Because the most attractive dam sites have, in many regions, already been used and because some potential sites may be off limits for political or environmental reasons, most of the sites that remain available for the construction of dams require an ever-increasing standard of care in the design of seepage and leakage control measures. In addition to safety considerations, the cost of the stored water has become an increasingly important consideration in the design of measures to reduce the potential for seepage loss. Therefore, it is increasingly important that the design of the grouting program incorporate due consideration of the geologic and hydrologic characteristics of the site and that the best available technology be used. In this regard, the available as-built data from other dam sites in comparable geologic terrain should be reviewed. Nonetheless, caution must be exercised when drawing conclusions from those data; subsurface conditions potentially conducive to seepage loss or structural settlement can vary greatly from one part of a geologic formation to another, so identical conditions are unlikely to be encountered at any two sites within the same geologic formation. Moreover, recognizing that more is likely to be learned about the subsurface geologic conditions at any given site as each hole is drilled and grouted, the project specifications should provide built-in flexibility for the grouting program to be modified appropriately during the course of the work to achieve the most effective results. Test GroutingEspecially where valid data from comparable sites are lacking, it is prudent to conduct a test grouting program at the site before finalizing the designs of the grouting program and of the dam. Among the more important design parameters that can be ascertained only by a test program is the average residual rock mass permeability that can be achieved by grouting. The information gained, in addition to helping refine the grout curtain design and the procedures for its construction, commonly will be a basis for realistically assessing the equipment and labor necessary for completing the work on schedule. The results also will help in estimat-Dam Foundation Grouting Downloaded from ascelibrary.org by New York University on 04/13/15.