Earthquake Engineering for Concrete Dams. Anil K. Chopra

Figure 1.1.2 Cross section of Koyna Dam showing water level during 1967 earthquake and regions where principal cracking at the upstream and downstream faces was observed.

      Source: Adapted from National Research Council (1990).

Figure 1.1.3 Hsinfengkiang Dam, China. Completed in 1959, this dam is 105 m high and 440 m long.

Figure 1.1.4 Cracking in Hsinfengkiang Dam, China, due to earthquake on March 19, 1962.

      Source: Adapted from Nuss et al. (2014).

Figure 1.1.5 Lower Crystal Springs Dam, California, USA. Built in 1888, this 45‐m‐high curved‐gravity dam is located within 350 m of the San Andreas Fault, which is under the reservoir, oriented roughly parallel to the dam.

Figure 1.1.6 Section view of the Lower Crystal Springs Dam (adapted from Nuss et al. [2014] and Wieland et al. [2004]).

Another example of a concrete dam subjected to very intense shaking is the 113‐m‐high Pacoima (arch) Dam (Figure 1.1.7). During the 1971 magnitude 6.6 San Fernando earthquake, an accelerograph located on the left abutment ridge recorded a peak acceleration of 1.2 g in both horizontal components and 0.7 g vertical, with strong shaking lasting for 8 sec, suggesting that the excitation at the dam–foundation^‡ interface – which was not recorded – must have been very intense. However, the only visible damage to the dam was a ⅜ in. opening of the contraction joint on the left thrust block and a crack in the thrust block. During the 1994 magnitude 6.7 Northridge earthquake, peak accelerations recorded ranged from 0.5 g at the base of the dam to about 2.0 g along the abutments near the crest. The damage sustained was more severe than in 1971. The contraction joint between the dam and the thrust block in the left abutment again opened, this time by 2 in. at the crest level (Figure 1.1.8), decreasing to ¼ in. at the bottom of the joint (60 ft below the crest), at which point a large crack extended down diagonally through the lower part of the thrust block to meet the foundation (Figure 1.1.9). The good performance of the dam can be attributed primarily to the low water level – 45 m below the dam crest – at the time of both earthquakes. Additional information is available in Scott et al. (1995).

Figure 1.1.7 Pacoima Dam, California, USA. Completed in 1929, this dam is 113 m high and 180 m long at the crest.

Figure 1.1.8 Two‐inch separation between Pacoima Dam Arch (left) and the thrust block (right) on the left abutment (Scott et al. 1995).

Figure 1.1.9 Crack at the joint between the Pacoima Dam arch and the thrust block and diagonal crack in the thrust block (Scott et al. 1995).

Figure 1.1.10 Shih‐Kang Dam, Taiwan, (a) before and after the Chi‐Chi earthquake; (b) close‐up of damaged bays. Completed in 1977, this gated spillway is 21 m high and 357 m long.

      (a) Two photos courtesy of C.‐H. Loh, National Taiwan University, Taiwan.

      (b) Photo courtesy of USSD.org.

Shih‐Kang Dam in Taiwan (Figure 1.1.10) – a 70‐ft (21.4‐m)‐high, 18‐bay gated spillway – located directly over a branch of the Che‐Lung‐Pu fault that caused the 1999 magnitude 7.6 Chi‐Chi earthquake represents the first known dam failure during an earthquake. However, this failure was caused primarily by fault rupture, not ground shaking, although it was very intense, as indicated by the peak ground acceleration of 0.5 g recorded at a location 500 m from the dam. During the Chi‐Chi earthquake the branch fault ruptured, with a vertical offset of 29 ft (9 m) and a horizontal offset diagonal to the dam axis of about 23 ft (7 m). As a result, bays 16–18 incurred extensive damage, but the damage to the other bays was surprisingly little; spillway piers sustained cracking, simply supported bridge girders came off their bearings, and six gates were inoperable after the earthquake.

      It is clear from the preceding observations that concrete dams can be significantly damaged by ground shaking due to earthquakes. They are not as immune to damage as had commonly been presumed prior to the 1967 experience at Koyna Dam. This fact is now universally recognized, and there is much interest in the earthquake performance of concrete dams.

1.2 COMPLEXITY OF THE PROBLEM

The ability to evaluate the effects of earthquake ground motion on concrete dams is essential in order to assess the safety of existing dams, to determine the adequacy of modifications planned to improve existing dams, and to evaluate proposed designs for new dams to be constructed. However, the prediction of performance of concrete dams during earthquakes is one of the most complex and challenging problems in structural dynamics because of the following factors:

      1 Dams and the impounded reservoirs^† are of complicated shapes, as dictated by the topography of the site (see Figures 1.2.1 and 1.2.2).

      2 The response of a dam is influenced greatly by the interaction of the motions of the dam with the impounded water and the foundation, both of which extend to large distances. Thus the mass, stiffness, material damping, radiation damping of the foundation (see Section