Dams constructed on recent volcanic rocks have many difficult problems on foundations and abutments such as strength, permeability and so on. Authors have been conducting case studies on 25 dams constructed on Neogene and Quaternary volcanic rocks in Kyushu District to find out the most relevant seepage control methods for such dams. Obtained results are as follows.
From a seepage control viewpoint, geological features of studied dams are classified into following six large groups: 1) Lava, 2) Alternation of lava and pyroclastic rocks, 3) Tuff breccia, 4) Densely welded tuff, 5) Partially welded tuff-“Shirasu”, 6) Old river deposits etc. On the other hand, permeability of rock mass estimating from ground water level at abutments is classified into following three large groups: A) Ground water level at abutments is relatively high, B) Ground water level at abutments is relatively low but higher than river bed elevation, C) Ground water level at abutments is lower than river bed elevation.
Considering these six geological features and three permeability characteristics, the most relevant seepage control methods proposed by authors are shown in Table 7. As shown in Table 7 curtain grouting can be applied for a wide variety of geological conditions. Detailed profile of curtain grouting is shown in Table 8. But both Table 7 and 8 are tentative standards at early planning stage, so practical seepage control methods should be selected taking full account of precise geological conditions of each dams and conducting various in situ tests such as grouting test, Lugeon test, pumping test, etc.
As recent volcanic rocks in Kyushu are often distributed laterally, problems on seepage control arise from not only dam sites but also overall storage areas. So it is important to investigate geological conditions of overall storage areas and to point out problems. Then the most relevant seepage control methods should be selected for various geological conditions such as consolidation grouting, limb grouting, branket, concrete cutoff, etc.