【正文】 o facilitate operation of paction equipment and to avoid differential settlement. Cracks, joints, and openings beneath the core and possibly elsewhere (see below) should be filled with mortar or lean concrete according to the width of opening. （2）The excavation of shallow exploration or core trenches by blasting may damage the rock. Where this may occur, exploration trenches are not remended, unless they can be excavated without blasting. Where core trenches disclose cavities, large cracks, and joints, the core trench should be backfilled with concrete to prevent possible erosion of core materials by water seeping through joints or other openings in the rock. （3）Shale foundations should not be permitted to dry out before placing embankment fill, nor should they be permitted to swell prior to fill placement. Consequently, it is desirable to defer removal of the last few feet of shale until just before embankment fill placement begins. （4）Where an earth dam is constructed on a jointed rock foundation, it is essential to prevent embankment fill from entering joints or other openings in the rock. This can be done in the core zone by extending the zone into sound rock and by treating the rock as discussed above. Where movement of shell materials into openings in the rock foundation is possible, joints and other openings should be filled, as discussed, beneath both upstream and downstream shells. An alternative is to provide filter layers between the foundation and the shells of the dam. Such treatment will generally not be necessary beneath shells of rockfill dams. （5）Limestone rock foundation may contain solution cavities and require detailed investigations, special observations when making borings (see EM 111011804), and careful study of aerial photographs, bined with surface reconnaissance to establish if surface sinks are present. However, the absence of surface sinks cannot be accepted as proof that a foundation does not contain solution features. The need for removing soil or deposed rock overlying jointed rock, beneath both upstream and downstream shells, to expose the joints for treatment, should receive detailed study. If joints are not exposed for treatment and are wide, material filling them may be washed from the joints when the reservoir pool rises, or the joint filling material may consolidate. In either case, embankment fill may be carried into the joint, which may result in excessive reservoir seepage or possible piping. This consideration applies to both earth and rockfill dams. （6）Where faults or wide joints occur in the embankment foundation, they should be dug out, cleaned and backfilled with lean concrete, or otherwise treated as previously discussed, to depths of at least three times their widths. This will provide a structural bridge over the fault or jointfilling materials and will prevent the embankment fill from being lost into the joint or fault. In addition, the space beneath the concrete plug should be grouted at various depths by grout holes drilled at an angle to intersect the space. This type of treatment is obviously required beneath cores of earth and rockfill dams and also beneath rockfill shells. c. Abutment treatment. The principal hazards that exist on rock abutments are due to irregularities in the cleaned surfaces and to cracks or fissures in the rock. Cleaned areas of the abutments should include all surfaces beneath the dam with particular attention given to areas in contact with the core and filters. It is good practice to require both a preliminary and final cleanup of these areas. The purpose of the preliminary cleanup is to facilitate inspection to identify areas that require additional preparation and treatment. Within these areas, all irregularities should be removed or trimmed back to form a reasonably uniform slope on the entire abutment. Overhangs must be eliminated by use of concrete backfill beneath the overhang or by barring and wedging to remove the overhanging rock. Concrete backfill may have to be placed by shotcrete, gunite, or similar methods to fill ers beneath overhangs. Vertical rock surfaces beneath the embankment should be avoided or, if permitted, should not be higher than 5 ft, and benches between vertical surfaces should be of such width as to provide a stepped slope parable to the uniform slope on adjacent areas. Relatively flat abutments are desirable to avoid possible tension zones and resultant cracking in the embankment, but this may not be economically possible where abutment slopes are steep. In some cases, however, it may be vertical on 1 horizontal or 1 vertical on 1 horizontal, thereby minimizing the possibility of cracking. Flattening of the abutment slope may reduce the effects of rebound cracking (., stress relief cracking) that may have acpanied the development of steep valley walls. The cost of abutment flattening may be offset by reductions in abutment grouting. The cost of foundation and abutment treatment may be large and should be considered when selecting damsites and type of dam.52. Strengthening the Foundationa. Weak rock. A weak rock foundation requires individual investigation and study, and dams on such foundations usually require flatter slopes. The possibility of artisan pressures developing in stratified rock may require installation of pressure relief wells. Liquefiable soil. Methods for improvement ofliquefiable soil foundation conditions include blasting, vibratory probe, vibropaction, paction piles, heavy tamping (dynamic paction), paction (displacement) grouting, surchargeibuttress, drains, particulate grouting, chemical grouting, pressureinjected lime, electrokinetic injection, jet grouting, mixinplace piles and walls, insitu vitrification, and vibroreplacement stone and sand columns (Ledbetter 1985, Hausmann 1990, Moseley 1993). b. Foundations. Foundations of pressible finegrained soils can be strengthe