The photograph shows part of the foundation of the apron downstream of Victoria Dam, Sri Lanka. It is taken looking diagonally upstream; the river flows from left to right behind the gabion wall (a) and part of the main dam foundation may be seen in the top left of the photograph (b). Sketches 1 to 3 show the location and layout of the site.

The weathered band dipped irregularly from (c) towards the gabion wall, below which, at a depth of some 15 m, it died out. The band was thickest at (c) where 3 m of weathered limestone was removed by surface excavation, forming faces (c) and (d). Beyond (d) the band became thinner and was partially removed by a mining operation, visible in the photograph (e).

The general geology at Victoria is summarized in the caption to the cover photograph on this issue. Although the weathered band is actually a marble the local terminology referring to it as a limestone has been retained. This particular band consisted of coarsely crystalline micaceous limestone within which were irregular masses of mica-schist. It was extensively weathered, the limestone weathering to a sensitive soft clay and the mica-schist to a stiff micaceous clay. The weathering products retained the original rock structure.

The apron is required to withstand the impact of water plunging from the dam crest gates approximately 100 m above, and it was clear that the majority of the weathered band would require treatment to provide a suitable foundation. Programme constraints

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