Introduction

Dams are constructed to retain or store water. To minimize the flow of water through the dam/foundation system special impervious zones or elements must be designed and constructed. Dams constructed of concrete can be considered practically impervious, except for possible leaky joints. Embankment or fill dams require a zone of low permeability soil, asphalt, or concrete, which can be placed either in the interior of the dam or with the latter two materials also on the upstream face. Flow of water through the foundation below the dam is controlled by the prevailing geological conditions. Seepage through pervious strata (alluvial deposits, residual soils, etc) can be controlled by a barrier or cutoff consisting of a sequence of impervious elements (piles, panels) reaching down to a stratum of much lower permeability, usually rock. Seepage through rock is usually controlled by single or multiple row grout curtains. Rock, however, can also be highly pervious, for example in the case of karst, which may reach to great depth, or when rock is intensely broken or crushed in regions of high tectonic stresses. For such cases cutoffs may be more appropriate than grout curtains. Grout curtains have been constructed in overburden materials using the so-called sleeve pipe (or tube-à-manchette) method. The largest grout curtain of this kind was constructed for the foundation treatment of the High Aswan Dam in Egypt (Shalaby, 1991). The curtain is up to 193 m deep, 40 m wide at the top and 20 m at the base. It consists of 15 rows at the top which are telescopically reduced to 8 rows in the lower part. The curtain has performed satisfactorily and no repair work has been required as of 2009. Installing grout curtains in alluvial deposits, however, can require more time than the construction of a cutoff. Moreover, the residual risk of some imperfections is considerably higher than with a cutoff. Today, diaphragm wall cutoffs with depths of 50 to 60 m can be constructed at rates of up to 250 to 300 m² per day (e.g. Naga Hammadi, Egypt).