Читать книгу Earthquake Engineering for Concrete Dams - Anil K. Chopra - Страница 12

As far as can be determined, no large concrete dam with full reservoir has been subjected to extremely intense ground shaking. The closest to such an event was the experience at Koyna (gravity) Dam (Figure 1.1.1), with the reservoir nearly full, during the 1967 earthquake (Chopra and Chakrabarti 1973). Ground accelerations recorded at the dam site during a nearby earthquake of magnitude 6.5 had a peak value of 0.38 g in the stream direction and strong shaking lasted for 4 sec. Significant horizontal cracking occurred through a number of taller non‐overflow monoliths at or near the elevation where the downstream face changes slope; however, the dam continued to retain the reservoir even though the water level was 25 m above the cracks (Figure 1.1.2). A similar experience had occurred in 1962 at Hsinfengkiang (buttress) Dam (Figure 1.1.3) during a magnitude 6.1 earthquake in close proximity. Although not recorded, ground motions were probably quite intense causing cracking at 16 m below the crest (Figure 1.1.4); the dam continued to retain the reservoir though the water level was 3 m above the cracks.

A few dams have withstood very intense ground shaking with little or no damage because of their unusual design or low water level. Perhaps the strongest shaking experienced by a concrete dam to date was that at Lower Crystal Springs Dam, a 42‐m‐high curved gravity structure (Figure 1.1.5) dam with nearly full reservoir, located within 350 m of the San Andreas fault that caused the magnitude 7.9 1906 San Francisco earthquake. Built with interlocking concrete blocks, the dam was undamaged, even though its reservoir was full. However, the earthquake resistance of this dam greatly exceeds that of typical gravity dams due to its curved plan and a cross section that was designed thicker than normal in anticipation of future heightening, which was never completed; a section view of this dam is shown in Figure 1.1.6.

Figure 1.1.1 Koyna Dam, India, constructed during 1954–1963; this dam is 103 m high and 853 m long.

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.