INTRODUCTION

lzarigawa Dam is a 45.5-m-high rockfill dam with a central impervious earth core which was constructed at the creek of Izarigawa, a tributary of the Ishikari river. It was completed in August 1979 by the Hokkaido Development Bureau of Japanese Government. The multipurpose dam provides for flood Control, municipal water, and irrigation. The rock forming the foundation of Izarigawa Dam is pumice flow deposit resulted from the Shikotsu volcanic activity of the late Pleistocene. The foundation is composed of two kinds of rocks --- welded one and non- welded one. Although the welded rock itself can be regarded as almost impermeable, the rock mass shows very high permeability because of an abundance of columnar and Platy joints caused by cooling. On the Contrary the nonwelded rock shows high permeability because of its porous nature and large void ratio.

This paper describes the investigations Of the permeability, the design and grputing for seepage control for this foundation, and the behavior after impounding.

GEOLOGY OF DAM FOUNDATION

The geologic profile of the damsite is shown in figure 1. The lithofacies and their geotechnical properties are shown in table 1.

(Figure in full paper)

The Neogene sedimentary rock (PNo) which can be regarded as impermeable underlies the present riverbed, almost horizontally at a depth of 50 meter. PNo is overlain by the older terrace deposit (Gv), which is overlain by the Shikotsu pyroclastics of about 150 meter thickness. The intervals of eruptions were estimated to be so short that each unit was influenced by the overlying flow unit and the change of each flow unit is gradual with an indistinct boundary. The Shikotsu pyroclastics are composed of welded tuff (Wt) and pumiceous tuff (Ps). Around the damsite Ps comprises most of the foundation underneath the riverbed. Although Ps itself is supposed to be a combination of several flow units, it is treated as a single element because there is not distinct boundaries, having the same constituents, and each flow unit has almost the same geotechnical properties. Wt which forms the abutments above the riverbed is subdivided into poorly welded tuff (Wtl),densely welded tuff (Wtml),extremely densely welded tuff (Wtm2),and nonwelded tuff (Wtu) from the oldest unit according to the degree of welding. Each unit is deposited almost horizontally with obscure interfaces. The dam crest is located at the upper part of Wtm2' Another nonwelded pumiceous tuff overlies Wt almost horizontally.

(Table in full paper)

The rocks which raises problems from the point of the seepage control of foundation are Gv, Ps and Wt. Among them, Gv is most permeable. However, since firstly it exists at a depth of 40 meter from the riverbed at the location of dam axis, secondly Ps underlain by Gv is expected to function as a blanket for Gv, and thirdly the depth to Gv from the riverbed increases in the downstream because the dip of Gv is slightly steeper than the inclination of the riverbed, Gv can be excluded from the kind of rocks which have engineering problems.

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