Shirakawa Dam is a 66-m-high rockfill dam with a central earth core constructed on sedimentary Neogene rock and its purposes are flood control, irrigation, industrial water supply and hydro-electric development. The crest length is about 350 meters and it has a spillway at the left abutment, as shown in Fig. 1. The cross section of the dam is shown in Fig. 2. The foundation of the dam consists of two types of soft rocks, especially softer rock and relatively harder rock. Moreover many open cracks existed in the rock foundation and treatment by grouting was very difficult. Construction of the dam began in 1971 and it was completed in 1979, meanwhile nearly 150,000 meters of borehole was drilled for the grouting. The reservoir began to be filled in November 1979 and reached its high water level in November 1980. The measurements of the deformation of the dam and the leakage during this period show that there has been no unusual behaviour and that treatment by grouting has been successful.

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The bedrock in the region surrounding the damsite consists of various kinds of sedimentary rocks of the Miocene Neogene, namely alternating sandstone, tuff, sandy tuff, siltstone, etc. shown in Fig. 1 and Fig. 2, and it is covered with deposits of talus, gravel of the river bed and terrace. The thickness of the stratum of each rock ranges from 10 to 50 meters near the dam axis and in parts the mudstone or conglomerate strata have a thickness of about 10 centimeters. This bedrock was created in the Miocene period and sank deeper after that. It was then consolidated under other rocks (with a thickness of about 1000 meters) and rose again to the present level after the Pliocene. For this reason these rocks are consolidated relatively well in Miocene rocks. Fine grained rocks are relatively well cemented but coarse grained rocks are not so well consolidated. The strike of the strata are nearly parallel to the dam axis. The dip is very steep (almost 80°) and the direction of the dip changes at the fault of the river bed. The faults found at the damsite are not so large and the fractured part of the faults are small.


The bedrocks of the damsite are all weakly consolidated as are the soft rocks. These rocks are classified into two groups; in particular softer rock and a relatively harder rock as shown in Table 1 which indicates that the two types of soft rocks exhibit different characteristics. The softer rock (ScS... see Fig. 1 or Table 1) is easily weathered, and the result of the slaking test showed that the unconfined compressive strength of the ScS rock in a wet condition decreased to less than 4 kgf/cm2 after one wetting and subsequent drying(original strength ranges from 14 to 18 kgf/cm2).the distribution of the coefficients of permeability at the section of dam axis is shown in Fig. 3.

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