 Extensive remedial works were undertaken between 1999 & 2001 to strengthen the dam wall . This work involved removing the top 3 @.@ 8 m ( 12 ft ) of the existing dam wall and drilling / blasting through the dam wall plus up to a further 70 m ( 230 ft ) into the bedrock below . The top section of the wall was then rebuilt using reinforced concrete . Finally , permanent , re @-@ <unk> ground anchors were then installed through the formed and drilled holes from the crest to be stressed and <unk> into the foundation rock . At time of completion , these were the largest capacity and longest permanent ground anchors ever installed in the world . 
