m) Fig (3) Relationship between dam height and Stream Prior initial natural period (Rockfill dams: Stream Directions) Earthfill direction Dam axial direction Vertical direction New T H T H T H st Natural Period *1) T represents initial natural period (seconds). H represents dam height (m). *2) Represents prior work according to Matsumoto et al (Analysis of Strong Motions Recorded at Dams during Earthquakes, 73 rd Annual Meeting of ICOLD) Dam Height (m) Fig (4) Relationship between dam height and initial natural period (Earthfill dams: Stream Directions) 21

22 2.4.2 Dam Length and Natural Period Numbers of relevant earthquake records are as shown in Table (3). Table (3) Relevant Earthquake Records Gravity Arch Rockfill Earth Stream sets 174 sets 93 sets Directions sets Dam Axial sets 174 sets 97 sets Direction sets Vertical sets 175 sets 96 sets Direction sets The correlation of dam width / dam height and 1st natural period / dam height for stream direction are shown for gravity dams in Fig (1), arch dams in Fig (2), rockfill dams in Fig (3), earth dams in Fig (4) for stream direction. Here, the dam width is a dam length at crest elevation. When viewing values for 1st natural period / dam height by dam type, those for concrete gravity dams and arch dams remain stable even when width at crest changes. In contrast, rockfill dams and earth dams show dispersal in results even for dams with similar widths at crest. When viewed according to dam types, there is no clear tendency for initial natural period concrete gravity dams and arch dams. When focusing on dams with a value of less than 10 for dam width / dam height, rockfill and earthfill dams show tendencies to increase as width at crest increases. For dams with a value of 10 or more for dam width / dam height, because of a lack of relevant locations heavily influences results, specific correlations are unclear. Furthermore, when focusing on dams for which records of multiple earthquakes are available, the same dams show dispersal in initial natural periods. As a result, it can be inferred that this has an influence on seismic motion level and other factors. Fig (1) Relationship between dam width at crest and natural period (Concrete gravity dam, stream directions) Fig (2) Relationship between dam width at crest and natural period (Arch dam, stream directions) Dam Width at Crest / Dam Height Fig (3) Relationship between dam width at crest and natural period (Rockfill dam, stream direction) Dam Width at Crest / Dam Height Dam Width at Crest / Dam Height Dam Width at Crest / Dam Height Fig (4) Relationship between dam width at crest and natural period (Earthfill dam, stream directions) 22

23 2.4.3 Peak Acceleration Level and Natural Period Using dams with 4 or more observed earthquake records, correlation diagrams were created by dam type for peak accelerations and natural periods. Correlation diagram vertical axes use natural period (s) / dam height (m) in order to exclude individual dam height differences from comparisons. Horizontal axes show seismic motion levels in over three types: peak foundation acceleration, peak crest acceleration, and response amplification ratio. a) Concrete gravity dams Relevant concrete gravity dam observed earthquake records are as shown in Table (1). In these records, the 528.5gal foundation acceleration and 2,051 crest acceleration observed at Kasho dam are the highest input seismic levels. Correlation diagrams are shown in Fig (1) through (3). Values for when each dam's natural period (s) was divided by dam height (m) are generally at the same level. Significant changes to natural periods with increases of input seismic motion levels were not observed, and no clear correlation can be identified between input seismic motion level and natural period. For maximum acceleration observations at Kasho dam, as acceleration rose natural period showed a tendency to change toward long periods, although changes were minor. (Fig (1)). Table (1) Relevant earthquake observed records (Gravity) Dam Height Number of Records name (m) Stream Dam Axis Vertical Direction Directions Direction Tase Miharu Tsuruda Kasho Tagokura Tase Miharu Tsuruda Kasyo Tagokur Foundation Peak Acceleration (gal) Fig (1) Relationship between foundation acceleration and natural period (Concrete gravity dam, stream directions) Tase Miharu Tsuruda Kasyo Tagokur Crest Peak Acceleration (gal) Fig (2) Relationship between dam crest acceleration and natural period (Concrete gravity dam, stream directions) Tase Miharu Tsuruda Kasyo Tagokur Response Amplification Ratio (T/F) Fig (3) Relationship between amplification ratio and natural period (Concrete gravity dam, stream directions) 23

24 b) Arch Dams Relevant arch dam observed earthquake records are as shown in Table (2). Input seismic activity levels are low compared to observational records for other dam types. Maximum foundation accelerations are around the 100gal level. As a result, correlation between input seismic motion levels and natural periods is unclear. Correlation diagrams are shown in Fig (4) through (6). Values for when each dam's natural period (s) was divided by dam height (m) are generally at the same level. Table (2) Relevant earthquake observational records (Arch) Dam Height Number of Records name (m) Stream Direction Dam Axis Directions Vertical Direction Naruko Yagisawa Kurobe Takane No Naruko Tagisawa Kurobe Takane Foundation Peak Acceleration (gal) Fig (4) Relationship between foundation acceleration and natural period (Arch dam, stream directions) Naruko Tagisawa Kurobe Takane Crest Peak Acceleration (gal) Fig (5) Relationship between dam crest acceleration and natural period (Arch dam, stream directions) Naruko Tagisawa Kurobe Takane Response Amplification Ratio (T/F) Fig (6) Relationship between amplification ratio and natural period (Arch dam, stream directions) 24

25 c) Rockfill Dams Relevant rockfill dam observed earthquake records are as shown in Table (3). In these records, the gal foundation acceleration and 525.3gal crest acceleration observed at Aratozawa dam are the highest input seismic activity levels. Correlation diagrams are shown in Fig (7) through (9). The values derived by dividing natural period (s) by dam height (m) for each dam are relatively large for horizontal direction elements in Tadami and Yamamoto No.2. As acceleration rises natural period show a tendency to change toward long periods, relation with response amplification ratio is, however, unclear Table (3) Relevant earthquake observational records (Rockfill dams) Dam Height Number of Records name (m) Stream Direction Dam Axis Directions Vertical Direction Kuzuryu Izarigawa Tarumizu Aratozawa Miho Makio Kisenyama Kuttari Tadami Yamamoto No Fig (7) Relationship between foundation acceleration and natural period (Rockfill dam, stream direction) Fig (8) Relationship between dam crest acceleration and natural period (Rockfill dam, stream direction) Foundation Peak Acceleration (gal) Crest Peak Acceleration (gal) Yamamoto No.2 Kuzuryu Tadami Izarigawa Kuttari Kisenyama Makio Miho Aratozawa Tarumizu Yamamoto No.2 Kuzuryu Tadami Izarigawa Kuttari Kisenyama Makio Miho Aratozawa Tarumizu Yamamoto No.2 Kuzuryu Tadami Izarigawa Kuttari Kisenyama Makio Miho Aratozawa Tarumizu Response Amplification Ratio (T/F) Fig (9) Relationship between amplification ratio and natural period (Rockfill dam, stream direction) 25

26 d) Earthfill dams Relevant earthfill dam observed earthquake records are as shown in Table (4). In these records, the 620.5gal foundation acceleration and 568.7gal crest acceleration observed at Kawanishi dam are the highest seismic motion levels. Correlation diagrams are shown in Fig (10) through (12). The values of natural period (s) divided by dam Yamamoto Asagawara Ono Togane Nagara Kawanishi Kejunuma Minamishio height (m) for each dam are highest for horizontal stream elements in Asagawara and Togane dam. As acceleration rises natural period show a tendency to change toward long periods, relation with response amplification ratio is, however, unclear. Foundation Peak Acceleration (gal) Fig (10) Relationship between foundation acceleration and natural period (Earthfill dam, stream directions) Table (4) Relevant earthquake observational records (Earthfill dams) Dam Height Number of Records name (m) Stream Direction Dam Axis Directions Vertical Direction Minamishio Kejonuma Kawanishi Nagara Togane Ono Asagawara Yamamoto Asagawara Ono Togane Nagara Kawanishi Kejunuma Minamishio Crest Peak Acceleration (gal) Fig (11) Relationship between dam crest acceleration and natural period (Earthfill dam, stream directions) Yamamoto Yamamoto Ono Nagara Kejunuma Asagawara Togane Kawanishi Minamishio Response Amplification Ratio (T/F) Fig (12) Relationship between amplification ratio and natural period (Earthfill dam, stream directions) 26

27 2.5 Damping Characteristics Using the Fourier spectra of stream direction record and initial natural frequencies calculated thus far, each dam's damping ratio was calculated using the half power method. The 27 selected dams, each with 4 or more earthquake records, are shown in Table-2.5-(1). An overview diagram of the half power method is shown in Fig.-2.5-(1). Calculation methods for damping ratios using the half power method are as shown below. (a) Read peak value (G max ) for a Fourier spectrum comparison of initial natural frequency (f n ). (b) Divide the G max in (a) by the square root of 2. (c) Read the frequencies f 1 and f 2 which will corresponds with the Fourier spectral ratio equals to the value obtained in (b). However, let f 1 < f 2. (d) Calculate damping ratios using the equation (2.5-1). Gravity Arch Rockfill Earthfill h f f 2 1 (2.5-1) 2 f f 1 Table-2.5-(1) Name of Dams Considered Tase Miharu Tsuruda Kasho Tagokura Narugo Kurobe Yagisawa Takane No.1 Kuzuryu Izarigawa Tarumizu Aratozawa Miho Makio Kisenyama Kuttari Tadami Yamamoto No.2 Minamishio Kejonuma Kawanishi Nagara Togane Ohno Asagawara Yamamoto Fourier Amplitude Frequency Fig.-2.5-(1) Overview Diagram for Half Power Method A 0.5Hz range Parzen window was used for concrete gravity and arch dams, and a 0.2Hz range Parzen window was used for rockfill and earthfill dams. This should be kept in mind, as Parzen window range has major influence on damping constant values. Additionally, in considering damping ratios, for the case in which there were multiple Fourier ratio peaks between frequencies f 1 and f 2. the damping could be not calculated by the abovementioned method. 27

28 Fig.-2.5-(2) shows relationships between dam height and damping ratio as calculated by the half power method, according to dam type. By dam type, tendencies for damping ratios to grow or alternately decline as dam height rises are somewhat visible. However, dispersion for calculated damping ratios is also high, and a clear relationship cannot be found between dam height and damping ratios. In Fig.-2.5-(3), relationships between maximum foundation acceleration and damping ratios according to the half power method are shown by dam type. Similarly, in Fig.-2.5-(4), relationships between maximum foundation acceleration by dam type and damping ratios according to the half power method are show, for maximum foundation accelerations up to 200gal. On viewing Fig.-2.5-(3) and Fig.-2.5-(4) on concrete gravity dams, Tagokura dam's damping constant tends to be larger than that of other concrete gravity dams. This is thought to be partially due to the gentle curve of Fourier spectrum near in 1st natural. For other concrete gravity dams, this is within a 1-7% range, and no tendencies dependent upon maximum acceleration could be acknowledged. On viewing Fig.-2.5-(3) and Fig.-2.5-(4) on arch dams, Naruko dam was at 6-10%, Yagisawa dam at 7-11%, Kurobe dam at 6-12%, and Takane Daiichi Dam at 4-6%. The 4 arch dams considered had maximum foundation accelerations of 128gal, lower than for other dam types, and no correlations between maximum acceleration and damping ratios could be acknowledged. On viewing Fig.-2.5-(3) and Fig.-2.5-(4) on rockfill dams, all are generally distributed in the 2-9% range, and no correlation with maximum foundation acceleration can be acknowledged. On viewing Fig.-2.5-(3) and Fig.-2.5-(4) on earthfill dams, all are generally distributed in the 1-10% range, and no correlation with maximum foundation acceleration can be acknowledged. For damping ratios calculated using the half power method, the selection of smoothing processing parameters, such as Parzen window width, must be carefully considered because of large effects on the calculated damping value. Damping Ratio Damping Ratio Damping Ratio Damping Ratio Concrete Gravity Dam Height (m) Arch Dam Height (m) Rockfill Dam Height (m) Earthfill Dam Height (m) Fig.-2.5-(2) Relationships between dam height by dam type and damping ratios according to the half power method 28

29 Damping Ratio (%) Concrete Gravity Tagokura Tase Miharu Tsuruda Kasyo Damping Ratio (%) Concrete Gravity Tagokura Tase Miharu Tsuruda Kasyo Foundation Peak Acceleration (gal) Foundation Peak Acceleration (gal) Yamamoto Aratozawa Kuzuryu Damping Ratio (%) Arch Narugo Yagisawa Kurobe Takane No.1 Damping Ratio (%) No.2 Izarigawa Taruzimu Tadami Miho Kisenyama Rockfill Foundation Peak Acceleration (gal) Foundation Peak Acceleration (gal) Rockfill Earthfill Minamishio Kejonuma Damping Ratio (%) Yamamoto No.2 Aratozawa Kuzuryu Izarigawa Taruzimu Miho Makio Tadami Kisenyama Damping Ratio (%) Kawanishi Togane Asagawara Nagara Ohno Yamamoto Foundation Peak Acceleration (gal) Foundation Peak Acceleration (gal) Damping Ratio (%) Earthfill Minamishio Kawanishi Togane Asagawara Kejonuma Nagara Ohno Yamamoto Fig.-2.5-(4) Relationships between maximum foundation acceleration by dam type and damping ratios according to the half power method (maximum foundation accelerations up to 200gal displayed) Foundation Peak Acceleration (gal) Fig.-2.5-(3) Relationships between maximum foundation acceleration by dam type and damping ratios according to the half power method 29

30 3. Summary Thus far, this report has addressed analysis results for seismic activity characteristics, dam response amplification ratios, and period characteristics. In summary, the primary results were as shown below. -Seismic motion characteristics -Maximum acceleration (F') in bedrock outside the dam body was compared against the same in the foundation (F1), and generally increasing tendencies were confirmed in all directions. -Distance attenuation characteristics can be seen between hypocenter distance and maximum acceleration for both dam foundations and soil (surface/underground). Also, dam foundation data is plotted as a center point between surface and underground data. However, in these results, dam foundation data generally overlaps with underground data. - Response amplification ratios -For concrete gravity dams, rockfill dams, and earthfill dams, a tendency for response ratio to decline as maximum foundation acceleration grows can be seen. -There are few observational earthquake records for arch dams, and correlations between maximum foundation accelerations and response amplification ratios were unclear. -Natural period characteristics -In accordance with previous research findings, dam height and initial natural period are generally proportionally related. -When viewing values for 1st natural period / dam height, those for concrete gravity dams and arch dams remain stable even when width at crest changes. In contrast, rockfill dams and earthfill dams show dispersal in results even for dams with similar widths at crest. -In concrete dams, no marked changed in natural period could be seen accompanying increases in input seismic motion level. However, in rockfill and earthfill dams, some changes to natural period can be seen. -Damping ratios - Damping was estimated using the half power method. Clear correlations between maximum acceleration and damping ratios could not be found. The above concludes a summary of the initial processing analysis results of dam earthquake observational records by JCOLD committee. 4. Conclusion Seismic observation on dams in Japan collected by JCOLD is unprecedented. Analysis results of ground motion and dam response using data observed at dams are thought to be extremely informative and highly valuable. Nevertheless, examples of observational data for strong seismic forces of over 300gal in dam foundations are by no means common and we believe it is vital that observation of seismic activity and analysis of data obtained continue, in order to advance the evaluation of performance of existing dams during. 30